Why does so much variation exist within species?

Why does so much variation exist within species?

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My last phrasing of this question did not go down well, so I will try again.

The genotype of species is not always the same. If you ask yourself why not all of these possible expressions except one have died out, a natural answer is each genotype occupies a specific niche: For example, different eye colors might be found attractive by different kinds of people. If the the genotype for one eye color became more rare for some reason, other individuals with this genotype had a higher mating chance; the frequency of that genotype would get pushed back up.

But there seem to be genotypes (for example, having zits) that don't occupy any niche at all. Why didn't they die out?

First of all, your assumption is incorrect: those are not genotypes and they do not occupy specific niches. You are talking about phenotypes (eye colour) or in the case of acne, not a trait at all.

I'll answer the question I think you're asking: why does biological variation exist?

Variation is constantly being generated by mutation, errors in the replication of the genome of an individual. If a mutation occurs in the gametic cells, it affects the genotype of the offspring. By affecting the genotype, it may or may not have a discernable effect on the phenotype. Where a gametic mutation isn't fatal, it has a chance of becoming established in the population when that mutant individual breeds. That chance is increased the further along the gradient from harmful to beneficial that effect of that mutation is.

Variation also exists because development responds to environmental factors, not just genetics. Different individuals grow in different environments, and have unique combinations of factors influencing many traits.

The reason variation persists is because, except under conditions of extreme selective pressure or genetic bottlenecks, many traits have no impact on survival or fecundity (reproductive success) or are determined only partly by genetics. Height, for example, only has a very slight impact on fecundity within quite a broad range of heights. It is partly genetically determined, and partly determined by environmental factors such as diet (Eckhardt et al. 2005), exposure to toxins during development, muscular development etc.

If height were the key factor in human mate selection, we might see a gradual reduction in the range of heights, and a gradual increase in average height. However, what we see in reality is that height has changed through human history in concordance with economic factors like social status, inflation and war (e.g. Steckel, 2001), suggesting that nutrition is a major determinant.

I know it wasn't your choice of example, but for more detail on the effects of natural selection on human height, see this question.

It sounds like you're also trying to ask the question, "how can non-adaptive or maladaptive traits evolve?" and that you're not necessarily interested in acne specifically. The following are just a few examples.

It is possible that a gene that contributes to an undesirable trait, might also play a role in a strongly desirable trait. The classic textbook example of this is the gene involved in sickle-cell anemia, which causes the disease in homozygous individuals and is almost asymptomatic in heterozygous individuals. Since the gene helps with resistance to malaria, heterozygous individuals are actually better of than those with no mutation of this gene in areas where malaria is prevalent.

However, this is not the only way a less than optimal genotype can persist. A gene may only produce an undesirable trait in combination with certain other genes, and there may not be enough selective pressure for it to be eliminated form a population.

I think there are two elements to this answer. To cut to the short answer skip to the bold summary at the bottom…

Firstly, genetic variation exists because of mutation. Genes get mutated every generation, the . Larger populations will have more mutants within them because: more individuals = more nucleotide base pairs (C's G's A's and T's) = more potential sites of mutation. However mutation is not likely to explain the persistence of variation because mutation rates are very low (1 in 100,000 to 1,000,000 gametes have a newly mutated loci at any individual locus) and singleton alleles have only a 50% chance of reproduction (assuming no selection) so are likely to be lost by drift (Falconer & Mackay, Intro to Quantitative Genetics 1996).

You also talk about acne, which is likely to have a large component of environmental variance. Therefore you should remember that not all phenotypic variance is genetic its source and it is highly likely that an individual trait has some degree of environmental variance component. Simplistically:

Phenotypic variance = genotypic variance + environmental variance

So the bigger question is why does variation persist? There are many potential causes of this which continue to be widely debated. Essentially it seems paradoxical because selection should reduce variation as it drives the fixation of all loci to the fittest allele. However, selection is transient, both spatially and temporally, and is not efficient against rare alleles (especially recessive alleles because they are hidden by dominant traits - e.g. disease "carriers"). Another important point is that some mutations will be neutral, therefore remain unaffected by selection.

In the spatial context, this means that selection is not always favouring the same allele in all places a species inhabits. Selection might be different based on the where it is occurring (within a species, traits like fur would be beneficial to populations in cold climates but not to those in warmer climates - here I am assuming that the sole effect of fur is to improve the ability of retaining heat).

Temporally there are also key elements. Principally, over time selection changes. Again sticking with my fur example, climates change. Ice ages come and go bringing with them different selection coefficients for fur growth.

Another variance in selection can be sexually antagonistic selection, where different alleles are favoured in either sex. In this case selection does not deplete variation but instead maintains it. It has recently been shown that sexual antagonism is prevalent throughout the genome.

the divergent reproductive strategies of the sexes could promote the maintenance of sexually-antagonistic variation (Sharp & Agrawal 2012… yesterday!)

Other hypotheses suggest mechanisms by with selection can maintain variation such as assortative mating.

Long story short, you stated that you expect variation to reduce as a consequence of selection. However genetic variation persists for many reasons, and can even be maintained by selection in several ways. Furthermore, phenotypic variation which is what you actually describe with your acne example (and I with my fur example) can be caused by non-genetic components of variation.


Suggested reading:

Cox & Calsbeek 2009, Sexually Antagonistic Selection, Sexual Dimorphism, and the Resolution of Intralocus Sexual Conflict.

Falconer & Mackay 1996, Introduction to Quantitative Genetics.

Singh & Krimbas 2000, Evolutionary Genetics: from molecules to morphology.

Sharp & Agrawal 2012 (in press, accepted on-line version released yesterday, print may be 2013) Male-biased fitness effects of spontaneous mutations in Drosophila melanogaster, Evolution.

Innocenti & Morrow 2011, The Sexually Antagonistic Genes of Drosophila melanogaster, PLoS Biology.

Arnqvist 2011 Assortative mating by fitness and sexually antagonistic genetic variation, Evolution. (also see his book sexual conflict).


In the last chapter for this term, we will be looking at variation within a species and what this means. Learners have already learned how to classify organisms using shared characteristics down to the species level. But, it is important for learners to understand that even within a species, the individuals are different. These differences are called variation. As we have not yet learned about cells and DNA, this chapter will not look at the genetic basis for variation, but rather focus on the fact that there are differences between individuals in the same species, and that some of those characteristics are inherited (passed down from one generation to the next). We will also introduce the concept of natural selection in which a particular variation can make an organism better suited (adapted) to a particular environment. This is crucial to the survival of the species, especially as environments can change. Learners will be introduced to DNA in Gr. 9, and only if they carry on with Life Sciences in Gr. 10-12 will they look at DNA, meiosis, variation, natural selection and human evolution in detail in Gr. 12.

4.1 Variation within a species (1.5 hours)

Activity: Small, big, long-haired, short-haired, black, white, brown or spotty?!

Remembering, identifying, describing, explaining,

Activity: The height of learners in your class

Measuring, recording, plotting graphs, comparing, calculating, discussing

4.2 Inheritance in humans (1.5 hours)

Activity: What is your inheritance?

Thinking, observing, recording, calculating, comparing, drawing, labeling

Activity: Natural selection in the peppered moth

  • Are all dogs part of the same species if there are so many different sizes, shapes and colours?
  • What about humans? What does it mean that we have different skin colours, heights and other differences if we are all part of Homo sapiens?
  • What does variation mean?
  • What causes variation?
  • Why is it important that we study variation?

Genetic Variation

Genetic variation is a measure of the genetic differences that exist within a population. The genetic variation of an entire species is often called genetic diversity. Genetic variations are the differences in DNA segments or genes between individuals and each variation of a gene is called an allele.For example, a population with many different alleles at a single chromosome locus has a high amount of genetic variation. Genetic variation is essential for natural selection because natural selection can only increase or decrease frequency of alleles that already exist in the population.

Genetic variation is caused by:

  • mutation
  • random mating between organisms
  • random fertilization
  • crossing over (or recombination) between chromatids of homologous chromosomes during meiosis

The last three of these factors reshuffle alleles within a population, giving offspring combinations which differ from their parents and from others.

Figure (PageIndex<1>): Genetic variation in the shells of Donax variabilis: An enormous amount of phenotypic variation exists in the shells of Donax varabilis, otherwise known as the coquina mollusc. This phenotypic variation is due at least partly to genetic variation within the coquina population.

Why Is Genetic Variation Important?

Genetic variation is important because a population has a better chance of surviving and flourishing than a population with limited genetic variation. Genetic diversity also decreases the occurrence of unfavorable inherited traits.

Genetic variation comes from mutations within DNA the movement of genes from one population to another, or gene flow and new genetic combinations resulting from sex. When a population contains genetics of individuals who vary significantly, some of the individuals in the group can possess traits that make them resistant to disease or cold, increasing the group's chance for survival when these individuals breed with the others. A small, isolated population's individuals may be forced to breed with close relatives, increasing the occurrence of genetic flaws. When inbreeding occurs, any genetic weaknesses found in the parents can be multiplied in future generations.

Genetic variation also helps organisms survive in different climates and environments. If the environment is unpredictable over time and includes a variety of diseases and predators, some differences among individuals increase the chances of some individuals surviving to reproduce, while others do not. In disease resistance, genetic diversity is important because a disease can decimate a homogeneous population in which all the individuals are equally susceptible to the disease.


Darwin's theory of evolution by natural selection was based on the observation that there is variation between individuals within the same species. This fundamental observation is a central concept in evolutionary biology. However, variation is only rarely treated directly. It has remained peripheral to the study of mechanisms of evolutionary change. The explosion of knowledge in genetics, developmental biology, and the ongoing synthesis of evolutionary and developmental biology has made it possible for us to study the factors that limit, enhance, or structure variation at the level of an animals' physical appearance and behavior. Knowledge of the significance of variability is crucial to this emerging synthesis. Variation situates the role of variability within this broad framework, bringing variation back to the center of the evolutionary stage.

Darwin's theory of evolution by natural selection was based on the observation that there is variation between individuals within the same species. This fundamental observation is a central concept in evolutionary biology. However, variation is only rarely treated directly. It has remained peripheral to the study of mechanisms of evolutionary change. The explosion of knowledge in genetics, developmental biology, and the ongoing synthesis of evolutionary and developmental biology has made it possible for us to study the factors that limit, enhance, or structure variation at the level of an animals' physical appearance and behavior. Knowledge of the significance of variability is crucial to this emerging synthesis. Variation situates the role of variability within this broad framework, bringing variation back to the center of the evolutionary stage.

Far from special: Humanity's tiny DNA differences are 'average' in animal kingdom

Today's study, "Why should mitochondria define species?" published as an open-access article in the journal Human Evolution,builds on earlier work by Drs. Stoeckle and Thayer, including an examination of the mitochondrial genetic diversity of humans vs. our closest living and extinct relatives. The amount of color variation within each red box of the Klee diagram illustrates the far greater mitochondrial diversity among chimpanzees and bonobos than among living humans. Credit: The Rockefeller University

Researchers report important new insights into evolution following a study of mitochondrial DNA from about 5 million specimens covering about 100,000 animal species.

Mining "big data" insights from the world's fast-growing genetic databases and reviewing a large literature in evolutionary theory, researchers at The Rockefeller University in New York City and the Biozentrum at the University of Basel in Switzerland, published several conclusions today in the journal Human Evolution. Among them:

  • In genetic diversity terms, Earth's 7.6 billion humans are anything but special in the animal kingdom. The tiny average genetic difference in mitochondrial sequences between any two individual people on the planet is about the same as the average genetic difference between a pair of the world's house sparrows, pigeons or robins. The typical difference within a species, including humans, is 0.1% or 1 in 1,000 of the "letters" that make up a DNA sequence.
  • Genetic variation—the average difference in mitochondria DNA between two individuals of the same species—does not increase with population size. Because evolution is relentless, however, the lack of genetic variation offers insights into the timing of a species' emergence and its maintenance.
  • The mass of evidence supports the hypothesis that most species, be it a bird or a moth or a fish, like modern humans, arose recently and have not had time to develop a lot of genetic diversity. The 0.1% average genetic diversity within humanity today corresponds to the divergence of modern humans as a distinct species about 100,000—200,000 years ago—not very long in evolutionary terms. The same is likely true of over 90% of species on Earth today.
  • Genetically the world "is not a blurry place." Each species has its own specific mitochondrial sequence and other members of the same species are identical or tightly similar. The research shows that species are "islands in sequence space" with few intermediate "stepping stones" surviving the evolutionary process.

Among 1st "big data" insights from a growing collection of mitochondrial DNA

"DNA barcoding" is a quick, simple technique to identify species reliably through a short DNA sequence from a particular region of an organism. For animals, the preferred barcode regions are in mitochondria—cellular organelles that power all animal life.

The new study, "Why should mitochondria define species?" relies largely on the accumulation of more than 5 million mitochondrial barcodes from more than 100,000 animal species, assembled by scientists worldwide over the past 15 years in the open access GenBank database maintained by the US National Center for Biotechnology Information.

The researchers have made novel use of the collection to examine the range of genetic differences within animal species ranging from bumblebees to birds and reveal surprisingly minute genetic variation within most animal species, and very clear genetic distinction between a given species and all others.

"If a Martian landed on Earth and met a flock of pigeons and a crowd of humans, one would not seem more diverse than the other according to the basic measure of mitochondrial DNA," says Jesse Ausubel, Director of the Program for the Human Environment at The Rockefeller University, where the research was led by Senior Research Associate Mark Stoeckle and Research Associate David Thaler of the University of Basel, Switzerland.

"At a time when humans place so much emphasis on individual and group differences, maybe we should spend more time on the ways in which we resemble one another and the rest of the animal kingdom."

Says Dr. Stoeckle: "Culture, life experience and other things can make people very different but in terms of basic biology, we're like the birds."

The study results represent a surprise given predictions found in textbooks, and based on mathematical models of evolution, that the bigger the population of a species, the greater the genetic variation one expects to find. In fact, the mitochondrial diversity within 7.6 billion humans or 500 million house sparrows or 100,000 sandpipers from around the world is about the same.The paper notes, however, that evolution is relentless, that species are always changing, and, therefore, the degree of variation within a given species offers a clue into how long ago it emerged distinctly -- in other words, the older the species the greater the average genetic variation between its members. Credit: The Rockefeller University

"By determining the genetic variety within species of the animal kingdom, made possible only recently by the burgeoning number of DNA sequences, we've documented the absence of human exceptionalism."

Says. Dr. Thaler: "Our approach combines DNA barcodes, which are broad but not deep, from the entire animal kingdom with more detailed sequence information available for the entire mitochondrial genome of modern humans and a few other species. We analyzed DNA barcode sequences from thousands of modern humans in the same way as those from other animal species."

"One might have thought that, due to their high population numbers and wide geographic distribution, humans might have led to greater genetic diversity than other animal species," he adds. "At least for mitochondrial DNA, humans turn out to be low to average in genetic diversity."

"Experts have interpreted low genetic variation among living humans as a result of our recent expansion from a small population in which a sequence from one mother became the ancestor for all modern human mitochondrial sequences," says Dr. Thaler.

"Our paper strengthens the argument that the low variation in the mitochondrial DNA of modern humans also explains the similar low variation found in over 90% of living animal species—we all likely originated by similar processes and most animal species are likely young."

Genetic variation does not increase with population

The study results represent a surprise given predictions found in textbooks, and based on mathematical models of evolution, that the bigger the population of a species, the greater the genetic variation one expects to find.

"Is genetic diversity related to the size of the population?" asks Dr. Stoeckle. "The answer is no. The mitochondrial diversity within 7.6 billion humans or 500 million house sparrows or 100,000 sandpipers from around the world is about the same."

The paper notes, however, that evolution is relentless, that species are always changing, and, therefore, the degree of variation within a given species offers a clue into how long ago it emerged distinctly—in other words, the older the species the greater the average genetic variation between its members.

Genetically, 'the world is not a blurry place.' It is hard to find 'intermediates' -- the evolutionary stepping stones between species. The intermediates disappear. The research is a new way to show that species are 'islands in sequence space.' Each species has its own narrow, very specific consensus sequence, just as our phone system has short, unique numeric codes to tell cities and countries apart. Credit: The Rockefeller University

Evolutionary bottlenecks: the fresh new beginning of a species

While asteroids and ice ages have played major roles in evolutionary history, scientists speculate that another great driver may have been the microbial world, notably viruses, which periodically cull populations, leaving behind only those able to survive the deadly challenge.

"Life is fragile, susceptible to reductions in population from ice ages and other forms of environmental change, infections, predation, competition from other species and for limited resources, and interactions among these forces," says Dr. Thaler. Adds Dr. Thaler, "The similar sequence variation in many species suggests that all of animal life experiences pulses of growth and stasis or near extinction on similar time scales."

"Scholars have previously argued that 99% of all animal species that ever lived are now extinct. Our work suggests that most species of animals alive today are like humans, descendants of ancestors who emerged from small populations possibly with near-extinction events within the last few hundred thousand years."

'Islands in sequence space'

Another intriguing insight from the study, says Mr. Ausubel, is that "genetically, the world is not a blurry place. It is hard to find 'intermediates' - the evolutionary stepping stones between species. The intermediates disappear."

Dr. Thaler notes: "Darwin struggled to understand the absence of intermediates and his questions remain fruitful."

"The research is a new way to show that species are 'islands in sequence space.' Each species has its own narrow, very specific consensus sequence, just as our phone system has short, unique numeric codes to tell cities and countries apart."

Adds Dr. Thaler: "If individuals are stars, then species are galaxies. They are compact clusters in the vastness of empty sequence space."

The researchers say that with the bones or teeth of an ancient hominid, like those found in southern France or northern Spain, scientists might shed further light on the rate of evolution of the human species.

"It would be very exciting if over the next few years physical anthropologists and others were able to compare mitochondrial DNA from hominid species over the last 500,000 years," says Dr. Stoeckle.

Race is a Social Concept, Not a Scientific One (Op-Ed)

Michael Hadjiargyrou is chair of the Department of Life Sciences at the New York Institute of Technology. He contributed this article to Live Science's Expert Voices: Op-Ed & Insights.

Beyond the Ferguson, Mo., media reports on the "racial divide," the facts require some correction: Despite notions to the contrary, there is only one human race. Our single race is independent of geographic origin, ethnicity, culture, color of skin or shape of eyes — we all share a single phenotype, the same or similar observable anatomical features and behavior.

Science highlights these similarities in our embryonic development, physiology (our organ-based systems), biochemistry (our metabolites and reactions), and more recently, genomics (our genetic makeup). As a molecular biologist, this last one is indeed the most important to me — data show that the DNA of any two human beings is 99.9 percent identical, and we all share the same set of genes, scientifically validating the existence of a single biological human race and one origin for all human beings. In short, we are all brothers and sisters. [What is the Difference between Race and Ethnicity? ]

Biologically speaking, one clear example is that most diseases afflict all of us — diseases like cancers and cardiovascular and neurological disorders, as well as viral, microbial and parasitic infections. Obviously, there are differences in how individual humans respond to various diseases or infections some never suffer from cancer and may be immune to assorted infections. This may be due to factors such as diet, exercise, overall health or environmental conditions. However, the fact that a human population, irrespective of geography or ethnicity is susceptible to the same diseases, coupled with the existence of multiple pandemics, is a clear indication of how identical we are.

Genetically speaking, studies have shown that there is much greater genetic variation within a given human population (e.g., Africans, Caucasians, or Asians) than between populations (Africans vs. Caucasions), indicating that human variation cannot be subdivided into discrete races.

It is history, not science,that reveals how the concept of different human "races" arose, how the term has become widely misused, and how it continues to pervade our planet. In fact, the word race has come to symbolize the division of humanity into segments, divisions that often lead to conflicts. Over centuries, people have used the word to divide us into black, white, yellow, red, and other distinctions in order to fulfill selfish goals and objectives. Whether those goals were to subjugate various groups of humans, deem them inferior or simply discriminate against them, the reality is that billions of people have been directly affected as a result of the misuse of the word race.

The end result, in its extreme form, has led to a plethora of existential crises such as segregation, slavery, violence, wars and genocides. One classic example is the dehumanization of millions of Jewish people by Germany and other European nations during the 1930s and 40s, and the colonization and slavery of Africans by European and North American nations is another.

The continual use of the word race, predominantly by the media and policy makers, perpetuates the myth of multiple human races and further polarizes our society. We must not allow the media or our lawmakers to hijack this issue and continue to misuse the word. We must hold them accountable and demand that they stop misusing it, especially for sensationalistic and factually false reporting. It is simply irresponsible and feeds into the hands of those that espouse discriminatory and unscientific ideas about the single human race. Society can certainly protect the rights of minorities without invoking the word race.

Scientists and educators have even a greater responsibility to speak out and present the scientific facts. From pre-kindergarten to graduate school, society must be relentless in our goal to eradicate the word as currently used. In fact, racism , the application of the word race, together with ethnocentrism and nationalism, are indeed the biggest enemies of humanity. Historically speaking, both have been used to justify the active domination of one group of humans over another, often with disastrous and deadly consequences — purely antithetical to scientific truths.

We must all realize that the faster we eliminate the use of the word that drives a wedge between people, the better our world will be: More peaceful and prosperous and with equality and mutual respect. And it all starts with accepting a simple scientific fact: We all evolved from the same ancestors and are, indeed, all virtually genetically identical to each other, making us a single race.

Follow all of the Expert Voices issues and debates — and become part of the discussion — on Facebook, Twitter and Google +. The views expressed are those of the author and do not necessarily reflect the views of the publisher. This version of the article was originally published on Live Science.

How many species concepts are there?

It's an old question in biology: what is a species? Many answers have been given over the years – I counted 26 in play, and recently a new one, the "polyphasic" concept (basically a consilience of many lines of evidence) has been introduced in bacterial and other microbial contexts, and which may apply to macrobial species too.

But on another count (where I asterisked what I thought were independent concepts in that list) there are 7 species concepts: agamospecies (asexuals), biospecies (reproductively isolated sexual species), ecospecies (ecological niche occupiers), evolutionary species (evolving lineages), genetic species (common gene pool), morphospecies (species defined by their form, or phenotypes), and taxonomic species (whatever a taxonomist calls a species).

So, to sound a bit like Chicago, 26 to 27, or 7.

But notice that some of these seven are in fact not concepts of what species are, that is, what makes them species, but instead how we identify species: by morphology, or the practices of taxonomists. A gene pool is defined as a population of genomes that can be exchanged, and so it is basically a reproductive (that is, biospecies) definition. And evolutionary species are not what species are so much as what happens when some processes (such as ecological adaptation or reproductive isolation) makes them species that persist over long time. A common "concept" of species, the so-called phylogenetic species concept, is likewise a mix either of morphospecies, biospecies or evospecies or all of them. The polyphasic concept is also a method for identifying species. So, what does that leave us?

Agamospecies are species that lack some property: sex. An agamospecies is a not-biospecies species. So what makes an agamospecies a species? It can't be reproductive isolation, for obvious reasons, so it must be the only thing that we have left on the list: ecological niche adaptation. [It could be chance too: things will tend to cluster about a genomic wild type for chance reasons as well, but if that happens by chance it is unlikely to be maintained by chance, and so we can ignore random clustering over time.]

So in the absence of sex, you are going to need ecological niche adaptation to keep the cluster from just randomly evaporating. Of course, few if any species are purely asexual in the sense that they don't ever exchange genes microbes have several mechanisms to do this even if they lack genders and fail to reproduce by any other means than division. Some genetic material can be exchanged through viral transportation, through picking up stray DNA in the medium after a cell has broken apart, or by deliberate insertion of small rings of DNA, called plasmids. "Horizontal" or "lateral" genetic transfer is probably as old as life itself. But while this might introduce some genetic variation into a population, it is selection for a local fitness peak that makes the genome not stray too far from that abstract genome biologists call the "wild type".

As sex becomes more frequent, rising from near zero recombination per generation up to the maximum of 50% exchanged for obligatorily sexual organisms, another factor comes into play. Increasingly, the compatibility of genomes, reproductive processes at the cellular, organ, and physiological level become important. In organisms with behavioural signalling (that is, with nervous systems and sensory organs), reproductive behaviours like calls and movements become important.

Sex acts to ensure that the organisms that can interbreed tend to be those whose genome and anatomy are consistent enough. I call this "reproductive reach": the closer two organisms are related to each other, the more likely they are within each other's reach as potential mates, and so the species is maintained by reproductive compatibility, and of course some ecological adaptation.

Consider lions and tigers. They separated from each other, evolutionarily, about 3.7 million years ago. They can interbreed, however, forming ligers (male lion, female tiger cross) and tiglons (male tiger, female lion cross). In the wild, though, they don't. Why not? In part it is ecology: lions are grassland cooperative hunters, while tigers are woodland individual hunters. They don't frequent the same bars as each other. But even when they do, they date differently. Tigers are in estrus only occasionally, while lions are polyestrous (the females are receptive, when they are not rearing their cubs, several times a year). Moreover, the genitalia are structured differently. So while it can happen, when lions and tigers share a geographical range, they tend not to interbreed. Ecology and reproductive reach keep them separate.

This is very similar to a definition of "species" by the geneticist Alan Templeton, who said that species were "the most inclusive population of individuals having the potential for phenotypic cohesion through intrinsic cohesion mechanisms", "that defines a species as the most inclusive group of organisms having the potential for genetic and/or demographic exchangeability." [1989, My emphasis.] "Genetic" exchangeability here means the ability to act in the same manner in reproduction - any two members of the species are (more or less) interchangeable. "Demographic" exchangeability means that any two members of the species behave the same, ecologically, behaviourally and so forth, and are interchangeable (more or less).

With these two causes of being a species, we can now narrow down the number of concepts to two: ecospecies or biospecies. To be honest, I don't like calling the reproductive concept "biological" – all species concepts in biology are biological, and so I call them "reproductive isolation concepts". Let's call them "reprospecies" for short.

So, back to Chicago: 26-27, or 7, or 2.

But wait! There's a philosophical matter to clear up. These causal explanations are just that: explanations. They are not the concept of species. There was a concept of species before we had any clear idea of what they might be. We identified species in the 15th century that are still regarded as species, and there wasn't the slightest hint of an explanation in the air at the time. And it's an old concept, too, although the first simply biological definition of "species" (a Latin word that means "form" or "appearance") waited until 1686 when John Ray defined it. Ray said of a species:

After long and considerable investigation, no surer criterion for determining species has occurred to me than the distinguishing features that perpetuate themselves in propagation from seed. Thus, no matter what variations occur in the individuals or the species, if they spring from the seed of one and the same plant, they are accidental variations and not such as to distinguish a species . Animals likewise that differ specifically preserve their distinct species permanently one species never springs from the seed of another nor vice versa.

Ray's definition was based on a simple observation: progeny resemble their parents. Species are those groups of organisms that resemble their parents. A version of it can be found in Epicurus' disciple, Lucretius:

If things could be created out of nothing, any kind of things could be produced from any source. In the first place, men could spring from the sea, squamous fish from the ground, and birds could be hatched from the sky cattle and other farm animals, and every kind of wild beast, would bear young of unpredictable species, and would make their home in cultivated and barren parts without discrimination. Moreover, the same fruits would not invariably grow on the same trees, but would change: any tree could bear any fruit. Seeing that there would be no elements with the capacity to generate each kind of thing, how could creatures constantly have a fixed mother? But, as it is, because all are formed from fixed seeds, each is born and issues out into the shores of light only from a source where the right ultimate particles exist. And this explains why all things cannot be produced from all things: any given thing possesses a distinct creative capacity. [On the Nature of Things (Lucretius 1969:38, Book I. 155-191)]

There is some power, a generative capacity, to make progeny resemble parents, and it seems to rely upon seeds. I call this venerable view, the generative conception of species, and I hold that it was not only the default view before Darwin, but Darwin himself held it, as do all modern biologists (exception below). I argue this in my two 2009 books (summarised in my recent paper). It is what it is that the explanations explain. So technically there is only one species "concept", of which all the others, the 2 or 7 or 27, are "conceptions".

The idea that there is one generic category into which there are put many "concepts" is a mistake made by Ernst Mayr, introduced in 1963. In ordinary philosophical usage, it is the concept that is the category, and the definitions define, in various ways, that concept. Another mistake often made by biologists is to think that if there is a concept/category, there has to be a specified rank or "level" at which all species arise. This is a big error that requires another essay another time, but it seems to rely on the idea that because Linnaeus took Ray's concept of species and made it the lowest rank in his classification scheme, there has to be something that all and only species have as properties. This has caused no end of confusion. That species all exist does not imply that all species have some essential property (any more than because we can usually identify what an organism is implies there is something that all and only organisms share). This philosophical error is called "essentialism", and it is a supreme irony that Mayr, the opponent of essentialism about individual species, was held in thrall to essentialism about taxonomic concepts.

So, back to Chicago: 26-27, or 7, or 2, or 1.

Are we there yet? Almost. Some people think that there are no species. Moreover, they wrongly think this view is a consequence of evolution and that Darwin himself denied there were any. Now what Darwin thought 150 years ago is of no real consequence to modern biology, but he didn't think species were unreal constructs he thought there was no single set of properties species had to have. He was not a taxonomic essentialist. But neither is it the case that species are unreal because they shade into each other. In modern philosophy, there is an ongoing debate over whether one can have vague and fuzzy sets or kinds, but for science we need only a little logic and metaphysics: If we can identify mountains, rivers, and organisms, we can identify species, and they will tend to have a "family resemblance" (Wittgenstein's most apt phrase in this context). What is a species among primates will tend to be like species in all other close relatives. What is a species among lizards will (usually) be like what a species is in close relatives (some lizards are parthenogens and have no males, where their nearest relatives are sexual, but in that case they are like their sexual cousins ecologically and morphologically see my 2003).

But some, like Jody Hey, think that species do not exist except in the minds of biologists and their public. So for them, zero.

Final score: 26-27, 7, 2, 1 or 0.

What to think? My solution is this:

There is one species concept (and it refers to real species).

There are two explanations of why real species are species (see my microbial paper, 2007): ecological adaptation and reproductive reach.

There are seven distinct definitions of "species", and 27 variations and mixtures.

And there are n+1 definitions of "species" in a room of n biologists.

Templeton, Alan R. 1989. The meaning of species and speciation: A genetic perspective. In Speciation and its consequences, edited by D. Otte and J. Endler. Sunderland, MA: Sinauer:3-27.

Wilkins, John S. 2003. How to be a chaste species pluralist-realist: The origins of species modes and the Synapomorphic Species Concept. Biology and Philosophy 18:621-638.

———. 2007. The Concept and Causes of Microbial Species. Studies in History and Philosophy of the Life Sciences History & Philosophy of the Life Sciences, 28, 389-408.

———. 2009. Species: a history of the idea, Species and Systematics. Berkeley: University of California Press.

———. 2009. Defining species: a sourcebook from antiquity to today, American University Studies. V, Philosophy. New York: Peter Lang.

John Wilkins is an Assistant Professor of Philosophy at Bond University on the Gold Coast in Queensland, Australia. Dr Wilkins has written two books, Species: A History of the Idea [2009, University of California Press Amazon UK Amazon US] and Defining Species: A Sourcebook from Antiquity to Today [2009, Peter Lang Publishing Amazon UK Amazon US] and has edited and contributed a chapter to a third book, Intelligent Design and Religion as a Natural Phenomenon [2010, Ashgate Press Amazon UK Amazon US]. He also writes the blog, Evolving Thoughts.

Why is Earth so biologically diverse? Mountains hold the answer

What determines global patterns of biodiversity has been a puzzle for scientists since the days of von Humboldt, Darwin, and Wallace. Yet, despite two centuries of research, this question remains unanswered. The global pattern of mountain biodiversity, and the extraordinarily high richness in tropical mountains in particular, is documented in two companion Science review papers this week. The papers focus on the fact that the high level of biodiversity found on mountains is far beyond what would be expected from prevailing hypotheses.

"The challenge is that, although it is evident that much of the global variation in biodiversity is so clearly driven by the extraordinary richness of tropical mountain regions, it is this very richness that current biodiversity models, based on contemporary climate, cannot explain: mountains are simply too rich in species, and we are falling short of explaining global hotspots of biodiversity," says Professor Carsten Rahbek, lead author of both review papers published in Science.

To confront the question of why mountains are so biologically diverse, scientists at the Center for Macroecology, Evolution and Climate (CMEC) at the GLOBE Institute of the University of Copenhagen work to synthesize understanding and data from the disparate fields of macroecology, evolutionary biology, earth sciences, and geology. The CMEC scientists are joined by individual collaborators from Oxford University, Kew Gardens, and University of Connecticut.

Part of the answer, these studies find, lies in understanding that the climate of rugged tropical mountain regions is fundamentally different in complexity and diversity compared to adjacent lowland regions. Uniquely heterogeneous mountain climates likely play a key role in generating and maintaining high diversity.

"People often think of mountain climates as bleak and harsh," says study co-leader Michael K. Borregaard. "But the most species-rich mountain region in the world, the Northern Andes, captures, for example, roughly half of the world's climate types in a relatively small region -- much more than is captured in nearby Amazon, a region that is more than 12 times larger."

Stressing another unique feature of mountain climate, Michael explains, "Tropical mountains, based in fertile and wet equatorial lowlands and extending into climatic conditions superficially similar to those found in the Arctic, span a gradient of annual mean temperatures over just a few km as large as that found over 10,000 km from the tropical lowlands at Equator to the arctic regions at the poles. It's pretty amazing if you think about it."

Another part of the explanation of the high biodiversity of certain mountains is linked to the geological dynamics of mountain building. These geological processes, interacting with complex climate changes through time, provide ample opportunities for evolutionary processes to act.

"The global pattern of biodiversity shows that mountain biodiversity exhibits a visible signature of past evolutionary processes. Mountains, with their uniquely complex environments and geology, have allowed the continued persistence of ancient species deeply rooted in the tree of life, as well as being cradles where new species have arisen at a much higher rate than in lowland areas, even in areas as amazingly biodiverse as the Amazonian rainforest," says Professor Carsten Rahbek.

From ocean crust, volcanism and bedrock to mountain biodiversity

Another explanation of mountain richness, says the study, may lie in the interaction between geology and biology. The scientists report a novel and surprising finding: the high diversity is in most tropical mountains tightly linked to bedrock geology -- especially mountain regions with obducted, ancient oceanic crust. To explain this relationship between geology and biodiversity, the scientists propose, as a working hypothesis, that mountains in the tropics with soil originating from oceanic bedrock provide exceptional environmental conditions that drive localized adaptive change in plants. Special adaptations that allow plants to tolerate these unusual soils, in turn, may drive speciation cascades (the speciation of one group leading to speciation in other groups), all the way to animals, and ultimately contribute to the shape of global patterns of biodiversity.

The legacy of von Humboldt -- his 250th anniversary

The two papers are part of Science's celebration of Alexander von Humboldt's 250th birth anniversary. In 1799, Alexander von Humboldt set sail on a 5-year, 8000-km voyage of scientific discovery through Latin America. His journey through the Andes Mountains, captured by his famous vegetation zonation figure featuring Mount Chimborazo, canonized the place of mountains in understanding Earth's biodiversity.

Acknowledging von Humboldt's contribution to our understanding of the living world, Professor Carsten Rahbek, one of the founding scientists of the newly established interdisciplinary GLOBE Institute at the University of Copenhagen says:

"Our papers in Science are a testimony to the work of von Humboldt, which truly revolutionized our thinking about the processes that determine the distribution of life. Our work today stands on the shoulders of his work, done centuries ago, and follows his approach of integrating data and knowledge of different scientific disciplines into a more holistic understanding of the natural world. It is our small contribution of respect to the legacy of von Humboldt."

Why “Race” Isn’t Biological

This speech by Charles Mills, which we’ve posted before, does an excellent job explaining the social construction of race:

Nicholas Wade’s new book on race and genetics, which takes the biological basis of race as a given, provides no consistent definition for “race.” During his debate with Wade, anthropologist Agustín Fuentes pointed out that “Wade uses cluster, population, group, race, sub-race, ethnicity in a range of ways with few concrete definitions, and occasionally interchangeably throughout the book.” In a response to Wade’s book, Fuertes explains how A Troublesome Inheritance gets race so wrong:

The originators of the computer program most often used to support the argument that humans divide into the continental genetic clusters (which Wade says are “races”) comment that their model (called structure) is not well-suited to data shaped by restricted gene flow with isolation by distance (as human genetic variation data on large scales are). They warn that if one does try to apply this model to those data, the inferred value of K (how many clusters emerge) can be rather arbitrary. For example, one article Wade cites shows not three, not five, not seven but 14 clusters, six of which are in Africa alone.

So when Wade states in chapter 5 of his book, “It might be reasonable to elevate the Indian and Middle Eastern groups to the level of major races, making seven in all,” he notices a problem: “But then, many more subpopulations could be declared races.” But he has a solution: “[T]o keep things simple, the 5-race continent based scheme seems the most practical for most purposes.”

Sure, it is practical if your purpose is to maintain the myth that black, white and Asian are really separable biological groups. But if your goal is to accurately reflect what we know about human biological variation, then no, it is a really not practical at all in fact, it is flat-out wrong. What we know about human genetic variation does not support dividing humans into three or five or seven “races.”

Other writers who argue that race is biological aren’t as sloppy as Wade. And, even though I do not believe that defining race biologically is correct, it’s best to engage with the strongest arguments of those who disagree. For starters, here is part of a 2012 post by Jerry Coyne that defends defining human races biologically:

What are races?

In my own field of evolutionary biology, races of animals (also called “subspecies” or “ecotypes”) are morphologically distinguishable populations that live in allopatry (i.e. are geographically separated). There is no firm criterion on how much morphological difference it takes to delimit a race. Races of mice, for example, are described solely on the basis of difference in coat color, which could involve only one or two genes.

Under that criterion, are there human races?

Yes. As we all know, there are morphologically different groups of people who live in different areas, though those differences are blurring due to recent innovations in transportation that have led to more admixture between human groups.

Coyne, in the midst of a scathing review of Wade’s book, writes that “Wade’s discussion of genetically differentiated subgroups, whether or not you want to call them ‘races’—is not too bad.” H. Allen Orr, who tears Wade’s book to shreds, likewise defends a genetic definition of race:

The central fact is that genetic differences among human beings who derive from different continents are statistical. Geneticists might find that a variant of a given gene is found in 79 percent of Europeans but in only, say, 58 percent of East Asians. Only rarely do all Europeans carry a genetic variant that does not appear in all East Asians. But across our vast genomes, these statistical differences add up, and geneticists have little difficulty concluding that one person’s genome looks European and another person’s looks East Asian. To put the conclusion more technically, the genomes of various human beings fall into several reasonably well-defined clusters when analyzed statistically, and these clusters generally correspond to continent of origin. In this statistical sense, races are real.

This is what I also claimed, and of course got slammed by the race-denialists who are motivated largely by politics. To a biologist, races are simply genetically differentiated populations, and human populations are genetically differentiated. Although it’s a subjective exercise to say how many races there are, human genetic differentiation seems to cluster largely by continent, as you’d expect if that differentiation evolved in allopatry (geographic isolation).

Relatedly, Razib Khan argues that “the modern American consensus that race is a social construct is true but trivial”:

It’s true because a de facto race such as “Latinos/Hispanics” were created in the 1960s by the American government and elite for purposes of implementing public policies such as affirmative action. Obviously this is a classic case of a social construct, as the quasi-racial category is based upon social, not biological, factors (Latinos/Hispanic can explicitly be of any race, though implicitly it’s transformed into a non-white class in the United States). A group like “black Americans” ranges from people with considerably less than 50% African ancestry to more than 90% African ancestry (though almost always black Americans who are not immigrants from Africa or first generation offspring of those immigrants have some segments of European ancestry). The problem is that people move from this non-controversial point, that some racial categories are social constructs, to the assertion that all racial categories are social constructs, and that phylogenetic clustering of human populations is irrelevant or impossible. It is not irrelevant, or impossible. Human populations vary, and that variation matters. Human populations have specific historical backgrounds, and phylogenetics can capture that history through methods of inference.

I disagree with Khan calling “phylogenetic clustering of human populations” races, but Razib is far more intelligible here than Wade is in most of his book. Nevertheless, the biological definitions of race outlined above are problematic because they are not the same as the social definitions of race. There is significant overlap between the biological and social definitions but defining “race” two ways only confuses matters. In an interview, Wade offers an explanation for why he uses the term “race” as he does:

It seems that the problem might be, as you said, that there is so much historical baggage associated with the term race. Is there a way to get around that? Do we just need a different term than race to talk about these genetic differences?

I’m not sure how that will play out. The geneticists, if you read their papers, have long been using code words. They sort of dropped the term “race” about 1980 or earlier, and instead you see code words like “population” or “population structure.” Now that they’re able to define race in genetic terms they tend to use other words, like “continental groups” or “continent of origin,” which does, indeed, correspond to the everyday conception of race. When I’m writing I prefer to use the word race because that’s the word that everyone understands. It’s a word with baggage, but it’s not necessarily a malign word. It all depends on the context in which it’s used, I guess.

Wade says that “everyone understands” the word race. But what everyone understands are the social definitions of race: White, Black, Latino, Asian, Native American, Samoan, and so on. Wade dismisses geneticists who use terms like “population structure,” “population stratification,” “ancestry” and “ancestry informative markers.” But those terms are useful when discussing genetics because they allow for far more complexity and specificity than our social definitions of race do.

Obviously, skin color and the other physical characteristics society uses to categorize individuals racially are biological. But skin color and other physical traits are not the same as race. And, as Khan noted recently, one “of the ironies of traits which we use to differentiate populations, such as skin color and facial features, is that these might actually have relatively shallow time depth within a given lineage.” So prioritizing skin color above all other ancestry informative markers finds little basis is biology. In a 2012 post, Fuentes argued against a biological understanding of race for related reasons:

Even something thought to be so ubiquitous as skin color works only in a limited way as dark or light skin tells us only about a human’s amount of ancestry relative to the equator, not anything about the specific population or part of the planet they might be descended from.

There is not a single biological element unique to any of the groups we call white, black, Asian, Latino, etc. In fact, no matter how hard people try, there has never been a successful scientific way to justify any racial classification, in biology. This is not to say that humans don’t vary biologically, we do, a lot. But rather that the variation is not racially distributed.

Alfred W. Clark, a strong defender of Wade’s book, has a useful round-up of commentary on A Troublesome Inheritance. In it, he dismisses Fuentes by arguing that he is suffering from a “slightly more sophisticated version of Lewontin’s Fallacy.” What is Lewontin’s Fallacy? In a 2005 NYT article arguing that race is biological, Armand Marie Leroi explained it:

The dominance of the social construct theory can be traced to a 1972 article by Dr. Richard Lewontin, a Harvard geneticist, who wrote that most human genetic variation can be found within any given “race.” If one looked at genes rather than faces, he claimed, the difference between an African and a European would be scarcely greater than the difference between any two Europeans. A few years later he wrote that the continued popularity of race as an idea was an “indication of the power of socioeconomically based ideology over the supposed objectivity of knowledge.” Most scientists are thoughtful, liberal-minded and socially aware people. It was just what they wanted to hear.

Three decades later, it seems that Dr. Lewontin’s facts were correct, and have been abundantly confirmed by ever better techniques of detecting genetic variety. His reasoning, however, was wrong. His error was an elementary one, but such was the appeal of his argument that it was only a couple of years ago that a Cambridge University statistician, A. W. F. Edwards, put his finger on it.

The error is easily illustrated. If one were asked to judge the ancestry of 100 New Yorkers, one could look at the color of their skin. That would do much to single out the Europeans, but little to distinguish the Senegalese from the Solomon Islanders. The same is true for any other feature of our bodies. The shapes of our eyes, noses and skulls the color of our eyes and our hair the heaviness, height and hairiness of our bodies are all, individually, poor guides to ancestry.

But this is not true when the features are taken together. Certain skin colors tend to go with certain kinds of eyes, noses, skulls and bodies. When we glance at a stranger’s face we use those associations to infer what continent, or even what country, he or his ancestors came from – and we usually get it right. To put it more abstractly, human physical variation is correlated and correlations contain information.

Genetic variants that aren’t written on our faces, but that can be detected only in the genome, show similar correlations. It is these correlations that Dr. Lewontin seems to have ignored. In essence, he looked at one gene at a time and failed to see races. But if many – a few hundred – variable genes are considered simultaneously, then it is very easy to do so.

But this still fails to prove that races are biological. Calling these populations “races” is a semantic rather than a scientific decision. Wikipedia provides useful context on this front:

Philosophers Jonathan Kaplan and Rasmus Winther have argued that while Edwards’s argument is correct it does not invalidate Lewontin’s original argument, because racial groups being genetically distinct on average does not mean that racial groups are the most basic biological divisions of the world’s population. Nor does it mean that races are not social constructs as is the prevailing view among anthropologists and social scientists, because the particular genetic differences that correspond to races only become salient when racial categories take on social importance. From this sociological perspective, Edwards and Lewontin are therefore both correct. [13] [14] [15]

Similarly, biological anthropologist Jonathan Marks agrees with Edwards that correlations between geographical areas and genetics obviously exist in human populations, but goes on to note that “What is unclear is what this has to do with ‘race’ as that term has been used through much in the twentieth century – the mere fact that we can find groups to be different and can reliably allot people to them is trivial. Again, the point of the theory of race was to discover large clusters of people that are principally homogeneous within and heterogeneous between, contrasting groups. Lewontin’s analysis shows that such groups do not exist in the human species, and Edwards’ critique does not contradict that interpretation.” [6]

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