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Thread: How to define a species ?

  1. #1
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    Question How to define a species ?



    The most common definition of 'species' is a group of organisms capable of interbreeding and producing fertile offspring. Notwithstanding, the way biologists (specifically zoologists and botanists) divide species often has little to do with the capability of interbreeding, but mostly with physical features, such as colour or size.

    In his book The Ancestor's Tale, Richard Dawkins states (p. 346) that "depending on whether your ichthyologist is a lumper or a splitter, the number of species of cichlid [fish] in Lake Victoria is between 200 and 500". He explains that species are actually divided either based upon their ability to interbreed OR their willingness to interbreed. Many species of fish and birds have evolved into many "races" sporting different colours, so that they typically choose to mate with individuals similar to them. That doesn't mean that they cannot produce fertile offspring with the different-looking races. They simply choose not too. Well, in most cases, as hybridisation does occur from time to time.

    The same thing actually happens a lot with humans. Geographical isolation has led us to evolved into different-looking races, and up to this day the majority of people choose to mate with people of their own racial group, even in cosmopolitan societies like the USA. That doesn' mean that we are different species. So why would there be hundreds of species of cichlid fish if they can all interbreed together ? Closer to us, why do we categorise dogs, wolves, jackals, dingos and foxes as different species if they can interbreed with each others but lack the opportunity (geographic isolation) or just choose not to (cultural difference) ?


    I have quoted a passage from Richard Dawkins' The Ancestor's Tale (p. 349-350) to illustrate my point :

    "According to the geographical isolation theory, speciation begins with the accidental geographical division of a single ancestral species into separate populations. No longer able to interbreed, the two populations drift apart, or are pushed by natural selection in different evolutionary directions. Then, if they subsequently meet after this divergence, they either can't interbreed or don't want to. They often recognise their own species by some particular feature, and studiously avoid similar species who lack it. Natural selection penalises mating with the wrong species, especially where the species are close enough for it to be a temptation, and close enough for hybrid offspring to survive, to consume costly parental resources, and then turn out to be sterile, like mules. Many zoologists have interpreted courtship displays as aimed mainly against miscegenation. This may be an exaggeration, and there are other important selection pressures bearing upon courtship. But it is still probably correct to interpret some courtship displays, and some bright colours and other conspicuous advertisements, as 'reproductive isolation mechanisms' evolved through selection against hybridisation.

    As it happens, a particularly neat experiment was done on cichlid fish by Ole Seehausen, now at the University of Hull, and his colleague Jacques van Alphen at the University of Leiden. They took two related species of Lake Victoria cichlids, Pundamilia pundamilia and P. nyererei (named after one of Africa's great leaders, Julius Nyerere of Tanzania). The two species are very similar, except that P. nyererei has a reddish colour, whereas P. pundamilia is bluish. Under normal conditions, females in choice tests prefer to mate with males of their own species. But now, Seehausen and van Alphen did their critical test. They gave females the same choice, but in artificial monochromatic light. This does dramatic things to perceived colour, as I remember vividly from schooldays in Salisbury, a city whose streets happened to be lit by sodium lights. Our bright red caps, and the bright red buses, all looked dirty brown. This is what happened to both the red and the blue Pundamilia males in Seehausen and van Alphen's experiment. Red or blue in white light, they all went dirty brown. And the result? The females no longer distinguished between them, and mated indiscriminately. Offspring of these matings were fully fertile, indicating that female choice is the only thing that stands between these species and hybridisation. The Grasshopper's Tale gives a similar example. If the two species were a bit more different, their offspring would probably be infertile, like mules. Later still in the process of divergence, isolated populations reach the point where they couldn't hybridise even if they wanted to. Whatever the basis of the separation, failure to hybridise defines a pair of populations as belonging to different species. Each of the two species is now free to evolve separately, free from contamination by the genes of the other, even though the original geographical barrier to such contamination is no more. Without the initial intervention of geographical barriers (or some equivalent), species could never become specialised to particular diets, habitats or behaviour patterns. Notice that 'intervention' does not necessarily mean it is geography itself that made the active change — as when a valley floods or a volcano erupts. The same effect is achieved if geographical barriers existed all along, wide enough to impede gene flow, but not so formidable that they are never crossed by occasional founder populations. In the Dodo's Tale we met the idea of sporadic individuals having the luck to cross to a remote island, where they then breed in isolation from their parent population.
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    1 out of 1 members found this post helpful.
    I think Dawkins is simply wrong here. The definition of species is quite consistent and doesn't need to be altered from "the ability to produce fertile offspring." The difficulties with it (such as the difficulties of observing it in nature, applying the definition to asexual organisms, etc.) are well known, and when an ambiguity arises, it's worthwhile to debate the particular ambiguity, not the definition of species.

    What Dawkins describes is clearly a combination of the concepts of a species (members of which have the ability to produce fertile offspring) and a subspecies (different groups within a species that do not typically produce offspring).

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    Quote Originally Posted by sparkey View Post
    I think Dawkins is simply wrong here. The definition of species is quite consistent and doesn't need to be altered from "the ability to produce fertile offspring." The difficulties with it (such as the difficulties of observing it in nature, applying the definition to asexual organisms, etc.) are well known, and when an ambiguity arises, it's worthwhile to debate the particular ambiguity, not the definition of species.

    What Dawkins describes is clearly a combination of the concepts of a species (members of which have the ability to produce fertile offspring) and a subspecies (different groups within a species that do not typically produce offspring).
    Richard Dawkins's quote is just an example. There is no doubt that tens of thousands of animals are officially classified as separate species (e.g. wolves, coyotes, jackals, foxes, etc.) when they shouldn't because they can interbreed. For example, Wikipedia says that members of about 37 species are referred to as foxes, of which only 12 species actually belong to the Vulpes genus of "true foxes." I think that most (if not all) members of the Caninae subfamily of the Canidae family should be able to interbreed with one another. In that case it shouldn't be a family nor a genus but simply a species, with plenty of subspecies and races.

    I think that the problem is that there is no unique, universally accepted definition of 'species'. It can mean something quite different to a lot of people, even among biologists and zoologists.

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    Here are three other short excerpt from The Ancestor's Tale :

    From page 408

    "There is a pair of European grasshopper species, Chorthippus brunneus and C. biguttulus, which are so similar that even expert entomologists can't tell them apart, yet they never cross-breed in the wild although they sometimes meet. This defines them to be 'good species'. But experiments have shown that you need only allow it female to hear the mating call of a male of her own species caged nearby and she will happily mate with a male of the wrong species, 'thinking', one is tempted to say, that he is the singer. When this happens, healthy and fertile hybrids are produced. It doesn't normally happen in the wild because a female doesn't normally find herself near, but unable to reach, a singing male of her own species at the same time as a male of the wrong species is courting her. Comparable experiments have been done on crickets, using temperature as an experimental variable. Different species of cricket chirp at different frequencies, but the chirp frequency is also temperature-dependent. If you know your crickets, you can use them as a reasonably accurate thermometer. Fortunately, not only the male's chirping frequency but also the female's perception of it is temperature-dependent: the two vary in lockstep, which normally precludes miscegenation. A female in an experiment, offered a choice of males singing at two different temperatures, chooses the one at her own temperature. The male singing at a different temperature is treated as if he belongs to the wrong species. If you heat up a female, her preference shifts to a 'hotter' song, even if that causes her to prefer a cool male of the wrong species."

    From pages 409-410

    "It is quite common to find pairs of related species mat never interbreed under natural conditions but that can do so if humans interfere. The case of Chorthippus brunneus and C. biguttulus is just one example. The Cichlid's Tale told of a comparable case in fish, where monochromatic light abolished the discrimination between a reddish and a bluish species. And it happens in zoos. Biologists normally classify animals that mate under artificial conditions but refuse to mate in the wild as separate species, as has happened with the grasshoppers. But unlike, say, lions and tigers, which can use in zoos to make (sterile) 'ligers' and 'tigrons', those grasshoppers look identical. Apparently the only difference is in their songs. And it is this, and only this, that stops them cross-breeding and therefore leads us to recognise them as separate species. Human beings are the other way round. It requires an almost superhuman feat of political zeal to overlook the conspicuous differences between our own local populations or races. Yet we happily interbreed across races and are unequivocally and uncontroversially defined as members of the same species. The Grasshopper's Tale is about races and species, about the difficulties in defining both, and what all this has to say about human races."

    From pages 414-415

    "As I said, zoologists define a species as a group whose members breed with each other under natural conditions — in the wild. It doesn't count if they breed only in zoos, or if we have to use artificial insemination, or if we fool female grasshoppers with caged singing males, even if the offspring produced are fertile. We might dispute whether this is the only sensible definition of a species, but it is the definition that most biologists use.
    If we wished to apply this definition to humans, however, there is a peculiar difficulty: how do we distinguish 'natural from artificial conditions for interbreeding? It is not an easy question to answer. Today, all surviving humans are firmly placed in the same species, and they do indeed happily interbreed. But the criterion, remember, is whether they choose to do so under natural conditions. What are natural conditions for humans? Do they even exist any more? If, in ancestral times, as sometimes today, two neighbouring tribes had different religions, different languages, different dietary customs, different cultural traditions and were continually at war with one another; if the members of each tribe were brought up to believe that the other tribe were subhuman 'animals' (as happens even today); if their religions taught that would-be sexual partners from the other tribe were taboo, 'shiksas', or unclean, there could well be no interbreeding between them. Yet anatomically, and genetically, they could be completely the same as each other. And it would take only a change of religious or other customs to break down the barriers to interbreeding. How, then, might somebody try to apply the interbreeding criterion to humans? If Chorthippus brunneus and C. biguttulus are separated as two distinct species of grasshoppers because they prefer not to interbreed although they physically could, might humans, at least in ancient times of tribal exclusivity, once have been separable in the same kind of way? Chorthippus brunneus and C. biguttulus, remember, in all detectable respects except their song, are identical , and when they are (easily) persuaded to hybridise their offspring are fully fertile."



    This obviously raises questions about the definition of the word species. In my opinion, the only genetically valid definition is the one of being able to interbreed and produce fertile offspring. It doesn't matter at all whether two races of the same species decide to interbreed or not. It is just convenient for zoologists to separate isolated and slightly different looking or behaving members of a same species into different species because that prevents them from having to test all cross-breeding possibilities in artificial conditions. Many animals are listed as separate species simply because they live in isolation from each others, on different continents or islands, but could very well be able to interbreed if they were given the opportunity to meet. The fact that it happens in zoos proves this. Similar species who cannot produce fertile offspring, like a horse and a mule, a lion and a tiger, or even a human and a chimpanzee, often have different numbers of chromosomes. This should be enough to draw the species line.


    Using one definition of the other Homo Sapiens and Homo Neanderthalensis, and Denisovans, which are all currently classified as different species under the genus Homo, should be reclassified as a single species. Not only has it been established through DNA tests that they could interbreed and produce fertile offspring, we also know that they chose to do it many times in different places. There is absolutely no scientific basis for considering them separate species. At best they could be subspecies. The different appearance of modern human races is actually the result of cross-breeding between these Paleolithic populations. In fact, there is even genetic evidence that Aboriginal Australians are partly descended from the relatives of the Java Man (Homo erectus erectus), which means that Homo Sapiens was also capable of sucessfully interbreeding with Homo Erectus.

    If someone should insist that unmistakably different looks in groups of closely related animals that have evolved in isolation for a few tens/hundreds of thousands of years is enough to list them as separate species, then they should also agree that the main human racial groups (Caucasoids, Negroids, Mongoloids, Dravidians, Melanesians, Australoids) should be seen as different species. One has to be consistent. Either Chorthippus brunneus and C. biguttulus are different species of grasshoppers, and human races become human species. Or Chorthippus brunneus and C. biguttulus are one species and so are all Homo Sapiens, Homo Erectus, Neanderthals, etc. At present we are treating humans with a different definition than all other animals.

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    1 out of 1 members found this post helpful.
    If the definition of species is a group whose members breed and produce fertile offspring, then there shouldn't be species for life beings that reproduce without the help of another individual, be it asexually (mitosis, sporogenesis, mycelial fragmentation, budding, etc.) or hermaphroditically (as is the case of many plants). Yet, if you check up the names of any bacterium on Wikipedia or in a biology book, they are divided into species like any sexual plants and animals. There are also animals that can reproduce either sexually and asexually, like sponges and jellyfish. It doesn't make sense to call those reproducing asexually "species". I think another term should be used instead to categorise different-looking members of a same genus or family reproducing by themselves. "Kind" might be appropriate. Why not say that E. coli is a kind of bacterium rather than a species of bacterium ?


    There are deeper issues in finding a good definition for species. If being able to pass on one's genes by interbreeding with another member of the group is what defines a species, how does that leave an individual with a genetic defect (not environmental) that leaves them sterile ? The DNA of such an individual cannot be passed on, and therefore it does not belong to the species of his/her parents. Strictly speaking such an individual could not even be considered a life being if the definition of life is to be able to reproduce, pass one one's genes... It show how simple definitions can lead to absurdities when applied intransigently. Yet, if we want science to be accurate, there shouldn't be any leeway in the most fundamental definitions used to classify life beings. You can't claim that a horse and a donkey are different species because their offspring is sterile and at the same time say that two grasshoppers are different species even though they can produce fertile offspring. It's double standards.

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    Quote Originally Posted by Maciamo View Post
    If the definition of species is a group whose members breed and produce fertile offspring, then there shouldn't be species for life beings that reproduce without the help of another individual, be it asexually (mitosis, sporogenesis, mycelial fragmentation, budding, etc.) or hermaphroditically (as is the case of many plants). Yet, if you check up the names of any bacterium on Wikipedia or in a biology book, they are divided into species like any sexual plants and animals. There are also animals that can reproduce either sexually and asexually, like sponges and jellyfish. It doesn't make sense to call those reproducing asexually "species". I think another term should be used instead to categorise different-looking members of a same genus or family reproducing by themselves. "Kind" might be appropriate. Why not say that E. coli is a kind of bacterium rather than a species of bacterium ?
    I would approve of changing the term used for "species" of asexual organisms, to something like "kind," to allow for stricter use of the term "species." As is, we have to recognize that "species" takes different meanings when applied to organisms that reproduce differently. When applied to asexual organisms, it's more of a judgment call, taking the form of something like "a group of closely-related individuals," often informed by how they cluster or their behavior.

    Quote Originally Posted by Maciamo View Post
    There are deeper issues in finding a good definition for species. If being able to pass on one's genes by interbreeding with another member of the group is what defines a species, how does that leave an individual with a genetic defect (not environmental) that leaves them sterile ? The DNA of such an individual cannot be passed on, and therefore it does not belong to the species of his/her parents. Strictly speaking such an individual could not even be considered a life being if the definition of life is to be able to reproduce, pass one one's genes... It show how simple definitions can lead to absurdities when applied intransigently.
    OK, but nobody is suggesting that species is defined on an individual basis, it is defined on a population basis. So a sterile individual can still be part of a species by virtue of being closely related to a population whose typical member can reproduce with the others. Since its parents can reproduce, obviously, assigning a sterile individual to a species is never a difficult judgment call for scientists to make.

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    Once we think we figured it out and categorized something correctly, the Life throws a curve ball with sterile individuals and asexual animals, lol. Good examples Sparkey. I think Maciamo's definition of species should work fine for mammals and birds at least. This is a very natural categorization.

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