(Futuyma pg 448)
Early taxonomists (predarwin) held what Ernst Mayr has called a "typological" or "essentialist" notion of species. Individuals were members of a given species if they sufficiently conformed to that "type" or ideal, in certain characters that were "essential", fixed properties, a concept descended fom Plato's "ideas".
Nevertheless populations ARE variable. Quite often the variation among specimens falls into discrete groups, or clusters, as in the case of the raven versus crow. These clusters, for early taxonomists, were species. Each had certain "essential" defining properies, such as the ravens's pointed tail, that varied only slightly. But often gradations between such clusters DID exist. For instance , carrion crows whcih are entirely black, differ from hooded crows which have a gary back and belly, EXCEPT in certain parts of Europe, where some specimens ARE intermediate.
the existence of such cases has led some authors to conclude that species are arbitrary constructs of the human mind imposed on a a continuuum of variation. Darwin for one, took this position.
There coexisted, even in the eighteenth century, another criterion: common descent. Offspring of the same parents were the same species, EVEN if they differed considerably (from each other or the parents). [this is important, and leads to subtle points].. the striped and banded forms of the California king snake known to be born to the same mother, represent a genetic polymorphism, not different species.
By the early 20th cent.. Mayr, in his Systematics and the Origin of Species (1942) noted:
1) many, perhaps most, characteristics vary among the members of a single population of interbreeding individuals. Sometimes continuus quantitative variation; sometimes discrete like the king snake.
2) populations in different geographic locations usually differ in the mean of one or more characteristics. very often intermediate forms are found where such populations meet, providing evidence [?] that they interbreed.
3) what APPEARS at first to be a single, MORPHOLOGICALLY uniform species often proves to include two or more subpopulations that occupy the same area, but do not interbreed. these cryptic sibling species might be recognized on the basis of some form of reproductive barrier (different genetalia, different breeding seasons, etc..) for instance two treecreepers (birds) in central europe almost identical but for very different songs.
the abundance of variation within and between COMBINED with Darwin's view that all characters can keep evolving into more and more different characters led to the abandonment of the typological species concept. the biological species concept became more and more widely held. promulgated by the likes of Dobzhansky (1937) and Mayr (1942). Mayr defined it: "species are groups of actually or POTENTIALLY interbreeding poplations whcih are reproductively isolated from other such groups"
It has some drawbacks.
1) it only works in a particular slice of time. if you look at a branching lieage over time.. you can't test whether the population at time A could interbreed with the population at time B. these may be seperated into what are called chronospecies.
2) if a lineage forks, we do not know which fork... is it one lineage with a side lineage or is it 3 chronospecies...
3) of course it doesn't work for asexual populations
4) this is a definition in terms of populations, not individuals. chihuahuas don't (i don't think) mate with great danes, but in a population of dogs, chihuahuas can mate with dachshunds, which mate with i don't know my dogs! which mate with ... which mate with great danes. this last point opens up the whole can of worms! for instance Ring Species:
Ring Species: Unusual Demonstrations of Speciation
By Darren E. Irwin
[blending! till they meet at the begining again. so all it takes is .... extinction in the middle!]
5) the definition of reproductive isolation is "those which are naturaly [?] reproductively isolated from each other by their own behavior" which is WHAT? their behavior depends on the environment no? so as individuals migrate to different environments what might happen? anyway there are plenty of populations who DON'T mate in their usual habitat who CAN be induced to mate in artificial environments and who's offspring is fertile.
these last two lead us to an important fact. many seemingly fixed boundaries CAN leak occasionaly either through modification of behavior (by environment or by mutation) or through slow leakage through roundabout intermediates (like the chihuahua and great dane, or like ring species.
6) many populations are geographically isolated and will never manage to interbreed even though if we brought individals from them together they WOULD. hmm what to say? this is like (5)!!! BSC says they are both part of the same species. but frankly i think that stinks of essentialism! the fact is that species maybe is a property of the whole biome not just property of the organisms's essence! hence i say if we are not to be essentialists we call those two different species. the environment is part of what defines the species. and certainly with time the two populations might diverge reproductively anyway.
hell, the fact is that species might be nothing at all!
Futuyma (pg 452) says it would be absurd to call every geographicaly isolated pocket of frogs a different species. this is similar to the problem with dandelions. who at some point in the past were sexual, but now have given it up and have split into thousands of pockets of nonbreeding slightly different populations. of course frogs occasionaly travel (by flood maybe) and these dandelions OCCASIONALY have sex...
Baum and Shaw's genealogical species concept adds to this idea of leaks. (p452)
one thing we gotta mention now is that organisms are mosaics. genetically we are mosaics of thousands of discrete (almost, everything has messy exceptions) genes each in pairs, one from each parent. for each gene position on the chormosomes, in the population there might be many versions, in fact since we are diploid, even an individual organism can have two versions of a gene, one from each parent. these versions are called alleles. the gene is called a locus.
(of course it's very messy, not only can genes get mutations so that one of an offspring's allele can differ from its parent, you can have all kinds of mutations. for instance a gene can get duplicated! now where a parent has one gene called A332, the offspring has two of them from one parent and one from the other. in time this gets passed down and one of the copies can get a mutation and now some members of the population can have A332 and A332...)
the point is, that two subpopulations can be similar for SOME genes and different for OTHERS! each gene can evolve INDEPENDANTLY to varying extents.
so, genealogical species concept: after a population splits in two, each half has a mix of these different alleles for each gene. now whether alleles make it to the next generation is a chancy event, or it might be selected for. so eventually for gene 12444 a member of pop A might have alleles similar to pop B, while for gene 24432 a member of pop A will have alleles similar to pop A. so which genes do we look at to tell whether these two populations are distinct species or not?
this by the way is the case for ALL human populations. can't tell if there are distinct races for ALL genes. we all split up TOO recently, and still too much MIXING.
the genealogical species concept is we wait till the populations keep going until enough alleles get lost through randomly not breeding into the next generation so that pop A has only alleles in common with it for ALL (or MOST?) genes and pop B has alleles in common with it.
more problems with BSC:
1) narrow hybrid zones. there are cases where what looks like different species meet at a border and for a narrow zone at that border you find hybrids. there is evidence of pairs like this existing for some time, yet remaining distinct. some kind of mechanism is allowing SOME hybridization to take place, but the hybrids are not as fit as the parent species and the zone of hybridization doesn't spread through the seperate populations. this does NOT mean that individual genes, cant travel from one population to the other! the overall mix in the hybrid might be rather unfit, but single genes from populoation A might still eventually spread throughout population B. LEAKS!
2) this can even happen with two populations sharing the same teritory. just that hybrids are VERY RARE.
the question is what mechanism maintains this!
plants however work different than animals. and in fact the BSC may not be as useful for plants as animals. for instance there are many plant groups that rampantly hybridize all over the place like Quercus (oaks) and even worse: Crataegus (hawthorns)
so here is the point since different genes can have different propensity to spread from one population to another... some genes might affect something minor like color. some genes might affect something major like adaption to new niches. and some genes might affect MATING BEHAVIOR.
and now we have to talk about reproductive isolation.
what are the things that can cause this:
1) geographical isolation
2) behavioral difference: what time of day to mate, on which plant to mate, which way to dance, which color plumage you like, what kind of song you learn...
3) anatomical diff: maybe your penis is shaped the wrong way..
4) biochemical mechanisms that block the viability of sperm or pollen
5) difference in genetics (chromosomes ) that make the zygote unable to form
6) differnece in genes that make development botch up somehwere down the road
7) differences in genetics that make the offspring unable to perform mieosis and thus make viable eggs or sperm and thus GRANDCHILDREN are impossible.
biology is fun, no?
each of these mechanisms CAN be the product of a single mutation so potentially this can happen in one generation. for instance in plants, it is common that an offspring gets the wrong number of chromosomes (double or 2/3 etc..) and can only therefore mate with another offspring with the same mutation.
What's the upshot? the dicotomy is: are species a property of the whole history of the whole population structure or is species a property of some innate properties of individual organisms?
the Eldredge and Gould claim that a large part of the fossil record consists of: for layer after layer fossils of a certain type wander around a mean of morphology for a coupla million years and then switch to a nother mean quickly (in geological time) and then stay wandering around the new mean. (punctuated equilibrium) or the type splits into a FEW distinct types which each then wander around their distinct means...
there are two poles and probably everything inbetween:
1) individuals reproduce to randomly varying offspring and the set of individuals wander higgledy piggledy all over the space of all possible varieties. but since each niche has finite carrying capacity there is arbitrary extinction. therefore after awhile gaps appear between lineages. i don't know how much natural selection is needed for this to happen.
2) the logic of development, physiology, ecology act as feedback loops which encourage mutations of only certain types to persist and rejects other mutations, until a combination of mutations creep in that throws the system into a new set of feedback loops keeping the system around a new norm (which would give punctuated equilibrium) This would correspond to typological species concept.