Speciation and then reproduction

[aychamo]

Ok. I know evolution works on populations, but what I don't understand is if you use the biological definition of species, being that a species is separated by reproductive barriers, how does speciation work?

I mean, would you just have an individual born that is changed to the point that it can not reproduce with the population from which it came? But who would it reproduce with?

[SkinWalker]

It would involve entire populations making the change to new species. Over many millions of years.

Orangutan's for instance come in one species with two sub-species: the Pongo pygmaeus abelii of Sumatra and the Pongo pygmaeus pygmaeus of Borneo.

Both sub-species are capable of interbreeding and producing fertile offspring (the primary marker of an individual species), but have diversified after 10k years of geographical separation. It's likely that in the future (another 10k years or another million...) that the genetic separation between the subspecies will be too great for production of fertile offspring.

The evolutionary forces on speciation is often hard to imagine due to the significant amount of time it takes for a species to evolve into another. When thinking about this, it's helpful to think about evolution among populations rather than individuals.

It's in populations that the major forces of evolution work: mutation, natural selection, gene flow and genetic drift. The less gene flow that exists between populations and the more environmental pressures placed upon populations, the more likely genetic separation is to occur. In addition, population size plays a large role as well, since the larger the population the less effect mutation can have.

Hope that was helpful... I tried not to be too technical.

[premjan]

The challenge in imagining new speciation is to imagine how systematic mutation permanently alters a population. Probably the mutation arises initially only in one individual. But that mutation becomes crucial to the species' survival for some reason, hence progeny of that mutant survive preferentially.

[arcangle]

A good example of this is the speciation found in isolated lakes. When a lake is formed by damming a river, or new lakes are formed by fluctuating water levels, all the inhabitants of that portion of river or lake are isolated, eventually leading to speciation.

Run a google on Lake Victoria, speciation, and fish.

Very interesting reading.


Lyn

[premjan]

rather than "mutation" being an unusual crisis phenomenon, we can conclude that genetic "radiation" is constantly happening and the only thing keeping organisms from deviating into different species is sexual reproduction which tends to diffuse specialized genes among the population. So the instant some isolation of a population occurs, it becomes likely that speciation would occur. Thus if, by chance, a genetic variant that can survive in an unusual environment is created, then that variant will most likely survive in the absence of competition and eventually radiate into a new species. This sort of steady genetic divergence makes more sense, with species dying or becoming eliminated only in rare cases of radical climatic change or predator pressure. Thus evolution actually occurs constantly, not in punctutated bursts. The illusion of "new" species only comes about when the ability to reproduce with a parent population is incidentally lost due to some physical architectural problem.

[nessa20x]

Generally reproductive isolation takes a long, long time to evolve. However if you do some research on colchicene experiments, you can see that after 1 generation the offspring can be genetically isolated from the parental plants and from any other members of the parental species. Of course there are significant barriers to this experiment in animals because of the alloploidy nature of the offspring...but the same results from the treatment occurs in nature and is one way (albeit a rare one) that new plant species evolve.

[Roland98]

This is an E/C issue.

-Roland98
S & S moderator

[RufusAtticus]

Originally posted by aychamo
I mean, would you just have an individual born that is changed to the point that it can not reproduce with the population from which it came? But who would it reproduce with?

That is not how reproductive isolation works in most cases. The following is what Mayr advocated. A single panmictic (all-breeding) population is split into two populations via some geographic barrier. That barrier prevents or greatly hinders individuals from migrating between populations. In isolation the two populations can diverge through both genetic drift and local adaptation. At some point traits will become fixed in both populations that create biological barriers to interbreeding and thus when the geographic barrier goes away and the populations come into contact with one another, they can no long interbreed.

There is no first individual since the process is gradual.

Now, some taxa can have first individuals whose isolation is caused by a mutation of large effect. These are taxa in which an individual can reproduces asexually or via selfing so that not initially having a partner does not stop it from spreading.

[MrDarwin]

Two things I've learned from studying plants are that (1) natural populations frequently exhibit a great deal of natural genetic variation (in general, the more widespread the species, the more the local populations will differ from each other), and (2) related species (including many that are morphologically quite different from each) retain the ability to interbreed, but don't interbreed because they have non-overlapping distributions, utilize different pollinators, bloom at different times of the day (or year), etc. The more distant the relationship, the stronger the intrinsic reproductive barriers.

This strongly suggests that (1) morphological (and presumably genetic) differences precede speciation, and (2) strict reproductive isolation--the utter inability to interbreed--is a byproduct of speciation, and not a cause of it.