Why species?

[pErvinalia]

Not "how species", but "why species"?

Let's talk about say humans and chimpanzees. What is the evolutionary benefit/adaptation/reason for one bag of genes (human) not being able to recombine with another bag of genes (chimpanzee)? I can understand the distinction between say plants and animals - different life strategies to pass on genes; the result leading to an obvious physical incompatibility for sexual genetic recombination. But animals, and even more specifically vertebrates, and even more specifically mammals etc etc..., share a similar mobile life strategy for ensuring the survival and replication of their genes. So in the case of closely related animals, there isn't a physical barrier to sexual recombination, only (presumably) a chemical one. The question is - Why has this chemical barrier come about, and what evolutionary purpose does it serve? Looking at this simplistically, it could make more sense that the genes of chimps and humans could recombine and replicate, than not. As a crude example, say we have only 100 humans and 100 chimps. The genes of either increase their chance of replication by double if sexual reproduction across species was possible.

Or is it a case that Chimps and Humans are sufficiently different (but only 2%!?) that "outbreeding" would cause a watering down of the genes most responsible for the behaviour of sexual attraction and mating?

[Xamonas Chegwé]

You're looking at it bass ackwards.

It is not that there is a reason for sexual incompatability between populations of different species. It is simply the case that, if there is sexual incompatability between two populations, they are defined as seperate species.

The sexual barrier comes about due to long-term isolation of populations eventually leading to genotypes that can't easily produce viable, fertile offspring.

Nature doesn't do stuff for a reason. Nature is simply the sum total of all the stuff that happens.

[pErvinalia]

You've answered the "what". Not the "why". The "why" doesn't imply any sentience or direction of nature. It is a question of the mechanisms that lead to a certain outcome. What is the evolutionary mechanism that lead to sexual incompatibility in species that aren't physically incapable of reproduction? The "why" of that "what" is - Why does genetic isolation of different populations lead to a chemical inability to reproduce? The basis of evolution is the replication of genes/alleles. I'm wondering how the mechanism that places a restriction on that core action came into being. Clearly(?) the answer is that the genes most responsible for sexual reproductive behaviour are favoured by that restriction. But i'm interested in what the mechanism responsible for that state of affairs is.

[pErvinalia]

Hang on, I seem to be answering my own question. The question I want to know is the "Clearly" bit from above. Is that the reason that has lead to the restriction on genetic recombination across species?

[pErvinalia]

Or... Is it just an inevitable effect of genetic adaptation to different environments? Basically, is it just a physical reality, and initially totally independent of the mechanisms and processes of evolution?

[Xamonas Chegwé]

It is mainly that, in isolated populations from a common stock, the chromosomes alter physically over time as more and more mutations and selection occurs, until a point is reached where the alleles on the two sets don't correspond enough to produce a viable offspring. Left long enough, even basic details like the number of chromosomes in a cell, or the lengths of the DNA chains, can change.

This is the genetic basis of speciation. Add to this the phenotype changes that can make sex between species physically difficult or undesirable (due to changes in mating behaviour, pheromones, shape of genitalia, etc.)

[Robert_S]

Kinda like you have a hard time when all your nuts and bolts are metric but your wrenches are normal!

[Jason]

Or when your bolt is metric but your nut is SAE. You can force it, but it won't be pretty.

[Horwood Beer-Master]

Not "how species", but "why species"?

Let's talk about say humans and chimpanzees. What is the evolutionary benefit/adaptation/reason for one bag of genes (human) not being able to recombine with another bag of genes (chimpanzee)?..

None. It's just the more different the two sets of genetic 'instructions' become (due to isolation) the less well they can operate side by side to build a viable organism.

Obviously this effect means that somewhere along the way it becomes a disadvantage to even try as your genes will work much better combined with those of an individual not so distantly related to you. At this point, all things being equal, individuals of the two groups, if in contact with each other will positively avoid interbreeding as they don't want to waste the investment on offspring that may very well develop abnormally (if at all). Also if the two lineages have begun to evolve to specialise on different food sources, an in-between individual is often less well adapted at exploiting either food source then it's 'pure blood' parents.

But it's still the initial isolation that causes the beginnings of the incompatibility, and does it for no "reason" at all. It's purely the effect of differences building-up.

It becomes far more obvious if you choose as your examples more distantly related organisms. Take a human and an oak tree, even if the genes from each do find themselves in the same gamete-cell, how the hell can any kind of viable organism possibly come of it.


Although I might just add on the human-chimp side of things that (rumoured Soviet projects aside) I'm not sure how well genetic incompatibility between the two species has ever really been tested.


There may be a number of spelling/grammar errors in the above post - I didn't get much sleep last night. I fixed all the ones I could find.

[Clinton Huxley]

If everything could reproduce with everything else I think we would not have got beyond bacteria.

[Mysturji]

[Gawdzilla Sama]

Evolution is unguided, so "why" is not a question.

[pErvinalia]

Non-sequitur.

see http://www.rationalia.com/forum/viewtop ... 9#p1238193

[Mysturji]

Shit happens.

[Xamonas Chegwé]

Shit happens.

I think that is the answer to "Why fæces?" Easy mistake to make.