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This is probably a paper of interest for all those surprised or otherwise intrigued by the gene flow now detected between H. neanderthalensis and H. sapiens.
Melanie L. Lancaster et al. Two behavioural traits promote fine-scale species segregation and moderate hybridisation in a recovering sympatric fur seal population. BMC Evolutionary Biology 2010. Open access.
The two fur seal species share the same reproductive spot at Macquarie island in New Zealand (and two other sites) and they do engage in fact in rather high interbreeding. However the two species remain neatly separated because of habitat preferences (colder/warmer waters, pebble/boulder beaches) and maybe also proposed fitness costs for the hybrid offspring.
20 comments:
Thanks for that interesting paper. And I agree it could be very relevant in the Neanderthal/modern human admixture scenario. It actually demonstrates a sort of ecological speciation although only for the actual breeding area, not the wider habitat.
"and maybe also proposed fitness costs for the hybrid offspring".
The authors don't actually propose any 'fitness costs' except the differing lactation lengths. The lactation length of hybrids would be an interesting study in itself.
The two species were obviously greatly disrupted by the sealing of the last two hundred years, and the species may be only now settling into a more stable situation. I suspect that as a result of the dramatic decrease in numbers (and so genetic variation) in the past the present gene flow between the species probably results in increased hybrid vigour, merely through the increased diversification of the gene pool. Perhaps once sufficient vigour has been re-introduced to each species hybridisation will cease. On the other hand the formation of hybrids and back-crossing may continue for some years yet, or even ultimately lead to a new species.
"The authors don't actually propose any 'fitness costs'"...
They do mention it though they are not very specific. I understand that, noticing that the genetic flow overall is low in spite of free hybridization and that both species remain neatly separated anyhow, they assume that hybrids have some sort of hidden cost.
"the present gene flow between the species probably results in increased hybrid vigour"...
There's not a single piece of evidence supporting that. While in highly inbred sub-specific populations, hybridation surely results in statistical improvement of fitness (individuals will vary), in inter-species hybridation this may not be the case at all, because the hybrids may have some fitness costs in way too many cases, if not all.
In the most extreme cases, mules for instance, the fitness cost is very clear: no fertility, but in other cases there may be other not so obvious costs as well. After all each species has already evolved for a very long time to optimal adaptation to certain circumstances and the other species is not fit for them. Also there can be illnesses derived from malfunction of genes that work perfectly within the pure species genome but fail when interacting with a different set, the strange development of hybrids such as the liger suggests that is common.
"Hybrid vigor" seems to be a banner you rally after but it's probably not always the case, in particular in inter-species reproduction.
"They do mention it though they are not very specific".
Not specific at all. In fact they simply assume there would be some disadvantage, but provide no evidence for any possible example.
"I understand that, noticing that the genetic flow overall is low in spite of free hybridization"
I'd hardly describe 21% as 'low'.
"in inter-species hybridation this may not be the case at all, because the hybrids may have some fitness costs in way too many cases, if not all".
As I mentioned above, there is no evidence for such a fitness cost in the present case. In fact 21% may hint at an advantage, in that the hybrids are able to exploit both environments.
"there can be illnesses derived from malfunction of genes that work perfectly within the pure species genome but fail when interacting with a different set, the strange development of hybrids such as the liger suggests that is common".
No. Just present in the liger, an infertile hybrid. The seals are quite fertile, with each other and with either species. In fact the authors don't even look at a third species involved in the hybridism complex on the island, merely mention it in passing. I agree that if two species are particularly well adapted to two different environments usually any hybrids that form will be at a disadvantage in either of those environments.
"'Hybrid vigor' seems to be a banner you rally after but it's probably not always the case, in particular in inter-species reproduction".
Of course not all species are capable of forming fertile hybrids with all other members of their genus. But I'm confident that future research will show that evolution generally proceeds through the complex interaction of inbreeding and hybrid vigour. Obviously any new mutation in an allele can only become fixed in a population through inbreeding. It is then able to spread through the wider population, often by the increased hybrid vigour in mixed offspring.
"Of course not all species are capable of forming fertile hybrids with all other members of their genus".
The only thing I will insist is that, even if they are fertile, fertility may be reduced in comparison with pure individuals and the fertile offspring may be largely non-competitive, not just for adaptation to specific environment but also for mating behavior, hormonal preferences, etc. There are a zillion things that can go wrong when you combine two different adapted organisms (species) in one with half the pieces of each: it may be the best innovative model but it has many chances of being a little disaster.
It is not hybridization what causes vigor but diversity within the population. In artificially bred species diversity is always so extremely low that hybridization is almost invariably good. But that's seldom the case in nature.
However you may be right in other aspects of your reply.
"even if they are fertile, fertility may be reduced in comparison with pure individuals"
But that doesn't seem to be the case with the seals.
"It is not hybridization what causes vigor but diversity within the population".
But, obviously, hybridization increases diversity. Even if not always to advantage.
"In artificially bred species diversity is always so extremely low that hybridization is almost invariably good. But that's seldom the case in nature".
I wouldn't be so certain of how different the two situations are. Many populations in nature become surprisingly inbred, as is the case with several bird species populations in NZ and the Pacific islands. Presumably fluctuations in the numbers of various populations eventually leads to spill-over into neighbouring regions, thus restoring vigour.
If the population is sufficiently large, diversity is generated from inside it, via novel mutations and via maintaining the pre-existent diversity.
Also, new species evolve largely because of inbreeding anyhow, as some genes are left out while others become dominant, driving speciation.
You exaggerate the value of hybrid vigor and I think it is because of two reasons: (1) your cattle-breeding professional bias and (2) your tendency to grab some "fetish" explanations and make them "absolute" instead of the relative value they usually have.
While there's some gene flow between closely related species often, and while this may help to transfer some adaptative or just random genes between them, it's highly questionable how important this is in each case and how "necessary" it is as well.
Surely totally isolated and inbred populations can and do evolve on their own towards well adapted species. In fact, I'd say it's the main drive of speciation: reproductive isolation. That's what all manuals of biology say. :)
Sorry to leave it so long before returning to this, but here goes:
"You exaggerate the value of hybrid vigor"
I see you're still not convinced regarding hybrid vigour. But, as my old textbook on 'Quantitative Genetics' says, 'The phenomenon of heterosis is simply inbreeding depression in reverse'. So it's basically a restoration of normality.
"Also, new species evolve largely because of inbreeding anyhow, as some genes are left out while others become dominant, driving speciation".
So you obviously accept that inbreeding is needed to produce double recessives of any new gene.
"If the population is sufficiently large, diversity is generated from inside it, via novel mutations and via maintaining the pre-existent diversity".
But by the time inbreeding is sufficient to produce double recessives of any particular (probably slightly) advantageous gene it would also be sufficient to produce a number of double recessive ancient disadvantageous genes. It's therefore unlikely that this inbred population would be able to expand on its own, no matter how advantageous the gene.
On the other hand single alleles would have spread randomly beyond the region of inbreeding. And the mutation may not even have originated in that region to start with. So, if the inbred population with the advantageous double allele were able to form hybrids with another population containing members with the single allele then the allele's expansion could become rapid. Especially considering that those with the double recessive allele would now have restored hybrid vigour. The population with restored hybrid vigour that carried the advantageous allele would be the one that expanded. Not the original inbred population. Presumably this method of allele expansion is a reasonably common pheneomenon. It certainly seems to explain our evolution right back to, and even beyond, Australopithecus. So hybrid vigour is just as important as inbreeding.
"'The phenomenon of heterosis is simply inbreeding depression in reverse'. So it's basically a restoration of normality. "
But for that you first need an inbreeding depression and also you need compatible genes, which are less likely to be so once the populations have already diverged into different species. Remember that there are studies suggesting that certain genetic closeness can be favorable, even within species and others demonstrating that populations which are inbred up to a point can be perfectly viable, that you need just a dozen or so of founders to make a population viable (maybe not ideal but viable anyhow).
The rest of what you say needs of highly different adaptative values for certain alleles, which may exist or not. I'm highly skeptic of the oversimplistic black&white evolutionary explanations of genetic drift, which in most cases have been proven wrong. I rather tend to think that alleles are normally neutral or just weakly adaptative/deleterious, what makes drift a zillion times more relevant than adaptative sweeps.
Caution: I don't say that adaptative sweeps do not exist, just that they seem to be extremely rare and at best are driven by very weak adaptative value in most cases. The fact that most traits are regulated not by one allele but by many many of them makes this point even more relevant.
"you need compatible genes, which are less likely to be so once the populations have already diverged into different species".
But it's extremely unlikely that one species will split into two overnight. Speciation is presumably a slow process, and even in the case where it's a result of geographic separation there are likely to be periods where the two variations are able to merge again to some extent. A few years ago some research was published that suggested the two species that eventually gave rise to chimps and humans had been able to form hybrids several times during their early separation. That would make sense to me.
"that you need just a dozen or so of founders to make a population viable"
I don't think so. In fact the greatest problem facing those trying to preserve threatened species is that the surviving population is too small to survive without extensive manipulation and control of breeding stock. In many such cases many more than just a dozen individuals survive. If left to their own whims the species would become extinct in virtually every case.
"I rather tend to think that alleles are normally neutral or just weakly adaptative/deleterious, what makes drift a zillion times more relevant than adaptative sweeps".
Probably true. But drift alone is unlikely to create two species from just one, unless we're dealing with very long periods of time. But we know that evolution proceeds quite rapidly at times. So drift is unlikely to be the explanation for change in such cases. Selection for particular genes or combinations of genes is a far more likely explanation.
"The fact that most traits are regulated not by one allele but by many many of them makes this point even more relevant".
And it makes it even less likely that any new species starts off as just a single, small, inbred population. The complex of genes involved in any change would usually have to reach fixation in several populations before they were able to form a new combination. Nature is a dynamic process, not fixed through time.
"Speciation is presumably a slow process"...
Of course. But if there is speciation at all, there must be neat and growing differences, otherwise the proto-species would tend to blend back into just one.
"A few years ago some research was published that suggested the two species that eventually gave rise to chimps and humans had been able to form hybrids several times during their early separation. That would make sense to me".
Sure. But that can only mean limited genetic flow or the speciation would have never been completed.
"I don't think so".
There is research on this. I am pretty sure it showed up in our discussion on the colonization of Crete c. 130 Ka ago at A Very Remote Period Indeed, discussion in which you also participated.
"If left to their own whims the species would become extinct in virtually every case".
Why? Middle Paleolithic Cretans survived for 90,000 years, Hobbits survived for much longer... Eurasian H. sapiens, with a restricted gene pool have not only survived but actually become hegemonic globally. The effects of the inbreeding episode only carry on as not too important nuisances in the overall picture.
"But drift alone is unlikely to create two species from just one"...
Why not? If there's physical (or also ecological) separation (and these are the real mechanisms of speciation: separation), in the long run, all alleles will tend to fixation in different variants in each of the populations, some of them being novel mutations that work ok in the affected population but can be incompatible with the other one(s). In the long run drift alone can and probably does create new species.
"... unless we're dealing with very long periods of time".
We are always dealing with very long periods of time when we talk evolution.
"But we know that evolution proceeds quite rapidly at times".
Examples?
"Selection for particular genes or combinations of genes is a far more likely explanation".
I don't want to exclude selection, which possibly plays a role too. But many (if not most) differences between species look pretty much neutral, capricious, so...
"And it makes it even less likely that any new species starts off as just a single, small, inbred population".
I don't say it necessarily does but I don't see why not.
"The complex of genes involved in any change would usually have to reach fixation in several populations before they were able to form a new combination".
The opposite is true in fact. The complexity and versatility of gene combos actually allows for most of the possible combos, if not all, to work well. So whatever the combo you have for eye color it works as well as the combo almost any other human has and, as long as you are not albino or something like that, it doesn't really matter if your eyes are brown or green.
What changes is the existence or not of alleles that make non-brown color possible at population level, and, at individual level, the different combos, all of whose results are similarly trivial and well fit.
I was actually going to let this be but I can't resist.
"Middle Paleolithic Cretans survived for 90,000 years"
I don't think the evidence supports that length of time. But anyway leaving aside any disagreement over whether their presence on the island necessarily proves boating ability, or whether the population was Neanderthal or modern, the evidence pretty conclusivley demonstrates that they ultimately failed to form a self-sustaining population.
"Hobbits survived for much longer..."
By becoming reduced in size, thus allowing a relatively numerous population to survive in that limited area.
"In the long run drift alone can and probably does create new species".
Are you claiming that the tiger's stripes and the lion's tawny colour are a product of random drift in each of these species? And to your favourite subject, the enlarged human brain: was that increase in size a result of random drift?
"Examples?"
Several New Zealand species that were divided by the rise of sea level just 10-12,000 years ago have already diversified into recognisably different subspecies. And doesn't a mere million years, at most, demonstrate rapid diversification of Neanderthals and modern humans? And on a geological time scale the separation of chimps and humans 5-8 million years ago suggests rapid evolution in at least one of the lines.
"But if there is speciation at all, there must be neat and growing differences, otherwise the proto-species would tend to blend back into just one".
The mixing need not be a complete intermingling of the two populations. And geographic boundary separating two subspecies is unlikely to be completely impermeable. And a single individual is quite capable of introducing a few new genes. If some new combination of genes is advantageous to the wider species, in even just the slightest manner, the combination would spread through that species, perhaps gradually. If the new genes spread rapidly the haplotype of the incoming individual would be carried along, at least to some extent.
"Eurasian H. sapiens, with a restricted gene pool have not only survived but actually become hegemonic globally".
As far as I'm aware the human gene pool is not significantly smaller than that of many other species. What is limited is the haplogroup diversity, but this can be easily explained and doesn't mean that the whole genotype is as limited.
Please, the evidence (so far) for the Cretan quartz industry was claimed to last from c. 130,000 to 40,000 BP. That's 90 Ka. and that's much longer than modern Europeans, excepting the tiny Neanderthal input, have been here.
Also the reason for their wipe out was probably no other than the dramatic Campanian supervolcano, which also helped to wipe out other cultures of Europe such as the Chatelperronian, the Uluzzian, the Neanderthals of Kostenki and maybe even North African Aterian. So it's not like you can argue at all that they died off because of "inbreeding".
Due to circumstances I have not blogged on this paper. But you may find interesting in order to expand your one-sided view of how diversity and relatedness (inbreeding) act in a competing adaptative way. Both have pros and cons but there's no absolute law.
What happens with pets and cattle is mostly extreme inbreeding, often repeated incest. Oddly enough this is done in order to enhance some characteristics that are considered desirable by breeders and that are rare in the genetic pool. So, following your somewhat far-fetched idea that breeders are like a force of nature, inbreeding may well be good in some cases and to some extent.
"Are you claiming that the tiger's stripes and the lion's tawny colour are a product of random drift in each of these species?"
These traits in particular are surely adaptative but the tiger (and possibly the lion) could well do with spots, as so many other felines or with other designs as the clouded leopard (or whatever). Cheetahs do not have the sand color of the lion and do well in the same habitat, leopards inhabit also the jungle and manage with spots, just like cheetahs.
So there are always many options, evolution, like good debates, is not just about A or B, there's always C, D, F and many other possibilities.
"And to your favourite subject, the enlarged human brain: was that increase in size a result of random drift?"
Probably the beginning of this process of brain enlargement was surely accidental, otherwise chimpanzees would have followed track, and so would have done all other animals.
The well-fit survival of chimpanzees and the demise of Neanderthals clearly shows that even what we consider adaptative traits like brain size are not necesarily best. Even what is clearly adaptative from some viewpoint has a lot of randomness to it. Tomorrow there might be a nuclear war and humans would be gone... while brainless grasses would survive.
Even adaptation is subject to chaos. Remember that any order is nothing but a subset of chaos. Let's abandon, at least partly, Newtonian mechanical linear logic. Reality does not work that way, at least not exactly so.
One of the main problems of evolutionarism or adaptativism is that most view things in black and white. A trait is expected to be fit or not, when in fact most are just more or less fit depending on circumstances (dynamic equilibrium) or are largely interchangeable (effectively neutral). The time of black and white cinema is long gone and even more the time of Newtonian mechanical thought.
We need to open our minds and embrace the amazing multidimensionality and complexity of Chaos, of Real Reality.
You can always step down to linear mechanics, to oversimplified A or B choices and the like for the sake of modeling or explaining but always aware that Reality is not that way.
Clouds are not spheres, mountains are not cones, coastlines are not circles, and bark is not smooth, nor does lightning travel in a straight line.
Benoît Mandelbrot.
Those who insist in looking for fitness as the only drive of evolution, similarly miss the real picture.
"Several New Zealand species that were divided by the rise of sea level just 10-12,000 years ago have already diversified into recognisably different subspecies".
Subspecies is an undefined scientific term and does not imply radical division. The fact that they are not yet different species, as are not humans nor are Asian and African lions shows it takes much more time.
For me millions of years (as you mention for the other examples) is not "rapid evolution" but quite slow in fact. If Neanderthals and us could still mate and form fertile hybrids (and even tigers and lions can), that means that evolution really takes its time not that it goes fast.
Of course it can be accelerated at some specific critical periods, when extinctions or other events open new niches earlier unavailable.
But the most normal way of evolution is gradual, largely stochastic and more often than not driven by geographic separation.
"And geographic boundary separating two subspecies is unlikely to be completely impermeable".
But it's likely to be effectively impermeable, drifting out most of the rare genes that manage to cross it, just because of the law of probability.
"And a single individual is quite capable of introducing a few new genes".
A single individual in a large population most likely won't introduce any gene because the law of probability plays against that (not counting other barriers such as differences in mating behavior or even biological barriers). It happens but it's rare and most often imply highly adaptative genes which can overcome sometimes the high odds against them for that very reason (introgression).
"If the new genes spread rapidly the haplotype of the incoming individual would be carried along, at least to some extent".
Not really. Because the introgression cases we know of imply intermediate buffer hybrid populations. It's not an individual who normally makes that but a gradual flow by means of the small buffer hybrid population (and probably other not really hybrid ones). It's a process similar to distillation, so really only the hyper-adaptative gene makes it, not the haplotype.
But guess it may depend and you may be right in some particular cases.
"As far as I'm aware the human gene pool is not significantly smaller than that of many other species".
I was talking of Eurasians, most of whose genetics went through a quite sharp bottleneck-like episode at the OoA migration, which is demonstrated to have caused genetic diseases, for example (sorry but can't find the link right now).
It is not only haploid diversity which was lost but overall genetic diversity and that can be seen in many different research articles. The loss of diversity was very notable but still we managed to survive quite well and make the best of that remaining diversity. Soon anyhow, because of population explosion we generated our own diversity but always a fraction of what is found in Africa.
"you may find interesting in order to expand your one-sided view of how diversity and relatedness (inbreeding) act in a competing adaptative way".
But hang on. If anything the paper shows that ultimately hybrid vigour is necessary for long term survival. 'We conclude that negative effects associated with competition are less manifest in diverse groups'. They claim that the most heavily populated regions are occupied by a divesity of genotypes.
The main claims are, 'survival was higher in low than in high density, [That makes complete sense, no matter whether inbred or not] that survival peaked at intermediate colour morph diversity in high density, [presumably at least partly a result of hybrid vigour] and that survival was independent of diversity in low density where competition was less intense'.
So inbred populations are able to survive for some generations where competition is less intense, but are unable to exploit the environment to the fullest. Their numbers remain small. And will do so until new genes are introduced from the wider population. Members of the wider population are no doubt able to occasionally enter regions where inbred populations survive. At such times they will restore some level of hybrid vigour. If that fails to happen the inbred group will ultimately die out. Unless it manages to produce sufficient diversity within its own population before it does so.
The paper also says, 'that there is conflicting selection for and against genetic diversity occurring simultaneously, and that diversity and relatedness may facilitate the productivity and ecological success of groups of interacting individuals'. In other words exactly as I have always calimed: evolution is the result of the complex interplay between inbreeding and hybrid vigour.
I firmly believe that human development is a product of the same process. Isolated populations have been given a boost by incoming groups. Often even the technology displays 'hybrid vigour'.
"So, following your somewhat far-fetched idea that breeders are like a force of nature, inbreeding may well be good in some cases and to some extent".
You misunderstand me. I have always claimed that inbreeding is necessary to fix any new gene, whether in nature or in farming. So, therefore, advantageous.
"One of the main problems of evolutionarism or adaptativism is that most view things in black and white. A trait is expected to be fit or not, when in fact most are just more or less fit depending on circumstances"
Survival, whether in nature or on the farm, is a product of a whole raft of traits, not just one. This holds true even for farmers trying to fix a particular desirable trait in their livestock. The inbreeding required for trait fixation is almost always sufficient to bring out some disadvantageous genes as well. Successful breeding requires a very complex balancing act.
"the tiger (and possibly the lion) could well do with spots"
Lions actually have spots if you look closely. Lions are presumed to be more closely related to leopards evidently. They are even capable of forming fertile offspring with them, unlike lions and tigers whose hybrid offspring are infertile. Do you really believe the reduced prominence of spots on a lion's coat is the product merely of drift?
"otherwise chimpanzees would have followed track, and so would have done all other animals".
Presumably whatever advantage the enlarged brain provided for humans did not apply to chimpanzees, or other species. But I'm sure that even if the genetic change that lead to 'this process of brain enlargement was surely accidental' its spread through the proto-human species surely was not.
"Subspecies is an undefined scientific term and does not imply radical division".
Obviously not radical, or it would already be defined as a species. On the other hand subspecies are obviously the first step along the road to speciation.
"But it's likely to be effectively impermeable, drifting out most of the rare genes that manage to cross it, just because of the law of probability".
Any rare genes will only drift out if they provide no advantage whatsoever to individuals with them.
"We need to open our minds and embrace the amazing multidimensionality and complexity of Chaos, of Real Reality".
But whatever events chaos throws up the response of the survivors is certainly not chaotic.
What the grasshoppers' paper shows is that both relatedness and diversity affect survival in a complex and dynamic way. It is an intra-species study, so it's not fully comparable with the inter-species hybridization, where genetic distances may be already too large and seem favor less your hybrid vigor proposal for that scenario (even if it can still have some restricted favorable influence in some particular cases).
The grasshopper scenario is only directly comparable with intra-species dynamics, such as those among variants of Homo sapiens, and not with inter-species ones, such as those of Neanderthals and Sapiens. And even in these conditions it shows that relatedness has also a positive selective effect, even if hybridization has it too.
"You misunderstand me. I have always claimed that inbreeding is necessary to fix any new gene, whether in nature or in farming. So, therefore, advantageous".
Maybe I misunderstood you but still I think you give way too much importance to selective evolution in comparison with neutral evolution and also that you seem to ignore that the complex "alchemy" of interdependent genes may be abruptly disturbed with the introduction of some (but maybe not others) genes from different populations, specially when these other populations are so distant that we call them different species.
"Successful breeding requires a very complex balancing act".
I don't doubt it. And this is also true in natural conditions surely. But in natural conditions both excess inbreeding and outbreeding across large genetic distances are generally much rarer, with the populations drawing mainly from their own internal diversity.
"Do you really believe the reduced prominence of spots on a lion's coat is the product merely of drift?"
I don't know for sure but the comparison with cheetahs, which share the same savanna environment really suggests that both designs are equally adaptative for these conditions. So neutral evolution looks more likely in fact.
"Any rare genes will only drift out if they provide no advantage whatsoever to individuals with them".
Rare genes with only slight advantages will also be drifted out unless they happen to be very lucky. Obviously, the more adaptative the novel gene is, the easiest it will have to overcome this statistical handicap but still I bet that a lot of highly adaptative genes have been lost because of mere drift. That's because fitness is not a simple but a highly complex value and single traits alone cannot really account for individual fitness overall. And also because of the stochastic nature of real selection, where a lot of unpredictable accidents can destroy any adaptative individual (or even whole communities) before the can bear offspring and consolidate the trait somewhat.
Also what now is adaptative, tomorrow may be a handicap and vice versa. Conditions change and that's why diversity is so important.
"But whatever events chaos throws up the response of the survivors is certainly not chaotic".
Everything, even the most elaborate order is chaotic. Orders are only subsets of Chaos.
While individuals or communities may behave in an apparent orderly manner, behaviors in fact vary a lot and are extremely unpredictable, specially in the long run. One could argue for instance that hydrodynamics introduce a law or principle in survival in watery environments such as the sea... but then you have species that have clearly dropped hydrodynamics in favor of other advantageous traits such as camouflage or whatever.
There's always another and yet another possibility. Evolution is anything but linear or pre-determined and very slight local variations that are stochastic in nature completely alter the pattern of evolution once and again. The fact that very few species remain relatively static is prove that Chaos is once and again twisting everything. Even best fit designs can evolve into equally best fit but very different ones.
There's no one single solution for the extremely complex algorithm of Nature. Probably solutions are infinite, though only a finite number actually come to existence, largely because of accidents.
Also you may be interested in reading this latest article from BMC Genetics (Cheng Xue et al.), which deals with how recombination and the fact that most genes are duplicated in the individual genome affects evolution (favoring the wild-type or ancestral alleles and, that way, also favoring the rare novel advantageous mutations, while removing the more common novel deleterious ones).
I do not mention it to support my points or counter yours but just to expand the scope of the analysis on how evolution happens, regardless of admixture.
Probably rather than argue on differences of emphasis we should read and assimilate more materials on this interesting matter, a very much open field of research.
(Note: the article is highly technical so I won't probably blog on it).
Thanks for that interesting, although rather long, link.
"Maybe I misunderstood you but still I think you give way too much importance to selective evolution in comparison with neutral evolution"
The first thing I noticed in the link is that they mention that even under weak positive selection 'the advantageous allele will fix before the nonfunctional allele will'. So even weak selection is capable of altering the genetic makeup of a population.
"Rare genes with only slight advantages will also be drifted out unless they happen to be very lucky".
That doesn't seem to be what the article is saying.
"Evolution is anything but linear or pre-determined"
I doubt if any but the most rabid creationist would claim evolution is 'pre-determined'.
"Probably rather than argue on differences of emphasis we should read and assimilate more materials on this interesting matter, a very much open field of research".
I agree. And research carried out on domestic animals will be very revealing. Already has been in fact. Much money has been put into the animal and plant breeding spheres.
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