We all (except a handful of fanatics) agree that evolution happens. What is not fully clear are the details of how it happens. These days two new researches challenge some conceptualizations and support others instead.
Evolution is not constant but punctual.
That is what Stephen A. Smith (left) and colleagues have discovered when analyzing the phylogeny of angiosperms (flowering plants). They found that the conceptualization of evolution as punctuated rather than constant seems to be correct, and also that the characteristics of a new taxon evolve in a single population or root species before diversification (radiation) occurs.
The research has been published in the American Journal of Botany (open access) and a synthesis appears at Science Daily.
Perfectly fit? Maybe not needed.
A simplistic school promotes the idea that given enough evolutionary time in a simple ecological niche only one competitor will survive: the fittest one.
However this does not seem to be the case. Research by Robert E. Beardmore and colleagues, published in Nature (ppv), vindicates previous research in which optimal efficiency coexists with suboptimal one:
The fit use food well but they aren't resilient to mutations, whereas the less efficient, unfit consumers are maintained by their resilience to mutation. If there's a low mutation rate, survival of the fittest rules, but if not, lots of diversity can be maintained.
Full story at Science Daily.
"They found that the conceptualization of evolution as punctuated rather than constant seems to be correct, and also that the characteristics of a new taxon evolve in a single population or root species before diversification (radiation) occurs".
ReplyDeleteThis is remarkably close to what I explained to either you orGerman concerning the expansion of newly-arrived species. They stay put for several generations and then spread prodigiously. It is at this later stage of a species' expansion that extinction of other species occurrs. In the earlier discussion I was explaining anthropogenic species-extinction during the Late Pleistocen, but the same process is more widely applicable.
But it's obvious why newly-evolved species don't expand immediately. A new species-defining mutation doesn't occur in a whole population simultaneously. and it must become established in a population before it can even give rise to a new species. So the individula in which the species-defining mutation first occurrs must be able to breed with neighbours that lack the mutation, at least for a few generations.
Ultimately the new species expands. After that expansion the same process can repeat itself in a regional population of that new species. And off we go again.
I thought I'd explained the process quite well in y essay "Hybrid Vigour and Inbreeding", specifically under the heading "Wave Theory of evolution".
They seem to be quite different things: you are talking about migrant species' expansion and also probably of a matter of years, decades or centuries at most. The article deals with speciation, radiation into many different species, not with mere geographic expansion of a single species.
ReplyDeleteI fail to see any connection.
"They seem to be quite different things"
ReplyDeleteNot really. The article deals with the expansion of a new species, which is surely comparable to the expansion of a newly-introduced species. Especially as it seems that both processes involve a period of slow increase of population numbers before a rapid expansion.
"The article deals with the expansion of a new species"...
ReplyDeleteYour misunderstanding: it deals with radiation into different species, not expansion of a single species.
"as it seems that both processes involve a period of slow increase of population numbers before a rapid expansion".
It does not say that either. It says that the root species tends to remain stable for a long while before radiating into diverse new species. Not a single mention to numbers.
"Your misunderstanding: it deals with radiation into different species, not expansion of a single species".
ReplyDeleteSo what is the difference? From your own notes:
"It says that the root species tends to remain stable for a long while before radiating into diverse new species".
Basically the new species tends to remain stable until it radiates into new species. Radiation will not happen until the species expands geographically. Your next entry starts off:
"A simplistic school promotes the idea that given enough evolutionary time in a simple ecological niche only one competitor will survive: the fittest one".
That holds for plants as well. Diversity is associated with specialisation for particular environments. As an ex gardener you should know that. So:
"the characteristics of a new taxon evolve in a single population or root species before diversification (radiation) occurs".
Therefore the characteristics of a new taxon evolve in a single population or root species before it is able to expand its geographic range. At which stage it will diversify.
It all depends on whether speciation is allopatric or sympatric. For example tigers expanded a lot but the main related speciation events did not happen at remote places: lions, leopards and snow leopards (close relatives) also live in South Asia.
ReplyDeleteThe speciation steps in this case look more of a sympatric phenomenon than anything else, where speciation is driven by ecological specialization rather than by geographical distance.
The same can be observed between Pan and Homo, where it was not remoteness but ecological specialization (jungle vs. savannah) what drove the divergence.
There may be other cases but it's clear that biological radiation is not a mere issue of geographical expansion but more one of geographical fragmentation by any means. The typical example are the turtles of Galapagos Islands, where a single ancestral taxon created many different (sub-)species only because each lived in a different island. In other cases, also in the Galapagos, it is ecological specialization (iguanas, finches) but in all Galapagos' cases it is not a matter of expansion because the islands are pretty much finite, but a matter of ecological division of some sort.
"For example tigers expanded a lot but the main related speciation events did not happen at remote places"
ReplyDeleteTiger subspecies are distributed geographically. So their diversification was geographic presumably after the expansion of virtually a single species.
"lions, leopards and snow leopards (close relatives) also live in South Asia".
And their diversification from tigers was presumably also originally geographic. Lions in savanah regions of Africa and leopards somewhere in some forested region. The four species now live together in India but they tend to divide their niches ecologically, but that is not the reason for their original diversification.
"The same can be observed between Pan and Homo, where it was not remoteness but ecological specialization (jungle vs. savannah) what drove the divergence".
But those ecological regions were geographically distinct.
"In other cases, also in the Galapagos, it is ecological specialization (iguanas, finches)"
I would hesitate to use these as examples of ecological speciation. They too may have originally separated geographically, on separate islands, and subsequent inter-island movement has brought them together in places.
"The speciation steps in this case look more of a sympatric phenomenon than anything else, where speciation is driven by ecological specialization rather than by geographical distance".
It is my guess that geographical distance is far more important in speciation than is ecology. Ecological specialisation takes place once two geographically differentiated species eventually meet up.
"There may be other cases but it's clear that biological radiation is not a mere issue of geographical expansion but more one of geographical fragmentation by any means".
I agree, but geographic
nevertheless. For example Hebe is a widespread New Zealand genus of more than eighty species, and almost certainly originated in New Zealand. One species is endemic to easter Island and two NZ species are also found in South America and the Falkland Islands.
But within NZ it is rare to find two species within the same region. And where you do find more than one species they are usually ecologically specific. Each species is isolated either geographically or ecologically from the other species.
Are you really claiming that all these separate species originally formed in just one small region and then expanded from there? That their present distribution is a product of drift, founder effect and bottlenecks? I don't think so. The diversification accumulated after a single species had expanded through New Zealand and adapted to different environments.
(continued)
"they tend to divide their niches ecologically, but that is not the reason for their original diversification".
ReplyDeleteI believe it is, as shown by other examples.
"Each species is isolated either geographically or ecologically from the other species".
What I am saying. This does not require expansion in mere extension as much as differences between ecologies, either by force, "choice" or a mix of both.
"Are you really claiming that all these separate species originally formed in just one small region and then expanded from there?"
I am actually saying that does happen at least often enough. The Galapagos' examples are a clear case.
(related observations)
ReplyDeleteI now perceive the difference between how you and I perceive the process of evolution, including the origin of present human haplogroup distribution.
You are trying to fit the haplogroup evidence to as small a number as possible of small populations carrying a whole collection of basal haplogroups. Whereas I am prepared to accept a much greater number of small populations carrying just a few haplogroups, basal or otherwise.
You envisage an early expansion of a single population carrying not only Y-haps C1, C2, C3, C4, C5, F1, F2 and F3 but goodness knows how many Ks. You're even prepared to envisage it as containing already diversified MNOPS. On the mtDNA side your hypothetical population contained all 14 basal N haplogroups and goodness knows how many basal M haplogroups. In order to then explain the modern haplogroup distribution you are forced to invoke a surprising level of drift, bottlnecks and founder effects. You avoid considering the haplogroup evidence as it stands.
I try to consider the haplogroup distribution evidence at face value before I invoke drift, bottlenecks and founder effects. I believe the evidence most easily fits a whole series of expansions of different haplogroups from different regions, right from when Y-hap CDEF and mtDNAs M and N first emerged from Africa. And presumably even before that.
I am prepared to accept that we could consider the population carrying just these three haplogroups as some sort of 'original' Eurasian population, but that population was almost certainly still connected geographically to the African populations. It was also connected geographically to Neanderthals in the northwest, Denisovans in the northeast and 'who knows what' in India. Perhaps 'near-modern' humans.
Perhaps the various marginal populations mixed with the OoA people, and basically just the separate haplogroups moved through the population by a process of drift, bottlenecks and founder effects? And possibly the expanding haplogroups replaced the older ones, leading to their extinction? We know that at times various descendant haplogroups have since expanded from some reasonably small geographic region and repeated the process. Why would the same process not recede into the distant past?
"I believe it is, as shown by other examples".
ReplyDeleteWhat 'other examples'? Don't forget we're talking just large cats here.
"This does not require expansion in mere extension as much as differences between ecologies"
It requires expansion into a new ecological region. Note: 'expansion'.
"The Galapagos' examples are a clear case".
And I've just pointed out that the Galapagos examples are not a 'clear case'.
Darwin's finches clearly illustrate sympatric speciation (though there are some cases in which mere dynamic equilibrium polymorphism has been proposed instead - not confirmed). They exist in a large number of species and genera, which inhabit the Galapagos (and in one case Cocos Island) and are different mostly on ecological specialization, not on geography.
ReplyDeleteYou have yet to understand the concept of sympatric speciation it seems to me.
"... we're talking just large cats here".
Not at all: they were just one example among several.
"You are trying to fit the haplogroup evidence to as small a number as possible of small populations carrying a whole collection of basal haplogroups. Whereas I am prepared to accept a much greater number of small populations carrying just a few haplogroups, basal or otherwise".
After you replace the meaningful verbs by neutral forms, yes. I am not "trying to fit" anything but I perceive genetics as doing that naturally. Whereas you are not "prepared to accept" anything but fanatically convinced of that idea of yours. I am the one prepared to accept as a possibility that various haplogroups represent various historical populations in some cases but not necessarily so - not when it is not the most parsimonious explanation (Occam
Razor forever!)
"I believe the evidence most easily fits a whole series of expansions of different haplogroups from different regions"...
As you know I have done my best to make mtDNA and Y-DNA history-reflecting distribution patterns fit each other. But in the cases you mention of the OoA and Eurasian expansion this is clearly not the case: the Y-DNA and mtDNA lineages are all too mixed for that solution to be acceptable: we must invoke total swaps of male-female lineage pairings once and again, what clearly indicate that we are not before various strictly distinct populations but before a mish-mash at best.
"And I've just pointed out that the Galapagos examples are not a 'clear case'".
No, you have not "just pointed out" anything, much less explained in any way.
You instead said:
"It is my guess that geographical distance is far more important in speciation than is ecology".
But we see how lions through Africa and Asia (and Europe in the past) can reproduce with each other without difficulty (other than being separated by thousands of kilometers), while they cannot reproduce effectively with their ecologically (and behaviorally) distinct relatives such as the leopard, with whom they often share geography and to whom they are very closely related (and occasionally even make friends, I saw in a documentary, when young - not anymore as adults).
So your guess is very wrong, at least for many real life cases. If you would at least allow real life to shape your ideas instead of trying to forcibly reshape real life around your ideas, things would be much better and scientific.
"If you would at least allow real life to shape your ideas instead of trying to forcibly reshape real life around your ideas, things would be much better and scientific".
ReplyDeleteMaju. That sums up exactly your problem. It is you who are trying to fit evidence to your preconceived beliefs.
"You have yet to understand the concept of sympatric speciation it seems to me".
I understand it perfectly. It's just that I am very far from convinced it is at all common. Allopatric speciation is much more common, and is usually the most obvious explanation.
"Not at all: they were just one example among several".
And their diversification is very unlikely to have been sympatric.
"But we see how lions through Africa and Asia (and Europe in the past) can reproduce with each other without difficulty (other than being separated by thousands of kilometers)"
Yes. They are the same species. They have become differentiated into subspecies by geography, not by ecological separation. Subspecies a simply the first step to becoming species and we can follow the pattern for the evolution of the big cats. You may find this link regarding lion evolution interesting. Seems they originated in Africa and relatively recently moved as far as India:
http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1000251
"while they cannot reproduce effectively with their ecologically (and behaviorally) distinct relatives such as the leopard, with whom they often share geography and to whom they are very closely related"
I accept they're closely related, and occassionally share geography. And when they share that geography they usually exploit different ecologies within that geography. But that is a totally different thing to arguing that it was ecology that first led to the split between the two species. And I'd be surprised if leopards actually evolved in Africa. So the split between Lions and leopards may not be sympatric at all. And certainly when we include tigers in the perspective we see that the allopatric explanation is far more likely. Tigers evolved in the east where lions never were present. A book I have on mammals claims:
"The tiger was originally a northern and probably arctic animal and appears to have originated in what is now eastern Siberia, whence it spread south and west in two separate emigrations."
So the three species evolution is an example of allopatry. They each developed in a different geographic region.
(continued)
"Darwin's finches clearly illustrate sympatric speciation (though there are some cases in which mere dynamic equilibrium polymorphism has been proposed instead - not confirmed). They exist in a large number of species and genera, which inhabit the Galapagos (and in one case Cocos Island) and are different mostly on ecological specialization, not on geography".
ReplyDeleteThat is a gross oversimplification. Many closely related species are separated simply
by being on a different island. And many species that are claimed to have evolved sympatrically are found on several islands. So their current ecological specialisation is quite possibly the result of their original allopatric evolution on a different island.
Anyway even if we concede sympatric evolution as being the cause in any particular case we still have the problem of explaining how that evolution occurred. The new species certainly would not have evolved before it began exploiting the new aspect of the environment. It would have become adapted once it began exploiting that ecological niche. So we still have geographic expansion involved. So even in the case of sympatric evolution we still have a large element of allopatry involved.
"As you know I have done my best to make mtDNA and Y-DNA history-reflecting distribution patterns fit each other".
Well yes. But you start from the perspective that they all started out from the same place at the same time. To me it seems much more likely that the distribution reflects some far more complicated process.
"the Y-DNA and mtDNA lineages are all too mixed for that solution to be acceptable"
They are now, what with 100,000 years of movement backwards and forwards around the world. But it's possible to unravel the threads.
"Whereas you are not 'prepared to accept' anything but fanatically convinced of that idea of yours"
I had no idea at all what had happened until I had examined the evidence. My only assumption was that human evolution would have followed much the same pattern as that of all other species. My conclusions are based totally on what the evidence tells us.
A bit more concerning Darwin's fiches:
ReplyDelete"They exist in a large number of species and genera"
Large number? I don't think so. A dozen, about:
http://en.wikipedia.org/wiki/Darwin's_finches
From that link:
"Peter and Rosemary Grant and their colleagues have made research trips to the Galápagos for about thirty years, particularly studying Darwin's finches".
I have read their book. And when you're claiming 'sympatric speciation' for the finches it is their research you're basing the claim on. However what we find with the two species involved here is an example of yoyr statement:
"though there are some cases in which mere dynamic equilibrium polymorphism has been proposed instead - not confirmed"
The Wiki article goes on:
"Males are dimorphic in song type: songs A and B are quite distinct. Also, males with song A have shorter bills than B males. This is also a clear difference. With these beaks males are able to feed differently on their favourite cactus, the prickly pear Opuntia. Those with long beaks are able to punch holes in the cactus fruit and eat the fleshy aril pulp which surrounds the seeds, whereas those with shorter beaks tear apart the cactus base and eat the pulp and any insect larvae and pupae (both groups eat flowers and buds). This dimorphism clearly maximises their feeding opportunities during the non-breeding season when food is scarce".
Fits their original idea. But:
"If the population is panmixic,[20][21] then Geospiza conirostris exhibits a balanced genetic polymorphism and not, as originally supposed, a case of nascent sympatric speciation. The selection maintaining the polymorphism maximises the species' niche by expanding its feeding opportunity. The genetics of this situation cannot be clarified in the absence of a detailed breeding program, but two loci with linkage disequilibrium[22] is a possibility".
So it's not 'a case of nascent sympatric speciation'. My guess is that because of low population numbers inbreeding becomes a problem so periodically the two 'species' are able to hybridise and so increase genetic variability in both populations.
Any other examples of sympatric speciation? I suggest you're most likely to find reasonably convincing examples if you research 'cichlid fishes'.
I even devoted a section of my essay "Change" to 'Galapagos
Finches'. You may like to read it:
http://humanevolutionontrial.blogspot.com/2009/06/human-evolution-on-trial-change.html
Ok, interesting about lions. Though it seems a study on viruses, not lion-specific genetic markers, I find hard to understand it or to take it too seriously. I recently read elsewhere that lions arrived to Africa in two waves, with Sahel lions being of the same subspecies as Asian and of a more recent origin in Africa. But the African origin could well be more parsimonious.
ReplyDeleteAs for tigers, I strongly doubt your idea of these being original from Siberia: most tiger subspecies are adapted to the tropical jungle, so it's a quite straightforward conclusion to think that they originated there in the Asian tropics and then a group or two migrated to Siberia and other non-tropical areas.
And finches, well, they are still a case of (incomplete if you wish) sympatric speciation. They are not the only case, in fact this way of speciation is very common in species that do not move such as plants or corals, or those with a limited range such as insects. Often a small change in reproductive traits is enough to cause a total or almost total speciation: for example a small change in flowering time or stuff like that.
I'd suggest that you study the matter in depth instead of making claims that may be wrong just because your guts say so.
PS- re. lions I see that there is mention of mtDNA and nDNA.
ReplyDelete"I find hard to understand it or to take it too seriously".
ReplyDeleteI understand exactly why you would say that.
"I strongly doubt your idea of these being original from Siberia: most tiger subspecies are adapted to the tropical jungle, so it's a quite straightforward conclusion to think that they originated there in the Asian tropics"
Even so they did not evolve anywhere near where lions evolved. Therefore no sympatric evolution. And the book I quoted earlier also claims:
"The largest tigers, with a thick undercoat and very long, pale overfur,come from the colder areas of Siberia ranging from the Altai right across to the Stanovoi Mountains. Despite the fact that tigers inhabit the true jungles of the south which are comparatively cool, they still suffer miserably from the heat, and whenever possible make a practice of bathing and swimming to cool off".
I'm sure you realise that is unusual behaviour for a cat. So here's a link to the siberian tiger:
http://en.wikipedia.org/wiki/Siberian_tiger
To paraphrase your comment:
"so it's a quite straightforward conclusion to think that they originated there in Siberia and then a group or two migrated the Asian tropics".
"And finches, well, they are still a case of (incomplete if you wish) sympatric speciation".
The article states specifically that the authors were wrong in that original idea.
"They are not the only case, in fact this way of speciation is very common in species that do not move such as plants or corals, or those with a limited range such as insects".
But very uncommon in larger species (apart, perhaps, from trees). And even where it seems to have occurred it is quite possible that some sort of allopatric divergence occurred BEFORE the apparent sympatric divergence. Besides which, if we accept the concept that:
"Subspecies a simply the first step to becoming species"
We can find endless examples of subspecies separated from others of their species by geography, but I can't think of a single example of subspecies separated from others of their species simply by ecology.
"Often a small change in reproductive traits is enough to cause a total or almost total speciation: for example a small change in flowering time or stuff like that".
Can you actually provide any examples?
"I'd suggest that you study the matter in depth instead of making claims that may be wrong just because your guts say so".
I'm very strongly suspicious that it is you who have not studied the subject in any way at all.
Back to the original post:
ReplyDelete"the characteristics of a new taxon evolve in a single population or root species before diversification (radiation) occurs".
So, as I said, the arrival of a new species in a new region is exactly the same as the evolution of a new species in a new region.
Bathing is an unusual behavior for a "desert cat" like common cats or lions, it is normal not just for tigers but also for jaguars, i.e. at least part of the cats which live in the jungle - the exception would seem to be the leopard, which is, according to your data, of African origin. On this data, the fear of water that African-bound felines express is probably caused for no other reason than crocs, which are much less of a problem in Asia or America.
ReplyDeleteCats through the world have a nocturnal tendency and tend to be relatively inactive through the day, specially in the hottest hours. This last clearly indicates to me tropical origins.
Whatever the case, it seems you are focusing on Pantera sp. because it is somewhat convenient to your reasoning.
"Can you actually provide any examples?"
Galapagos iguanas: they live side by side and are generally believed to have diverged from a common ancestor, yet speciated sympatrically, as each ancestor group took upon one way of feeding: some in the sea and some in land.
There are many other examples but I'm no biologist. In the Wikipedia page you have examples like Darwin's finches (divergence driven by beak shape and feeding habits), insects specialized in different plants to whose toxins they have to adapt, NE pacific orcas.
But even if you want to stick to felines, aren't leopards and lions just different because of habits rather than because of geography? Isn't this sympatry? A sympatric divergence so similar to that between Pan sp. and Homo sp. that hurts.
"Whatever the case, it seems you are focusing on Pantera sp. because it is somewhat convenient to your reasoning".
ReplyDeleteNo. I am focusing on it because that was a genus you suggested was split by ecological factors rather than geographic. It was obviously split geographically, not ecologically.
"Galapagos iguanas: they live side by side and are generally believed to have diverged from a common ancestor, yet speciated sympatrically"
There is not the slightest evidence that they 'speciated sympatrically'. Sea iguanas can obviously travel from island to island, so their original diversification from the land iguanas could easily have been allopatric, just on one (presumably very small) island. And certainly the land iguanas' speciation was allopatric.
"There are many other examples but I'm no biologist".
That's a very strange comment coming from one who, a few days ago, wrote: 'I'd suggest that you study the matter in depth instead of making claims that may be wrong just because your guts say so'. I thought from that that you were an expart on the subject.
"In the Wikipedia page you have examples like Darwin's finches (divergence driven by beak shape and feeding habits)"
I'm almost prepared to accept the diversification of the Daphne Island Geospiza as sympatric, but even in this case it is not necessarily so. In fact the link claims they are not an example of such a phenomenon.
"insects specialized in different plants to whose toxins they have to adapt"
But these insect species are usually found in geographically distinct regions. Their speciation is my no means certainly sympatric.
"NE pacific orcas"
What on earth do they have to do with sympatric speciation?
"aren't leopards and lions just different because of habits rather than because of geography?"
Possibly. But unlikely. The lion is more likely to be the more recent species, and I grant it developed on the open savanah. But that represents a geographical shift rather than simply sympatric speciation. And I have no idea on which continent the leopard originated.
"A sympatric divergence so similar to that between Pan sp. and Homo sp. that hurts".
What makes you so sure that that divergence was sympatric and not allopatric?
More information on the relevant genus of Darwin's finches, the Geospiza:
ReplyDeletehttp://en.wikipedia.org/wiki/Geospiza
Six species, not dozens.
The 'large cactus finch':
http://en.wikipedia.org/wiki/Large_Cactus_Finch
From the link:
"It is one of Darwin's finches, and is endemic to the Galápagos islands, Ecuador, where it is restricted to Española, Genovesa, Darwin, and Wolf Islands. This rather dark bird resembles the smaller and finer-beaked Common Cactus Finch, but the two species do not co-inhabit any island. There are significant differences between the subspecies of the Large Cactus Finch; G. c. conirostris of Española has a far larger beak than G. c. darwinii and G. c. propinqua of the remaining three islands".
So it's not an example of sympatric diversification but rather of allopatric. Let's consider another example. Perhaps we'll strike it lucky:
http://en.wikipedia.org/wiki/Sharp-beaked_Ground_Finch
From the link:
"This rather small, slender-billed bird is endemic to the Galápagos Islands, where found on Fernandina, Santiago, Pinta, Genovesa, Darwin and Wolf".
So it shares several islands with the previous species but it is quite likely still be a product of allopatric speciation. It shares islands simply because it moved to new islands after it evolved. And now to the first of the two species that are supposed to be a shining example of sympatric speciation, Geospiza fortis:
"The Medium Ground Finch (Geospiza fortis) is a species of bird in the Emberizidae family. It is endemic to the Galapagos Islands. Its natural habitats are subtropical or tropical dry forests and subtropical or tropical dry shrubland. One of Darwin's finches, the species was the first which scientists have observed evolving in real-time".
Sounds as though it's widely spread through the islands, so its capacity to form hybrids with the cactus finch is hardly evidence against it having developed through allopatric speciation. And now the species with which it can form hybrids on Daphne Island in some circumstances, the large ground finch:
"It is endemic to the Galapagos Islands, and is found in the arid zone of most of the archipelago, though it is absent from the southeastern islands (Floreana, Española, San Cristóbal and Santa Fé). It is the largest species of Darwin's finch both in total size and size of beak".
So it could have first developed on any of the islands. The fact it can hybridise with Geospiza fortis is more an indication of 'recent' separation rather than 'sympatric' separation. There is no relevant information concerning the medium ground finch so, lastly:
"The Common Cactus Finch or Small Cactus Finch (Geospiza scandens) is a species of bird in the Darwin's finch group of the tanager family Thraupidae. It is endemic to the Galapagos Islands, where found on most islands, with the notable exception of Fernandina, Española, Genovesa, Darwin and Wolf. Most of these islands are inhabited by its close relative, the Large Cactus Finch".
So the separation of the large cactus finch and the common cactus finch is extremely unlikely to have been a product of sympatric speciation. And one is forced to concede no two species of geospiza can be said, with any level of certainty, to have split through ecological factors. Each almost certainly developed on a separate island and then spread.
http://en.wikipedia.org/wiki/Sympatric_speciation
ReplyDeleteThanks for the link, although I already knew the ideas behind the concept. I'd like to bring your attention to some quotes in the link:
ReplyDelete"In multicellular eukaryotic organisms, sympatric speciation is thought to be an uncommon but plausible process"
Note: a 'plausible' process, and certainly 'uncommon'. I'm quite prepared to accept that it may be possible. But I am absolutely certain it is far from common.
"The Medium Ground Finch (Geospiza fortis) is showing gene pool divergence in a population on Santa Cruz Island. Beak morphology conforms to two different size ideals, while intermediate individuals are selected against".
That is not actually correct. Geospiza fortis is the 'medium' ground finch and the one with the larger beak is the 'large' cactus finch. They are able to breed together during times of plenty, at which time 'intermediate' individuals survive and breed. But usually they behave as separate species. Sympatry is unlikely to be the explanation for their separation.
"A well studied circumstance of sympatric speciation is when insects feed on more than one species of host plant. In this case insects become specialized as they struggle to overcome the various plants' defense mechanisms".
But, as far as I'm aware in no case have they become identified as becoming separate species.
"For example, Nicaragua crater lake cichlid fishes include at least one species that has evolved by sympatric speciation"
I did suggest you check them out as being the most reliable example of sympatric diversification.
"By 1980 the theory was largely unfavourable given the void of empirical evidence available"
And that is still the case in spite of the claim:
"well documented empirical evidence now exists"
They don't provide any. One fact alone argues against sympatry as being at all common in speciation. We would surely expect examples of evolution in action to be relatively common. And that is the case. We find multiple examples of subspecies separated by geography, but very few examples indeed of subspecies separated by ecology. And when it comes to the separtaion of 'Pan sp. and Homo sp' we find the ecological specialisation of the two species does not occur until long after the split. Australopithecus seems to have had much the same ecological requirements as do modern chimpanzees. It's not until the genus Homo evolved that we have a movement into more open habitat. Today chimpanzees are largely found north of the equator but Australopithecus was found well into South Africa. I suggest that the original Pan/Homo split was allopatric, not sympatric.
Terry: I do not want to debate this further because I know I lack the basis. I just know that sympatric speciation has been documented often enough and I think that the Wikipedia article is written in a tone that is much like yours. However it also includes a number of references of papers that support sympatric speciation.
ReplyDeleteIf you prefer to think in terms of parapatric speciation (say polar bears or snow leopard or even chimpanzees and us), I do not really care but it's clear that not all (or even most) of speciation is merely allopatric: difference of ecology seems to be at least as important as mere geographical distance, if not much more.
"Sympatry is unlikely to be the explanation for [finches'] separation".
Why? It is obvious that they diverge only in ecology, not geography. Aren't you in denial?
"... in no case have they become identified as becoming separate species".
They are giving you the example of the apple maggot, a clear case of ongoing sympatric speciation.
"They don't provide any".
Yes, they do.
"I do not want to debate this further because I know I lack the basis".
ReplyDeleteFair enough. But I'll reply to some of your latest comments.
"I think that the Wikipedia article is written in a tone that is much like yours".
Presumably because the authors know quite a bit about the basis of the subject.
"it's clear that not all (or even most) of speciation is merely allopatric"
I disagree, as do the authors of the Wiki link. Sympatric speciation may or may not occur, but allopatric speciation definitely does occur quite commonly.
"difference of ecology seems to be at least as important as mere geographical distance, if not much more".
Obviously it serves to keep closely related species separate once they have evolved. But that says nothing about the method by which they separated in the first place. Most pairs of species that divide a particular region ecologically are each also found in regions where the other species is absent. So it is more than likely that in virtually all cases their original separation was geographic.
"Why? It is obvious that they diverge only in ecology, not geography. Aren't you in denial?"
Certainly not. Because what you say is not correct. All the Geospiza finches that share an island, but split it ecologically, are also found on islands where the other species is absent. Again it looks as though separate islands were involved in the original diversification. Sympatry is a result of the species separate arrival on the various islands.
"They are giving you the example of the apple maggot, a clear case of ongoing sympatric speciation".
One example, in which the separation into two separate species is yet to be demonstrated. Besides which they specifically state, 'Rhagoletis pomonella, the apple maggot, may be currently undergoing sympatric or, more precisely, heteropatric (see heteropatry) speciation'.
http://en.wikipedia.org/wiki/Heteropatry
From the link:
"Heteropatric speciation is a special case of sympatric speciation that occurs when different ecotypes or races of the same species geographically coexist but exploit different niches in the same patchy or heterogeneous environment ... Specifically, from a population perspective, the process looks sympatric, but from an individual’s perspective, the process looks allopatric, once the time spent flying over or moving quickly through intervening non-preferred niches is taken into account".
So it's a special case of allopatric speciation as well.