Homo floresiensis evolutionary path

[Blake]

@Stranger On A Train

A typical island gigantized species occurs due to a lack of predation…

I’m not sure this is the case. Certainly there are plenty of giant island species that have evolved in environments with an overabundance of predators. Your own example, the dodo, co-evolved with numerous predatory hawks. Indeed the species appears to have become large specifically to allow it to outgrow its predators, just as giant mainland species have done.

Do you have any evidence that island species typically occur due to alack of predation? Ii fact can you name even one giant island species that evolved in the absence of predation, rather than evolved to large size to avoid predation?

(virtually all gigantic species are either herbivores or insectivores, verging to scavengers, but not primary predators) and minimal interspecies competition.

I don’t believe this to be true. There are more gigantised herbivores than carnivores, but there are more herbivores than carnivores, period. I’m not seeing any evidence that gigantised species are more likely to be herbivores. Certainly I could name numerous species like giant New Caledonian geckos, giant New Zealand Eagles and giant Australian kingfishers and giant Madagascan mongooses that are all primary predators.

Do you have any evidence to support your claim that virtually all gigantic species are herbivores?

However, gigantism is often a balance between selective pressure for large size and biomechanical limitation for skeletal support and locomotion. This places the species at an extrema from which it is difficult to “evolve back” to a more moderate size, particularly if it has adopted physiological mechanisms to support said gigantism.

But surely this is true of any and all adaptations isn’t it? I can see no reason why a needle beaked finch is going to find it any easier to evolve back to a short beaked finch than a giant rat is to evolve back to a smaller form.

Even in isolation, a gigantic species might eat itself out of house and home; in competition with a smaller species, and especially in an environment in which the conditions which initially favored gigantism have changed (i.e. less food, greater predation, et cetera) a species of more moderate or even diminished size is generally more adaptable and will flourish at the expense of the giant species.

This really seems like your supporting your argument simply by rephrasing it: giant species are more at risk because giant species find it harder to evolve to avoid risk. A giant species is more likely to die n the face of change because giant species are less adaptable in the face of change. I appreciate your input but I’m just not seeing any reasoning or evidence here, just repetition of the original premise: that giant species are less likely to survive change.

Despite the examples you list, the vast bulk of gigantic species fare very poorly even with faced with only a moderate adaptive challenge or competition, and it’s rare (I think) that a species classified as gigantic–that is, directly derived from a previous species with essesntially all phenotypical features intact and enlarged–continues onto a new, distinct lineage of species.

Yes, but do you have any evidence or logical reasoning to support such an assertion? Can you for example provide a list of species on even a single island that indicates that the giants fared worse than the conservative? You’ve stated several times now that giants fare poorly when faced with change, but I’ve not seen any evidence or reasoning to suggest that is the case, just reiteration that it is the case.

Hence, the claim that gigantism is an evolutionary dead end. Derivative species of larger size certainly does happen, but this is generally accompanied by other significant modifications in phenotypes which preclude the classification of “gigantic”.

No offense intended, but this seems to be simply moving the goalposts. Regardless of what evolutionary course it heads down, any species will acquire modifications over time which preclude classification as the original. Thus we are forced to conclude by your argument that all evolutionary traits are dead ends.

So while a giant bird will often also acquire flightlessness, a flying lizard will also acquire toothlessness. By your argument the acquisition of flightlessness preclude the classification of gigantic, yet surely the acquisition of toothlessness precludes the classification of “flying”.

Can you explain for example why this argument doesn’t also force us to conclude that flight, dwarfism, thermoregulation, sight and any and all other evolutionary modifications aren’t also evolutionary dead ends? Since all those things are also inevitably accompanied by other significant modifications in phenotypes in derived species it seems we are forced to conclude that all adaptations are dead ends.

To me personally, and in all the literature I’ve ever read, an evolutionary dead end in one which left no derived species at all. Not necessarily extinct, but stagnant. Hence cockroaches were a dead end insofar as they have essentially stagnated. In contrast early therapsids weren’t a dead end since they radiated into novel forms. But by your definition the early therapsids were a dead end simply because all their derivative species have acquired modifications which preclude the definition of “therapsid”.

Before I will buy that island giants are more at risk than any other island group I would need to see some evidence or at least some reasoning that supports such a conclusion. A list of endemic species on an island, along with a list of extinctions broken down into giants and non-giants would be ideal. Failing that some logically supportable reason why giants should be more at risk or less adaptable (rather than simply stating that they are) would convince me. Until then I’m going to view the concept with extreme skepticism

[What_Exit]

Just curious, would not the Komodo count as a gigantic predator? In their isolated island habitats, they are the only large predators and appear to be gigantic next to the monitor lizards from which they likely evolved.

I think the Flores islands of Indonesia are also home to a giant rat. I think it was described as 2-3 times larger than it mainland relatives.

Jim

[Ludovic]

@Blake

But as it is Ockham’s Razor demands we reject it.

Ockham’s Razor “demands” nothing. There are plenty of intermediate steps that must have happened in nearly every evolutionary path we’ve founf that have a “total lack of evidence”: does this falsify the theory of evolution?

If there is no evidence for gradualism in Floriensis’ path, and gradualism would prove to be problematic in other ways, and there is no reason to doubt a swift mutation can occur, there is no reason to summarily reject the possibility of an already-dimunitive species colonization.

No where did I claim it was a leading theory or the most probable one.

And we’ve no evidence of normal sized erectus on Flores

. Hmmm. You’d think we would have, if normal sized erectus migrated to Flores, then evolved into hobbits?

[Blake]

@What Exit?

]Just curious, would not the Komodo count as a gigantic predator? In their isolated island habitats, they are the only large predators and appear to be gigantic next to the monitor lizards from which they likely evolved.

In order of appearance:

No they are not giant predators in the sense of being giantised descendants of more conservative mainland species.

They are not the only large predators in their environments. They share their homes with saltwater crocodiles which are much larger animals.

Komodo dragons come from a very ancient lineage of large monitors, none of which managed to survive human predation. The Komodo dragons are notably smaller than sister species once found on Flores itself, and less than half the size of several Megalania species of Australasia. So you could make a fairly convincing argument that Komodo dragons are dwarfed version of the ancestral forms.

@Ludovic

Ockham’s Razor “demands” nothing. There are plenty of intermediate steps that must have happened in nearly every evolutionary path we’ve founf that have a “total lack of evidence”: does this falsify the theory of evolution?

No, because evolution is the simplest explanation that fits the observations. In contrast your hypothesis is more complex than other explanations with no additional predictive power. Thus Ockham’s razor does indeed demand that we reject it.

If there is no evidence for gradualism in Floriensis’ path, and gradualism would prove to be problematic in other ways, and there is no reason to doubt a swift mutation can occur, there is no reason to summarily reject the possibility of an already-dimunitive species colonization.

Yes, there is: Ockham’s razor. The theory requires us to embrace numerous baseless and illogical stipulations in order to support it. In contrast simply accepting that it is a case of island dwarfism requires no such assumptions. Thus we have two competing theories, both of which explain exactly the same observation, but one if far, far simpler.

No where did I claim it was a leading theory or the most probable one.

My point is that it is entirely an argument form ignorance. Since we can’t prove it didn’t happen we have to accept that it might have happened. Neither logical nor scientific.

Hmmm. You’d think we would have, if normal sized erectus migrated to Flores, then evolved into hobbits?

No, that is precisely the point. If we accept your hypothesis of a long term population of Erectus displaced by Floresiensis they should have left far more evidence of their existence than Floresiensis. However if Erectus maintained only a small population and rapidly evolved into more numerous Floresiensis we would expect to see exactly what we do see.

[Ludovic]

@Blake

No, that is precisely the point. If we accept your hypothesis of a long term population of Erectus displaced by Floresiensis they should have left far more evidence of their existence than Floresiensis. However if Erectus maintained only a small population and rapidly evolved into more numerous Floresiensis we would expect to see exactly what we do see.

I just don’t want people to think that I hold this argument, which I never made. Other than that I don’t have anything to add to your mischaracterization of my posts.

[Chronos]

But surely this is true of any and all adaptations isn’t it? I can see no reason why a needle beaked finch is going to find it any easier to evolve back to a short beaked finch than a giant rat is to evolve back to a smaller form.

Well, larger animals are going to have a harder time adapting than smaller ones, since both their lesser numbers and their longer generations are going to slow down evolution.