From: the other end of the sidewalk
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Message 46 of 47 (494033)
01-12-2009 9:56 PM
Reply to: Message 45 by onifre
01-12-2009 7:06 PM
Moving on again.
Thank onifre, brief asides are not a problem.
|I think I understand all of the variables now thanks to you and Ned so that won't be needed. |
Now what we want to look at on this thread is what happens after speciation (however it happens, however you define it). What we have are these somewhat unique conditions that don't apply to the normal evolution of species over time:
(1) near or overlapping ecology
(2) near or overlapping behavior
(3) smaller difference between the daughter populations than between either one and all other species
What this implies to me is high competition for survival, and thus high selection pressure to become more different from the other population, or the probability that one or the other will go extinct is high.
When we look at speciation (and evolution in it's aftermath) chronologically, as shown by Pelycodus we see:
Where I have taken the liberty of drawing my own lines over fig.10 from Gingrich's paper. This is more representative, imho, of the actual course of events: not a linear trend, not a "punk-eek" picture, but one of adaptation from generation to generation under changing conditions and different rates of evolution, and some back and forth (non-directional) adaptations.
Part of the changing conditions is the existence of other daughter populations after speciation, with competition for living space and competition for food sources.
In this picture we see 3 different speciation events. The first one has a short lived branch to Copelemur praetutus (in green), probably because there was not enough separation between it and Pelycodus trigonodus, and going smaller was too difficult for survival. The second branch (in purple) is similar, except that Copelemur feretutus seems to have had room to go smaller, to Copelemur consortutus. There also seems to be a small branch in between that was reabsorbed into the species population by the next level.
It appears that as the basal Pelycodus group tends towards larger body size (See Cope's Law), that there is more opportunity for a smaller primate to live in the same area, perhaps taking advantage of food sources high in the trees or at the ends of tree branches that the larger primates cannot reach. It is also likely that branching to a smaller form is easier than a larger one as (a) the trend already is to get larger, and we may already be seeing the maximum rate for this species to grow in size, and (b) the population is very likely already "pre-adapted" to take advantage of the smaller form, as they have inherited traits from smaller forms that could still be useful.
Next we see another speciation branch, where Pelycodus abditus divides into Pelycodus jarrovii (still in red) and the smaller Pelycodus frugivorus (in blue). P. frugivorus is in the middle between P. jarrovii and C. consortutus, but is able to drive C. consortutus out, and then keep getting smaller to diverge more from P. jarrovii.
All very exciting, however we are still left with two similar primates, P. frugivorus and P. jarrovii. P. jarrovii continues to grow in size and gets reclassified as a new genus, Notharctus (another arbitrary distinction, similar to arbitrary speciation, as are all higher taxon designations). P. frugivorus continues as a smaller primate.
The question is: what becomes of the P. frugivorus and P. jarrovii lineages later in time? Just how diverse do they end up?
Well one answer is all the primates alive today, from monkeys to humans, but that is for later.
When we look at speciation (and evolution in it's aftermath) spatially, as shown by the Asia Greenish Warbler we see:
Greenish warblers (Phylloscopus trochiloides) inhabit forests across much of northern and central Asia. In central Siberia, two distinct forms of greenish warbler coexist without interbreeding, and therefore these forms can be considered distinct species.
Map of Asia showing the six subspecies of the greenish warbler described by Ticehurst in 1938. The crosshatched blue and red area in central Siberia shows the contact zone between viridanus and plumbeitarsus, which do not interbreed. Colors grade together where Ticehurst described gradual morphological change. The gap in northern China is most likely the result of habitat destruction.
Ecology and body shape/size
Given the differences between the two northern forms viridanus and plumbeitarsus in plumage, songs, and genetics, we might expect them to also differ in ecological and morphological traits. Surprisingly, the two northern forms of greenish warbler differ little in habitat preference and body shape and size. However, viridanus and plumbeitarsus do differ from southern forms in these traits. The northern forms are about 10% smaller in body size than the southern forms, and the northern forms inhabit much denser forest at lower elevation than the southern forms. The northern forms also must migrate much further to their wintering grounds in southern Asia. So it appears that during both northward expansions there was parallel evolution of smaller body size and different habitat preferences, even though there was divergence in traits used in communication (plumage and song).
So the two forms are similar in size, habitat and basic ecology (food sources, predators etc). What strikes me here, is that in the absence of the other variety, each would be able to take advantage of the opportunity of the other's habitat, but that the amount of overlap is small: as narrow or narrower than the hybrid zones between the other varieties. It clearly appears, imho, that they see each other as competitors, and thus block such spreading and co-habitation.
Again, we see two species just after speciation, and the question is: what becomes of the P.t.viridanus and P.t.plumbeitarsus lineages later in time? Just how diverse will they end up? Will one drive the other out? Will the other evolve to take advantage of a different ecology so that there is less competition?
Edited by RAZD, : added
Edited by RAZD, : narrow vs small
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