Speciation Rate:
Speciation can occur rapidly no matter what the driving force. Transition times are generally shorter whenever the process involves an unstable intermediate stage; the forming species must either cross that unstable stage quickly or dissolve (involves sympatric phase or crossing adaptive valley). Transition times for allopatric speciation can be longer since the groups are separated by geographic barriers and are prevented from interbreeding anyway. However, one should not conclude that all cases of rapid speciation occurred sympatrically. It is unsafe to infer biogeographic or genetic modes of speciation from rates of speciation. A good fossil record is rare, but by far the most reliable.
One fairly solid prediction from the authors: For a given divergence time, the amount of reproductive isolation between taxa with overlapping ranges should be greater than that of completely allopatric taxa.
Calculating speciation intervals seems shaky at best at this point. Most estimates vary by millions of years at least.
I find it quite interesting that some groups have apparently not experienced much noticeable morphologic changes for hundreds of millions of years.
Biogeography: Theory and experimental data predict that transition times for speciation events involving a sympatric phase will typically be shorter than for those that are purely allopatric. Reinforcement probably artificially lowers the apparent transition time, though.
Currently the best way to estimate biological speciation intervals is to correlate the divergence time between sister taxa with their degree of reproductive isolation.
Factors Affecting Speciation Rates:
“key innovations”: factors that increase the rate at which new species arise
- Properties of organisms that facilitate speciation: things that speed up evolution of reproductive isolation; traits that promote sexual selection in animals, for instance. Also when organisms are involved in biological interactions with other species — adapting to other species which are evolving themselves can cause faster change
- Properties of organisms that prevent extinction: If the group that possesses it does not die, the trait and perhaps some that accompany it will be spread.
- Properties of organisms that open up new “adaptive zones”: Traits that allow rapid invasion of new habitats can trigger massive adaptive radiations. Preadaptations? Usually impossible to determine exactly what the trait was.
- “Species-level traits” that affect speciation or extinction rates: Species selection instead of selection on individuals?
Traits that increase rates of extinction could also increase rates of speciation. The opposite could also be true. There is a great deal of speculation and inferences in this field of study…
Species Selection: might produce evolutionary trends not predictable from selection acting within species. Daughter species would acquire traits of ancestor species.
What? Is this like group selection? I thought this was disproved / out of style…? Rather confused.
Done with Speciation by Coyne and Orr!
