This is the first of two posts about speciation. A lot of biologists are mostly into natural selection … me, I’m goofy about speciation. It’s lovely, juicy, full of problems and unjustified assumptions, badly taught, and otherwise ripe for meat-axing the whole history of debate. My kind of thing.You can’t even start without realizing that your feet are mired in goo. The damn term isn’t even defined!
This is the one, Mishler and Donoghue, 1982. Print it, read it today, and read it again tomorrow. Drive it into your brain.
Let’s make sure we’re focused on sister species. For example, this isn’t speciation, but rather the remnants of extinction; it depicts several relicts amidst a swarm of invisible ghosts. Each one speciated, sure, but not from any of the others shown. “Sister species” means we’re talking about this species evolving from that one.
Now for the process or processes. Here the credit belongs to Ernst Mayr for his term peripatry.
The pedagogy for these concepts is a bit muddled. The following image, for example, confounds the current distribution with how speciation happened. The terms allopatric, parapatric, and sympatric are properly applied to the former – e.g., if two species’ currently ranges overlap, they’re sympatric. That designation doesn’t care where and how the speciation occurred. For example, species B could have arisen allopatrically, and in the fullness of time and circumstance, A and B could now be parapatric, or any other combo you care to imagine.
By contrast, the term peripatric does apply to speciation as a process, and as Mayr argued, it apparently applies extremely frequently. It doesn’t apply to current distribution. Let’s say, for instance in a given case, that the current distribution does reflect what happened during speciation. If so, then it’s most likely that the speciation was peripatric in some fashion: based on substantial (really isolating) vicariance or dispersal to result in allopatry, or based on minor versions of either to result in parapatry; or either, followed by changes in population growth and movement to result in sympatry.
I hope it’s clear why the common geographic circumstance of peripatric speciation directly supports a cladogenic rather than anagenic understanding of multiple speciation events. Not that shocking, right? If anagenic speciation were the norm, there wouldn’t be very many species on the planet at once …
In a nutshell: reproductive isolation, my pink patootie. It’s obviously something that happens, but also obviously eventually, several speciations down the road as far as any given two species are concerned. The same goes for those hoary inhabitants of bio textbooks, treating “barriers to reproduction” as if they were adaptations instead of mere case by case outcomes of where speciation happens to have landed.
If you think all this required a major conceptual blowtorching from the guy who studied how genital diversity evolves, you’d be right. Those posts are going to be interesting …
Well, genitals or no genitals, the whole “isolation” puzzle leads directly into the next problem: given the reduced frequency or absence of reproductive contact between subgroups of the same species, what happens then? Just how much selection is involved? And for (or rather against) what? Here we move into the thickets of macroevolution in the sequel post, coming soon.
Next: Should we talk about religion