Here’s my next step in discussing speciation, following Where do little species come from? As I explained in that post, the answer to that question is, from subsets of existing populations, as opposed to the transformation of an entire population. That’s the geographic context which strongly implies some different context for new species’ appearance besides whole-population, within-population, “ordinary” selection.
The core question now concerns continuity between two most-closely-related species, which is actually a long-standing intellectual crisis point in evolutionary theory. After all, the very title [On] The Origin of Species would suggest that this exact issue is what Darwin was talking about … but as it turns out, that’s not the case. His text is concerned almost solely with (what we now call) microevolution, the changes within a population. As it turned out, too, he and Alfred Russell Wallace disagreed about species’ biological continuity with one another, with the latter focusing strongly on reproductive capacity.
Ernst Mayr’s insight concerning peripatry, which I discussed in the previous post, throws this controversy into higher tension, and although Mayr’s own term (the biological species concept) and his extensive footnotes to The Origin in 1963 tried to resolve it, I don’t think he succeeded. Let’s take a look at the idea that although speciation is geographically discontinuous, it’s “still just” selection in action, with species differences arising from immediate differences in the local environments. The above cartoon captures it pretty well, especially the concept that a given immediate environment has in it something new for an arriving or isolated population to do. This idea is called adaptive radiation.
Here’s the image I’m talking about, which is also the featured image for this post on the main page: see the geographic history with its vicariance events, and see the associated diversity of these birds at what is now a higher taxonomic level. According to adaptive radiation, “there’s nothing to see here,” because selection accounts for whatever occurs. This logic invokes as well the long-standing fallacy that opposes adaptation and extinction, positing that since the creature isn’t extinct from developmental defects or from environmental factors, its new appearance must have been selection which saved it from this fate.
Speciation, or rather its analysis, has always been iffy regarding when and whether it has to happen, and yet is obviously so damn prevalent that we know it must be happening. It’s quite frustrating in contrast to selection, which is nicely locked down: when the three necessary conditions prevail, selection happens, and when any or all of the three do not, then it doesn’t. I call attention to the philosophical difficulty people have with something being not dysfunctional or disastrous and yet also not purposeful (as I discussed in Graven images), as well as to the confusions between “chance” and physically-caused.
Fair warning: what follows now is not standard biology pedagogy or the most-favored position in the discipline. It’s also advanced reading in blog terms, as I’m drawing on multiple previous posts, and wouldn’t mind if you reviewed them.
You can tag my position on speciation straight back to Stephen Jay Gould, that trouble-maker. Until the late 1990s, although I considered Mayr’s analysis of Darwin to be flawed in some ways (and verified that I understood him in person), I tended to agree with him that speciation was a matter of local selection “doing the job” on a subpopulation. My personal heresy therein was to challenge the idea that evolving genital diversity served in some directed way toward species isolation,. Little did I know that this particular break with Wallace’s position would ultimately lead to a general one.
The culprit in that moment would be Daniel Brooks, from the U of Toronto, via his books Evolution and Entropy and, with Deborah McClennan, Phylogeny, Ecology and Evolution. The latter book influenced my research and dissertation profoundly. I’ll refer to his alternative concept to adaptive radiation by the title of the former.
Briefly, the notion is that populations’ genetic profiles and physical forms are thrown into discontinuity via geographic isolation (of any kind) by default, thus making speciation the norm, rather than iffy. Selection is recast as the conserving effect counter to this constant – and undirected, extremely non-niche-y or “adapted” – and splitting and changing.
My thinking comes in two parts, beginning with Brooks’ idea, and refining it based on my experience with both developmental genetics and with phylogenetic systematics.
To clarify the difficult phrasing in that last sentence: in adaptive radiation, selection generates differences among taxonomic groups; in evolutionary and entropy, selection conserves the similarity within each taxonomic group.
That just throws the ordinary textbook view out the window. Reproductive isolation is irrelevant, and can end up any which way? (you know, like it observably does) Selection is what keeps species alike, rather than what drives their diversity? Whoa! I’ve noted that colleagues tend to dismiss the book on the basis that “there’s too much math,” “didn’t have time to really look at it,” … perhaps a little too consistently, given that he makes a rather excellent case for reversing a cherished and pedagogically established concept for which the research is always churning up exceptions and almosts. I think evolution and entropy provides a much more coherent case for the very same image of the birds’ thisaway-thataway diversification, given that it’s an astronomically common phenomenon across living things.
Developmental work at this same time – mid-1990s – seemed to me to be especially supportive of evolution and entropy. These biologists like to say that selection is the frosting of evolutionary change, not the cake; however, and all respect to them, few Evo Devo gene-jocks have practical training in phylogenetic systematics and taxonomy. I was well-placed during my dissertation process, at least at the theoretical level, to integrate developmental genetics with speciation biogeography. The question this raises is, what is the variation which is impacted by the geographic isolation alone?
To prepare for my thoughts on this, if you have the time, review my posts Same thing(s) and We are, in fact, Devo. Then consider the difficulty of positing for each and every instance of speciation, that a nifty mutation just happens to appear, zingo, ready-made for the new species to slot nicely into the available niche, for which the only real support lies in the romantic notions that I criticized in Adapt this. And finally, consider the stew of alleles available among the genetics of any population, and how their frequencies are affected by population dynamics as I described in Do the drift.
Consider two cases of diversification resulting from any form of peripatry, i.e., geographic outlier status. If the changes are due to selection only, whether diet-based, sexual selection, predator composition, whatever, then the extent of the change is frosting only: end-of-cascade, details-based effects. Good examples would be the Galápagos finches or the soapberry bugs or perhaps the striking quantity of speciation among certain groups of rodents. How reversible it may be in any given case, who knows, but at least in the finches things seem to remain pretty labile. Ring species offer a good example too, especially insofar that if and when reproductive incompatibility occurs, it’s only after many steps of sequential change.
Whereas when the changes are due to the entropic effect, the immediate species shift is much more discontinuous, based on earlier-acting steps in the genetic cascades, resulting in more striking changes in body form and also in a plethora of changes in the later steps due to the “bend” in the preceding ones. Across examples, the shifts are also oddly uncategorizable in their diversity of specific features, as what happens is completely case by case – big or little being the most obvious example of “whatever, man.” I stress too that selection still “frosts” the outcome, and I submit almost always does, so this isn’t anti-selection – but the selection isn’t what made the new species happen.
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