When I was a bright-eyed, attentive freshman in my first biology survey course in college, I thought the definition of species was cut-and-dry with few exceptions. Species was the most basic unit of taxonomic structure, outlining individuals that, under natural conditions, could mate and rear viable offspring. Great, I thought, clapping metaphorical dust off my hands, let’s store this somewhere for an exam and not worry about it. Fast forward a bit: in the past few weeks, I’ve realized that I don’t understand the first thing about species or speciation, nor do I know if we’ll ever have an understanding that doesn’t involve a fair deal of hand-waving and/or philosophical reasoning. In this post, I’ll give an example of the species problem, and I promise nothing will be resolved.
Oscar Puebla’s lecture at Bocas del Toro considered reef-dwelling hamlet fish as models of adaptive radiation. Hamlet (Hypoplectrus spp) populations diverge in marine environments, where speciation within a local environment cannot be so easily attributed to geographical isolation. Though hamlets are similar in an ecological sense, they’ve radiated to give rise to a number of colorful phenotypes, somehow having reproductively isolated and branched off to form new species. Using RAD-seq, an inexpensive way of comparing genomes while skirting around the issue of not having a complete assembly, Puebla compared two species that rarely hybridized, but were able to produce viable offspring upon doing so. Perhaps to be expected, the divergence was ridiculously low, to a point that fancy statistics were required to pick out a lowly SNP that stood out as the only real difference between the two. And it was in a Hox gene. Not some master regulator transcription factor, just a developmental patterning gene that I probably learned about in that same class where they lied to me about what species meant. Could such outstanding phenotypic and behavioral divergence be attributed to a change in developmental patterning? No. I don’t think so. But these findings are by no means uninteresting; they point to much bigger questions we need to ask.
Okay, sure, the genomes aren’t complete, maybe the divergence is actually higher. Still, I am coming to terms with how poorly I understand the question of what makes two species distinct, at what point do two populations of organisms go from mating to hybridization to reproduction isolation? The non-mating rule, which I once held fast as unbreakable, is so commonly violated in natural settings, so scratch that one (even some ligers aren’t sterile, ya’ll). What level of divergence should we require before we say one species is not another, and when can we safely conclude that?