I was recently looking at a practice AP biology test question on evolution, and sparing you the details, I found it interesting that two of the four parts dealt with genetic variation and speciation in such a way that it was difficult to tell them apart. As expected, students who answered these questions got confused as well, and tended to give perfectly good answers to Part B, but unfortunately, this was their answer to Part A. By the time they got to Part B they seemed a little confused, perhaps realizing that there was some overlap and conflation of concepts.
Inter and intra-specific variation is probably patterned such that the sum of variation among several species is greater than the partitioned variation within a given species. That’s pretty obvious.
(Just in case it is not: Imagine measuring the mass of several elephants. The variation can be represented by the standard deviation, range, or whatever you like, among your measurements. It is such and such. Now do the same thing with a bunch of mice. Again, you have some measure of variation. Now do it for the mice and elephants combined. Here, the variation will be larger than for either. This is not the same as if you want to compare variation or patterns of variation between mice and elephants. Do do that, you need to scale the variation, say by using the coefficient of variation. In this case, combining the coefficients of variation might show less variation when combined than for either group simply because of sample size effects. But what I’m talking about here is total variation. Mice are tiny, elephants are huge, so their total size variation runs all the way from tiny to huge, so that would be a lot.)
But what is not obvious and possibly not well patterned is the relationship between variation within a species’ genome, and variation within a species phenome (ah, by that I mean total phenotype). Or if it is well patterned I don’t think we have good theory on this (please comment below to correct me on this if I am wrong!)
I would assert that there is probably not a pattern to this relationship across life in general, though there may be patterns for certain kinds of life. This relates to some extent to our perception. We are visual organisms, so differences in visual appearance mean something to us. Several individuals within a species may all look the same to us, but they may all SMELL very different to each other! On the other hand, striking visual variation among individuals within one species may be apparent to us, but at some other level, perhaps a level or in a modality more important to the individuals we are looking at themselves, this visual variation is meaningless, and some other variation is more important.
One might predict that among related clades of terrestrial vertebrates, certain visual signal related variations may be very stable among individuals in diurnal species but more randomly varying among nocturnal species where vision matters less. Overall body shape of, say, fossorial organisms or organisms with flight may be so tightly engineered that they simply do not vary much even between species, but, possibly, slow moving aquatic forms that don’t require refined aqua-dynamic engineering may have a great deal of body shape variation because it matters less.
Two recently reported discoveries speak to this issue. First, is a new species of skink (lizard) found by Chris Austin of LSU under some logs in Borneo.
Apparently, this new species looks a lot like a related form from the Philippines, but is genetically very different. I can’t interpret “very” and “different” here because I have not read the original papers, and even if I had, there probably is no current methodology for making that kind of comparison. Let’s just say that provisionally, there are some species of skink that are visually almost identical to the non-skink expert (or, in fact, the non-skink organism) but that are clearly different species, based on genetics, biogeography, and scale count (the key morphology for skink taxonomy, apparently).
In contrast to this is a recent report by Laurie Godfrey, Umass Amherst, about coat color variation in mouse lemurs.
In short, what were previously thought to be different species of mouse lemur, classified based on obvious difference in coat color, are not different species at all. These lemurs simply vary a lot in coat color. They vary in a way that if they were diurnal primates, this coat color variation would clearly indicate different species to us humans, and probably would serve as a species recognition mechanism among the primates themselves.
Hey, didn’t I predict this? Diurnal vs. nocturnal differences in differences. Brilliant.
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