I once offered nonlin.org the ‘fridge-o-matic’ challenge. The idea was to give me a number from 1 to 9 (a shelf in my fridge/freezer), and a lateral location Left, Middle or Right, and a depth locator Front, Middle, Back. That gives 81 sectors. I go to the named sector and fumble for the nearest organism (he’d have to trust me not to cheat). If it’s a stew or pizza I’d need more info. Then I’d need a chromosome number in that organism, and a gene number on the chromosome. We could shortcut the back’n’forth by giving numbers 1-50 and 1-10,000 which I’d then normalise to the actual counts.
Armed with this target of a random gene in a random organism in my fridge, I’d go to a public database and (if it’s in) run a BLAST on the gene to find the sequence alignments, arranged by closeness of match. I offered high confidence that this would recover a reasonable approximation of the Linneaean hierarchy – closest matches in species, then genus, then Family etc. This, I feel, nicely fits with the hypothesis of Common Descent of taxonomic ranks.
Now, the Linnaean hierarchy was based on morphology. He did not have the technology to ‘see’ genes directly. So we’d have to wonder why I could stick my random thumb in and pull out a Linnaean plum pretty much every time, on genes invisible to him.
Nonlin declined to play (he is, frankly, no fun at all). “It’s expected”, he said. “Like organisms will have like genes”. But for that to be a relevant objection, every single genetic difference between every single species pair must be involved in every morphological distinction between them. That requires quite a lot of heavy lifting from a gene. Its job might be to … I dunno, ligate a strand break in DNA let’s say. But it’s also got to do it in a squid way, and a dandelion way, and a yeast way that feeds through to morphology for every gene. We’re always being told how brittly unchangeable genes are, yet we seem to have a huge range of latitude in genes that are notionally pinned in place both by primary function and by multiple morphological roles across species.
I bring this up now for two reasons related to nonlin’s ‘Sexual selection’ thread.
1) If every difference between every gene really were morphologically significant, there’d be a constant stream of mutants not possessed of the vital characteristics that render the sexes mutually attractive to each other but not to members of other species.
2) Sexual dimorphism is a particularly striking morphological difference, which, on the ‘genes=morphology’ view, would require similar genetic differences between the sexes to those observed between like species. But most genes, even those involved in sexual differences, reside on the autosomes, which pass through both genders of offspring.
The answer, of course, is that genetic differences on the broad scale are not the key to morphological difference. Those differences are under the control of relatively few genes, and a lot of regulation (itself genetic in origin). This is as true between species as it is within (don’t fight me on this; it’s an established fact!).
Anyway, anyone fancy taking the Fridge-o-matic for a spin?