Posted: Sep 03, 2010 1:55 am
by Calilasseia
Actually, my old diagram that I used to illustrate the inheritance of ERVs can be pressed into service here too:


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Originally, I used the coloured markers to illustrate endogenous retroviral insertions in chromosomes, in order to illustrate the point about how inheritance from common ancestors explains the observed patterns seen in real genomes, but "special creation" has NO explanation for this phenomenon. However, the diagram can be used to illustrate the appearance of new features in new lineages as well. The species at the bottom of the tree starts off with simple features, the next species add a small degree of extra complexity, and the next species add yet another small degree of complexity, building up incrementally with each new cladogenesis event. Of course, the "size" of each incremental stage (however that is defined) need not be a fixed constant between any given pair of ancestors and descendants. One lineage could proceed in very small steps, incrementally acquiring small changes in the genome that do not manifest any major phenotypic changes, until that lineage suddenly gives rise to a descendant that acquires a mutation facilitating new building upon what has gone before, whereupon, in that most recent descendant, we see what appears to be a major phenotypic change. On the other hand, a second lineage could acquire genotype changes that result in fairly major phenotypic changes in a short period of time.

And, of course, we have an extant example of major phenotypic change available in pet shops throughout the developed world, in the form of Betta splendens, the Siamese Fighting Fish. Which, in around 100 years of selective breeding by humans, has gone from this:


to this:


More to the point, however, aquarium bred specimens in the 1970s turned up exhibiting a radical new mutation, the Double Tail mutation:


This mutation results in the appearance of:

[1] Increased numbers of dorsal fin rays over the 'standard' fish;

[2] Twin tails arranged in "over-under" shotgun fashion.

The two tails are not merely a single tail split to the caudal peduncle - anatomically, fishes with this mutation are true two-tailed fishes. They possess two caudal plates from which the fin rays emanate, and hence a double caudal peduncle.

The fun part, though, is that the mutation responsible takes place in a single gene, and that gene exhibits classic Mendelian recessive inheritance. So we have here an instance of a mutation in a single gene resulting in a major phenotypic change.

I've known about this mutation since it first appeared in the 1970s - the first specimen I saw was on page 9 of the November 1977 issue of Tropical Fish Hobbyist magazine (yes, I still have the magazine!) - but the Double Tail mutation was in circulation amongst specialist Betta breeders for some years before this. I know this for a fact, because the specimen exhibited in that photograph is a "melano black" Double Tail, an extremely rare combination that even today requires special effort to produce, and Double Tails of other colours certainly existed beforehand in order to produce that "melano black" specimen. For those unfamiliar with this topic, "melano black" Bettas are difficult to produce because females homozygous for this recessive mutation are infertile (in some cases, the mutation is lethal in females), though there exists a "fertile black" mutation which is less intense, and consequently less desirable colour wise than the intense "melano black". Consequently, you have to go to the trouble of producing individuals that are heterozygous for both the "melano black" gene and the Double Tail gene, then cross them to produce a "melano black" Double Tail male, which is hard. Turn up at a Betta show with a good quality "melano black" Double Tail, and you'll walk away with a prize.