mrh@cybvax0.UUCP (Mike Huybensz) (01/07/87)
In article <124@bcsaic.UUCP> michaelm@bcsaic.UUCP (Michael Maxwell) writes: > I suggested this [genetic drift] in answer to an exam question once, as the > origin of blind (eyeless) cave fish. (The incidence of eyeless fish is quite > high, but for obvious reasons the eyeless ones seldom make it very far in > life up here.) The professor didn't like my answer...his point was that > there had to be a selective advantage to blindness in cave life. I didn't > believe so at the time, and I'm still skeptical. Anyone care to comment? Neither of these explanations strike me as correct. I would attribute blindness of cave animals (something which has arisen many times independently) to accumulation of mutations which normally would be selected against. When selection pressure is eliminated by a dark habitat, developmental mutations that cause blindness can accumulate, where normally they would be immediately eliminated. Without selection pressure, there is no limit to the number of mutations that can accumulate. It is also possible that there is some advantage to eliminating eyes, in terms of vulnerability to injury, disease, etc., or simply in reduced energy costs. Both of these are independent of population size, and thus not the same as genetic drift. -- Mike Huybensz ...decvax!genrad!mit-eddie!cybvax0!mrh
emigh@ecsvax.UUCP (Ted Emigh) (01/08/87)
In article <1340@cybvax0.UUCP> mrh@cybvax0.UUCP (Mike Huybensz) writes: > >I would attribute blindness of cave animals (something which has arisen many >times independently) to accumulation of mutations which normally would be >selected against. When selection pressure is eliminated by a dark habitat, >developmental mutations that cause blindness can accumulate, where normally >they would be immediately eliminated. Without selection pressure, there is >no limit to the number of mutations that can accumulate. > >It is also possible that there is some advantage to eliminating eyes, in >terms of vulnerability to injury, disease, etc., or simply in reduced >energy costs. > So far, OK. >Both of these are independent of population size, and thus not the same as >genetic drift. The second explanation -- selection for eyelessness -- works independently of population size. The first explanation -- lack of selection FOR eyes -- needs to have a small population size in order to be effective (random drift). An example I use in my Human Genetics class is to compute how long it takes for an allele to decrease in frequency from 100% to 20% BY MUTATION ALONE. For a mutation rate of 1X10^(-5) (considerably higher than most mutation rates in humans) it takes about 150,000 generations. The time it needs to operate by mutation alone is too long for the various cave animals to lose eyes by mutation alone. -- Ted H. Emigh Genetics and Statistics, North Carolina State U, Raleigh NC USENET: emigh@ecsvax.uucp ARPA: mcnc!ecsvax!emigh@BERKELEY BITNET: NEMIGH@TUCC
mrh@cybvax0.UUCP (Mike Huybensz) (01/09/87)
In article <2526@ecsvax.UUCP> emigh@ecsvax.UUCP (Ted Emigh) writes: > The first explanation -- lack of selection FOR eyes -- > needs to have a small population size in order to be effective (random > drift). An example I use in my Human Genetics class is to compute how > long it takes for an allele to decrease in frequency from 100% to 20% > BY MUTATION ALONE. For a mutation rate of 1X10^(-5) (considerably higher > than most mutation rates in humans) it takes about 150,000 generations. > The time it needs to operate by mutation alone is too long for the various > cave animals to lose eyes by mutation alone. There are two reasons why I don't think you can dismiss a degradation hypothesis so simply. First, because some cave populations might be ancient enough, and second because your counter-example makes the assumption of a single gene when the actual number of genes necessary to the developmental pathway might be quite large. These hypotheses could be distinguished by genetic comparison of cave animals with closely related outside populations. If blindness is favored by natural selection, then we would expect to see a very few alleles responsible for the blindness. If it is a result of degradation, then we would expect to see many alleles. -- Mike Huybensz ...decvax!genrad!mit-eddie!cybvax0!mrh
lew@ihlpa.UUCP (01/11/87)
Eyes could be a liability in a totally dark environment because of their vulnerability to injury. This vulnerability is amplified of course since the fish can't see! Cave animals also tend to lack pigment. I don't know what liability pigment might represent to an animal, except possibly resource consumption. I personally lean towards simple regression as the explanation for the loss of eyes and pigment. I think failure to select in favor of a character results in its loss by random variation. I'm no expert, though. Lew Mammel, Jr.