parle@ditsyda.oz (Andrew Parle) (04/19/89)
in article <273@acates.UUCP>, dan@acates.UUCP (Dan Ford) says: > Xref: ditsyda sci.bio:1089 talk.origins:42 > > It seems to me that not all evolution can be explained in terms of simple > environmental selection. My example is the albino cave fish, shrimp, and > various varied darkness dwellers. These critters ware decendants from colored > animals, and yet that have become albino. This cannot be due to environmental > selection, since you cannot have selection for (lack of) color in the black > caves. It is a repeatedly seen pattern of change, and not a single odd > species mutation. You seem to be making an assumption here that color cannot be selected for in caves because there is no light. What you forget is that color MAY be selected for directly (as is camouflage, mating plumage, etc) or it may be a by-product of other selection (such as the dark skin of peoples living in the tropics). As the skin is a body organ with many more properties than just color, it would be difficult to say that there is no selection pressure for a type of skin which, just accidentally, happens to be white. It is not a problem that this is a general rule across different genera, as skin is a pretty basic development and is similar across large areas of animal species. > What might be going on: In the above-ground population there is a constant > (but small) mutation rate of albinos. On the surface this is very bad, and > is quickly selected against. Underground, however, it is not a disadvantage > and may continue in the gene pool. But, not only does it continue, but it > apparently is selected FOR since you end up with all these albino animals. > How can coloring possibly be selected for in a dark environment? Why a > repeated selection of albino in an environment where color is meaningless? I am not sure that cave albinism (literally "whiteness") is the same as the genetic mutation of albinism. however, if it is related, there is no real problem. This is because albinism, although a single mutation, has a wide ranging effect on the organism, NOT limited to skin color. For example, the next best known feature of albinos is their pink eyes. Less well known is the fact that the eyes are pink because of various changes in the physiology of the eyes which also lead to increased vision in the dark, with the disadvantage of difficulty in seeing in bright light. Albinos above ground also are selected against because their skin is too sensitive to UV, so they get sunburn, skin cancer, etc. > Odd theory, which I call evolutionaly entropy: complex structures must be > selected for in order to continue, or they suffer from random mutation > and are degraded, possibly disappearing from the gene pool. This is not > to say that such things are selected against, but just that lack of selection > FOR them is enough to have the trait leave. Complexity is good only if it > solves a problem and improves surviveability, otherwise "keep-it-simple-stupid" I would prefer to put this as "use it or lose it". There are of course many cases of atrophy of organs and structures which are no longer of use, but again this is in the mainstream of evolutionary thought. If a structure is no longer useful, then it usually follows that it is a handicap (like feet on a whale). Mutations which diminish or remove this structure will be encouraged, and if those members of the species which have these mutations can compete more successfully, there numbers will increase. However, speciation is rather more complex than this, in that mutations not only effect the organism, but also effect the environmental niche that it is capable of occupying. As an example, if proto-whales still had feet (and so presumably still lived on the shore line (perhaps like seals), then if a mutant arrived which had no feet, then obviously it would spend all its time in the water. If this mode of life is successful, eventually a new species would emerge which occupies a different habitat from the original footed whale. Mind you, some vestigial remnants of its original footed status might remain... such as the few small bones, unconnected to muscle tissue, which modern whales still have where their feet used to be... Similarly, perhaps cave dwelling species actually lived in the light, but their albino mutations were happier living further in. -- _______________________________________________________________ "Even paranoids have enemies..." Andrew Parle CSIRO DIT
gary@percival.UUCP (Gary Wells) (04/22/89)
>in article <273@acates.UUCP>, dan@acates.UUCP (Dan Ford) says: >> >> It seems to me that not all evolution can be explained in terms of simple >> environmental selection. My example is the albino cave fish, shrimp, and >> various varied darkness dwellers. These critters ware decendants from colored >> animals, and yet that have become albino. This cannot be due to environmental >> selection, since you cannot have selection for (lack of) color in the black >> caves. It is a repeatedly seen pattern of change, and not a single odd >> species mutation. It could also be that the coloring agents in the skin need exposure to UV (or some other component) to "darken". I think that myolin is the pigment agent in human skin, and that is definitely sensitive to UV (thus sun tans). I may be wrong on the name of that pigmenting agent, but the fact remains. Has anyone tried exposing any of the cave-albinos to normal levels of UV and checking what happens to the skin color? Just thinking out loud, here. -- -------------------------------------------------------------------------------- Still working on _natural_ intelligence. gary@percival (...!tektronix!percival!gary)