Re: How does natural selection apply to loss of organs or faculties?

Colin,

Your question is an interesting one and I'd like to answer it using the same example of cave fish. Every animal has limited resources available to it throughout its lifetime, this is one of the conditions under which the theory of natural selection operates. There is a cost to the organism in developing organs or increasing in size; it is selectively advantageous to that organism to keep the costs as low as possible in relation to its competitors.

Taking your example of cave fish which have evolved from fully sighted ancestors - if, once they entered the cave habitat, a mutation occurred which shortened the developmental period of the eye at the expense of a loss of visual acuity that mutation would be favoured. In this dark habitat good eyesight is irrelevant to the survival or reproduction of the fish.

If various adaptations were not costly then presumably some kind of "superorganism" could evolve which had excellent hearing, vision, speed etc. and outcompete everything else. Due to limited resources this fish with the mutation for a shorter eye development period may have a larger number of offspring than its competitors who use up proportionately more of their resources in developing a useless organ.

Presumably the cost of the adaptation should be proportional to the speed at which it is lost, with a very costly organ being lost quickly and a less costly adaptation being lost at a slower rate. This would be a good test but I am not aware of any studies which have investigated this (if anyone knows of any I would be interested to find out).

I hope this answers your question,

Yvonne Buckley (y.buckley@ic.ac.uk)