Evolution of Snake Venom

The principle of natural selection has two components to it. First is the generation of diversity, which as far as we can tell is a random process. The second part is non-random--the survival and reproduction or death of an organism. The generation of diversity is often driven by mutation of genes. The definition of "mutation" that immediately comes to mind is usually the simplest kind--single base-pair changes. Critics of evolution often express incredulity that such a process could be responsible for the diversity of life. One out-dated argument calculates the probablility of a particular amino acid sequence coming into existence at random and comes up with some astronimical number, thus showing that mutation could not be responsible for the existence of the protein.

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However, if we broaden the definition of mutation to be any changes in the genome, the picture becomes more plausible. New genes can be created in several ways, one of which is gene duplication. There are at least a couple of ways a gene can be duplicated--the specifics are not important here. Once a gene has been duplicated, the forces of mutation (in the stricter sense) and natural selection can play on one copy while the other copy retains its original function. Often mutation will render the new copy junk, resulting in a pseudogene. However, sometimes a new function can be gained.

This is apparently how snakes evolved venom. New research indicates that most venomous proteins made by snakes are very similar to proteins contained elsewhere in the snake's body.

From Science News:

By comparing the venom gene sequences with other known sequences, Fry was able, in three-quarters of the cases, to infer which gene had duplicated. He found that the various toxins traced their way back to proteins that play diverse roles in an assortment of organs, from the brain to the testes. For example, proteins in the acetylcholinesterase group are common neurotransmitters for muscle control, but the snake's venomous form induces paralysis and difficulty breathing; similarly, kallikrein proteins regulate blood pressure, but the toxic versions induce a fatal drop in blood pressure.

From Nature News:

"Despite the incredible changes in bioactivity that occur, the basic molecular scaffold and three-dimensional shape doesn't change notably," Fry says. The results of his analysis appear this week in Genome Research.

So here we have an example of a novel biological function arising from existing genetic information.