The nature of natural selection

If you've gone through the Schol Bio exam papers from previous years, you'll have noticed that evolution is one of the key themes in every paper. So I thought it could be useful to spend a bit of time on concepts relating to natural selection, one of the drivers of evolutionary change.

Natural selection acts on genotypes; evolution consists of changes in allele frequencies

A beneficial feature won't be passed on to offspring if it is not heritable i.e. if it does not have a genetic component. If a change in a phenotypic feature, such as beak length, is retained in a population after the selection event, this indicates it had a genotypic basis. And remember – natural selection acts on individuals, but its results occur in populations.

Natural selection does not look to the future.

This means that natural selection can't produce a trait that might possibly be useful at some point in the future. It acts only through current environmental conditions to produce individuals adapted to those particular conditions. If the environment should change, those individuals wouldn't necessarily have the adaptations that would allow them to survive.

Remember that although the traits known as exaptations (or preadaptations) do appear to contradict this, an exaptation is a trait, of survival value in one environment, which also has survival value in a new environment. The fact that a feature may have value in a changed environment does not override the fact that it must be of survival value in the current environment to have evolved at all.

Lungs are an excellent example of an exaptation. Lungs are a feature that evolved early in the history of fish (there's an image here). We know this because lungs are found in all but two of the modern fish lineages. This meant that when the ancestors of amphibians first ventured onto land, they were preadapted to breathe air. (It's hard to see how a lung could have evolved after those early tetrapods left the water!)

New traits can evolve, even though natural selection acts on existing traits.

Natural selection can act only on existing variation. However, it can produce new traits over time, as mutations generate new alleles and sexual reproduction produces new genotypes.The new traits produced by these 2 events are available for natural selection to work on. Artificial selection provides a good example of how this can operate.

Natural selection is not “perfect”

Natural selection must work on existing characteristics. This means that many of its outcomes are not “perfect”, in the sense of being exactly what the organism might require. The panda’s thumb is a good example. In primates, the first digit on the hand has evolved into an opposable thumb. In the panda, the forepaw was completely dedicated to locomotion (as in all other bears) & so the first digit could not be used. The radial sesamoid “thumb” is a compromise solution.

Natural selection is non-random, but not progressive

The result of natural selection is adaptation to the prevailing environment. Thus, natural selection is non-random.

Nor is it progressive i.e. it doesn’t lead towards a predetermined goal. Organisms are “improved” only in the sense of becoming better adapted to their environment. This does not always equate to increased complexity e.g. both tapeworms & snakes have lost complex structures when compared to their ancestors. And a barnacle called Saccculina, which parasitises crabs, is reduced to a simple mass of cells in its adult form. Yet its larvae look like those of any other barnacle.



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