Environmental Factors Reveal Genetic Variation


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One of the common misunderstandings about evolution centers around the way organisms always seem to be able to express a trait that fits an environment, even if the same kind of organism elsewhere looks different. The classic example of this is dark and light peppered moths. Another example is white winter coats and brown summer coats of arctic hares. It appears as though organisms can take stock of their surroundings, decide what traits would be most suitable and express them. In reality, this adaptive ability is based on two ideas: variation within a population and selective pressure.

Traits that change under different environmental conditions are called polyphenisms. Scientists studying horn worms, a common biological model, showed that a population of tobacco horn worms carried a hidden genetic ability to change color – the source of variation. This ability was only expressed under certain selective pressures, in this case heat shocking larvae to mimic known natural environmental conditions. By selecting for two different traits (black or green worms), the scientist were able to develop two distinct populations. The key was a mutation in the expression level of a juvenile hormone and the proposal is that mutations in hormones or similar developmental proteins might be an important source of "quick" adaptations to new environmental pressures. This concept is called "genetic accommodation" and refers to the idea that at different levels of expression the effects of the hormone are influenced differently by the environment.