Saturday, 17 August 2013

Edith's Butterfly Shows How Evolution Works

Ediths Checkerspot
I recently came across this striking example of how the ability to evolve fairly rapidly in response to environmental change can be both a benefit and a threat to a species. It also illustrates how a basic understanding of evolution can help us understand distribution patterns for species like this butterfly.

Edith's checkerspot butterfly (Euphydryas editha) is a group of very many subspecies of butterfly from north-western North America. Wikipedia lists 30 subspecies which have so far been identified, many of which are critically endangered. Each tends to be a specialist feeder in that adult females lay their eggs on one specific plant species on which the caterpillars feed. This makes it vulnerable to environmental changes affecting the occurrence and distribution of its host plant species.

A clue to why there are so many subspecies can be found in this abstract to a letter published in Nature from 1993:
RAPID evolution of host association is now occurring independently in two populations of the host-specialist butterfly Euphydryas editha, each of which has recently incorporated a novel host species into its diet. The reasons for these episodes of rapid evolution lie in human land use practices: logging in one case and cattle ranching in the other. In contrast to other insects that have used tolerance of human activities to expand their ranges into disturbed habitats, these rare butterflies have remained at their original sites and evolved adaptations to the changes occurring at those sites. At both sites, the proportion of insects preferring the novel host has increased, in one case clearly because of genetic changes in the insect population. This process is now starting to generate insects that refuse to accept their ancestral host, foreshadowing a new problem in conservation biology. By adapting genetically to human-induced changes in their habitat, the insects risk becoming dependent on continuation of the same practices. This is a serious risk, because human cultural evolution can be even faster than the rapid genetic adaptation that the insects can evidently achieve.

It seems that Euphydryas editha is able to switch it's dependence on one food species to dependence on another but isn't able to extend the number of plant species on which it is dependent. Hence it is easily directed down an evolutionary dead end by its environment and so is vulnerable to short-term changes with which it simply can't keep up.

The environment can change very quickly, in as little as a year or two, but it takes at least a few generations at one or two generation per year for the genepool to acquire the necessary level of allele frequency to make a new subspecies viable, then the species needs to evolve mechanisms to prevent interbreeding to prevent dilution of the new alleles in a larger genepool.

This can only be understood by an appreciation of how evolution is an adaptive response to environmental change but necessarily lags behind it. When the environment is subject to rapid change, as it is under human interference for example, the process of evolution can be too slow and the species can go extinct.

There is simply no way this can be presented as the result of directed evolution by some benevolent intelligence, or as a single act of creation just a few thousand years ago. The reason the distribution of Euphydryas editha, and the large number of subspecies into which it is divided, looks for all the world like it was produced by Darwinian Evolution is because that's exactly what produced it.

Simple little examples such as this entirely refute the infantile notion that a magic man created everything, of course, and yet almost half of American adults continue to believe it to be the best available explanation for the diversity of life on Earth, and of the distribution patterns and existence of so many subspecies of species like the Edith's checkerspot butterfly. This can only be because of ignorance, willful or otherwise, of the facts, or a refusal to acknowledge them.

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1 comment :

  1. " This can only be
    because of ignorance, willful or otherwise, of
    the facts, or a refusal to acknowledge them."

    ...or stupidity. This is a nation whose people can be induced to cheer on wars of choice against innocent countries on the other side of the planet by transparent, obvious lies. This is a nation whose people think taking guns to class is a good idea, that the Metric system is "too difficult", and that using public transport is "socialism". What do you expect?


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