Tuesday, 1 May 2012

The Good Shepherd's Purse Is Bad News For Creationists

Capsella grandiflora
Creationists will always wave aside examples of observed evolution as 'micro-evolution' but not 'macro-evolution', which is what they call the evolution of new species. For some reason they seem to find this a threat to their favourite easy answer of the creation of all living things out of nothing by a magic man, so it must be devastating for them to hear of yet another example of it.

This one has recently been found in the family of common garden weeds, which I have always known as 'Shepherd's purse', with the generic name Capsella. They have a characteristic heart-shaped seed capsule which gives them their English name. These plants are something of a garden pest because they can harbour the fungal root infection of the cabbage family known as club root disease.

Capsella grandiflora, like many species of plant, has a mechanism which prevents self-pollination or 'selfing', known as self-incompatibility. Loss of this mechanism is fairly common in some plants because if offers a short-term solution where pollinators are rare. In that situation the disadvantage of not exchanging genes with another individual can be outweighed by the advantage of being able to produce fertile seeds, so natural selection can favour it in the right circumstances and work against in in other circumstances.

Capsella rubella
A very closely related species, Capsella rubella (red shepherd's purse), however is self-compatible.

By comparing the genomes of the two species, a team of researchers working at the Max Planck Institute for Developmental Biology, Germany, found that the two species diversified only 30-50,000 years ago and, because of the low number of different alleles in C. rubella, it must have gone through a very narrow bottleneck and probably arose from a single incidence of selfing, probably in Greece. Its spread, and maybe its survival, was facilitated by the spread of agriculture in the area at about the same time.

So, human agency at the dawn of agriculture may well have created the right environmental conditions in which 'selfing' became an advantage. That allowed a new species to arise and prosper, and one which continues to be an agricultural weed.

Recent speciation of Capsella rubella from Capsella grandiflora, associated with loss of self-incompatibility and an extreme bottleneck. Guo YL, Bechsgaard JS, Slotte T, Neuffer B, Lascoux M, Weigel D, Schierup MH.
Department of Molecular Biology, Max Planck Institute for Developmental Biology, 72076 Tübingen, Germany.
Proc Natl Acad Sci U S A.;2009 Mar 31;106(13):5246-51. Epub 2009 Mar 23.

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