Sunday 23 November 2014

Rapid Darwinian Evolution In Agricultural Weeds

Wheat Field; Vincent van Gogh, 1888. Weeds and all.
Dawn of farming sparked speed-evolution in weeds - life - 19 November 2014 - New Scientist

One of my recurrent themes in my blogs about evolution is the difference between information held in the DNA and the meaning of that information as interpreted by the environment. This is something that creationist frauds traditionally ignore of course, especially when they try to fool their credulous victims with the claim that no new information can arise because of the Second Law of Thermodynamics (sic), so evolution is impossible.

Not only is that nonsensical claim refuted by simple observation but evolution often doesn't need new information anyway; evolution often comes about because a change in the environment give new meaning to existing information.

This was nicely illustrated by a paper by Ehud Weiss and his colleagues of Bar-Ilan University, Ramat-Gan, Israel, published in Vegetation History and Archaeobotany a few days ago. They showed how within about two thousand years, after humans first began cultivating crops, several 'weed' species had evolved to exploit this new environment of regularly cultivated soil and some had already become completely dependent on human agriculture, as many are today.

A characteristic group of obligatory weeds was found in the well of the submerged Pre-Pottery Neolithic C site of Atlit-Yam, Israel. Identifying these finds to species level was crucial for defining them as obligatory weeds. We deal here with the earliest and largest assemblage of obligatory and facultative weeds in the southwest Asian Neolithic. Atlit-Yam may reflect a stage in the establishment of weeds in cultivated fields. Weeds are an important resource for reconstructing the agricultural situation in archaeological sites, as weed-crop interactions reflect an agricultural lifestyle. Some of the weeds of Atlit-Yam grow in fields as well as in Mediterranean herbaceous habitats. This may indicate that the local herbaceous ecosystem was the original habitat of the weeds and the place where the first fields were planted. Presence in a single context of the earliest identified obligatory grain pest beetle (Sitophilus granarius) along with obligatory weeds reflects a novel change made to the ecosystem by the farmers, in which stored crops were invaded by pests.

Anat Hartmann-Shenkman, Mordechai E. Kislev, Ehud Galili, Yoel Melamed, Ehud Weiss;
Invading a new niche: obligatory weeds at Neolithic Atlit-Yam, Israel;
Vegetation History and Archaeobotany
, 08 Nov 2014

The development of agriculture would have opened up a new niche, and species that were already adapted to naturally disturbed habitats would be well positioned to rapidly spread to agricultural habitats.

Kenneth Olsen, Washington University, St Louis, Missouri, USA
I wrote a couple of years ago about how human agriculture had provided just the right conditions for a chance mutation in Capsella grandiflora, or Shepherd's Purse, to give rise to an entirely new species, Capsella rubella (Red Shepherd's Purse) which spread from its origins, possibly from a single specimen, in Greece, with human agriculture.

This of course gives the lie to creationist claims that no new species can arise by evolution because something they call 'macro-evolution' is impossible and anyway all mutations are invariably harmful. In this argument, in contrast to the 'no new information' argument, something called 'micro-evolution' is not only possible but explains all the diversity from two survivors of a mythical flood in a few thousand years. It's only the evolution of new species which becomes impossible when taxonomists decide that an identifiable isolated gene pool constitutes a new species as defined by taxonomists. Understandably, they never explain how this works exactly and normally become abusive and/or try to change the subject when asked.

Anyway, as the above paper shows, Capsella rubella is not the only example of the environmental changes brought about by human agriculture providing the conditions for new species to arise and for existing species to evolve to move into the new niches this change opened up. The researchers studied plant remains from about 9000 year ago which had been submerged by rising sea-levels which inundated a coastal settlement at Atlit-Yam on Israel's Mediterranean coast. Amongst the agricultural species which included durum wheat, chickpeas, figs and culinary herbs they found thirty-five species of agricultural weeds, including five which have become dependent on human agriculture.

One of these weeds, darnel (Lolium temulentum), had even adapted to being a weed in wheat fields by coming to resemble it, so making removal by hand harder. All of this is entirely consistent with a paper published by C.C.Vigueira, K.M.Olsen and A.L.Caicedo in Heredity in November 2012 which cited agricultural weeds as examples of rapid Darwinian evolution.

The Darwinian evolutionary explanation is perfectly straightforward of course. Humans first created new niches for plants to move into from amongst those already adapted to exploiting newly-broken ground, and then set up an intense selection process by selectively removing those plants, so creating conditions in which those able to come to resemble the crops they grew amongst, or able to set seeds very quickly after germination, or even to be able to regenerate from fragments of roots left in the soil were at an advantage. An especially effective adaptation is to set seed as the crop is being harvested so getting seeds into the next season's sowing.

No new information need have arisen for any of this to occur, since in every stage the process was one of natural selection from amongst already present variation, so changing the proportion of those variants in the species gene pool.

In other words human agricultural activity set the conditions for an adaptive arms race powered by a selection process which favoured certain species and certain variations in those species. Because the rate of evolution is directly related to the intensity of the selection pressure, the evolution of agricultural weeds was rapid and appears to have taken as few as 2000 years in this location.

Is it worth asking creationists how this fits in with their notion of intelligent design by a benevolent designer? Probably not, because the answer would almost certainly involve gibberish about Adam being condemned to clear weeds from fields because he ate an apple, and claims that this is a valid alternative scientific argument entirely consistent with modern biological science.

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