Friday, 1 June 2012

Evolution Gave Us This Day Our Daily Bread

An interesting recent occurrence of speciation gave rise to bread wheat, i.e. the variety of wheat (Triticum aestivum) used to make the flour we in the west use to make bread. It was not a straightforward event and needed an earlier speciation events to occur in the wheat family to produce the two parent plants which hybridised to make our daily bread.

This is a good example of speciation due to a change in the number of chromosomes to produce a condition known by biologists as polyploidy. A brief explanation of polyploidy follows for those non-biologists who may be reading this blog, so skip the next three or four paragraph if you know this already.

Each species has a basic number of chromosomes in its cells. in most normal adult forms each chromosome is paired so that we have two of each. So the base number is 2n where n is number of pairs. 'n' is known at the haploid number so normal adult cells are diploid (i.e. 2n).

The cells from which most individuals are made at fertilization are haploid. These are produced in the germ cells when the reproductive cells or gametes are produced by a special form of cell division in which one of each pair of chromosomes goes to each pair of cells produced instead of the chromosomes doubling first as in normal cell division. This is called meiosis.

In the case of mammals, meiosis produces sperm in males and ova or eggs in females. In the case of flowering plants it produces pollen (male) and the ovules (female) reproductive cells. When these gametes fuse together they create a diploid cell which develops into the normal individual.

Ploidy then refers to the number of chromosomes in relation to this normal haploid number. In normal humans we have 23 pairs of chromosomes (including the x and y chromosomes which determine gender but which are not really pairs as such in males, who have one of each)

Particularly in plants, polyploidy, i.e. where the normal individual had a multiple of the haploid number, often, but not always, tetraploid (4n) is fairly common and is a frequent cause of speciation. This occurs when there is a fault with meiosis and one gamete gets both pairs of chromosomes. If this happens in a pollen grain and in an ovule which happen to come together in fertilization, the resulting cell can be viable. The production of diploid gametes if probably fairly common but what is rare is for them to find another diploid gamete to fuse with, and for that to produce a viable seed.

When this happens between two related species is it known as allopolyploidy and the resulting hybrid is said to be amphidiploid (i.e double diploid) or tetraploid.

Anyway, from an examination of the genome of bread wheat, it is now clear that, not only did it result from just such an allopolyploid hybrid speciation event but that one of the parent species which hybridized was itself produced in an earlier allopolyploid hybridization. The result of this is that bread wheat is hexaploid (i.e. 6n) rather than the normal 2n or less frequent 4n arrangement.

At some time in the past the cultivated form of Triticum monococcum allopolyploidily (not a word to say after a couple of G & T's) hybridized with T.saersii to produce the wild T.turgidum. T.monococcum had been cultivated as Einkorn wheat. Then about 10,000 years ago, wild T.turgidum began to be cultivated as Emmer wheat. This in turn then hybridized with another wild wheat, T.tauschii, to produce T.aestivum which has been cultivated for about 8,000 years.

And, of course, not having a chromosome number which will normally successfully pair up with any other species of wheat other than itself in reproduction, including the parent species, bread wheat is genetically a fully isolated species in it's own right, filling all the criteria of a distinct species.

So, there are a couple of serious problems here for Creationists, the major one being why an intelligent designer would cause different species to hybridize in such a way as to produce a huge number of chromosomes, with all the in-built redundancy that entails, and spread this over several thousand years, when a single act of creation could have achieved the same result with only the normal number of chromosomes and far less, if any, genetic redundancy.

Creationists also need to explain why it looks like human agency was involved if only by producing a large number of cultivated plants in a small area which would have facilitated this hybridization and made it much more likely than mere chance, and of course by producing the highly favourable selective environment which ensured reproductive success of the hybrids.

Those Creationists who insist that speciation cannot occur by a natural process, especially by an evolutionary one, and that magic done by a special magician must be involved to produce new species, have to explain why the process described above is impossible and why magic must have been involved.

And of course, as with all creationist explanations, there is always the essential ingredient of magic to be explained away, along with the origin and nature of the magician and how exactly it works, so their explanation is infinitely complex and hopelessly incomplete and inadequate.

For evolutionary theorists, of course, there is no mystery to be solved; there is no magic to be explained and no infinite complexity. The explanation is entirely complete and includes nothing which is included merely to satisfy the superstition of the proposer. It's all due to a fully understandable natural process. As always, the best explanation is the least complex and most vicarious one, as William of Occam showed. His trusty razor once again pairs gods and magic out of the explanation and produces a fully worked-out theory which explains the evolution of bread wheat.

One really has to pity the poor Creationists as they struggle to fit observable reality into a primitive superstition so singularly ill-fitted for purpose.

More details of this process can be read on the National Center for Biotechnology Information website in the article entitled Changes in Chromosome Number from the book, Modern Genetic Analysis by Griffiths AJF, Gelbart WM, Miller JH, et al. New York: W. H. Freeman; 1999.

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1 comment :

  1. Nice post, Rosa. As always, you use the sharp knife of your intellect to slice up the chaff of ancient myths and separate it from the nourishing wheat of honest scientific endeavors.


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