Wednesday, 8 June 2016

How Evolution Works - Suffering Saxifrage!

I noticed this saxifrage which has been struggling for about three years to survive on a the patch of gravel which constitutes my front garden and it suddenly dawned on me how neatly it illustrates some aspects of evolution. It shows how a species fits itself into an available niche and how it diversifies as it spreads its range, not necessarily because it changes but because the environment in which it finds itself changes. The genetic information doesn't need to change for the meaning of that information to change because meaning is given to the information by environmental context.

If that idea seems a little obscure, consider the word karutis. It is probably meaningless to the average reader of this blog. Show it to a Latvian speaker however, and the meaning will be obvious - wheelbarrow. Of course, if you showed the word 'wheelbarrow' to a Latvian, it would be as meaningless as 'karutis' is to an English speaker. The information in the words is the same in either language; only the meaning changes with the language environment

Saxifrage is good ground cover for gravel because it can survive in shallow soil and spreads mostly vegetatively by short runners and so forms colonies of what are essentially clones of the parent plant. It can also survive being scuffed and kicked occasionally, which is useful when postmen and others going door to door walk to your neighbours door across your gravel garden as though it's a public footpath. It's there partly for ground cover but also for the pretty red flowers it puts up at the right time of the year.

In fact, this particular patch of saxifrage didn't start in that position but has developed from a little 'satellite' clump which grew from a small piece kicked off by someone and which quickly took root. The original died off. The gravel is on a bed of hard compacted clay that looks like something the builders dug out to make the foundations. It is completely useless for growing much on - which is why, together with our neighbours, we have covered it in gravel and are trying to make the most of it.

Look closely at the photograph and you will see the plant colony looks quite healthy and fresh in the centre of the picture but towards the bottom and especially to the right some satellite colonies look distinctly sickly and struggling. Genetically, these are all clones so the difference must have something to do with the local environment. There is either something that the saxifrage doesn't like there, or something is lacking that the saxifrage needs.

Either way, with the same genetic information, over a very short distance, there is a very different growth pattern caused not by the genes but by the way they interact with the environment.

So, from my initial planting of the (now extinct) parent plant a few years ago, the saxifrage colony has not only moved but it has happened to find a small niche in which it can thrive. Meanwhile, other satellite colonies have branched off, just as local populations of a species across its range can become isolated. As it extends its range the species may find itself in an environment in which it thrives or in one in which it fails. The net result of this process is that the species finds itself inhabiting a niche that best suits it just as water finds itself in a hole perfectly shaped for the puddle it forms in it.

There was no intelligence involved here, no plan and no direction. The whole thing was the result of random processes with the environment giving it direction and the result is that the plant has actually physically moved into the best niche in the neighbourhood.

Although this isn't strictly an evolutionary change as such because there was no change in allele frequency, it does illustrate the role of the environment in natural selection and in fitting a species to the best available ones. Imagine though, if one of these colonies inherited a mutation which meant they were better suited to the area where my saxifrage is now failing! It would not take long before that variant occupied a different niche and began to behave like a local variant. Scale this situation up to cover thousands of square miles and run the process for a few million years!

For my creationist readers: in this situation what would prevent these different regional variants evolving in their own different environment-moderated direction and eventually becoming different species to their cousin colonies a few hundred miles away, and all equally well 'designed' by natural processes to fit their particular niches?

Answers below please.


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