The Journey of Man: A Genetic Odyssey Read Online Free PDF Page B

got in a discussion with a guy in the lobby. He was talking to her as if he knew her. She kept saying ‘I’m sorry, sir, you’ve mistaken me for someone else.’ Finally he said, ‘Oh, I’m sorry madam – you all look alike to me.’ That really had a big effect on my thinking – they really are different from us, and we’re all alike.
    But the result was there in the statistical analysis, and it has been confirmed by many other studies over the past three decades. The small proportion of the genetic variation that distinguishes
between
human populations has been debated endlessly (is it higher within or between races?), but the fact remains that a small population of humans still retains around 85 per cent of the total genetic diversity found in our species. Lewontin likes to give the example that if a nuclear war were to happen, and only the Kikuyu of Kenya (or the Tamils, or the Balinese …) survived, then that group would still have85 per cent of the genetic variation found in the species as a whole. A strong argument indeed against ‘scientific’ theories of racism – and clear support for Darwin’s assessment of human diversity in the 1830s. It really was a case of ‘out of many, one’, as the title of this chapter says in Latin. But does this mean that the study of human groups is meaningless – can genetics really tell us anything about human diversity?
Forcing the issue
    For the next step on our journey, we need to cover some basic population genetics. The theory of how genes in a population behave over time is fairly complicated, and makes use of many related branches of quantitative science. Statistical mechanics, probability theory and biogeography have all contributed to our understanding of population genetics. But many of the theoretical frameworks are based on a few key concepts that can be understood by anyone, reflecting the relative simplicity of the forces involved.
    The most basic force is mutation, and without it polymorphism would not exist. By mutation I mean a random change in a DNA sequence – these occur at a rate of around thirty per genome per generation. Looking at it another way, each person alive today is carrying around thirty completely novel mutations that distinguish them from their parents. Mutations are random because they arise as copying mistakes during the process of cell division, with no particular rhyme or reason as to where those mistakes might occur – our genomes do not appear to favour certain types of mutation based on what the effect might be. Rather, we are like Heath Robinson engineers, forced to make use of what we are given in the mutational lottery. The blood group variants discovered by Landsteiner originated as mutations, as do all other polymorphisms.
    The second force is known as selection, in particular natural selection. This is the force that Darwin got so excited about, and it has certainly played a critical role in the evolution of
Homo sapiens
. Selection acts by favouring certain traits over others by conferring a reproductive advantage on their bearers. For example, in cold climatesanimals with thick fur would have an advantage over hairless ones, and their offspring would be more likely to survive. Selection is certainly what has made us the sentient, cultured apes we are today. It is what produced the important traits of speech, bipedalism and opposable thumbs. Without natural selection we would still be very similar to the relatively unsophisticated ape-like ancestor we would encounter if we could go back in time 5 million years or so.
    The third force is known as genetic drift. This is a rather specialized term for something we have an innate sense of – the tendency of small samples to reflect a biased view of the population from which they are drawn. If you flip a coin 1,000 times, you expect to get around 500 heads and 500 tails. If, on the other hand, you flip the coin only 10 times, it is quite likely that you will get something