杏吧原创

ARE humans still evolving? It鈥檚 a controversial and exciting question. And according to and , the answer is a resounding 鈥測es鈥�. Rebelling against the received wisdom of many evolutionary biologists who insist that we have reached an evolutionary plateau, the authors argue that not only are we still evolving, but also that we are actually evolving at an accelerating rate.

Cochran and Harpending draw on the past decade of genetic data for our species, including new information about natural selection鈥檚 effects on individual genes that influence features such as metabolism, disease resistance and the colour of our skin and eyes. They benefit from a marvellous new bird鈥檚-eye view of our entire genome, courtesy of the .

To measure how rapidly we have been evolving, the authors looked to the 鈥� a growing database that details patterns of human genetic variation. These variations most commonly occur as single-letter changes in DNA, known as or SNPs. By tracking their occurrences, we get a substantial look at the true extent of genetic variation in human populations. Most of these SNPs have no discernible effect, but they can be used as markers to trace the spread of other, more important, gene variations () in our chromosomes that do influence our survival and happen to be located near 鈥� are 鈥渓inked鈥� with 鈥� those SNPs.

The trick to finding evidence of recent evolution is to search for regions of the genome in which one tightly linked set of SNPs is commonly found in the population while the rest of the SNPs in that region of the genome have been mixed up by a long period of genetic recombination. The most likely explanation for such a pattern is that some advantageous alleles embedded in the region have been spreading rapidly through a population, towing linked SNPs along with them. Geneticists call this process a 鈥渟elective sweep鈥�.

Cochran, Harpending and their colleagues examined about 2000 examples of such sweeps in an African population and 2000 in the European population. They found strong evidence that advantageous alleles in these regions are sweeping through the human population 鈥� evidence of natural selection at work. These sweeps began earlier in Africa than in Europe, perhaps 80,000 and 40,000 years ago, respectively. They can be dated by looking at the degree to which the linked SNPs have started to break down through the continuing process of genetic recombination.

The authors conclude that these and other signs of more recent human evolution are the result of rapid cultural changes, triggered by the invention of agriculture and the urbanisation of our species. This can be seen in evidence of selection on specific genes such as those that influence our physical ability to form words or to metabolise new foods.

鈥淩ecent human evolution may have been triggered by the invention of agriculture鈥�

The evidence the authors present builds an overwhelming case that natural selection has recently acted strongly on us and may be continuing unabated, though the authors鈥� estimate of a 100-fold increase in the rate of recent evolution seems too high.

They spoil their argument by contending that the majority of the differences that have an impact on individuals are differences between one racial group and another, rather than those within individual racial groups. As they acknowledge, the evolutionary biologist has pointed out that at least 85 per cent of human genetic variation is within racial groups. They do not deny this, but they do make the entirely specious argument that human evolution is like that of domesticated dogs, where between-breed differences are far more important than within-breed differences.

The history of domesticated dogs, however, is very different from that of our species. For us, while there has been selection for superficial differences like skin colour that enable groups to adapt to different environments, there has also been strong selection for high levels of diversity within each group. By de-emphasising within-group variation, the authors ignore some of the most powerful trends in our recent rapid evolution and resurrect dangerous arguments about the importance of racial differences.

What is sadly lacking in the book is any discussion of what recent and continuing evolution means for our species as a whole. Instead, the authors address once again their highly controversial idea 鈥� in 2005 鈥� that Ashkenazi Jews have higher IQs than other populations. They present some interesting data suggesting that people who carry recessive alleles for the neurological diseases common among the Ashkenazim, such as Tay-Sachs and Niemann-Pick disease, may have higher IQs than the average Ashkenazi. They suggest that rapid selection for high IQ may have dragged these potentially harmful genes to higher frequency. This is an intriguing possibility, but it does not explain the paucity of these same genes in other Middle Eastern populations. The authors contend that the differences arose because Ashkenazim were selected for commercial abilities, but offer no concrete evidence.

Ultimately, the book offers a limited and biased interpretation of some very exciting research 鈥� research that shows clearly that our evolution is a continuing story in which we all play a part.