July 18, 2007
Evolution-11: The rise of population genetics and the neo-Darwinian synthesis
(Please see here for previous posts in this series.)
The joining of Darwin's theory of natural selection with the Mendelian theory of genetics is one of the great triumphs of biology, now called somewhat grandly the 'neo-Darwinian synthesis'. It forms the basis of all modern biology, and was strengthened by the discovery of DNA as the structure of genetic information and which explained how Mendelian genetics worked on a microscopic scale. The modern ability to map out the entire genome of humans and other species has produced overwhelming evidence in support of Darwin's theory of how organisms evolve and branch out into different forms. The rough tree of life that Darwin sketched out in his book based on the anatomy of biological species has now been made more precise and detailed by the mapping of the DNA of species, showing ever more clearly how species are related to one another and when they separated from a common ancestor.
Mendel showed that genes were discrete objects that retained their identity as they were handed down from generation to generation and that thus any changes in genes, however small, did not get blended away in a regression towards the mean. So you would have thought that the rediscovery and rapid popularization of Mendel's ideas in 1900 would have signaled a resurgence of Darwin's idea that natural selection worked on very small, almost continuous changes, and the defeat of those who argued that one needed discontinuous changes for evolution to occur.
Ironically, the exact opposite happened. Because Mendelian genetics was a discrete mechanism with the genetic information seeming to occur in small lumps that remained intact, it superficially seemed to support the discontinuous model of natural selection, and the proponents of discontinuous changes were able to co-opt Mendel's theory to their cause. By around 1908 or so, it seemed like Darwin's own favored model of small continuous changes leading to large changes was in almost total retreat, actually doomed by the arrival of Mendel. While there was a sprinkling of mathematicians like Udny Yule (what a wonderful name!) who argued that Mendel's theory was compatible with Darwin's model of continuous evolution, their voices were lost in the volume of controversy generated by the competing biological schools. (The Origins of Theoretical Population Genetics, William B. Provine, 2001, p. 85)
Part of the problem was that scientists were still struggling to understand the workings of both Darwin's theory and Mendelian genetics and many misunderstanding of each were then prevalent. For example, one thing that was puzzling about genetics (and puzzled me for a long time too) was this whole business of dominant and recessive genes and how it affected population distributions.
It was Mendel's work that argued for the existence of these two types of characteristics, the actual mechanism of which became better understood with the discovery on DNA and increased understanding of the way that chromosomal information was handed down from parent to child.
Simply put, each person has pairs of genes, one from each parent on the respective inherited chromosome. To be concrete, we can look at the gene for eye color. Each gene may be of a dominant type (denoted by A, for say brown eyes) or a recessive type (denoted by a, for say blue eyes). So a person would have one of the pairs AA, Aa, or aa on the pair of chromosomes that contain the genes for eye color. Since A is the dominant one, it always wins, and so those people with either AA or Aa will have the characteristic A (brown eyes) manifest itself in their features, and only the person who has aa will display the characteristic a (blue eyes). Each parent will also have AA, Aa, or aa, and will randomly pass on just one of the pair of genes it possesses to the child.
It seems intuitive that if a population starts out with some distribution of AA, Aa, and aa types, and there is random mating in the population, then the number of people displaying the dominant characteristic A will steadily increase in the population, while the manifestation of the recessive characteristic a will decrease and perhaps eventually even disappear altogether, since only someone possessing the relatively unlikely combination aa will manifest it. Since regressive characteristics did not seem to be disappearing in real life populations, and in fact seem quite stable in their numbers, early geneticists had some doubts about whether they were interpreting Mendel's model correctly.
But starting around 1908, things started to change as better experiments were done and more mathematical versions of the two theories started being used. Mendelian and Darwinian theories started to get quantified and people began to realize that Darwin's version of natural selection with continuous changes was in fact compatible with Mendel's theory. By 1918, the reversal was complete and Darwin was ascendant and has remained so ever since. This was largely due to the rise of the field now known as population genetics, whose practitioners developed mathematical models that looked at the consequences of Mendelian genetics in natural selection.
What started the shift was the result now known as the Hardy-Weinberg law, which will be discussed in the next posting in this series.
POST SCRIPT: American beliefs about evolution
Gallup has done one of its periodic surveys about Americans views on evolution
These results show that:
- 24% of Americans believe that both the theory of evolution and the theory of creationism are probably or definitely true.
- 41% believe that creationism is true, and that evolution is false.
- 28% believe that evolution is true, but that creationism is false.
- 3% either believe that both are false or have no opinion about at least one of the theories.
That first group of 24% is definitely confused, since there is no way that evolution could have occurred in the 10,000 years or less allowed by creationism. The survey creators speculate that these were people who believed that god influenced evolution and somehow wanted to incorporate that view and responded in contradictory ways depending on which question was asked.
- The reasons for rejecting evolution were mostly religious.
- The more regularly you attended church, the less likely you were to believe in evolution.
- Republicans were less likely to believe evolution (30%) than Democrats (57%).
It is apparent that many Americans simply do not like the idea that humans evolved from lower forms of life. This appears to be substantially based on a belief in the story of creation as outlined in the Bible -- that God created humans in a process that, taking the Bible literally, occurred about 10,000 years ago.