There were also other essential updates summarized by the Modern Evolutionary Synthesis. Darwin signified the importance of natural selection as the force of evolutionary movement, but the Modern Evolutionary Synthesis identifies the significance of three more evolutionary forces: mutation, gene flow and genetic drift. These came from advances in population genetics, the study of how evolution affects the frequency of alleles in populations of different sizes, and with different rates of immigration and emigration. This field was founded by the work of Ronald A. Fisher, John B.S. Haldane and Sewall G. Wright.
In On the Origin of Species, Darwin struggled to explain how variation could be formed and maintained. The Modern Evolutionary Synthesis identifies mutation as being central to natural selection. Mutation provides and maintains genetic variation amongst individuals in a population, a prime factor for natural selection to occur.
Gene flow is the movement of genetic information from one population to another. The most common example is the migration of individuals between populations which provides the receiving population with access to more alleles. This is important in determining the gene pool of the population and hence drives evolution by encouraging natural selection.
Genetic drift is the random fluctuation in frequency of different alleles of comparable fitness. One allele cannot “outcompete” the other, but random sampling errors can have a significant impact especially upon smaller populations; the smaller the population the more likely that one allele is completely lost due to an environmental catastrophe despite, there being no significant disadvantage in bearing one allele over another.
The modern synthesis combines these three factors identified by population genetics together with natural selection into a mathematical framework.
Another key contribution was made by the naturalist Ernst Mayr with his extensions of Darwin’s speciation work and introduction of a biological species concept where species are defined as ‘groups of interbreeding natural populations that are reproductively isolated from other such groups’. This work help resolve the problem of how species multiply as well as providing a more clear definition of a species for biologists to work with.
One outstanding issue was to bring palaeontologists into the fold as, prior to the synthesis, biologists who worked on the fossil record and on living species differed greatly in their views of evolution. Many paleobiologists argued that there was little evidence for natural selection being the dominant force in evolution. In 1944 however an American, George G. Simpson published work showing that the findings of palaeontology were fully compatible with the ideas of population genetics and natural selection.