Darwin’s theory of natural selection was based on the idea of the now much clichéd ‘survival of the fittest’. An individual that can out-compete another individual is relatively fitter, and has a greater chance of surviving and passing on its genes by reproduction. After many generations, this will result in a greater prevalence of organisms with the more successful genes. For natural selection to take place, several factors have to be present: variation in a population, a difference in fitness between the different variations, and heredity (the ability to pass on these variations to offspring).

The widely accepted mechanism for heredity when Darwin was alive was genetic blending inheritance: an individual’s genes are an intermediate mixture of their parents’. Although this may be visibly evident in many cases such as mongrel interbreeding, it does not explain why favourable variations are not rapidly lost within a few generations of breeding with the “normal” populace.

The work of Gregor Mendel provides a solution to this problem. Mendel discovered that the alleles (different forms of the same gene) of genes are not changed when inherited; they are not 'blended' together, but instead remain distinct and separate in the offspring. Thus, variation is not lost and persists throughout generations maintaining the raw material for natural selection to take place. This is known as Mendel’s particulate theory of inheritance. Mendel’s classic experiments and results were later built on by the early population geneticists.

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.

The modern synthesis united geneticists, naturalists and palaeontologists for the first time and laid the foundations for the explosion of research into evolution by natural selection that occurred after the Second World War. The key points that were settled were:

1. Population and Mendel’s principles of genetics are compatible with evolution
2. Evolution occurs by the accumulation of small genetic changes; it is a gradual process in this sense, though can vary in its actual rate
3. Natural selection is the dominant evolutionary force, which works on variation in individuals' characters
4. Evolution occurs within populations, where gene flow and genetic drift can have effects additional to those of selection
5. Evolutionary studies of the fossil record relate to what is observed within species. Evolution within a population is referred to as micro-evolution, the fossil record records macro-evolution which is evolution of species.

Our understanding of evolution is always being updated, and new ideas arise and are tested frequently. One example is epigenetic inheritance.

Throughout the era before the Modern Evolutionary Synthesis (known as The Eclipse of Darwin), there were many alternative theories of evolution instead of Darwin’s natural selection. The major challenger of natural selection was Lamarckism. Lamarckism was a popular mechanism for Evolution once generally accepted. Lamarckism is considered to be the passing of characteristics acquired in an organism’s lifetime to its offspring. Classic examples are the stretching of giraffe necks and the muscle development of blacksmith’s sons. The argument goes that the giraffe has to stretch its neck in order to reach the higher leaves of a tree to feed. This stretching causes the neck to grow longer over the giraffe’s lifetime, and this acquired long neck is passed on to the next generation, where it is stretched even further, and so on. Analogously, blacksmiths tend to have muscular sons, because blacksmiths acquire great strength to work iron, and this trait of muscularity is passed on.

After the death of Lamarck, Neo-Lamarckism (bearing the same major tenets as Lamarckism) evolved in the late 19th century and early 20th century as an eclectic collection of theories on animal evolution. Neo-Lamarckism suggests that the main evolutionary mechanism is evolution by the inheritance of acquired characteristics. This differs from Lamarckism, which suggests that animals evolve solely on the basis of inheritance of acquired characteristics. However, the evidence for Neo-Lamarckism is very limited and has been widely acknowledged as largely false. Lamark also did not believe in speciation or common descent, invoking a “special creation event” for the origin of each species.

Despite this, ideas superficially similar to Neo-Lamarkism may have some truth. "Epigenetics" is inheritance of the expression of genetic systems by non genetic change. For example, DNA is wrapped around proteins to help organise the cell nucleus. Some of these proteins can be modified by environmental change to make access to the DNA easier or more difficult. When regulatory proteins, which control the expression of a gene by binding to stretches of DNA which turn the gene on or off, have restricted access, gene expression levels may fall. Such changes can be brought about by environmental efects (and therefore acquired by the organism in its lifetime and in its germ cells), and can then be passed on through the generations even though the DNA itself has not changed.

Because selection acts at the level of an individual’s characteristics, epigenetic inheritance is of course subject to natural selection, genetic drift, mutation and gene flow. This will only be an addition to the main evolutionary theory as dictated in the Modern Evolutionary Synthesis as opposed to a complete revolution of the mechanisms of evolution. In the future, it is very likely that the Modern Evolutionary Synthesis will be reworked to incorporate some aspects of Neo-Lamarckism. Even then, this will be by no means the last update in our understanding of evolution.

Written by Buyun Zhao

Evolution: The History of an Idea
by Peter Bowler, University of California Press: 1983

The Evolution of Darwinism
by Timothy Shanahan, CUP: 2004

This Is Biology
by Ernst Mayr, Harvard University Press: 1997