The central stumbling block for Darwin's understanding of evolution by natural selection was the ignorance of the underlying laws of inheritance and variation. Yet before Darwin had produced the final edition of Origin, a monk working in obscurity in a Brünn (now Brno, Czech Republic) monastery began to unlock the secrets of heredity. Between 1856 and 1863 Gregor Mendel (1822-84) cultivated and tested at least 28,000 pea plants, carefully analysing seven pairs of seed and plant characteristics. Wise - or fortunate - enough to choose a plant that showed non- continuous variation (i.e. short or tall, round or wrinkled etc.), Mendel concluded that the inheritance was governed by a discrete system of alleles (later to be referred to as genes) that were either dominant (for a trait that shows up in offspring) or recessive, (for a trait masked by a dominant gene.) Mendel published his findings in 1866, but they remained almost totally ignored for the next thirty-four years.
However, even when rediscovered in 1900, Mendel's work initially proved problematic for Darwinism. Had Darwin himself have read Mendel's work (and there is evidence that he could have had access to the paper) its significance may well have been lost on Darwin: for, unlike Mendel's pea plants, nature often shows continuous variation - people are not either tall or short. The relationship between genes and an organism's characteristics is often, of course, much more complicated than in the pea- plants: the effect of a particular gene can be modified by the environment or the action of other genes. In other words, the apparent characteristics of an organism - its phenotype - are not necessarily a picture of underlying genotype. When a gene is referred to as being 'for' a particular trait, what is meant is that all else being equal, an organism with this gene is more likely to display the trait in question than an organism without it.
A more important problem that Mendel's work caused for Darwinism was that while it explained the continuity in nature, it did not fully explain the variation or the process of change. After Mendel's work was rediscovered in 1900, a school of thought developed that maintained that evolutionary change operated through a series of radical mutations in the genotype and hence phenotype. For the Mendelians, evolution therefore operated in a series of random and sudden leaps, a position which seemed irreconcilable with the Darwinian doctrine of continuous variation.
However, after 1930, a New Synthesis emerged through the mathematical genetics of R.A. Fisher, J.B.S. Haldane and Sewell Wright, which reconciled the two camps. Whilst any mutation large enough to cause dramatic change in the phenotype was shown to be fatally disruptive to the organism, it became clear that there had been an underestimation of the frequency and the utility of small mutations. Even the slightest selective advantage could lead to the rapid spread and permanent establishment of a novel feature. Furthermore, the recombination afforded by sexual reproduction was shown to be extremely effective in generating variation: even without mutation, the potential for natural selection to act would remain.
The growth in understanding of how genes operate to produce phenotype has also affected how natural selection is perceived and defined. "We are survival machines - robot vehicles blindly programmed to preserve the selfish molecules known as genes." So writes Richard Dawkins in The Selfish Gene (1989). In other words, whilst natural selection can only act on the phenotype that is expressed, the level at which this selection ultimately operates is the genetic level. Such an approach has provided answers to many of the questions that perplexed Darwin, such as the apparent altruism of social insects. However, it is important to remember that the 'fitness' of an individual gene is wrapped up in the success of gene- complexes and the whole co-adapted genotype within an individual organism.
The recognition of the central role of sexual reproduction coupled with the new understanding of genetics has shifted the focus from the individual to the population and therefore the gene pool that this population represents. Within this population an individual organism is merely a short-lived representative of a sample of the gene pool's contents. This is not the same as group-selection - a colony of bees is 'motivated' to survive not for some pre-defined worthiness of the colony but for the continued success of genes and gene-complexes. The new perspective has also redefined the concept of species: whereas for Darwin species were seen as sets of individuals sharing the same physical and behavioural characteristics, today they are defined as reproductive networks within which there is a free-flow of genes, and between which there exist significant barriers to such an exchange.
Given all this, nearly a century and a half after the publication of The Origin of Species, the process of natural selection can be summarised as follows: The underlying genotype of an organism, mediated by environmental factors, expresses itself in a phenotype. However, this genotype represents only a sample of the all genes existing within a single interbreeding population. Within such a population there may exist various forms - or alleles - of a particular gene. The presence of one or other of these alleles may confer a selective advantage on the individual that carries this allele (or set of alleles). Given that there remains a struggle for existence and reproduction, the alternative versions of each gene are constantly competing for a place in the next generation of organisms. Any gene which confers a selective advantage on an individual will therefore tend to make more surviving copies of itself than the other competing alleles. Therefore, although natural selection acts on the survival and reproductive success of individuals, the change that occurs during the course of evolution is the relative frequency of genes - which is, of course, expressed in the phenotype of individuals. Given this, the same overall processes of change and speciation that Darwin observed, continue to take place.
For more information on all of these topics, see Further Reading.