Genes provide both the continuity and the differences between parents and offspring. The differences (‘variations’) are caused as follows:
1. Combination of half the genes from each parent.
2. Reassortment of the genes inherited from each parent. Genes borne along the same chromosome tend to be inherited together (they are said to be ‘linked’) but during meiosis
there is normally some ‘crossing over’, or exchange of pieces of the split chromosomes.
3. The presence of a gene does not guarantee the appearance of the character with which it is associated, because gene effects may depend on the action of other genes present. The simplest example is dominance within an allelomorphic pair, but other genes may promote, suppress or alter the effect of a gene. A character may be the product of many different genes
acting together, and one gene may affect many characters: for example, genes acting early in development may transform the effects of other genes acting later. It is a dangerous
oversimplification to equate a character with the gene that in part governs it. Mendel has been mistakenly described as ‘lucky’ because his choice of the peas gave a simple picture: in fact he spent a very long time experimenting to find suitable material.
4. Mutations occur. These may be chromosome changes, or more frequently errors in gene copying as cells divide. Sudden change in a phenotype due to mutation is rarely advantageous, as large changes tend to be lethal, but small changes may accumulate in the genotype, undetected until some change in circumstances gives them a selective advantage. Clearly, mutation is the only one of these causes of variation that operates in asexual reproduction, where otherwise parent and offspring are genetically identical.
With the mechanism producing heritable variations understood, the picture of evolution caused by natural selection acting on random variations became firmly established. Due mainly to R. A. Fisher, the emphasis fell not on the sudden change in form of an individual but on the spread of that variation through a population. The study of natural selection at work became a matter of statistics rather than qualitative descriptions. The synthesis of Mendelian genetics and natural selection was called Neo-Darwinism or ‘The Evolutionary Synthesis’, and by the 1930s it was widely accepted. It became the unifying principle underlying all branches of biology.