Genetics - the theory which explains how inherited characteristics are passed from one generation to the next, and natural selection, which allows fitter individuals to survive and breed more often than others, together provide a coherent account of evolution.
When Charles Darwin (1809-1882) formulated his theory of evolution by natural selection, he had no firm idea how characteristics advantageous to an organism could be inherited: he simply assumed that organisms possessing such characteristics would be more likely to survive and reproduce, passing on their advantageous characteristics.
This led to a difficulty for the theory of evolution: if an organism spontaneously developed a new characteristic (say, it could see much better in the dark), it would be less likely to starve to death (if it were a predator) or to be eaten (if it were prey). It would therefore be more likely to survive to reproductive age and have offspring. However, its partner would be extremely unlikely to have spontaneously developed the same characteristic, and so the next generation would be able to see only half as well in the dark. The generation after this would have only a quarter of the advantage. Soon the new characteristic, advantageous or otherwise, would have been blended out of existence. Darwin himself was deeply worried about this.
Discovery of genetics
The discovery of genetics provided the solution that evolution needed. Gregor Mendel (1822-1884), an Augustine monk, was interested in the inheritance of characteristics in plants. He experimented with pea plants in the grounds of his abbey, crossing plants that had visibly different characteristics to see whether subsequent generations would inherit these characteristics. These experiments led him to discover several of the key laws of modern genetics. The crucial one for the resolution of Darwin’s problem is that hereditary variation, advantageous or otherwise, is caused by genes (although Mendel did not use the word gene) and that genes do not blend with one another.
In genetic language, then, we would say that evolution proceeds as follows. Every so often, organisms randomly develop genetic mutations. Some of these mutations confer on the organism a competitive advantage that makes it more likely to survive to reproductive age and breed successfully. These mutated genes are therefore more likely to be passed on to the next generation. Those individuals that inherit the mutated gene are at a competitive advantage relative to other organisms that do not have this gene (there is a slight assumption here – that the organism will benefit from inheriting the new mutation from one parent: some genes, known as recessive alleles, only have an effect if inherited from both parents). They are therefore more likely to survive and to reproduce successfully. This advantageous gene will therefore appear in a larger proportion in the next generation, and eventually this gene will spread throughout the population. Taking the original example of a gene that improves night vision, we can see that eventually the entire population will have better eyesight.
Modern evolutionary synthesis
This is the crucial link between natural selection and genetics: genetic variation is entirely random, but natural selection constantly eliminates variations that confer disadvantages on the organism and promotes those that confer advantages. The unification of inheritance by genetics with natural selection to provide a clear account of evolution is known as the modern evolutionary synthesis.