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.

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