Ernst Mayr
What Evolution Is
When sorting out the collections he had made on the voyage of the Beagle, Darwin encountered the same question again and again: Are some slightly different specimens merely variants within a population or are they different species? Indeed, in the 1840s when he wrote his monographs on the classification of the barnacles, Darwin came to the conclusion that no two specimens in a collection from a single population were exactly identical. They all were as uniquely different from each other as are human individuals. And the animal and plant breeders, with whom Darwin was associated since his Cambridge student days, told him the same. They always knew which individuals in their herds they should select as the breeding stock for the next generation. Individuality made this possible.
Since the terms 'transmutationism' and 'transformationism' are not suitable for this new theory, Darwin's theory of evolution through natural selection is best referred to as the theory of variational evolution. According to this theory, an enormous amount of genetic variation is produced in every generation, but only a few individuals of the vast number of offspring will survive to produce the next generation. The theory postulates that those individuals with the highest probability of surviving and reproducing successfully are the ones best adapted, owing to their possession of a particular combination of attributes. Since these attributes are largely determined by genes, the genotypes of these individuals will be favored during the process of selection. As a consequence of the continuous survival of individuals (phenotypes) with genotypes best able to cope with the changes of the environment, there will be a continuing change in the genetic composition of every population. This unequal survival of individuals is due in part to competition among the new recombinant genotypes within the population, and in part to chance processes affecting the frequency of genes. The resulting change of a population is called evolution. Since all changes take place in populations of genetically unique individuals, evolution is by necessity a gradual and continuous process.
