Classification alone does not confirm the principle of common descent, but when combined with other lines of evidence, the relationships are clear. For example, genetic analysis of species that are thought to be closely related on the basis of anatomical features and other criteria reveals a common hereditary background. Natural selection and adaptation Darwin's book focused on the role of natural selection in adaptation. All species have such great potential fertility that their population size would increase exponentially if all individuals that are born reproduced successfully. Populations tend to remain stable in size, except for seasonal fluctuations.
Environmental resources are limited.
Production of more individuals than the environment can support leads to a struggle for existence among individuals of a population, with only a fraction of offspring surviving each generati on. Survival in the struggle for existence is not random, but depends in part on the hereditary constitution of the surviving individuals. Those individuals whose inherited characteristics fit them best to their environment are likely to leave more offspring than less fit individuals. Natural selection is this differential success in reproduction, and its product is adaptation of organisms to their environment. Natural selection occurs from the interaction between the environment and the inherent variability in a population.