Malthus' essay on population. Darwin's theory of descent with modification through variation and natural selection. Darwin struck by competition and waste in natural world, as stressed by Malthus. Stressed that variation is reality and subject to natural selection. Lineages change in frequency over time.
Evolution is tree-like as lineages split and extinction occurs.
Current theory adheres to much of Darwin's thinking. Nature of the species. Evidence for Evolution by Natural Selection Increaseed evidence for variability within species, and response to selection. Size of male house sparrows in North America. Size is measured as a principal component of score, derived from skeletal measurements.