The diverse forms of life have arisen by descent with modification from ancestral species. The mechanism of modification has been natural selection working gradually over long periods of time. Some subtleties of natural selection Populations are important in evolutionary theory, since a population is the smallest unit that can evolve. A group of interbreeding individuals belonging to a particular species and sharing a common geographic area Natural selection is a consequence of interactions between individual organisms and their environment, but individuals do not evolve. Evolution can only be measured as change in relative proportions of variations in a population over several generations.
Natural selection can only amplify or diminish heritable variations.
Organisms can adapt to changes in their immediate environment and can be otherwise modified by life experiences, but these acquired characteristics cannot be inherited. Evolutionists must distinguish between adaptations an organism acquires during its lifetime and those inherited adaptations that evolve in a population over many generations as a result of natural selection. In wet years, birds preferentially feed on small seeds, and average beak depth decreases. In dry years, small seeds are less plentiful, so survival depends on the finches being able to crack the less preferred larger seeds. Average beak depth increases during dry years.