Through sexual reproduction, new combinations of chromosomes result. One member of each pair of chromosomes is inherited from each parent. This sexual genetic recombination leads to variability within the species. Mutations are simply chemical changes to the DNA molecule. Mutations produce much of the variability on which natural selection operates.
Variability is the raw material for natural selection.
Specialized features of organisms which allow them to perform useful functions are called adaptations . Similar body plans suggest common origins. Analogy - body parts with similar function , without regard to origin. Similar functions but different structures suggest different origins. These slits are found in the embryos of all vertebrates because they share as common ancestors the fish in which these structures first evolved.