The fundamental lesson from the work of Mendel is how to recognize the existence of genes. Mendel taught us that by crossing two individuals that differ in a trait and observing how that trait segregates in subsequent generations, it is possible to discover the existence of and certain properties of the gene responsible for that trait. Because members of the same species are usually quite similar, Mendel concentrated on the differences that plant breeders had recognized in peas and was limited to the study of a relatively small number of differences. Darwin and Natural Selection Darwin was the first to recognize that natural selection, commonly referred to as survival of the fittest, was the driving force of evolution. Minor differences arise between individuals in a species through natural processes of mutation.
Most mutations are harmful and decrease the chance of survival.
However, for those few mutations that cause a favorable change that enhance survival, the mutations will spread throughout the population due to the enhanced survival of the individuals with the mutation. The accumulation of mutations over time leads to the creation of new species. In theory, one could discover the genes responsible for the differences between breeds if it were possible to cross two members of a different species and observe the segregation of the trait in the offspring. For example the genes controlling wing length in birds would be recognized in the offspring of a cross between condors and hummingbirds. Similarly, the genes controlling the length of the neck in mammals would segregate in a cross between giraffes and warthogs.