This is the theory that every group of organisms descended from a common ancestor, and that all groups of organisms, including animals, plants, and microorganisms, ultimately go back to a single origin of life on earth. Multiplication of species. This theory explains the origin of the enormous organic diversity. According to this theory, evolutionary change comes about throught the abundant production of genetic variation in every generation. The relatively few individuals who survive, owing to a particularly well-adapted combination of inheritable characters, give rise to the next generation.
Survival is necessary, but insufficient as a criterion for judging the adaptiveness of a trait.
Note that an individual may change during its lifetime, but those are developmental changes, not evolution. The unit of change in biological evolution is the population . Macroevolution refers to changes that occur in a population that lead to the formation of new species . This is the topic of lectures 24-26. Carriers have an advantage over homozygous dominant indivduals in the presence of certain infectious diseases.