Here Darwin made a break with all previous suggestions on the subject. Darwin noted that species in nature had similar potential for modification. By Darwin’s time, striking variation in domestic animals had already been produced by selective breeding. Each individual must compete for food and must cope successfully with every facet of the environment-both physical, such as climate extremes, and biological, such as diseases and predators-in order to live to produce progeny.Įach individual differs from virtually all others in its species. Although most populations fluctuate year by year, they remain essentially constant over the long term.Ī very real struggle for existence occurs in nature. Spectacular progressive increases in population size do not, in fact, occur. Populations of animals and plants produce progeny at such a rate that were they all to survive, they would increase rapidly year after year. Organic evolution is the theory that more recent types of plants and animals have their origins in other pre-existing forms and that the distinguishable differences between ancestors and descendants are due to modifications in successive generations.
