EVOLUTION Artificial and Natural Selection Natural Variation l

EVOLUTION Artificial and Natural Selection

Natural Variation l l Darwin observed that there is natural variation among species. He also believed that because more organisms were born than could possibly survive, the struggle for survival kept Earth from being overpopulated by plants and animals.

Natural Variation l l Natural selection is the process that allows only the organisms with the best fitness to survive. In nature, the environmental conditions choose the adaptations that are most beneficial, so that only those organisms with the best traits are reproducing.

Selective Breeding l Darwin noticed that when farmers and breeders choose which animals or plants to mate they are relying on the fact that there are different traits in the population l No two individuals are exactly alike, and variation is inheritable, meaning it can be passed on to future generations

Artificial Selection l l Farmers chose best characteristics Nature provided variation, humans controlled the breeding This is called artificial selection Has resulted in a wide range of plants and animals that are very different from their ancestors

Selection in Nature l l l Inheritable variation exists in nature as well Because each organism is slightly different, each has slightly different advantages and disadvantages Some individuals are more fit than others (better suited to survive and reproduce, or survival of the fittest)

Selection in Nature l l Generation after generation, the struggle for existence selects the fittest individuals to survive and reproduce Species in general become better suited to their environments as they respond to selection pressures

Where does variation come from? l l Variations in traits are variations in phenotypes Traits are controlled by genes Inheritable variation comes from differences in genes (genetic diversity) Differences in genes come from mutations and gene shuffling

Mutations and Variation l Mutation rates vary by species – Humans have a low rate, bacteria have a high rate l Can be positive or negative

Gene Shuffling and Variation l l l Sexual reproduction is an important source of variation in nature Genes are shuffled during meiosis when gametes are formed Even the same genes combined in different ways produce different results

Principles of Natural Selection l l Individuals in a population show variations among others of the same species. Variations are inherited. Animals have more young than can survive on available resources. Variations that increase reproductive success will be more common in the next generation.

Inheritable Variation l l l Variation occurs at all levels (physical and biochemical) Many traits are also polygenic, creating a range of phenotypes Natural selection does not act on genotype – affects variations in phenotype only!