Natural Selection

Biology 1-2

Natural Selection

The primary mechanism of evolution is natural selection. Natural selection-differential success in the

reproduction of different phenotypes resulting from interactions with the environment.• Over time, favorable phenotypes reproduce in greater

numbers.• Unfavorable phenotypes are reduced in number.• The population is shaped to fit its environment.

Natural Selection cont’d

Mutations and recombination create variations in phenotype in a population. Mutations produce new alleles (variations on

existing traits) or new genes (new traits). Recombination produces variation in existing

traits.

Natural Selection cont’d

Evolution occurs when natural selection creates a change in a population’s gene pool. Gene pool-all of the genes in a population at a

given time. Through natural selection, individuals (and their

genes) are added or removed from the population, changing the gene pool.• Causes the population to evolve.

Natural Selection cont’d

Some variations provide an advantage in survival and reproduction. Adaptation-an inherited characteristic that

enhances an organisms ability to survive and reproduce.• Adaptations include physical traits as well as

behaviors.

Natural Selection cont’d

Organisms with adaptations to their environment survive longer and produce more offspring. The offspring have the traits of their parents.

• Each offspring is unique, based on recombination.

Natural Selection cont’d

Over time, more individuals are born with the adaptation.• Fitness-the measure of the contribution of an individual

to the population’s gene pool.• Alleles that produce unfavorable traits slowly disappear

from the gene pool.• Alleles that produce adaptations increase in frequency

in the gene pool.• Gene pool changes, population evolves.

Natural Selection cont’d

When conditions change, new/different traits become favorable. Different traits are considered adaptations.

• More variation in a population increases the chances of some individuals having the necessary traits for survival.

Natural Selection cont’d

Heterozygotes maintain variation in a population. They can “hide” alleles.

• They show the dominant (favorable) phenotype, while maintaining a different (recessive) allele that can be passed on.

“Hidden” alleles allow offspring with different traits to be born.• Increases the chances of having traits needed in a

new/different environment.

Natural Selection cont’d

Natural selection only works on traits already present in a population. Traits must be in the population before they are

selected for or against. If no favorable traits are present, the population

faces extinction.