15 2 Mechanisms of Evolution Can Individuals Evolve

15. 2 Mechanisms of Evolution

Can Individuals Evolve? n n n Genes determine most of an individual’s features. Individuals cannot evolve a new phenotype in response to their environment. Populations, not individuals, evolve.

Evolution of a Population n n Natural selection acts on the range of phenotypes in a population. Evolution occurs as a population’s genes and their frequencies change over time.

How can a population’s genes change over time? n n n All of the alleles of a population’s genes together make up a gene pool. Allelic frequency - % of any specific allele in the gene pool. Genetic equilibrium – a population in which the frequency of alleles remains the same over generations.

Changes in Genetic Equilibrium n n n A population in genetic equilibrium is not evolving. Mutations are one cause of genetic change. Lethal mutations disappear quickly, but mutations that cause a useful variation become part of the gene pool

Changes in Genetic Equilibrium cont. n n Genetic drift – the alteration of allelic frequencies by chance events. Gene flow – transport of genes into or out of a population by migrating individuals. Genetic drift, gene flow, and mutations can greatly affect small populations. Natural selection is usually the most significant cause of changes in any gene pool.

Natural Selection Acts on Variation n Some variations increase or decrease an organism’s chance of survival in an environment. Variations are controlled by alleles. Allelic frequencies in a gene pool will change due to natural selection of variations.

Three Types of Natural Selection n Stabilizing Selection – favors average individuals and reduces variation in a population.

Three Types of Natural Selection cont. n Directional Selection – favors one of the extreme variations of a trait and can lead to rapid evolution of a population.

Three Types of Natural Selection cont. n Disruptive Selection – favors both extreme variations of a trait, leading to the evolution of two new species.

Evolution of Species n n Speciation – occurs when members of a similar population no longer interbreed to produce fertile offspring. There are three different ways that organisms can become isolated from each other to form a new species.

Geographic Isolation n Physical barriers can break large populations into smaller ones.

Reproductive Isolation Occurs when formerly interbreeding organisms can no longer mate and produce fertile offspring. n May occur because genetic material becomes so different that fertilization cannot occur. n May be isolated by behavior, such as mating at different times. n

Change in Chromosome Number n n Mistakes in mitosis or meiosis can lead to polyploidy. Polypolids within a population may interbreed and form a new species.

Rate of Speciation – Two Hypotheses n n Gradualism – species originate through a gradual change of adaptations. Punctuated Equilibrium – speciation occurs relatively quickly with long periods of genetic equilibrium in between.

Patterns of Evolution n Adaptive Radiation – an ancestral species evolves into an array of species to fit a number of diverse habitats.

Patterns of Evolution cont. n n Divergent Evolution – pattern in which species that once were similar to an ancestral species diverge. Populations become less alike as they adapt, resulting in new species.

Patterns of Evolution cont. n Convergent Evolution – pattern in which distantly related organisms evolve similar traits.