Can Individuals Evolve? • 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 • 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? • 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 • 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. • 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 • 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 • Stabilizing Selection – favors average individuals and reduces variation in a population.
Three Types of Natural Selection cont. • 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. • Disruptive Selection – favors both extreme variations of a trait, leading to the evolution of two new species.
Evolution of Species • 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 • Physical barriers can break large populations into smaller ones.
Reproductive Isolation • Occurs when formerly interbreeding organisms can no longer mate and produce fertile offspring. • May occur because genetic material becomes so different that fertilization cannot occur. • May be isolated by behavior, such as mating at different times.
Change in Chromosome Number • 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 • 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 • Adaptive Radiation – an ancestral species evolves into an array of species to fit a number of diverse habitats.
Patterns of Evolution cont. • 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. • Convergent Evolution – pattern in which distantly related organisms evolve similar traits.