Charles Darwin Natural Selection

Natural Selection “Survival of the fittest” *Natural Selection 1. There is genetic variation in populations 2. There is competition/a struggle for existence in nature—for food, resources, mates, etc. Invidiuals which are more fit in a specific environment have a better chance of surviving and reproducing than less fit organisms. 3. Over time, generations will consist of organisms whose genetic variations aid survival and reproduction in that environment.

Natural Selection Important note: The fittest does not always mean the largest, strongest, or smartest. Fitness depends upon specific environmental conditions. Sometimes fittest means smallest, or best camouflaged, or most adaptable, etc.

Evolution Natural Selection leads to evolution, because natural selection causes change Evolution—change in a population/species over time

Artificial Selection Sometimes evolution can be “man-made” Selective breeding— When humans breed exceptional plants and animals in order to produce desired offspring. Ex: racing horses, purebred dogs Genetic engineering – Evolution by design

Mechanisms of Evolution—how it happens Populations (species) evolve, not individuals. Evolution occurs when a population’s gene frequencies change over time. Small populations that interbreed will have many mutations and therefore will see greatest potential for evolutionary changes.

The genes Gene pool - a population’s collective genes Allelic frequency—The % of an allele in a population (ex: 25% “t” in a pea plant population) Genetic Equilibrium—When the allelic frequency remains the same over generations (no evolution) This can happen when there is no movement into or out of a population.

Mechanisms of Evolution So, when a population is NOT in genetic equilibrium, when the gene pool changes, populations evolve. When mutations occur, creating new alleles, and individuals with that mutation survive and reproduce, the gene pool is altered, leading to evolution.

Natural Selection Revisited There are 3 types of natural selection: Stabilizing Selection—the average individual is selected for Disruptive Selection—Both extreme variations of a trait are selected for Directional Selection—One extreme variation of a trait is selected for

3 types of selection graphs

Genetic Drift “Evolution by chance” Genetic drift—Changes in the allelic frequency of a population due to chance events. Founder effect—evolution as the result of a migration of a small subgroup of a population to a new environment. Population Bottleneck—When an environmental catastrophe drastically reduces the numbers of a population, eliminating huge amounts of genetic diversity. Ex: humans 70,000 years ago

Speciation Speciation—Evolution of a new species

Reproductive Isolation Reproductive isolation—When subgroups of a population can no longer interbreed and produce fertile offspring Behavioral isolation—isolation due to different courtship rituals or reproductive strategies Ex: Eastern & Western Meadowlark songs

Reproductive Isolation Temporal Isolation—Isolation due to population groups reproducing at different times. Ex: flowers being pollinated at different times of the year

Reproductive Isolation Geographic isolation—Isolation of populations due to geographic barriers Ex: South American and African monkeys

Adaptive Radiation Adaptive Radiation—When one ancestral species spreads out geographically, and many new species form over time. One species radiates outward and each subgroup adapts to its own unique environment Speciation of multiple species from one ancestral species—a type of divergent evolution Ex: “Darwin’s Finches” of the Galapagos Islands

Darwin’s Finches—Adaptive Radiation

Mutations lead to Evolution DNA mutations can lead to new genes/new traits in individuals within a population. If these new traits are favorable in the organisms’ environment, and help it survive and reproduce, then the genes will be passed on to offspring. The greatest potential for evolutionary change would involve a small population, with many mutations

Mutations and bacteria Antibiotic Resistance—DNA mutations in bacteria can prevent them from being killed by medicine. They survive, then continue to grow and divide as bacteria do. Soon, all of that type of bacteria has evolved resistance.

Gradualism vs. Punctuated Equilibrium Gradualism— Speciation occurs at a slow, steady pace Punctuated Equilibrium— Speciation occurs quickly, with long periods of genetic equilibrium in between Most likely both are true, both are supported by fossil evidence