Evolution of Populations

Population vs. Species Population-a group of organisms of the same species that live in the same area Species-organisms that are capable of breeding AND producing fertile offspring Ex: Lion+ Tiger= Liger Liger=infertile Therefore lion and tiger are different species

Gene Pool Consists of all the genes that are present in a population Relative frequency of an allele=the number of times an allele appears in a gene pool compared to other alleles of the same gene Genetic Equilibrium-a population in which the frequency of alleles remains the same over generations

Key concepts An individual itself CANNOT evolve Only populations can evolve Evolution is a change in the gene pool of populations over time If the allele frequency does not change, the population will not evolve 5 conditions are required to maintain genetic equilibrium Random mating Population must be very large No movement into or out of the population No mutations No natural selection

Change in allele frequency Evolution occurred!

Causes of Evolution Genetic Drift Change in the gene pool of a population that occurs by random chance Random chance=natural disasters, disease, etc Has a greater effect on small populations Often causes reduction in alleles present in the gene pool Bottleneck Effect Founder Effect

Causes of Evolution-Genetic Drift Bottleneck Effect Results from an event/disaster that drastically reduces population size

Causes of Evolution-Genetic Drift Founder Effect (Migration) A situation in which allele frequencies change as a result of the migration of a small subgroup of a population

Causes of Evolution Mutations Change in an organism’s DNA Only source of brand new genes within a population The majority of mutations are deadly, thus are eliminated from a population

Causes of Evolution Gene Flow (Migration) Genes migrate in and out of the gene pool

Causes of Evolution Non-random mating (sexual selection) Inbreeding-mating between closely related partners Selective mating-organisms prefer some individual mates over others, typically those who are similar to themselves

Causes of Evolution Natural Selection Individuals better suited to their environment live to produce offspring who inherit their genes Favored genes become more frequent in a population over time

3 General Types of Natural Selection 1. Stabilizing Selection -nature selects the average form of a trait 2. Directional Selection -nature selects one extreme form of a trait 3. Disruptive Selection -nature selects both the extreme forms of a trait

Human babies born at an average mass are more likely to survive than babies born either much smaller or much larger than average

Individuals at one end of the curve have higher fitness than individuals in the middle or at the other end. A population of seed-eating birds have a food shortage that causes the supply of small seeds to run low. The beak size would change over time to beaker beaks since there are more big seeds now than little.

When individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle. Average sized seed becomes less common while the larger and smaller seeds become more common. The bird population then splits into two subgroups specializing in eating different sized seeds

How do new species form? Speciation occurs when the gene pools of two populations become separated As new species evolve, populations become reproductively isolated from each other Isolation Mechanisms Reproductive Geographic Temporal Behavioral Ecological

Reproductive Isolation When members of two populations cannot interbreed and produce fertile offspring

Geographic Isolation Physical separation of members of a population By formation of a canyon, mountain, river, etc Leads to different adaptations on each side of the barrier and eventually a new species

Temporal Isolation Species are isolated because they reproduce in different seasons or times of the day

Behavioral Isolation Species differ in their mating rituals Different bird songs, mating colors, dances, pheromones, etc

Ecological Isolation Species inhabit the same area, but different habitats, so they don’t encounter each other

How quickly do new species form? 2 Hypothesis Gradualism Many transitional forms between common ancestors and modern organisms Punctuated Equilibrium Sudden shifts in form from an ancestor to newer organism, so not many transitional forms between common ancestor and modern organisms

Patterns of Evolution Convergent Evolution Divergent Evolution Adaptive Radiation Artificial Selection Coevolution

Convergent Evolution Organisms appear similar, but aren’t closely related Why? Similar environment selects for similar traits

Divergent Evolution Related populations or species become less and less alike Adaptive radiation-type of divergent evolution that results in several variations of the species Adaptive Radiation

Divergent Evolution Artificial Selection Humans, rather than nature, select traits

Coevolution Two or more species in close association with one another change together Predator/prey Mollusks, like snails, have evolved thick shells and spines to avoid being eating by animals such as crabs and fish. These predators have then evolved powerful claws and jaws that compensate for the snails’ thick shells and spines Parasite/host Milkweed plant has leaves that contain a chemical very poisonous to most herbivores. Herbivores don’t want to eat milkweed. However, monarch butterflies have evolved the ability to tolerate these toxins. Because of this, predators don’t want to eat the monarchs since their bodies contain the poison that they eat from the milkweed Plant/pollinator Many plants often have long tubes that lead to their nectar. Over time, pollinators (bats, insects, etc) have evolved a long tube-like mouth that is long enough to reach in and get the nectar, while pollinating the plant at the same time.