15.3 Shaping Evolutionary Theory Main idea: The theory of evolution continues to be refined as scientists learn new information. Objectives: Discuss patterns observed in evolution. Describe factors that influence speciation. Compare gradualism with punctuated equilibrium. Review Vocabulary Allele: alternative forms of a character trait that can be inherited.

15.3 Shaping Evolutionary Theory New Vocabulary Hardy-Weinberg Principle Genetic drift Founder effect Bottleneck Punctuated Equilibrium Sexual selection

Mechanisms of Evolution Darwin’s theory of natural selection remains the central theme of evolution Scientists of today know that natural selection is not the only mechanism of evolution Evolution occurs at the population level, with genes as the raw material.

Mechanisms of Evolution Population Genetics Hardy-Weinberg Principle states that when allelic frequencies remain constant, a population is in genetic equilibrium. Even though the number of owls doubled, the ratio of gray to red owls remained the same.

Mechanisms of Evolution Population Genetics p2 + 2pq + q2 = 1 This equation allows us to determine the equilibrium frequency of each genotype in the population: homozygous dominant: p2 Heterozygous: 2pq homozygous recessive: q2 Note the sum of these frequencies equals one.

Mechanisms of Evolution Population Genetics A population in genetic equilibrium must meet all 5 conditions The population is very large No immigration or emigration Mating is random Mutations do not occur (no new variations) Natural selection does not occur

Mechanisms of Evolution Genetic Drift Genetic Drift is a change in the allelic frequencies in a population that is due to chance. In smaller populations, the effects of genetic drift becomes more pronounced, and the chance of losing an allele becomes greater. Examples: Founder Effect & Bottleneck

Mechanisms of Evolution Genetic Drift Founder Effect occurs when a small sample of a population settles in a location separated from the rest of the population Alleles that were uncommon in the original population might be common in the new population.

Mechanisms of Evolution Genetic Drift Bottleneck occurs when a population declines to a very low number and then rebounds Can decrease genetic variety.

Mechanisms of Evolution Gene Flow No new genes enter the population and no genes leave the population. A population in genetic equilibrium experiences no gene flow, however, few populations are isolated Random movement of individuals between populations reduces differences between populations

Mechanisms of Evolution Nonrandom mating Rarely is mating completely random in a population. Usually individuals mate with individuals in close proximity. This promotes inbreeding and could lead to a change in allelic proportions favoring individuals that are homozygous for particular traits.

Mechanisms of Evolution Mutation A mutation is a random change in genetic material. Occasionally mutations provide an advantage to organisms and become more common in subsequent generations. Basis upon which natural selection works.

Mechanisms of Evolution Natural Selection Select the individuals that are best adapted for survival and reproduction Stabilizing selection eliminate extreme expressions Birth Weights Directional selection Increases the expression of the extreme version of a trait. Peppered Moths Disruptive selection process that splits a population into two groups. Lake Erie Water Snakes

Mechanisms of Evolution Natural Selection Sexual selection operates in populations where males and females differ significantly in appearance. Males are largest and most colorful in the group More likely target for predators yet bodies enhance reproductive success Examples: Peacocks & Deer

Reproductive Isolation Prezygotic Isolation prevents reproduction by making fertilization unlikely. Eastern and Western meadowlarks similar in appearance but their mating songs separate them behaviorally.

Reproductive Isolation Postzygotic Isolation Occurs when fertilization has occurred but a hybrid offspring cannot develop or reproduce. A Liger is the offspring of a male lion and a female tiger. Ligers are sterile.

Speciation For speciation to occur, a population must diverge and then be reproductively isolated. Allopatric speciation A physical barrier divides one population into two or more populations.

Speciation For speciation to occur, a population must diverge and then be reproductively isolated. Sympatric speciation Species evolves into a new species without a physical barrier Apple maggot flies lay eggs on apples or hawthorns depending on the type of fruit the grew up on.

Patterns of Evolution Adaptive Radiation (aka divergent evolution) Can occur in a relatively short time when one species gives rise to many different species in response to the creation of new habitat or some other ecological opportunity. Follows large-scale extinction events Cichlid fish of Lake Victoria in Africa.

Patterns of Evolution Coevolution The relationship between two species might be so close that the evolution of one species affects the evolution of the other species. Mutualism – occurs when two species benefit each other. Comet orchids and the moths that pollinate them

Patterns of Evolution Coevolution Coevolutionary arms race – one species can evolve a parasitic dependency on another species. A plant and an insect that is dependent on the plant for food The plant evolves a chemical defense The insects evolve the biochemistry to resist the defense Response continues to escalate.

Patterns of Evolution Convergent Evolution Unrelated species evolve similar traits even though they live in different parts of the world. Occurs in environments that are geographically far apart but have similar ecology and climate.

Patterns of Evolution Rate of Speciation Punctuated equilibrium explains rapid spurts of genetic change causing species to diverge quickly. Gradualism: Evolution proceeds in small, gradual steps.