Evolution of Populations

Variation and Gene Pools  Genetic variation is studied in populations. A population is a group of individuals of the same species that interbreed. A population is a group of individuals of the same species that interbreed. A gene pool consists of all genes, including all the different alleles, that are present in a population. A gene pool consists of all genes, including all the different alleles, that are present in a population. The relative frequency of an allele is the number of times the allele occurs in a gene pool, compared with the number of times other alleles for the same gene occur. The relative frequency of an allele is the number of times the allele occurs in a gene pool, compared with the number of times other alleles for the same gene occur. Relative frequency is often expressed as a percentage. Relative frequency is often expressed as a percentage.

Gene Pools:

Allele Frequency: Gene Pool for Fur Color in Mice: Gene Pool for Fur Color in Mice:

Microevolution: Evolution as Genetic Change Natural selection affects which individuals survive and reproduce and which do not. Natural selection affects which individuals survive and reproduce and which do not. If an individual dies without reproducing, it does not contribute its alleles to the population’s gene pool. If an individual dies without reproducing, it does not contribute its alleles to the population’s gene pool. If an individual produces many offspring, its alleles stay in the gene pool and may increase in frequency. If an individual produces many offspring, its alleles stay in the gene pool and may increase in frequency.

Genetic Drift: –A random change in allele frequency is called genetic drift –Random change means that an allele might become more common in a population by chance, not because it provides an advantage. –In small populations, individuals that carry a particular allele may leave more descendants than other individuals do, just by chance. –Gene flow

The Founder & Bottleneck Effects If genetic drift occurs when a small group of individuals colonizes a new habitat it is often called a founder effect; if it occurs after a natural disaster wipes out a large proportion of the original population it is often called a bottleneck effect. If genetic drift occurs when a small group of individuals colonizes a new habitat it is often called a founder effect; if it occurs after a natural disaster wipes out a large proportion of the original population it is often called a bottleneck effect. Individuals may carry alleles in different relative frequencies than did the larger population from which they came. Individuals may carry alleles in different relative frequencies than did the larger population from which they came. The new population will be genetically different from the parent population. The new population will be genetically different from the parent population.

Genetic Drift Genetic Drift

Sources of Genetic Variation In genetic terms, evolution is any change in the relative frequency of alleles in a population. In genetic terms, evolution is any change in the relative frequency of alleles in a population. Genetic Variation (2 or more alleles in the gene pool) in a population is essential for natural selection to work: Genetic Variation (2 or more alleles in the gene pool) in a population is essential for natural selection to work: –mutations –genetic shuffling that results from sexual reproduction.

Mutations: Any change in a sequence of DNAAny change in a sequence of DNA Occur because of mistakes in DNA replication or as a result of radiation or chemicals in the environmentOccur because of mistakes in DNA replication or as a result of radiation or chemicals in the environment Do not always affect an organisms phenotypeDo not always affect an organisms phenotype

Gene Shuffling: Most heritable differences are due to gene shuffling. Most heritable differences are due to gene shuffling. Crossing-over increases the number of genotypes that can appear in offspring. Crossing-over increases the number of genotypes that can appear in offspring. Sexual reproduction produces different phenotypes, but it does not change the relative frequency of alleles in a population. Sexual reproduction produces different phenotypes, but it does not change the relative frequency of alleles in a population.

Gene Shuffling:

Genetic Equilibrium A population is in genetic equilibrium if allele frequencies are not changing from one generation to the next A population is in genetic equilibrium if allele frequencies are not changing from one generation to the next According to the Hardy-Weinberg theory, a population is in genetic equilibrium if the following conditions are met simultaneously: According to the Hardy-Weinberg theory, a population is in genetic equilibrium if the following conditions are met simultaneously: –Large population size –Random mating –No mutations –No migration –No natural selection

The Hardy-Weinberg Equations Although genetic equilibrium cannot be maintained, it can be assumed to be occurring at a particular moment. Although genetic equilibrium cannot be maintained, it can be assumed to be occurring at a particular moment. Based on this premise, the H-W equations can be used to estimate allele frequency and/or percentage of the population that is either homozygous recessive, homozygous dominant or heterozygous. Based on this premise, the H-W equations can be used to estimate allele frequency and/or percentage of the population that is either homozygous recessive, homozygous dominant or heterozygous. The equations: The equations: –p + q = 1 (p = dominant allele frq; q = recessive allele frq.) –p 2 + 2pq + q 2 = 1 (hom. dom.; hetero; hom. rec.)

Single-Gene and Polygenic Traits Many traits are controlled by two or more genes and are called polygenic traits. Many traits are controlled by two or more genes and are called polygenic traits. One polygenic trait can have many possible genotypes and phenotypes. One polygenic trait can have many possible genotypes and phenotypes. Height in humans is a polygenic trait. Height in humans is a polygenic trait. A bell-shaped curve is typical of polygenic traits. A bell-shaped curve is typical of polygenic traits. A bell-shaped curve is also called normal distribution. A bell-shaped curve is also called normal distribution.

Natural Selection on Polygenic Traits 3 categories: 3 categories: Directional: favors one extreme Directional: favors one extreme Stabilizing: favors the middle Stabilizing: favors the middle Disruptive: favors both extremes Disruptive: favors both extremes

Types of Natural Selection

What type of selection?

Divergent v. Convergent Evolution Divergent One species gives rise to many species One species gives rise to many species Also known as adaptive radiation Also known as adaptive radiation Many species with common ancestor Many species with common ancestor Many homologous structures Many homologous structures Convergent Similar looking species that do not have a common ancestor Similar looking species that do not have a common ancestor Similar behavior and appearance due to environmental similarities Similar behavior and appearance due to environmental similarities Many analogous structures Many analogous structures

Convergent Evolution

Coevolution The evolution of one species is directly influenced by the evolution of another

Punctuated Equilibrium Slow background evolution (stasis) is interrupted by rapid bursts of change Slow background evolution (stasis) is interrupted by rapid bursts of change Rapid bursts of change usually occur after a mass extinction Rapid bursts of change usually occur after a mass extinction

Speciation: Speciation is the formation of new species. Speciation is the formation of new species. A species is a group of organisms that breed with one another and produce fertile offspring. A species is a group of organisms that breed with one another and produce fertile offspring. The gene pools of two populations must become separated for them to become new species The gene pools of two populations must become separated for them to become new species When the members of two populations cannot interbreed and produce fertile offspring, reproductive isolation has occurred and speciation will result. When the members of two populations cannot interbreed and produce fertile offspring, reproductive isolation has occurred and speciation will result.

Types of Reproductive Isolation Behavioral Isolation – Different mating rituals prevent reproduction Behavioral Isolation – Different mating rituals prevent reproduction Geographic Isolation – barriers such as rivers or mountains prevent reproduction Geographic Isolation – barriers such as rivers or mountains prevent reproduction Temporal Isolation – different mating times (seasonal, nocturnal v. diurnal) prevent reproduction Temporal Isolation – different mating times (seasonal, nocturnal v. diurnal) prevent reproduction