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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 gene pool consists of all genes, including all the different alleles , that are present in a population.
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Variation and Gene Pools • Genetic variation is studied in populations. • 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. • 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.
Allele Frequency: • Gene Pool for Fur Color in Mice:
Microevolution: Evolution as Genetic Change • 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 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
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. The new population will be genetically different from the parent population. The Founder & Bottleneck Effects
Sources of Genetic Variation • 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: • mutations • genetic shuffling that results from sexual reproduction.
Any change in a sequence of DNA Occur because of mistakes in DNA replication or as a result of radiation or chemicals in the environment Do not always affect an organisms phenotype Mutations:
Most heritable differences are due to gene shuffling. 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. Gene Shuffling:
Genetic Equilibrium • 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: • 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. • 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: • p + q = 1 (p = dominant allele frq; q = recessive allele frq.) • p2 + 2pq + q2 = 1 (hom. dom.; hetero; hom. rec.)
Many traits are controlled by two or more genes and are called polygenic traits. One polygenic trait can have many possible genotypes and phenotypes. Height in humans is a polygenic trait. A bell-shaped curve is typical of polygenic traits. A bell-shaped curve is also called normal distribution. Single-Gene and Polygenic Traits
Natural Selection on Polygenic Traits • 3 categories: • Directional: favors one extreme • Stabilizing: favors the middle • Disruptive: favors both extremes
Divergent v. Convergent Evolution Divergent Convergent Similar looking species that do not have a common ancestor Similar behavior and appearance due to environmental similarities Many analogous structures • One species gives rise to many species • Also known as adaptive radiation • Many species with common ancestor • Many homologous structures
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 • Rapid bursts of change usually occur after a mass extinction
Speciation: • Speciation is the formation of new species. • 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 • 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 • Geographic Isolation – barriers such as rivers or mountains prevent reproduction • Temporal Isolation – different mating times (seasonal, nocturnal v. diurnal) prevent reproduction