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Evolution of Populations

Evolution of Populations. Chapter 16. The study of genetics helps scientists understand the relationship between inheritance and evolution Scientists know that… genes control traits and that many genes have at least two forms, or alleles

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Evolution of Populations

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  1. Evolution of Populations Chapter 16

  2. The study of genetics helps scientists understand the relationship between inheritance and evolution • Scientists know that… • genes control traits and that many genes have at least two forms, or alleles -members of all species are heterozygous for many genes

  3. In genetic terms, evolution is any change in the relative frequency of alleles in a population

  4. A population is a group of individuals of the same species that can interbreed • Members of a population share a gene pool - all the genes, and their different alleles, in the population

  5. The number of times the alleles occur in a gene pool compared to the number of times that other alleles for the same gene occur is the relative frequency of the allele

  6. The two main sources of genetic variation are mutations and gene shuffling • A mutation is any change in a sequence of DNA • Gene shuffling occurs during gamete formation (crossing-over)

  7. It can produce millions of different gene combinations, but doesn’t change the allele frequency in the population • Both mutations and gene shuffling increase genetic variation by increasing the number of different genotypes

  8. The number of phenotypes for a trait depends on how many genes control the trait • A single-gene trait is a trait controlled by only one gene • If there are two alleles for the gene, two genotypes are possible

  9. In humans, a single-gene trait is the presence of a widow’s peak • Allele for a widow’s peak is dominant over the allele for hairline with no peak. • As a result, there are only two phenotypes – having a widow’s peak or not

  10. A polygenic trait is controlled by two or more genes • Each gene of a polygenic trait may have more than one allele • Polygenic traits form many phenotypes • Variation in a polygenic trait in a population often forms a bell-shaped curve with most members near the middle • example: height in humans

  11. 16.2 Genetic Change • Natural selection on single-gene traits can lead to changes in allele frequencies…evolution

  12. Natural Selection on Polygenic Traits • Directional Selection

  13. Stabilizing Selection

  14. Disruptive Selection

  15. Genetic Drift/Founder Effect

  16. Hardy-Weinberg Principle • Allele frequencies in a population will remain constant unless one or more factors cause them to change • Genetic Equilibrium

  17. 5 Conditions to Maintain G.E. • Must be random mating • Population must be very large • No movement into or out of the population • No mutations • No natural selection

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