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DAY 4

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DAY 4

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  1. DAY 4

  2. The Hardy-Weinberg Equilibrium • The Hardy-Weinberg law states that the frequencies of alleles in a population will remain constant unless acted upon by outside agents or forces. In other words, the proportion of dominant to recessive genes remains the same. • The Hardy-Weinberg law describes the genetics of non-evolving populations. A non-evolving population is said to be in Hardy-Weinberg equilibrium.

  3. continued • A population will remain in genetic equilibrium if, and only if, all of the following conditions are met: • No mutations occur. • Individuals neither enter nor leave the population through migration. • The population is large. • Individuals mate randomly. • Natural selection does not occur.

  4. Applications • to calculate the percentage of a population that is carrying a gene for a disease. • to determine if a population is in genetic equilibrium.

  5. Altering Genetic Variation • Natural selection affects variation in a population as the better adapted (more fit) individuals survive and reproduce, passing on their genes to successive generations. • Immigration and emigration of individuals from a population will affect allele frequencies and therefore gene flow. • The change in the gene pool of a small population due to random chance is genetic drift. The bottleneck effect is a form of genetic drift that results from the near extinction of a population. The founder effect is a form of genetic drift that results from a small number of individuals colonizing a new area. In both cases, allele frequencies can change dramatically.

  6. Altering Genetic Variation • In animals, non-random mating is more often the case as the choice of mates is often an important part of behaviour (e.g. courtship rituals). Many plants self-pollinate, which is a form of inbreeding or non-random mating. • Mutations, although rare, do constantly occur. They provide the source of new alleles, or variation upon which natural selection can take place

  7. Darwin’s Finches

  8. Speciation and Barriers • How does one species, whose members can all interbreed, become two species, whose members cannot interbreed? • speciation - the evolution of a new species occurs when members of similar populations no longer interbreed to produce fertile offspring within their natural environment

  9. Other Barriers • A change in chromosome number – many new species of plants and some of animals have evolved in the same region as a result of polyploidy – any individual or species with a multiple of the normal set of chromosomes is know as a polyploid. • Gradualism is the idea that species originate though a gradual change of adaptations • Punctuated Equilibrium – 1972 – Niles Eldregde and Stephen Gould – argued that speciation occurs rather quickly, in rapid bursts.

  10. The Evolution of Species • Speciation – the evolution of a new species • Occurs when members of similar populations no longer interbreed to produce fertile offspring within their natural environment

  11. Physical Barriers • Geographic Isolation • Occurs whenever a physical barrier divides a population

  12. Reproductive Isolation • Occurs when formerly interbreeding organisms can no longer mate and produce fertile offspring. • Genetic material of the populations become so different that fertilization cannot occur. • Behavioural reasons – mating seasons

  13. Change in Chromosome Numbers • Chromosomes can also play a role in speciation. • Polyploidy (an individual or species with a multiple of the normal set of chromosomes) • If polybloids within a population interbreed, they may produce a new species (wheat, cotton, apples)

  14. Speciation rates • Gradualism – the idea that species originate through a gradual change of adaptations. • Sea lilies • Punctuated equlibrium – speciation occurs relatively quickly – in rapid bursts, with long periods of genetic equlibrium in between. • Environmental changes, the introduction of a competitive species • 10,000 years or less

  15. Patterns of Evolution • Adaptive radiation – when an ancestral species evolves into a number of species to fit a number of diverse habitats • A form of divergent evolution – the pattern of evolution in which species that once were similar to an ancestral species diverge, or become increasingly distinct.

  16. Different Species can look alike • Convergent evolution – a pattern of evolution in which distantly related organisms evolve similar traits • Occurs when unrelated species occupy similar environments in different parts of the world • Cactus plants

  17. Additional Activities • www.pbs.org/wgbh/evolution/darwin/origin. • www.pbs.org/wgbh/evolution/sex/guppy