Day 4
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DAY 4. 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.

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DAY 4

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Day 4

DAY 4


The hardy weinberg equilibrium

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.


Continued

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.


Applications

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.


Altering genetic variation

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.


Altering genetic variation1

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


Darwin s finches

Darwin’s Finches


Speciation and barriers

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


Other barriers

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.


The evolution of species

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


Physical barriers

Physical Barriers

  • Geographic Isolation

    • Occurs whenever a physical barrier divides a population


Reproductive isolation

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


Change in chromosome numbers

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)


Speciation rates

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


Patterns of evolution

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.


Different species can look alike

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


Additional activities

Additional Activities

  • www.pbs.org/wgbh/evolution/darwin/origin.

  • www.pbs.org/wgbh/evolution/sex/guppy


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