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NATURAL SELECTION

NATURAL SELECTION. http://www.youtube.com/watch?v=nS1tEnfkk6M. ***READ 8.1***. Types of Selection. Directional Selection: when selection favour individuals with a more extreme variation of a trait; selection that favours an increase or decrease in the

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NATURAL SELECTION

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  1. NATURAL SELECTION http://www.youtube.com/watch?v=nS1tEnfkk6M ***READ 8.1***

  2. Types of Selection Directional Selection: when selection favour individuals with a more extreme variation of a trait; selection that favours an increase or decrease in the value of a trait from the current population average.

  3. Types of Selection Stabilizing Selection: occurs when the average phenotype within a population is favoured by the environment; selection against individuals exhibiting traits that deviate from the current population average.

  4. Types of Selection

  5. Types of Selection Disruptive Selection: selection that favours two or more variations of a trait that differ from the current population average; favours individuals with variations at opposite extremes of a trait over individuals with intermediate variations.

  6. Types of Selection

  7. Types of Selection Sexual Selection: differential reproductive success caused by variation in the ability to obtain mates; results in sexual dimorphism, and mating and courtship behaviours.

  8. Types of Selection (Sexual) • Favours any trait that enhances the mating success of an individual • Ie. Evolving appearances and behaviours that are quite different in males and females • Ex. Male-male competition • These traits might be a disadvantage when it comes to predators (bright colours, distinctive song) • Also with plants: attract pollinators

  9. Altruism • At times, animals behave in ways that reduce their individual fitness and increase the fitness of others • “helping others” • There are two types of Altruism: • Kin Selection • Group Selection

  10. Kin Selection • Helping relatives to survive and reproduce even if it means forgoing breeding yourself • You share 50% of your genes with each parents and 50% with a sibling • If an individual forgoes breeding and instead helps raise a close relative, it is helping pass on a portion of its own genes to the next generation • The more closely related an individual is to another, the more likely it is to forego a breeding attempt

  11. Group Selection • “for the good of the group” • In social populations, small groups will compete with other small groups for resources • Any adaptation that would improve a group’s survival over another group would also improve the individuals survival even if it doesn’t directly benefit the individual • By cooperating with each other they ensure each other’s survival

  12. Evolutionary Change without Selection Genetic Drift: • changes to allele frequency as a result of chance; such changes are much more pronounced in small populations • Random shifting of the genetic makeup of the next generation • When individuals produce offspring, the chances of passing on a particular allele is subject to random chance • In small populations • Can result in a particular allele can be very common or disappear entirely • When it disappears, it reduces genetic diversity

  13. 1. BOTTLENECK EFFECT Bottleneck: a dramatic, often temporary reduction in population size, usually resulting in significant genetic drift • Loss in genetic diversity after an extreme reduction in population size • Can have adverse consequences

  14. 2. FOUNDER EFFECT Founder effect: genetic drift that results when a small number of individuals separate from their original population and establish a new population • Initial population would have a different mix of alleles, by chance alone • Populations from bottleneck and founder effects are subject to the effects genetic drift • Further increases the chances that their gene pool will differ from the original population

  15. Big Horn Sheep 1975: 20 sheep 1999: ~650 sheep

  16. HARDY-WEINBERG PRINCIPLE The Hardy-Weinberg Principle: in large populations in which only random chance is at work, allele frequencies are expected to remain constant from generation to generation • Any factor that causes allele frequencies to chance leads to evolutionary change

  17. HARDY-WEINBERG PRINCIPLE • These conditions lead to evolution • Natural selection: favours the passing of some alleles over others • Small population size: increases the likelihood of genetic drift • Mutation: introduced new alleles to a population • Immigration/emigration: introduces or removes alleles in a population • Horizontal gene transfer: the gaining of new alleles from a different species

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