Microevolution: How Does a Population Evolve?

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Microevolution How Does a Population Evolve?

• Chapter 16

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The Evolution of Evolution.

• Blending inheritance
• Inheritance of Acquired Characteristics
• Genetics alone causes evolution
• Modern synthesis evolution is due to natural
  selection working on inherited traits

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• Population genetics
• Microevolution changes in the frequency of the
  alleles of genes in a population.
• Industrial melanism
• Macroevolution - the process by which species of
  organisms originate, change and go extinct.

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• What is the source of variation within a
  population?
• Either point mutations of genes or chromosomal
  mutations
• If there is only one allele for a gene, the
  population is homozygous for that gene.
• If there are two or more alleles for a gene, the
  population is polymorphic for that gene.
• If the members of a population come in two or
  more forms, the population is polymorphic.

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• Most human traits are polygenic controlled by
  many genes
• These traits vary smoothly and continuously
  within a population.
• The graph of these traits is a bell curve.

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• In a changing environment, highly variable
  populations evolve more rapidly than less
  variable populations.
• The factors that determine the genetic
  variability of a population are
• The rate at which mutations accumulate in the DNA
• The rate at which changes spread through a
  population
• The rate at which deleterious mutations are
  eliminated from a population by natural selection.

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How much genetic variation is there?

• In humans about 25 of all proteins have an
  alternate form which is present in at least 5 of
  the population.
• In humans about 7 of our genes are
  heterozygous.
• Invertebrates -13
• Plants -17
• Drosophila 25

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• Remember Natural selection works only on the
  Phenotype which is an interaction of the
  geneotype and the influences of the environment.
• Genetic variation is fuel for evolution
• Yet, natural selection favors those traits best
  suited to the environment and weeds out the rest.

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• All of the genes of all the individuals in a
  population is called the gene pool.
• Hardy-Weinberg principle sexual reproduction by
  itself does not change the frequencies of alleles
  within a population. Genotype frequencies stay
  the same from generation to generation as long as
  certain conditions are met.
• Hardy-Weinberg equilibrium
• p q 1 and p2 2pq q2 1

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Conditions

1. There was random mating
2. There is a large population size
3. There are no mutations
4. There is no breeding with other populations
5. There is no selection, either natural or
   artificial

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• In reality, these conditions are hardly ever met,
  but it gives us a standard against which to
  measure evolution.
• Of all the conditions mentioned, only natural
  selection leads to adaptive change.
• The rest cause changes in gene frequency which
  may or may not be adaptive.

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Random mating

• Only practiced by organism which release gametes
  on the wind or in the water.
• Assortative mating based on choice
• May be without regard to ones phenotype
• Positive assortative mating choose individuals
  like ourselves
• Inbreeding increases the incidence of recessive
  disorders, leading to a less healthy population
• Negative assortative mating - outbreeding

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Large population size

• Random drift or genetic drift is a change in the
  allele frequency due to random events. This is
  more likely in a small pop.
• Founder effect a small subset of a population
  founds a new population.
• Bottleneck effect the population is reduced to
  a few individuals by some random disaster or
  harsh selection pressure (such as over hunting).
• Causes new mutations to spread or be removed.

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No interbreeding between populations

• Gene flow occurs as the result of interbreeding
  between two populations.
• Individuals immigrate and bring new alleles into
  the population.
• It increases the variation within a population.
• It makes adjacent populations more alike.

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No selection

• Natural selection
• Harmful genes are selected against
• Useful genes accumulate

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Types of Natural Selection

• Directional selection selects for one end of
  the bell curve

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Types of Natural Selection

• Stabilizing selection the extremes of a
  population are selected against and the average
  is favored.

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Types of Natural Selection

• Disruptive selection selects for extremes and
  against the average.

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Disruptive selection
Taste good
Taste bad
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Sexual selection

• Male competition
• Male competes against other males for territory,
  or access to females
• Anything that gives him an advantage makes him
  more likely to pass on his genes
• Female selection ( or male selection)
• Leads to sexual dimorphism
• Male must prove he is genetically good enough
• Plumage, gifts, nesting site or mating rituals

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• Natural selection can also encourage genetic
  variation when different alleles of a gene are
  equally useful.
• Different local environment
• One allele is better at a certain time of year
• Balanced polymorphism
• Sometimes the superiority of the heterozygote may
  maintain a high incidence of an allele which is
  harmful to the homozygote
• Sickle cell anemia and malaria

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Blue malaria Red sickle cell anemia Purple
overlap