microevolution: how does a population evolve? chapter 16
TRANSCRIPT
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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 choice (allele) for a gene, the population is homozygous for that gene.
• If there are two or more choices (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:
1. The rate at which mutations accumulate in the DNA.
2. The rate at which changes spread through a population (reproductive rate).
3. 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 genotype and the influences of the environment (basically what the individual looks like).
• 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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1) 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 one’s 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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2) 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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3) No Mutations
• Does not happen in nature.
• The DNA copying mechanism is nearly perfect, but mistakes are made.
• These mistakes result in new phenotypes which are subjected to selection and the basis for adaptive change,
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4) 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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4) 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