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Chapter Four

Chapter Four. Population Genetics. A Model of Population Genetics. Although individuals are the unit of selection, individuals do not evolve. The unit of evolution is the reproductive population. The largest reproductive population is the species.

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Chapter Four

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  1. Chapter Four Population Genetics

  2. A Model of Population Genetics • Although individuals are the unit of selection, individuals do not evolve. The unit of evolution is the reproductive population. The largest reproductive population is the species. • Just as one can speak of the phenotype and genotype of an individual, one also can speak of the phenotype and genotype of a population. • The sum of all alleles carried by members of a population is known as the gene pool.

  3. Genetic Equilibrium • Evolution can be defined as a change in the gene pool of a population, specifically an alteration in allele frequencies. • If not evolving, these frequencies remain constant, a situation termed genetic equilibrium. • The calculation for genetic equilibrium is known as Hardy-Weinberg equilibrium. • The formula is: p2+2pq+q2=1, where p is equal to the frequency of the dominant allele and q is equal to the frequency of the recessive allele.

  4. Mechanisms of Evolutionary Change • Certain conditions must be assumed for a population to remain in genetic equilibrium. • Mutation must not be taking place. • The population must be infinitely large. • Individuals from neighboring populations must not introduce alleles into the population. • Matings must take place at random and be equally fertile. • Natural selection must not be occurring.

  5. Mutations • A mutation is any alteration in the genetic material. • Mutations are random, arising with no design, predetermined reason, or purpose. • Mutations create new alleles. When the mutation occurs in sex cells, new alleles in the ova and sperm are produced, changing the gene pool in the next generation. • The ultimate source of all biological variation is mutation. • Mutations can occur due to mistakes in DNA replication, exposure to naturally occurring chemicals and temperature fluctuations, or by various chemicals and exposure to radiation.

  6. Genetic Drift, Population Bottlenecking, and Founders Principle • With small populations, changes in gene frequencies often occur due to chance effects. This is a case of sampling error. • A chance deviation in the frequency of alleles in a small population is known as genetic drift. • Population bottlenecking and founders principle are forms of sampling error.

  7. Nonrandom Mating • The equilibrium model assumes that individuals in the population are mating at random. • A consanguineous mating is a mating between relatives. Such matings may have a significant effect on rare alleles. • Another example of nonrandom mating would be assortive mating.

  8. Natural Selection • The model of genetic equilibrium assumes that all matings are equally fertile, but this is obviously not the case. • Differential fertility and mortality are both examples of natural selection, the heart of evolutionary theory. • Factors that result in greater fertility, if genetically determined, will be passed on to the next generation with higher frequency. • Conditions that result in decreased fertility or higher mortality, such as genetic abnormalities, will tend to be eliminated.

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