Chap 23 The Evolution of Populations Evolution Reminders

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Chap 23 The Evolution of Populations

Chap 23 The Evolution of Populations

Evolution Reminders 1. Individuals DO NOT evolve, a population evolves through its interaction with

Evolution Reminders 1. Individuals DO NOT evolve, a population evolves through its interaction with the environment. 2. Evolution can only be measured as changes in relative proportions of heritable variations in a population over a succession of generations. 3. Natural selection can amplify or diminish heritable traits only. 4. Environmental factors vary from place to place and time to time.

 • Natural selection produces evolutionary change when some individuals in a population possess

• Natural selection produces evolutionary change when some individuals in a population possess certain inherited characteristics and then produce more surviving offspring than individuals lacking these characteristics. • As a result, the population gradually comes to produce more and more individuals with advantageous characteristics. • Therefore the population evolves and becomes better adapted to the local circumstances.

 • A population is defined as a localized group of organisms that are

• A population is defined as a localized group of organisms that are capable of interbreeding and producing fertile offspring. • The study of how populations change over time Population Genetics. • The collection of genes in a population at any one time Gene Pool.

 • 1908 – Godfrey Hardy and Wilhelm Weinberg were confused as to why,

• 1908 – Godfrey Hardy and Wilhelm Weinberg were confused as to why, after many generations, populations didn’t become solely composed of individuals with the dominant phenotype.

 • Their theorem is used as a way to provide a benchmark against

• Their theorem is used as a way to provide a benchmark against which to measure frequency changes. • This theorem states that the frequencies of alleles and genotypes in a populations gene pool remain constant from generation to generation, provided certain assumptions are met.

 • This theorem describes a hypothetical population that is NOT evolving. • This

• This theorem describes a hypothetical population that is NOT evolving. • This theorem can be used to obtain approximate estimates of allele and genotype frequencies.

Conditions for Hardy-Weinberg Equilibrium 1. Extremely large population size. 2. No gene flow. 3.

Conditions for Hardy-Weinberg Equilibrium 1. Extremely large population size. 2. No gene flow. 3. No mutations. 4. Random mating. 5. No natural selection. ** this is normally unrealistic which means that most natural populations result in evolution. **

Theorem: p 2 + 2 pq + q 2 = 1 p = the

Theorem: p 2 + 2 pq + q 2 = 1 p = the frequency of an allele at a loci (dominant) q = the frequency of an allele at a loci (recessive) ** (p + q = 1)

 • The equation for the Hardy-Weinberg equilibrium states that at a locus with

• The equation for the Hardy-Weinberg equilibrium states that at a locus with 2 alleles, the three genotypes will appear in proportions to the sum of 1. • If a population were in H-W equilibrium, and its members continued to mate randomly generation after generation, the allele and genotype frequencies would remain constant.

3 Factors that Alter Allelic Frequencies Ø Natural Selection Ø Genetic Drift : The

3 Factors that Alter Allelic Frequencies Ø Natural Selection Ø Genetic Drift : The Bottleneck effect and The Founder effect. Ø Gene Flow These factors not only affect allelic frequencies but also cause the most evolutionary change.

Natural Selection as a Mechanism • Natural selection can alter the frequency of heritable

Natural Selection as a Mechanism • Natural selection can alter the frequency of heritable traits depending on which phenotypes in a population are favored. Ø Directional selection : shifts the frequency curve for some phenotypic character in one direction or the other by favoring individuals that deviate form the average.

Ø Disruptive selection: favors those individuals on both extremes of a phenotype over those

Ø Disruptive selection: favors those individuals on both extremes of a phenotype over those with an intermediate. Ø Stabilizing Selection: Acts against extreme phenotypes and favors those that are intermediate.

Male African Long-tailed Widow bird

Male African Long-tailed Widow bird

 • Evolution is limited by historical restraints. • Adaptations are often compromises. •

• Evolution is limited by historical restraints. • Adaptations are often compromises. • Chance and natural selection interact. • Selection can edit only existing variations. As a result evolution cannot fashion the “perfect” organism. As a result we see numerous examples of organisms that are less than “ideally engineered” for their lifestyle.