Genes within Populations - Animal Form, Function and Diversity - Lecture Slides, Slides of Biology

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Genes Within

Populations

Chapter 20

• Darwin : Evolution is descent with modification
• Evolution : changes in gene frequencies through time 1. Species accumulate difference 2. Descendants differ from their ancestors 3. New species arise from existing ones

Genetic Variation and Evolution

Natural selection: mechanism of

evolutionary change

Inheritance of acquired characteristics: Proposed by Jean-Baptiste Lamarck

• Individuals passed on physical and behavioral changes to their offspring
• Variation by experience…not genetic
• Darwin’s natural selection: variation a result of preexisting genetic differences

Mechanisms of Evolution

• Mutation - replication errors
• Natural selection - differential reproduction of genotypes
• Migration - movement of individuals among populations
• Genetic drift - random chance events in small populations
• Genetic Recombination - shuffling novel combinations of existing genes

Godfrey H. Hardy: English mathematician

Wilhelm Weinberg: German physician

Concluded that :

The original proportions of the genotypes in a

population will remain constant from

generation to generation as long as five

assumptions are met

Hardy-Weinberg Principle

Five assumptions :

1. No mutation takes place
2. No genes are transferred to or from

other sources, no migration

3. Random mating is occurring
4. The population size is very large
5. No natural selection occurs

Hardy-Weinberg Principle

Agents of Evolutionary Change

• Mutation: A change in a cell’s DNA
  • Mutation rates are generally so low they have little effect on Hardy-Weinberg proportions of common alleles.
  • Ultimate source of genetic variation
• Migration (Gene flow): A movement of alleles from one population to another - Powerful agent of change - Tends to homogenize allele frequencies

Agents of Evolutionary Change

• Genetic Recombination (Nonrandom Mating): mating with specific genotypes - Shifts genotype frequencies - Assortative Mating : does not change frequency of individual alleles; increases the proportion of homozygous individuals - Disassortative Mating : phenotypically different individuals mate; produce excess of heterozygotes

Selection

• Artificial selection : a breeder selects for

desired characteristics

Selection

• Natural selection: environmental conditions determine which individuals in a population produce the most offspring
• 3 conditions for natural selection to occur
  • Variation must exist among individuals in a population
  • Variation must be genetically inherited
  • Variation among individuals must result in differences in the number of offspring surviving

Fitness and Its Measurement

• Fitness: A phenotype with greater

fitness usually increases in frequency

– Most fit is given a value of 1

• Fitness is a combination of:

– Survival: how long does an

organism live

– Mating success: how often it mates

– Number of offspring per mating that

survive

Interactions Among Evolutionary

Forces

• Mutation and genetic drift may counter

selection

• The magnitude of drift is inversely related to

population size

Maintenance of Variation

• Frequency-dependent selection : depends on how frequently or infrequently a phenotype occurs in a population - Negative frequency-dependent selection: rare phenotypes are favored by selection - Positive frequency-dependent selection: common phenotypes are favored; variation is eliminated from the population
• Strength of selection changes through time

• Oscillating selection: selection favors

one phenotype at one time, and a different phenotype at another time

• Galápagos Islands ground finches
  • Wet conditions favor small bills (abundant seeds)
  • Dry conditions favor big bills

Maintenance of Variation