Evolutionary Mechanisms

Mechanisms of Evolution

• Definition: Mechanism - the process by which it occurs; the way it works.

1. Natural Selection

• Alleles become more or less common in populations.

• The allele influences an individual's ability to survive and reproduce, affecting survival in the environment.

2. Sexual Selection

• Involves increased reproduction, often at a survival cost.

• The allele affects reproduction in the environment.

3. Genetic Drift

• Alleles become more or less common in a population due to random occurrences.

• Effect is stronger in smaller populations.

• Allele does not affect survival or reproduction in the environment.

4. Gene Flow

• Changes in allele frequency due to migration; involves at least two populations.

5. Mutations

• Introduces new alleles into the population.

6. Darwin's Insight

• Evolution is a gradual process, always happening.

• Similar to selective breeding: individuals with undesired traits are not permitted to reproduce, while those with desired traits are allowed.

Natural Selection Observations and Inferences

Observations:

1. Members of a population vary in their inherited traits.

2. More offspring are produced than can survive.

Inferences:

1. Individuals with inherited traits that increase the probability of survival and reproduction will leave more offspring.

2. Over many generations, favorable traits will accumulate.

Effects of Mutations

• Some mutations impair survival and reproduction.

• Some mutations enhance these abilities.

• Some mutations have no effect.

• Mutations are usually random, while natural selection is not.

Survival of the Fittest

• Evolutionary Fitness: Ability to survive and reproduce based on an individual's DNA, not purely on physical fitness.

• Survival alone is not sufficient if reproduction is not optimal.

Sexual Selection

• Definition: Changes in allele frequency due to effects on reproduction.

Types of Sexual Selection:

1. Intersexual Selection: Mate choice.

2. Intrasexual Selection: Competition among individuals.

Gene Flow

• Allele frequency changes due to migration in and out of a population.

• The gene pool of both populations is affected by gene flow.

Genetic Drift

• Allele frequency changes in a population due to random events; effects are stronger in small populations.

Hardy-Weinberg Principle

• A population not evolving will have a constant allele frequency.

Conditions for Hardy-Weinberg:

1. No mutation.

2. Random mating.

3. No natural selection.

4. Extremely large population size.

5. No migration (gene flow).

Mathematical Representation:

• p + q = 1 (where p is the frequency of one allele and q of another).

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  Genotype frequencies are represented as:p² + 2pq + q² = 1 (where:

  • p² = frequency of homozygous dominant individuals,

  • 2pq = frequency of heterozygous individuals,

  • q² = frequency of homozygous recessive individuals).

Example: Shell Color in Snails

• Genotypes:

  • BB (black shell) = 550

  • Bb (gray shell) = 50

  • bb (white shell) = 400

• Total Snails (N) = 1000

• Total Alleles (2N) = 2000.

• Calculations:

  • p² = prevalence of BB = 0.331

  • q² = prevalence of bb = 0.1817

  • Expected if in HWE: 2pq = 0.489.

• Actual frequencies:

  • Freq[BB] = 550/1000 = 0.55

  • Freq[Bb] = 50/1000 = 0.050

  • Freq[bb] = 400/1000 = 0.400.

• Conclusion: The population is not in Hardy-Weinberg Equilibrium.

Identifying Evolution

• Evolution: Descent with modification; change in allele frequencies over time.

Evidence of Evolution:

1. Fossil Record: Evidence of past changes over time.

2. Homology: Similarity due to common ancestry (e.g., skeletal structures, molecular homologies such as pig and human insulin).

3. Vestigial Structures: Features that were functional in ancestors but reduced in descendants (e.g., whale hips and femurs).

4. Biogeography: Distribution of species explained by shared ancestry.

5. Direct Observation: Visualizing evolution within populations with short generation times (e.g., bacteria and antibiotic resistance).