The Process of Evolution

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Learning Objectives

1. Understand the process of evolution.
2. Understand the factors that impact allele frequency.
3. Understand how speciation occurs.
4. Understand how neutral or non-favorable traits can remain in a population.

Solutions

1. Evolution is sexual or reproductive fitness. The more something can reproduce, the stronger evolution will be. More reproduction means more of its alleles in the gene pool.
2. The factors that affect allele frequency are outlined in the Hardy-Weinberg Law. Allele frequency is influenced by:
   • No mutations (versus mutations)
   • Isolation from other populations (versus gene flow)
   • Large population size (versus genetic drift)
   • Random mating (versus no-random mating)
   • No natural selection (versus natural selection)
3. Speciation occurs in two primary ways:
• Geographic: two species are separated by geography for an extended period of time, to the point where they are no long able to reproduce.
• Temporal: two species do not have any overlapping seasons in which they reproduce, to the point where they are no longer able to reproduce.
4. Neutral or unfavorable traits can remain in a population for three main reasons:
• They are linked to a favorable trait (close together on the chromosome)
• They are favorable in certain environments (sickle cell anemia more common in areas with higher malaria incidence as sickle cell creates resistance to it)
• If the heterozygous phenotype has higher fitness (keeps the recessive allele in the population)

Notes

Evolutionary Considerations

• What is the main goal of all organisms? They live long enough to reproduce.
• Can every organism reproduce to it's fullest capacity? Only if there are enough resources for every organism, which there usually aren't.
• Given genetic variability in a population, which organisms will be more successful? Whichever will reproduce the most.
• Over time, which individuals will make up a higher percentage of a population? Whichever is best suited to the environment over time.
• How do some individuals become better adapted to an environment? More suitable adaptations come from mutations.
• All genetic variation is due to mutations.

Gene Pool

• All of the genes of all of the individuals in a population of a given species
• Different alleles make up different proportions of the gene pool
• Genetic fitness if the ability to produce offspring

Hardy-Weinberg Law

• Allele frequencies in a given population stay the same provided that:
• No mutations
• Isolation for other populations
• Large population size
• Random mating
• No universal selection
• This gives scientists a baseline expectation to compare observations to
• If allele frequencies start to change, one of these five things isn't true for the population
• Standard for detecting evolutionary change

Genetic Variation

• Change in allele frequency: evolution
• Things that change allele frequency
• Mutations
• Gene flow
• Genetic drift
• Non-random mating
• Natural selection
• Inverse of the Hardy-Weinberg Law

Gene Flow

• Pollen or seeds from one population entering another population
• New alleles introduced
• Change in allele frequency
• Can have new alleles or different proportions of same alleles

Genetic Drift

• Occurs in small populations
• Even if allele frequencies are the same, random chance could result in the loss of one allele

Comparison of Gene Flow versus Genetic Drift

• Gene flow
• Change across space
• Multiple populations
• Population size doesn't matter
• Genetic drift
• Change across time
• Single initial population
• Mainly in small populations

Genetic Drift: Founder Effect

• Proportion of plants founding a new population isn't equal to the parent population

Genetic Drift: Bottleneck Effect

• Any event that kills a large proportion of population
• Dinosaurs + asteroid = chickens

Non-Random Mating

• Upsets Hardy-Weinberg equilibrium
• Non-random mating patterns:
• Inbreeding: mating between related individuals
• Positive assortive and negative assortive mating with respect to specific phenotypes mating together
• Plants will more likely mate with other plants in close proximity
• Results in more homozygotes and less heterozygotes

Speciation

• Separation of one population into two
• Can be:
• Geographic: separated by space
• Temporal: separated by time
• Occurs when two different populations mate at different times
• Makes mating between populations impossible
• ex: winter versus summer pansies

Evolution

• When might a neutral or non-favorable trait remain in a population?
• When it's linked to a favorable (close to each other on the chromosome)
• When it's favorable in certain environments (having sickle cell anemia provides much greater resistance to malaria)
• If the heterozygous phenotype has a higher evolutionary fitness (keeps the homozygous recessive gene in the population)