Lecture 14 - Microevolution

Key Concepts

Natural Selection

• Definition: Process whereby individuals with traits more suited to their environment have a higher chance of survival and will most likely reproduce more successfully than others.

• Significance: Primary mechanism of evolution.

Microevolution vs. Macroevolution

• evolution is a change in allele frequency over time

• Microevolution: occurs within species (in the allele frequencies)

• Macroevolution: Evolutionary changes resulting in the formation of new species.

• Mechanisms influencing allele frequency:

  • Natural selection

  • Genetic drift

  • Gene flow

  • Mutation

Allele Frequency

• consider a population where gene A has 2 alleles (A1 and A2)

• Example of Allele A with two variants, A1 and A2:

  • Initial Frequencies:

    • Freq A1 = 10/20 = 0.5

    • Freq A2 = 10/20 = 0.5

  • After 10 years:

    • Freq A1 = 4/20 = 0.2

    • Freq A2 = 16/20 = 0.8

• Demonstrates alteration of genetic composition over time.

Origin of New Alleles

• Methods for generating new alleles:

  • Mutation: Random, heritable changes in DNA that can introduce new alleles into a gene pool

  • mutation rates are generally low (e.g., humans ~1.1 x 10^-8 per site per generation)

    • Types of mutations:

      • Deleterious

      • Neutral

      • Beneficial

    • Only inherited mutations occur in germ line cells.

  • Horizontal Gene Transfer: Transfer of genes between organisms, particularly common in bacteria.

    • likely origin of eukaryotic chloroplasts and mitochondria

Mutation and Evolution

• Discussion of the mutation rate in pathogens like COVID-19, highlighting higher than previously thought rates and potential implications for diversity among genomes:

  • Example from reports showing significant mutations in the COVID-19 genome over time.

  • deemed to be harmful, higher mutation rates = antibodies + immune system cannot keep up with new mutations/ variants

Types of Mutations

• Point Mutation: Involves change in a single base pair affecting a single amino acid (e.g., normal vs. sickle-cell mutation).

• example - Bird Flu

  • A single mutation with the virus can target humans + infect humans , leading to increased virulence and potential outbreaks. (no evidence of human transfer of the virus)

• Gene duplication

  • if a gene is duplicated, the second copy can accumulate mutations freely

  • think: why would a duplicated gene accumulate mutations more quickly than a non-duplicated gene

    • This is because the original gene can maintain its essential function while the duplicate is free to evolve new functions or become non-functional without detrimental effects on the organism.

• Genome Duplication

  • extra copies of all chromosomes above the diploid level

  • triploid, tetraploid etc.

  • mainly common in plants , where polyploidy can lead to increased genetic diversity and adaptability.

  • Polyploidy

    • can be induced artificially

    • agricultural plants often triploid - seedless, more vigorous

    • in trout and salmon - triploidy induces sterility

What affects allele frequency?

• adaptive evolution results in a better fit between individuals and their environment —> natural selection

• non-adaptive evolution is random and may have a positive, enutral or negative effect on fitness —> genetic drift + gene flow

Genetic Drift

• Definition: Random fluctuations in allele frequencies due to chance events, distinct from selection:

  • Most significant (larger effect) in small populations.

  • can fix alleles

    • an allele is fixed when it has a frequency of 1.0

• 

• Founder Effect: Example of limited genetic diversity arising from a small group founding a new population (e.g., Huntington's disease in Venezuela).

  • migrants are random sample of available alleles

  • new population may be over or under represented for some traits relative to the source population

  • Huntington’s disease example

    • frequency of disease, a degenerative nerve disease (autosomal dominant)

    • more common in Maracaibo Venezuela (700 infected per 100,000 people) compared to the US and Europe (6 infected per 100,000)

    • discovered that the villagers in Venezuela discovered the heritage led to a women who was infected by the disease who would then passed it down to her 10 kids

• Bottleneck Effect: Example of genetic diversity loss due to a sudden reduction in population size (e.g., Northern elephant seals).

• allele frequencies may change due to the chance event

• effect can occur within populations (not because of migration)

• North elephant seals

  • population reduced to 20 individuals in 1890

  • current population is 180,000

  • scientist examined 24 genes and found no allelic variation (each gene only had 1 allele) , indicating a significant reduction in genetic diversity due to the population bottleneck.

  • high homozygosity

• Cheetahs

  • historic range: North America, Europe, Asia, Africa

  • Bottleneck effect occurred near the end of the last ice age (10,000 years ago)

  • 90% homozygosity today

Gene Flow

• Definition: Transfer of genetic material between populations through migration (immigration/emigration).

• Impact: Helps maintain genetic similarity between populations and reduce divergence.

• transfer of alleles from one population to another

• immigration and emigration / accidental movement

• important in mobile organisms

• can reduce potential for genetic divergence

Types of Natural Selection

• Directional Selection: Changes in phenotype favouring one extreme (e.g., size in salmon and coloration in guppies).

• Disruptive Selection: Favors two or more extreme phenotypes over the average (e.g., beak size in Galapagos finches).

• Stabilizing Selection: Favors average traits while extreme variants are removed (e.g., human birth weight).

Examples of Directional Selection

• Trinidadian guppies: Differing phenotypes above and below waterfalls due to predation.

• African Elephants: Tusked vs. tuskless females’ prevalence due to poaching.

Conclusion

• Microevolution is influenced by various mechanisms including natural selection, genetic drift, gene flow, and mutation. Understanding these concepts is crucial for comprehending evolutionary processes. Next lecture: Phylogeny.