microevolution

Lecture on Microevolution

1. Definition of Microevolution

• Microevolution is defined as evolution viewed at the smallest scale.

• It involves generation-to-generation changes in allele frequencies within a population.

• Key Concept: A population, not an individual, is the smallest unit capable of evolving.

2. Overview of the Lecture

• Exploration of microevolution, focusing on the foundational piece of population genetics.

• Review general concepts of evolution and natural selection as the driving force behind evolution.

• Examination of population genetics and the changes in allele frequencies within a population of organisms.

• Discussion of various mechanisms of evolution, including:

  • Natural selection

  • Genetic drift

  • Gene flow

• Concluding with sexual selection, which is related to mate choice and its impact on evolution.

3. Review of Evolution and Natural Selection

3.1 Definition of Evolution

• Broad definition of evolution according to Darwin: "descent with modification".

• Implication that modern organisms are modified descendants of common ancestors.

3.2 Driving Force of Evolution

• Natural Selection: The primary process driving evolution.

• It selects individuals that are better adapted to their environment, impacting survival and reproduction.

• Example: In a population of insects, individuals with a specific allele confer resistance to pesticides.

• Insects lacking the resistance allele are eliminated, thus changing the population over time as resistant individuals reproduce.

3.3 Microevolutionary Example

• Reference to a small population of insects where natural selection leads to survival and reproduction favoring those with advantageous traits.

• Result: A shift in allele frequencies within the population over time.

4. Introduction to Population Genetics

4.1 Measuring Evolution

• Investigation of how to measure if natural selection is acting on specific traits, such as flower color in a population.

4.2 Gene Pool

• Defined as the total collection of alleles in a population, encompassing all versions of genes present in that population.

4.3 Allele Frequencies

• Allele Frequency: The percentage of all loci for a particular gene within the population.

• Locus: The specific physical location of a gene on a chromosome.

• Calculation of allele frequency example:

  • Given 10 flowers with 20 total R gene copies, allele frequency for a recessive gene can be determined; if 6 out of 20 are white alleles, then the frequency is approximately 25%.

5. Practical Application of Population Genetics

5.1 Larger Population Example

• Consideration of a diploid population of 500 plants (1000 total R gene copies).

• Genotype Breakdown:

  • 320 homozygous dominant (Big R)

  • 160 heterozygous (Big R, little r)

  • 20 homozygous recessive (little r)

• Allele frequency calculations:

  • Calculate contributions from each genotype to find allele frequencies (p for dominant allele and q for recessive).

  • $p = 0.8$; $q = 0.2$

5.2 Hardy-Weinberg Equilibrium

• Condition for a non-evolving population where allele and genotype frequencies remain constant across generations.

• Hard-Weinberg Equation:

  • $p^2 + 2pq + q^2 = 1$

  • Where:

  • $p^2$: frequency of homozygous dominant genotypes

  • $2pq$: frequency of heterozygous genotypes

  • $q^2$: frequency of homozygous recessive genotypes

6. Application of Hardy-Weinberg Equation

6.1 Example Problem: PKU in Humans

• PKU prevalence as an example: 1 in 10,000 babies born with PKU (homozygous recessive).

• Calculate carrier percentages using the Hardy-Weinberg equation:

  • Given: $q^2 = 0.0001$ (1/10,000)

  • $q = 0.01$, $p = 0.99$, and carrier frequency calculation:
    $2pq = 0.0198$

6.2 Example Problem: Butterflies

• Brown (Big B) dominant over white (little b) with 4% white butterflies (homozygous recessive).

• Find:

  • Percentage of heterozygous butterflies ($2pq$).

  • Frequency of homozygous dominant individuals ($p^2$).

7. Mechanisms of Evolution

7.1 Genetic Drift

• Genetic drift refers to changes in allele frequencies due to random chance, not adaptation.

7.2 Bottleneck Effect

• A form of genetic drift where a population experiences a drastic reduction in size, leading to reduced genetic variability.

• Example: Cheetahs experienced a bottleneck due to historical events (hunting, drought, disease).

7.3 Founder Effect

• Occurs when a small group starts a new population, resulting in limited genetic diversity based on the genes of the founding members.

• Example: Amish communities with genetic diseases due to a restricted gene pool.

7.4 Gene Flow

• Refers to the exchange of alleles between neighboring populations, increasing genetic diversity in a population.

• Example: Pollen movement in plants or human interbreeding across diverse populations.

8. Natural Selection and Its Outcomes

8.1 Modes of Selection

• Three types of selection based on environmental pressures:

8.1.1 Directional Selection

• Shift in one direction; population favors one extreme phenotype.

8.1.2 Disruptive Selection

• Split in the population; both extremes are favored over the average phenotype.

8.1.3 Stabilizing Selection

• Intermediate phenotypes are favored; population becomes homogeneous over generations.

8.2 Examples of Selection

• Directional selection impacts mouse populations based on coat color in changing environments.

• Disruptive selection can occur in patchy environments.

• Stabilizing selection can be observed in human birth weights to favor a middle weight range for infants.

8.3 Sexual Selection

• Selection based on mate preferences, often influencing male traits to attract females (sexual dimorphism).

• Example: Brightly colored male birds are more attractive to females for mating purposes.

• Highlights the importance of reproductive success in addition to survival.