(3) Unit 7 Topics 4 5 and 12 Part 1 Packet 1

Unit 7: Population Genetics and Hardy-Weinberg Equilibrium

Definition of Population

• A population is a group of individuals of the same species living in the same area.

• These individuals can interbreed and produce fertile offspring capable of further reproduction.

• Example: A group of cheetahs in a specific area that can breed and produce viable offspring.

Gene Pool

• A gene pool represents a population's genetic makeup, consisting of all the different alleles present in a population.

• It's a collective pool of all the genotypes, providing a way to analyze allele frequencies.

• If only one allele is present for a particular locus, it is termed fixed, reducing genetic diversity.

  • Example: If a tornado wipes out all turtles with a dominant allele, the remaining turtles with only the recessive allele result in a fixed allele for that gene.

• Consequences of fixed alleles include:

  • Increased risk of endangerment and extinction.

  • Decreased ability to resist ecological stresses, like diseases.

Understanding Allele Frequencies

• Population’s allele frequencies can change over time, leading to microevolution.

• Microevolution occurs as genetic changes due to:

  • Mutations: Alterations in DNA, can create new alleles, providing variation.

  • Genetic Drift: Random events causing changes in allele frequencies, especially significant in small populations.

  • Migration (Gene Flow): Transfer of alleles between populations, altering genetic makeup.

  • Natural Selection: Affects allele frequencies and can drive evolution.

Mutations

• Defined as changes to DNA that can result in genetic variation.

• Can be classified into three categories:

  • Harmful: Negative effects on survival.

  • Neutral: No significant effect on phenotype.

  • Beneficial: Enhances survival and reproductive capabilities. Rare, but focal point for natural selection.

• Mutation rates vary by organism, faster in prokaryotes due to quicker generation times.

Genetic Drift

• A chance event altering allele frequencies, more impactful in small populations, leading to:

  • Loss of genetic variation.

  • Fixation of harmful alleles.

Types of Genetic Drift

1. Bottleneck Effect:

   • Occurs when a large population is dramatically reduced by non-selective disasters (e.g., floods, earthquakes).

   • Results in overrepresentation, underrepresentation, or absence of certain alleles.

2. Founder Effect:

   • Happens when a small number of individuals become isolated from a larger population, starting a new population with a different gene pool.

   • Can lead to changes in allele frequencies over time, affecting genetic diversity.

Gene Flow

• Involves the transfer of alleles into or out of a population due to the movement of individuals or gametes.

• Example: Migration of birds or pollen transfer due to wind can introduce new alleles to populations.

Natural Selection

• Relative fitness measures reproductive success based on the number of surviving offspring produced relative to others in the population.

• There are three different modes of natural selection:

  • Directional Selection: Selection favors one extreme phenotype (e.g., larger beaks selected during a food scarcity).

  • Stabilizing Selection: Selection favors the mean phenotype, reducing extremes (e.g., average beak size).

  • Disruptive Selection: Selection favors both extremes, against the mean (e.g., short and long beaks favored, average size not).

Sexual Selection

• A specialized form of natural selection driving the development of showy or unique traits, often involving female choice.

• Example: Peacocks with extravagant feathers may attract mates but could be more easily spotted by predators due to their striking appearance.

• This highlights the balance between survival and reproductive strategy in evolutionary contexts.