7.4 - 7.5, 7.12 - Population Genetics

Genetic Drift

Definition: Genetic drift refers to chance events that cause changes in allele frequencies from one generation to the next. It is particularly significant in small populations and can lead to a loss of genetic variation and the fixation of harmful alleles.

Types of Genetic Drift

1. Bottleneck Effect:

   • Description: Occurs when a large population is drastically reduced by a non-selective disaster such as floods, famine, fires, hurricanes, or hunting.

   • Impact: Some alleles may become overrepresented, underrepresented, or completely absent in the surviving population, leading to reduced genetic diversity.

2. Founder Effect:

   • Description: Happens when a small group of individuals becomes isolated from a larger population and establishes a new population.

   • Impact: The new population may have reduced genetic diversity compared to the original population, and the gene pool is significantly changed.

Random Events Affecting Genetic Makeup

• Migration/Gene Flow:

  • Definition: The transfer of alleles into or out of a population due to the movement of individuals or their gametes.

  • Impact: Gene flow can introduce new alleles into a population or remove alleles, altering allele frequencies and increasing genetic diversity.

Natural Selection

Definition: Natural selection is the process by which certain traits become more or less common in a population due to their impact on the survival and reproduction of organisms.

Measuring the Effects of Natural Selection

• Phenotypic Changes: The effects of natural selection can be measured by examining changes in phenotypes over time.

• Genetic Variance: Natural selection can cause an increase, decrease, or shift in the genetic variance of a population.

Modes of Natural Selection

1. Directional Selection:

   • Description: Favors individuals at one end of the phenotypic range.

   • Impact: Causes a shift in the population’s phenotypic distribution toward the favored trait.

2. Stabilizing Selection:

   • Description: Favors intermediate variants by acting against extreme phenotypes.

   • Impact: Reduces phenotypic variation and maintains the status quo for a particular trait.

3. Disruptive Selection:

   • Description: Favors individuals at both extremes of the phenotypic range.

   • Impact: Increases phenotypic variation and can lead to the formation of distinct subpopulations.

Examples of Genetic Drift and Gene Flow

1. Black Robin Population Decline

• Scenario: The black robin, a small bird native to the Chatham Islands in New Zealand, experienced a dramatic population decline due to habitat conversion to farmland and the introduction of non-native predators. The population dwindled to only 5 individuals.

• Mechanism: Bottleneck Effect

  • Explanation: Human activities caused a severe reduction in the population, drastically reducing genetic diversity. Conservation efforts have since increased the population to around 230 individuals, but the genetic diversity remains low.

2. Amish Population in Pennsylvania

• Scenario: In the 1720s, an Amish population settled in Pennsylvania. A small number of early settlers from Germany carried several mutations, including one for polydactyly. This trait is now much more common in the Amish population than in the general population.

• Mechanism: Founder Effect

  • Explanation: A small group of settlers established a new population, bringing with them a higher frequency of certain mutations, including polydactyly. This trait became more prevalent in the Amish population due to their relative isolation and small gene pool.

3. Marble Trout Population and Flash Floods

• Scenario: Recurrent flash floods have caused mass mortalities in the marble trout population. Scientists found that the genetic diversity of the remaining population has decreased significantly.

• Mechanism: Bottleneck Effect

  • Explanation: Natural disasters, such as flash floods, drastically reduced the marble trout population, leading to a significant loss of genetic diversity.

4. Gene Flow in Flower Populations

• Scenario: Increased wind in a local area caused pollen from one population of flowers to travel further and pollinate another population. The offspring of these flowers now exhibit traits from both populations.

• Mechanism: Gene Flow

  • Explanation: The movement of pollen between populations resulted in the transfer of alleles, introducing new genetic variations and traits to the offspring.