17.05 Genetic Drift, Founder Effect, and Evolutionary Bottlenecks

Genetic Drift

• Definition:
  • Genetic drift refers to random changes in the frequencies of alleles (different versions of a gene) within a small population.
  • These changes occur by chance rather than through natural selection, leading to alterations in the genetic makeup of the population over generations.

Characteristics:

• Random Nature: Unlike natural selection, genetic drift does not necessarily favor alleles that enhance survival or reproduction. The changes in allele frequencies are purely stochastic.
• Population Size Dependency: Genetic drift has a more pronounced effect in small populations where random events can significantly impact allele frequencies. In larger populations, the effect of drift is minimized due to the larger gene pool.
• Allele Fixation and Loss:
  • Fixation: An allele becomes fixed when its frequency reaches 100% in the population, meaning all individuals carry that allele.
  • Loss: An allele may be lost entirely from the population if it falls below detectable levels.
• Reduced Genetic Variation: Over time, genetic drift can lead to a decrease in genetic variation within a population, potentially reducing its ability to adapt to new environmental challenges.

Mechanisms of Genetic Drift:

1. Bottleneck Effect: Occurs when a population is drastically reduced in size due to a catastrophic event (e.g., natural disaster, disease), resulting in a loss of genetic diversity.
2. Founder Effect: Happens when a new population is established by a small number of individuals from a larger population, leading to a limited gene pool.

Example of Genetic Drift:

• Consider a small population of plants with two flower color alleles: white and yellow. Suppose two white-flowered and three yellow-flowered plants reproduce. If, by chance, white-flowered plants’ seeds land in poor soil and fail to germinate, while yellow-flowered seeds thrive in fertile soil, the next generation may have a higher frequency of yellow alleles. Over successive generations, the white allele may be lost entirely, not because it was disadvantageous, but purely due to random chance.

Founder Effect

• Definition:
  • The founder effect is a specific type of genetic drift that occurs when a new population is established by a small number of individuals from a larger population. This small group carries only a subset of the original population’s genetic diversity, which can result in significant differences in allele frequencies compared to the source population.

Impact on Gene Pool:

• Reduced Genetic Diversity: The new population has fewer alleles than the original population, limiting genetic variation.
• Allele Frequency Differences: The allele frequencies in the new population may differ significantly from those in the original population purely by chance, potentially leading to traits that are rare or absent in the source population.

Example: Anole Lizards in the Caribbean

• Scenario: After a hurricane, several small islands are formed, and Anolis sagrei lizards from a larger island are carried to these new islands via floating vegetation.
• Genetic Outcome: If only a few green-colored lizards establish populations on new islands, the resulting populations may lack brown alleles if none of the founding individuals carried them. Over time, the populations on these islands remain predominantly green, not because green is more advantageous, but due to the limited genetic variation of the founders.

Case Study: 2004 Hurricane Event in the Bahamas

• Event: A hurricane submerged several small islands, eliminating existing Anolis sagrei lizard populations.
• Experiment Setup:
  • Scientists introduced one male and one female lizard from a larger, nearby island to each of seven newly formed islands.
  • The source population had lizards with long legs, advantageous for climbing large branches in their original habitat.
  • The new islands featured low, small vegetation where long legs were less beneficial.
• Findings:
  • Natural Selection: Over four years, lizards on each island evolved shorter leg lengths to better navigate the smaller vegetation.
  • Founder Effect: The initial leg lengths of the founding pair influenced the descendants’ traits, demonstrating that the founder’s genetics played a significant role alongside natural selection.

Evolutionary Bottleneck

• Definition:
  • An evolutionary bottleneck is a sharp reduction in the size of a population due to environmental events (such as earthquakes, floods, fires, disease, or human activities) or other pressures, leading to a loss of genetic diversity. This event can drastically alter the gene pool of a population, making it less genetically varied and potentially more vulnerable to future environmental changes.

Causes:

• Environmental Disasters: Natural events like volcanic eruptions, hurricanes, or tsunamis.
• Disease: Epidemics can drastically reduce population sizes.
• Habitat Destruction: Human activities like deforestation, urbanization, or pollution.

Consequences:

• Loss of Genetic Variation: The surviving population may have a limited number of alleles, reducing genetic diversity.
• Increased Inbreeding: With fewer individuals, the likelihood of breeding between relatives increases, leading to higher homozygosity and the expression of deleterious alleles.
• Reduced Adaptability: A less diverse gene pool limits the population’s ability to adapt to new environmental challenges, increasing the risk of extinction.

Example: Cheetah Population Bottleneck

• Historical Event: Approximately 10,000 years ago, climate changes following the last Ice Age likely caused a bottleneck in cheetah populations, reducing their numbers drastically.
• Current Impact:
  • Low Genetic Diversity: Modern cheetahs possess less than 5% of the genetic diversity typical of other animal species.
  • Homozygosity: Most gene loci are homozygous, meaning there is little variation in their genetic makeup.
• Conservation Concerns:
  • Vulnerability to Disease: Limited genetic diversity makes cheetahs more susceptible to diseases, as there is less variation in immune system genes.
  • Adaptation Challenges: Cheetahs struggle to adapt to environmental changes, such as habitat loss or climate shifts, due to their reduced genetic toolkit.
  • Risk of Extinction: The combination of these factors increases the likelihood of extinction, emphasizing the need for conservation efforts to preserve remaining genetic diversity.

Key Terms

• Genetic Drift: Random changes in allele frequencies in a population, particularly significant in small populations, leading to allele loss or fixation without regard to the alleles’ impact on survival or reproduction.
• Founder Effect: A form of genetic drift that occurs when a new population is established by a small number of individuals from a larger population, resulting in reduced genetic diversity and potentially different allele frequencies compared to the original population.
• Evolutionary Bottleneck: A drastic reduction in population size due to environmental or other pressures, leading to a significant loss of genetic diversity and reduced adaptability.
• Gene Pool: The complete set of genetic information (all alleles) present in a population. It represents the genetic diversity available for evolution and adaptation.
• Allele Fixation: The process by which one allele becomes the only variant in the gene pool for a particular gene, reaching 100% frequency.
• Homozygosity: The state of having two identical alleles for a particular gene, which can increase the expression of recessive traits, including deleterious alleles.
• Heterozygosity: The presence of different alleles at a gene locus, contributing to genetic diversity within a population.

Summary and Importance for Exams

• Understanding genetic drift, including the founder effect and evolutionary bottlenecks, is crucial for comprehending how populations evolve over time, especially in response to random events. These concepts highlight the role of chance in evolution, distinct from natural selection, and underscore the importance of genetic diversity for the survival and adaptability of species.

Exam Tips:

• Define Clearly: Ensure you can accurately define genetic drift, founder effect, and evolutionary bottleneck, highlighting their differences and relationships.
• Use Examples: Be prepared to explain examples, such as the Anole lizards or cheetahs, to illustrate these concepts effectively.
• Explain Mechanisms: Understand and articulate the mechanisms by which these processes affect allele frequencies and genetic diversity.
• Compare and Contrast: Be able to differentiate between genetic drift and natural selection, and understand how they interact in shaping populations.

Suggested Diagrams:

• Gene Pool Illustration: Showing allele frequencies before and after a genetic drift event.
• Founder Effect Scenario: Depicting a small group establishing a new population with limited genetic diversity.
• Bottleneck Effect Example: Illustrating a population before and after a drastic reduction in size.