GENETIC DRIFT

Source: Phys.org

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Context

• Recent research conducted by scientists from Purdue University and other institutions has revealed that genetic drift, rather than natural selection, is the primary force behind the speciation of the endangered White Sands pupfish in New Mexico.
• This discovery challenges the traditional view that natural selection is the predominant driver of speciation, particularly in such unique ecological settings.

Details

Key Findings

• Genetic drift, the random fluctuation of allele frequencies, was identified as the main evolutionary force driving the speciation.
• This process was significantly influenced by population bottlenecks and geographic isolation, particularly over the last 5,000 years.
• Contrary to expectations, no genes linked to environmental adaptations (e.g., salt tolerance) were identified, supporting the conclusion that genetic drift, not selection, was responsible for the divergence.
• The Carrizozo lava flow, occurring about 5,000 years ago, acted as a physical barrier, contributing to the separation and isolation of pupfish populations.
• Both the Malpais Spring and Salt Creek populations experienced significant reductions in size approximately 2,500 years ago.
• These bottlenecks enhanced the effects of genetic drift due to the smaller population sizes.

Implications for Conservation

• The discovery necessitates a revision of the pupfish classification, potentially designating two species rather than one.
• The researchers propose naming the new species "the enchanted pupfish," reflecting New Mexico's moniker, "the Land of Enchantment."
• Enhanced conservation efforts are recommended for both populations to protect their distinct genetic identities and ensure their survival.
• This case highlights the potential for genetic drift to play a significant role in speciation, particularly in isolated and small populations.

About Genetic Drift

• Genetic drift is a fundamental mechanism of evolution, contributing to the changes in allele frequencies within a population.
• Unlike natural selection, which favors traits that increase fitness, genetic drift occurs due to random sampling effects, especially in small populations.
• This randomness can lead to significant genetic changes over generations.

Key Concepts

• Alleles and Gene Pool:
  • Alleles are different forms of a gene found at the same place on a chromosome.
  • The gene pool is the set of all alleles in a population.
• Population Size:
  • Genetic drift has a more pronounced effect in small populations.
  • In large populations, random changes in allele frequencies are usually averaged out.
• Founder Effect:
  • Occurs when a new population is started by a small number of individuals.
  • This new population may have different allele frequencies than the original population.
• Bottleneck Effect:
  • Happens when a population undergoes a dramatic reduction in size.
  • The surviving population's gene pool may no longer reflect the original population's gene pool.

Mechanisms of Genetic Drift

• Random Sampling of Alleles:
  • Allele frequencies change due to the random sampling of gametes.
  • Over time, this can lead to the fixation or loss of alleles.
• Fixation:
  • An allele reaches a frequency of 1 (100%) in the population.
  • This means the allele is present in all individuals.
• Loss:
  • An allele reaches a frequency of 0 in the population.
  • This means the allele is completely lost from the gene pool.

Consequences of Genetic Drift

• Reduction in Genetic Variation:
  • Drift tends to reduce genetic variation within a population.
  • This can make the population more susceptible to diseases and environmental changes.
• Increased Genetic Differentiation:
  • Populations that are geographically isolated can become genetically different over time.
  • This can eventually lead to speciation.
• Neutral Evolution:
  • Genetic drift supports the neutral theory of molecular evolution, which states that most evolutionary changes are the result of random drift of neutral mutations.

Case Studies

• Island Populations:
  • Island populations often exhibit high levels of genetic drift due to their small size and isolation.
  • Example: The Galápagos finches.
• Conservation Biology:
  • Understanding genetic drift is crucial in conservation efforts, especially for endangered species.
  • Small, isolated populations are managed to maintain genetic diversity and reduce the risk of inbreeding depression.

Genetic Drift vs Natural Selection

Aspect	Genetic Drift	Natural Selection
Definition	Random changes in allele frequencies in a population.	Differential survival and reproduction of individuals due to differences in phenotype.
Mechanism	Chance events, especially in small populations.	Non-random, based on fitness and adaptation.
Effect on Alleles	Can lead to random fixation or loss of alleles.	Favors alleles that increase fitness; disadvantageous alleles decrease.
Population Size Impact	More pronounced in small populations.	Effective in populations of all sizes but stronger in large populations.
Genetic Variation	Decreases genetic variation within a population over time.	Can increase genetic variation by promoting advantageous mutations.
Adaptive Change	Non-adaptive, changes are due to random chance.	Adaptive, leads to evolution of traits that improve survival and reproduction.
Speed of Process	Generally slower but significant over many generations.	Can be fast if there is strong selective pressure.
Examples	Bottleneck effect, founder effect.	Evolution of antibiotic resistance in bacteria, beak size in finches.
Predictability	Unpredictable, random.	Predictable direction based on environmental pressures.
Influence on Population	Can lead to genetic differentiation between populations.	Leads to adaptation to specific environments.
Role in Evolution	Neutral changes, supports neutral theory of molecular evolution.	Driving force of adaptive evolution.

Key Points

• Genetic Driftis a stochastic process that has a more significant impact on small populations, leading to random changes in allele frequencies that are non-adaptive.
• Natural Selectionis a deterministic process that results in adaptive changes by favoring alleles that increase an organism’s fitness, operating effectively across all population sizes.

Sources:

Phys.org

PRACTICE QUESTION Q.  Both genetic drift and natural selection processes are crucial for understanding the dynamics of evolution and how genetic diversity within and between populations is shaped over time. Critically analyse. (250 Words)