Rucete ✏ Campbell Biology In a Nutshell
Unit 4 MECHANISMS OF EVOLUTION — Concept 23.3 Natural Selection, Genetic Drift, and Gene Flow Can Alter Allele Frequencies in a Population
Evolutionary changes in populations are primarily driven by three main mechanisms: natural selection, genetic drift, and gene flow. Each of these mechanisms alters allele frequencies and thus leads to evolution.
Natural Selection
Natural selection occurs through differential survival and reproduction:
Individuals with traits better suited to their environment produce more offspring.
Over generations, these favorable traits increase in frequency.
Example: DDT resistance allele in fruit flies (Drosophila melanogaster) was rare initially but increased after extensive pesticide use.
Genetic Drift
Genetic drift describes random fluctuations in allele frequencies due to chance events, especially impactful in small populations.
Unlike selection, drift does not consistently favor any specific allele.
Two forms of genetic drift:
Founder Effect:
Small group becomes isolated, forming a new population with different allele frequencies than the original.
Example: High frequency of blindness allele on Tristan da Cunha island due to a founding colonist carrying the allele.
Bottleneck Effect:
Sudden environmental change drastically reduces population size.
Survivors may not represent original allele frequencies, reducing genetic variation.
Example: Greater prairie chickens in Illinois experienced severe genetic drift, reducing genetic variation and reproductive success.
Gene Flow
Gene flow is the transfer of alleles into or out of populations due to migration of individuals or gametes.
Gene flow reduces genetic differences between populations.
It can introduce beneficial alleles, improving local adaptation (e.g., insecticide resistance in mosquitoes).
Alternatively, gene flow can prevent complete local adaptation by introducing less favorable alleles (e.g., banded snakes on Lake Erie islands).
Effects of Genetic Drift: Summary
Significant in small populations.
Changes allele frequencies unpredictably.
Can lead to loss of genetic variation.
May fix harmful alleles, threatening population survival.
Comparing Mechanisms
Natural Selection: Predictable, consistent increase in adaptive alleles.
Genetic Drift: Random fluctuations, especially significant in small populations.
Gene Flow: Allele movement between populations; can promote or hinder local adaptation.
In a Nutshell
Natural selection, genetic drift, and gene flow are major evolutionary forces altering allele frequencies. Natural selection consistently favors adaptive traits, while genetic drift randomly impacts small populations. Gene flow exchanges genetic material among populations, influencing adaptation and genetic diversity.