11.2 Mechanisms of Evolution

Evolution shapes life through various mechanisms. Natural selection favors beneficial traits, while mutations introduce new genetic variations. Genetic drift causes random changes in small populations, and gene flow transfers genes between groups.

These processes alter allele frequencies over time. Natural selection increases beneficial alleles, mutations add variety, drift causes random changes, and gene flow maintains diversity. Population size, barriers, and environmental factors influence these evolutionary mechanisms.

Mechanisms of Evolution

Mechanisms of evolution

• Natural selection favors organisms with beneficial traits, increasing their chances of survival and reproduction, leading to an increase in the frequency of advantageous alleles over generations (Darwin's finches)
• Mutations are random changes in DNA sequences that introduce new alleles into populations, providing the raw material for evolution, although most are neutral or harmful (sickle cell anemia)
• Genetic drift causes random fluctuations in allele frequencies due to chance events, particularly in small populations, potentially leading to the fixation or loss of alleles regardless of their adaptive value (founder effect)
• Gene flow is the transfer of alleles between populations through migration or interbreeding, introducing new alleles or changing existing frequencies, reducing genetic differences between populations (Great Britain and Ireland)
• Selection pressure, such as environmental factors or predation, influences the direction and strength of natural selection

Allele frequency changes over time

• Natural selection increases the frequency of beneficial alleles and decreases the frequency of deleterious ones, resulting in adaptation and the development of new traits (antibiotic resistance in bacteria)
• Mutations introduce new alleles into populations, increasing genetic variation, although most have little effect on frequencies, rare beneficial mutations can spread (nylon-eating bacteria)
• Genetic drift causes random changes in allele frequencies, especially in small populations, potentially leading to the fixation (reaching 100%) or loss (reaching 0%) of alleles by chance (Pingelap Island)
• Gene flow introduces new alleles or changes existing frequencies through migration or interbreeding, counteracting the effects of drift and selection by maintaining genetic variation (wolf-dog hybridization)
• Fitness, the relative ability of an organism to survive and reproduce, is a key factor in determining which alleles are passed on to future generations

Factors affecting genetic drift

• Smaller populations are more susceptible to genetic drift because chance events have a greater impact on allele frequencies, leading to more rapid fixation or loss of alleles (island species)
• Physical barriers like mountains or rivers can limit gene flow between populations, while habitat fragmentation can reduce gene flow and increase genetic differentiation (Galapagos Islands)
• Environmental changes such as climate change can alter migration patterns and affect gene flow between populations (migratory birds)
• Gene flow can counteract the effects of genetic drift by introducing new alleles and maintaining genetic variation, but in small, isolated populations, drift can overwhelm the effects of gene flow, leading to greater genetic differentiation (Tristan da Cunha)

Population Genetics and Microevolution

• Population genetics studies the changes in allele frequencies within populations over time
• The Hardy-Weinberg equilibrium provides a theoretical framework for understanding genetic variation in populations under ideal conditions
• Microevolution refers to small-scale changes in allele frequencies within a population over relatively short periods of time
• These processes can eventually lead to speciation, the formation of new species, when populations become reproductively isolated