7.3 Evolution and speciation Flashcards
What is evolution?
• Change in allele frequency over many generations in a population
• Occurring through the process of natural selection
What factors may drive natural selection?
• Predation
• Disease
• Competition
How does natural selection occur?
• Random gene mutations result in new NAMED allele
• In certain environments, this new allele may benefit possessor, giving organism a selective advantage
• Possessors are more likely to survive and reproduce (increased reproductive success)
• Advantageous allele is inherited by offspring
• Over many generations, NAMED allele increases in frequency in the population
Directional selection
Organisms with an extreme variation of a trait have a selective advantage
Increased frequency of alleles for extreme trait
Normal distribution curve shifts towards extreme trait
Stabilising selection
Organisms with average variation of a trait have a selective advantage
Increased frequency of alleles for average trait
Normal distribution curve similar but less variation around the mean
Disruptive selection
Organisms with alleles coding for either extreme variation of a trait have a selective advantage
Increased frequency of alleles for both extreme traits
Normal distribution curve splits into two (speciation)
How does speciation occur?
• Reproductive isolation of two populations
• Resulting in very different gene pools
• Members of the populations can no longer interbreed to produce fertile offspring
Allopatric speciation
• Population is split due to geographical isolation
• Leads to reproductive isolation, separating gene pools by preventing interbreeding
• Random mutations cause genetic variation within each population
• Different environments have different selection pressures
• So different alleles are selected for and passed on in each population
• So allele frequencies within each gene pool change over many generations
• Eventually different populations can no longer interbreed to produce fertile offspring
Sympatric speciation
• Population is not geographically isolated
• Random mutations lead to reproductive isolation (eg. Different flowering times), separating gene pools by proving interbreeding
• Different selection pressures act on each population
• So different alleles are selected for and passed on in each population
• So allele frequencies within each gene pool change over many generations
• Eventually different populations can no longer interbreed to produce fertile offspring
What is genetic drift?
• A mechanism of evolution in which allele frequencies in a population change over generations due to chance
• Greatest effect on small populations with no interbreeding with other populations as gene pool is small and chance has greater influence
• Can reduce genetic diversity
