7.3 Evolution

● Genetic factors ○ Mutations = primary source of genetic variation ○ Crossing over between homologous chromosomes during meiosis ○ Independent segregation of homologous chromosomes during meiosis ○ Random fertilisation of gametes during sexual reproduction ● Environmental factors (depends on context - eg. food availability, light intensity)

● Change in allele frequency over time / many generations in a population ● Occurring through the process of natural selection

● Predation, disease and competition for the means of survival ● These result in differential survival and reproduction, ie. natural selection

• Mutations: Random gene mutations can result in [named] new alleles of a gene 2.Advantage: Due to [named] selection pressure, the new allele might benefit its possessor [explain why] → organism has a selective advantage • Reproductive success: Possessors are more likely to survive and have increased reproductive success 4.Inheritance: Advantageous allele is inherited by members of the next generation (offspring) • Allele frequency: Over many generations, [named] allele increases in frequency in the gene pool

● Organisms with alleles coding for average / modal variations of a trait have a selective advantage (eg. babies with an average weight) ● So frequency of alleles coding for average variations of a trait increase and those coding for extreme variations of a trait decrease ● So range / standard deviation is reduced

● Organisms with alleles coding for one extreme variation of a trait have a selective advantage (eg. bacteria with high resistance to an antibiotic) ● So frequency of alleles coding for this extreme variation of the trait increase and those coding for the other extreme variation of the trait decrease

● Organisms with alleles coding for either extreme variation of a trait have a selective advantage ● So frequency of alleles coding for both extreme variations of the trait increase and those coding for the average variation of the trait decrease ● This can lead to speciation

• Reproductive separation of two populations (of the same species) • This can result in accumulation of differences in their gene pools • New species arise when these genetic differences lead to an inability of members of the populations to interbreed and produce fertile offspring

• Population is split due to geographical isolation (eg. new river formed) • This leads to reproductive isolation, separating gene pools by preventing interbreeding / gene flow between populations • Random mutations cause genetic variation within each population • Different selection pressures / environments act on each population • So different advantageous alleles are selected for / passed on in each population • So allele frequencies within each gene pool change over many generations • Eventually different populations cannot interbreed to produce fertile offspring

• Population is not geographically isolated • Mutations lead to reproductive isolation, separating gene pools by preventing interbreeding / gene flow within one population, eg. ● Gamete incompatibility ● Different breeding seasons (eg. different flowering times) ● Different courtship behaviour preventing mating ● Body shape / size changes preventing mating • Different selection pressures act on each population • So different advantageous alleles are selected for / passed on in each population • So allele frequencies within each gene pool change over many generations • Eventually different populations cannot interbreed to produce fertile offspring

● Genetic drift = a mechanism of evolution in which allele frequencies in a population change over generations due to chance ● As some alleles are passed onto offspring more / less often by chance ○ Regardless of selection pressures and whether alleles give a selective advantage ● So strongest effects in small populations as gene pool is small and chance has a greater influence ○ Eg. when a population is sharply reduced in size (bottleneck effect) ○ Eg. when a small, new colony forms from a main population (founder effect) ● This can reduce genetic diversity - some alleles can become fixed or lost entirely