7.3 evolution may lead to speciation

• why individuals within a population of a species may show a wide range of variation in phenotype

  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)
• what is evolution

  change in allele frequency over time / many generations in a population

  occurring through the process of natural selection
• factors that may drive natural selection

  predation, disease and competition for the means of survival

  these result in differential survival and reproduction

  eg natural selection
• explain the principles of natural selection in the evolution of populations

  mutations -> random gene mutations can result in [named] new alleles of a gene

  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

  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
• effects of stabilising selection

  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
• effects of directional selection

  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
• disruptive selection

  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
• describe speciation (how new species arise from existing species)

  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
• allopatric speciation

  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
• sympatric speciation

  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, different courtship behaviour preventing mating, body shape preventing mating

  different selection pressures act on each population

  so different advantageous alleles 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 and its importance in small populations

  genetic drift = mechanism of evolution which allele frequencies in a population change over generations due to chance

  some alleles are passed onto offspring more / less often by chance

  regardless of selection pressures and whether alleles give selective advantages

  strongest effects in small populations as gene pool is small and chance has greater influence

  eg when a population is sharply reduced in size or when small new colony forms from a main population

  this can reduce genetic diversity - some alleles become fixed or lost entirely