DG7- evolution

• Evolution is the changes in organisms over generations as a result of genomic variation
• Natural selection is the non random increase in the frequency of DNA sequences that increase survival and the non random reduction in the frequency of deleterious DNA sequences
• Organisms with advantageous alleles survive and reproduce, passing their advantageous alleles on to their offspring
• Natural selection is more rapid in prokaryotes
• Prokaryotes can exchange genetic material horizontally resulting in faster evolutionary change than in organisms that only use vertical transfer
• Horizontal gene transfer is where genes are transferred between individuals in the same generation
• Vertical gene transfer is where genes are transferred from parent to offspring as a result of sexual or asexual reproduction
• Stabilising, directional and disruptive selection can all result in changes in the phenotype frequency of a species
• In stabilising selection an average phenotype is selected for and extremes of the phenotype range are selected against
• In directional selection one extreme of the phenotype range is selected for
• In disruptive selection two or more phenotypes are selected for
• Speciation is the generation of new biological species by evolution as a result of isolation, mutation and selection
• Isolation barriers play a vital role in speciation- preventing gene flow between sub-populations
• There are three types of isolation barrier- geographical, ecological and behavioural
• Features such as deserts, oceans, rivers and mountain ranges result in geographical isolation of populations
• Although populations are not geographically isolated from each other they can be ecologically isolated by occupying different habitats or breeding areas
• Behavioural isolation results when populations that are not geographically isolated develop different courtship and mating behaviours- preventing them from interbreeding
• Stage one of speciation

  Different mutations occur on either side of the isolation barrier
• Stage two of speciation

  Different selection pressures operate on either side of the isolation barrier
• Stage three of speciation

  Some of these mutations will be favourable and provide a selective advantage
• Stage four of speciation

  As a result of natural selection, individuals with these advantageous mutations will survive and pass their alleles to their offspring
• Stage five of speciation

  After many generations, populations from either side of the barrier can no longer interbreed to produce fertile offspring
• A species is a group of organisms capable of interbreeding and producing fertile offspring and which does not normally breed with other groups
• Geographical isolation barriers lead to allopatric speciation
• Sympatric speciation is when new species arise despite occupying the same geographical area
• Ecological and behavioural isolation barriers lead to sympatric speciation