7.3

• Evolution?

  
  

  Evolution is a change in allele frequencies over time / many generations in a population occurring through the process of natural selection
• 4. These organisms are more likely to survive and reproduce, producing more offspring and passing on their favourable allele(s) to the next generation i.e. differential levels of survival and reproductive success

  • E.g. those able to hide from / escape predators, compete for light / prey / mates, or resist a disease

  5. Allele frequencies in gene pool change over many generations (time)
• The effects of stabilising, directional and disruptive selection

  Directional selection

  Selective pressures / environment favours individuals with a favourable combination of alleles one direction from the mean

  Mean shifts in direction of favourable allele / phenotype

  • Example: Pepper moth. Originally a higher population of white moths. Change in environment – pollution resulted in a higher population of black moths
• Stabilising selection

  Selective pressures favour the mean / act against the two extremes of a characteristic

  Individuals with extreme phenotypes less likely to survive à standard deviation smaller over time

  Mean stays the same

  • Example: Clutch size in birds. Robins lay eggs. If they lay too many eggs, chicks become malnourished and die. If they lay not enough eggs, they may all die and not hatch.
• Disruptive selection

  Selection against the mean

  Population becomes phenotypically divided – favours both extremes of phenotypes

  • Most important type of selection for evolutionary change as could result in two separate species
• Speciation is the development of a new species from an existing species

  Formation of a new species from an existing species…

  Reproductive separation of two populations (of the same species) can result in the 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

  Geographical isolation

  = Separate gene pools; no interbreeding / gene flow (between populations)

  Mutations create genetic variation in each population

  Different selection pressures (e.g. predation, disease, competition) act on each population

  • Leading to natural selection of different favourable alleles / characteristics
• 
  

  Differential survival and reproductive success

  Leads to change of allele frequencies within gene pools (favourable allele increases) over a long time

  • Members of different populations can’t interbreed to produce fertile offspring = new species arises from existing species = speciation
• Sympatric speciation

  Populations aren’t geographically isolated / population in the same area

  Genetic variation within the population due to mutations

  • Resulting in a mechanism that makes individuals reproductively isolated (gene flow is restricted), for example…
• Gamete incompatibility

  Temporal – different breeding/mating seasons

  Behavioural – different courtship behaviour preventing mating

  Mechanical – incompatible genitalia

  Different selection pressures operate

  Leads to change of allele frequencies within gene pools / divergence of gene pools

  • Members of different populations can’t interbreed to produce fertile offspring = new species arises from existing species = speciation
• Genetic drift and its importance in small populations

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

  Strongest effects in small populations as chance has a greater influence

  Unlike natural selection, genetic drift doesn’t take into account how favourable or harmful an allele is

  • Genetic drift has major effects when a population is sharply reduced in size (bottleneck effect) or when a small, new colony forms from a main population (founder effect)
• wide range of variation in phenotype is due to ?

  • mutations: primary source of genetic variation
  • Gene mutation; change in base sequence of DNA à new allele
  • Crossing over between homologous chromosomes during meiosis
  • Independent segregation of homologous chromosomes during meiosis
  • Random fertilisation of gametes during sexual reproduction
  • Environmental factors -Example: climate, food, lifestyle – apply these to the specific exam question