speciation

• Natural selection - change in allele frequencies in a population over time as a result of selection pressures and reproduction
• > mutations and migration introduce new alleles into populations.

  > individuals with better adapted characteristics to the environment due to their phenotypes and genotypes are more likely to reproduce and pass on their advantageous alleles

  • 3 main types of selection : stabilising, directional and disruptive
• Stabilising selection


  > occurs when organisms' environment remains unchanged

  > it favours intermediate phenotypes

  
• Directional selection


  > where a single phenotype is favoured causing the allele frequency to continuously shift in one direction

  > used by breeders to produce desirable traits

  
• Disruptive selection


  > favours the two extremes of a phenotype

  > selection pressures select against the average phenotype

  > takes place in an environment that favours more than one phenotype

  
• Genetic drift

  > frequencies of a population change over generations due to chance

  > chance events may include a natural disaster

  • therefore, the alleles that may be lost from the population aren't a result of selection pressures but due to genetic drift

  > occurs in populations with a small gene pool and therefore lack genetic variation

  > can arise after a genetic bottleneck or as a result of the founder effect
• Genetic bottleneck 


  > when a population size shrinks and then increases

  > the genetic diversity within population is reduced

  > there may be a loss of advantageous alleles or disproportionate frequency of harmful alleles placing populations long-term survival at risk

  > after a genetic bottleneck a population may shrink too small so that its fertility is affected leading to extinction
• Founder effect 


  > when a new population is established by a small number of individuals originating from a larger parent population

  > this population is not fully genetically representative of parent population and have a small gene pool

  • may be a result of cultural differences or geographical isolation
  • type of genetic drift
• population - a group of organisms of the same species that can interbreed living in same habitat
• Speciation

  > when two populations of the same species become so genetically different they can no longer interbreed and produce fertile offspring.

  > split into two isolated populations and accumulate different allele frequencies

  > sub-species - when the two populations are different but can still interbreed

  > when there is enough behavioural and physiological changes in populations and they no longer interbreed they have undergone speciation
• Geographical isolation

  Allopatric speciation:

  > populations separated from each other from physical barrier restricting gene flow

  > isolated populations subjected to different selection pressures in two different environments

  > then undergo changes to allele frequencies in their gene pool as a result of mutations, selection and genetic drift

  > two species become so genetically dissimilar that they cannot interbreed to produce fertile offspring
• Reproductive isolation

  Sympatric speciation

  > Genetic changes may also lead to reproductive isolation. A change in their chromosome number may:

  • prevent gamete fusion
  • make zygotes less viable so they fail to develop
  • lead to infertile hybrid offspring with an odd number of chromosomes, so pairing during meiosis cant occur

  > Mating between members of the reproductively isolated populations may be preventing by mutations leading to changes in

  • courtship behaviour e.g time of year for mating or courtship rituals that precede mating
  • animal genitalia or plant flower structure
• Outline how a fossa could have evolved from a much smaller, mongoose-like ancestor

  • allopatric speciation
  • different selection pressure / environmental conditions
  • random mutations
  • those with new feature survive pass on advantageous alleles
  • directional selection
• Explain how sexual reproduction in Hydra leads to genetic variation in the offspring

  • genetic variation is variety of alleles
  • offspring have alleles from more than one parent
  • random fertilisation
  • meiosis produces genetically unique gametes
  • crossing over in p1
  • alleles swapped between non sister chromatids
  • base sequence of chromosomes altered
  • independent assortment in M1
  • sperm of one hydra can fertilise egg of another
  • may have different alleles
  • sperm carried in water
  • could travel and fertilise an unrelated hydra egg
• selection alters allele frequency