populations and evolution

• $p+$$q=$$1$

  p= frequency of dominant alleles

  q= frequency of recessive alleles
• $p^2 +$$2pq +$$q^2 =$$1$

  $p^2$ = homozygous dominant

  $2pq$ = heterozygous

  $q^2$ = homozygous recessive
• phenotypic variation is differences in physical characteristics between the same species
• phenotypic variation can be genetic or environmental
• genetic variation is differences in DNA base sequence between individuals within a population of species
• genetic variation can be transferred from one generation to the next and can generate phenotypic variation
• genetic variation is due to mutation and new combination of alleles
• mutation can generate new generation of alleles that might be harmful, no effect on phenotype or remain not expressed at all
• genetic variation due to new combination of alleles is due to the gamete of individual. this happens due to

  • crossover
  • independent assortment
  • random fusion gametes during fertilisation
• cross over is non sister chromatids exchanging alleles during meiosis prophase 1. cross over point is called the chiasmata and sections of DNA are exchanged
• independent assortment is the production of different combinations of alleles in daughter cells due to random alignment of homologous pairs on the equator during metaphase 1
• natural selection is when individuals with the best adapted phenotype is more likely to survive and pass on beneficial allele to offspring so that beneficial alleles increases in frequency over time and generations
• environmental and genetic factors effect the ability of an organism to survive
• environmental abiotic and biotic factors effect the ability for a species to survive
• environmental factors which affect chance survival are called selection pressures
• individuals with better adapted phenotypes for survival have higher fitness to survive and pass on beneficial allele
• fitter individuals are better adapted to their environment
• allele frequency and phenotype frequency in a population can change due to isolation. this is when individuals in a species become isolated from others
• genetic drift is random and can be the phasing out of a particular allele
• genetic drift in small populations, change can affect alleles that get passed onto next generations. overtime some alleles are lost of favoured due to random chance, not natural selection
• selective advantages are alleles which give a better chance of survival
• non favourable alleles will decrease in frequency in a gene pool
• types of selection

  • stabilising
  • disruptive
  • directional
• stabilising selection

  • keep allele frequencies constant
  • birth weight
  • favours the average not extremes
• directional selection

  • produces gradual change in allele frequency over generations
  • this usually happens due to selection pressures, environment and new alleles appearing in a gene pool that is advantages
  • selection against a specific phenotype
  • prefers more extreme phenotypes
• disruptive selection

  • maintains high frequency of two different phenotypes/alleles
  • this occurs in a environment with variation
  • selection against the average phenotypes
  • eg different beak sizes on birds
• evolution is continuous speciation but over a long period of time and generations
• reproductive speciation is when a group of individuals are genetically and reproductively separated from pre existing species population. this is due to prevention of breeding of individuals in a population so alleles aren't exchanged
• reproductive speciation can be due to

  • seasonal changes
  • mechanical changes
  • behavioural changes
• seasonal changes cause reproductive speciation due to different reproductive timings in the year
• mechanical changes cause reproductive speciation because individuals may have developed changes in the shape of genitals preventing successful mating
• behavioural changes can cause reproductive speciation as courtship behaviours may not attract a mate