B18 populations + evolution

• gene pool is all of the alleles of all of the genes of all of the individuals in a population at a given time
• allelic frequency is the number of times an allele occurs within the gene pool
• the hardy-weinberg equation is used to calculate the frequencies of alleles of a particular gene in a population
• the hardy weinberg equation is:

  p^2 + 2pq + q^2 = 1 and p + q = 1
• in the hardy weinberg equation,

  p represents dominant allele frequency

  q represents recessive allele frequency

  p^2 represents homozygous dominant frequency

  q^2 represents homozygous recessive frequency

  2pq represents heterozygous frequency
• the hardy-weinberg principle is: the proportion of dominant and recessive alleles of any gene in a population remain the same from one generation to the next
• the hardy-weinberg principle has 5 assumptions:

  • no mutations arise
  • the population is isolated
  • no selection
  • large population size
  • mating is random
• individuals within a population show a wide range of variation in phenotype, due to genetic and environmental factors
• genetic variation arises as a result of:

  • mutations
  • meiosis
  • random fertilisation of gametes
• mutations are sudden, random changes to genes and chromosomes which may or may not be passed onto the next generation
• meiosis is a form of nuclear division which produces new combinations of alleles before they pass into the gametes
• random fertilisation of gametes produces new combinations of alleles so the offspring are different to the parents
• environmental variation arises as a result of many different factors, depending on the organism
• in most cases, variation is due to the combined effects of genetic and environmental factors, and their specific effects are hard to separate
• selection pressures are the environmental factors which limit the population of a species
• selection pressures vary between species
• selection pressures determine the frequency of each allele in the gene pool
• evolution by natural selection depends on a number of factors:

  • organisms produce more offspring than can be supported by the available food, space, light, etc
  • there is genetic variety within the populations of all species
  • there are a variety of phenotypes that selection operates against
• the process of evolution by natural selection is:

  • there is a variety of phenotypes in a population
  • an environmental change occurs, causing the selection pressure to change
  • some individuals have advantageous alleles so are more likely to survive and reproduce
  • advantageous alleles are passed down to offspring
  • frequency of alleles changes over time, leading to evolution
• selection is the process by which individuals that are better adapted to their environment are more likely to survive and reproduce, therefore passing on advantageous alleles
• there are three types of selection:

  • directional
  • stabilising
  • disruptive
• directional selection occurs when environmental conditions change
• stabilising selection occurs when environmental conditions are constant over long periods of time
• disruptive selection occurs when an environment has two or more distinct forms
• in directional selection, phenotypes most suited to the new conditions, furthest from the mean in one direction, are favoured, so are more likely to survive and reproduce, so over time the mean moves in the direction of these individuals
• in stabilising selection, phenotypes closest to the mean are favoured, so are more likely to survive and reproduce, so over time the mean stays the same but the range decreases as extreme phenotypes are selected against
• in disruptive selection, phenotypes furthest from the mean in both directions are favoured, so are more likely to survive and reproduce, so over time the two phenotypes split, forming two separate species with smaller ranges
• speciation is the evolution of new species from existing ones
• there are two types of speciation:

  • allopatric
  • sympatric
• a species is a group of individuals that have a common ancestry, and share the same genes but different alleles, and are capable of breeding with one another to produce fertile offspring
• allopatric speciation is the evolution of new species when two populations become geographically isolated
• in allopatric speciation, if environmental conditions either side of the physical barrier are different, natural selection will influence the populations differently, so they will evolve differently, eventually becoming two separate species
• sympatric speciation is the evolution of new species when two populations become reproductively isolated
• in sympatric speciation, if a population becomes less likely to mate with another population, different alleles will be passed down in the different populations, so they will evolve differently, eventually becoming two separate species
• once two populations evolve to a point where they can no longer reproduce to produce fertile offspring, they are two separate species