18 Isolation & Speciation

  • Created by: lee8444
  • Created on: 06-02-20 19:07

Allele frequencies and how selection affects them

  • In theory, any organism sexually mature can reproduce with any other and alleles can all be combined with any other allele within the gene pool
  • The number of times an allele occurs within a gene pool is referred to as the allelic frequency
  • The allelic frequency is affected by natural selection
  • Natural selection is due to environmental factors
  • Environmental actors have no effect on the probability of a particular mutation allele arising, they just affect the frequency of alleles that already occur
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  • Speciation is the evolution of a new species from an existing one
  • A species is a group of individuals that have a common ancestry and so share the same genes but different alleles and can, therefore, breed with each other
  • Members of a species are reproductively separated from other species
  • Within a species, there are one or more populations
  • Reproduction mainly happens within a population however they sometimes breed with an individual from another population
  • If populations are separated then different mutations will happen and depending on the new environments, different mutations will be advantageous
  • This is because of different selection pressures
  • This changes the allelic frequency of the alleles
  • This is called adaptive radiation
  • Eventually, the two populations will be unable to successfully breed with each other

Genetic drift

  • This is when a few individuals are separated from a population and so the allelic frequencies dramatically change due to chance as the gene pool becomes restricted
  • Likely to cause speciation quickly due to quick change in alleles present
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Allopatric Speciation

  • Two populations become geographically separated
  • This could be due to a physical barrier preventing interbreeding such as oceans, rivers, mountains and deserts. This will change depending on the type of species
  • If the environmental conditions are different on either side of the barrier, different mutations will be either advantageous or disadvantageous
  • This will lead to natural selection as the selection pressures are different either side of the barrier
  • This can take hundreds or thousands of generations to happen as this is not a quick process
  • This happened in the Galapagos Islands as one Galapagos finch colonised the Island and without competition, thrived and populations became established. Each population on the different islands evolved slightly differently depending on the selection pressures such as the beaks changing depending on the types of diets offered by each island. They became so different that they can no longer interbreed
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Sympatric Speciation

  • This happens when speciation happens within a population within the same area
  • This happens in the apple maggot fly
  • Originally it only laid its eggs in the native American hawthorns
  • When apple trees were introduced, flys also laid their eggs here
  • Females tended to lay their eggs in the same plant as where they developed and males tend to look for mates on the same plant that they developed
  • This caused sympatric speciation as hawthorns males mated with hawthorns females and the same happened in apples
  • Whilst they are not yet separate species, they are very genetically different which in the long run could result in speciation
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Isolating Mechanisms

  • Geographical - Populations are isolated by physical barriers such as mountains, river, oceans and deserts
  • Ecological - Populations inhabit different habitats within the same area and so individuals rarely meet e.g. lion (grasslands) and tiger (forest) in the same area
  • Temporal - Breeding seasons don't coincide such as types of frogs
  • Behavioural - Courtship behaviour is impacted by mutations which can change colours which are found in sticklebacks
  • Mechanical - Anatomical differences such as a Great Dane and a Chihuahua
  • Gametic - Gametes can be prevented from meeting due to genetic or biochemical incompatibility which happens with pollen landing on stigma with different genetic makeup or sperm that is destroyed by chemicals in the female
  • Hybrid sterility - Hybrids cannot produce viable gametes such as a horse (64) and a donkey (62) leading to a mule with 63 chromosomes
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