6.1.2(g) - Role of Isolating Mechanisms

selection pressures

density dependant

  • competition

  • predation

  • parasitism

  • communicable disease

  • food availability

density independant

  • climate change/climate

  • natural disasters

  • seasonal changes

  • human activities (deforestation/habitat destruction/pollution)

gene pool: all genes and their different alleles present in an interbreeding population (i.e. a species)

  • some populations of same species may be geographically isolated from each other so for the same speces there can be different gene pools

genome: whole of genetic information of organism i.e. all the genes/alleles that an organism has in its cells

reproductive isolation

for new species to form individuals must be reproductively isolated from one another (i.e. unable to interbreed + produce fertile offspring)

prezygotic isolation

  • geographical isolation (large scale or microhabitat)

    • e.g. cichlid fish found in east african lakes, isolated most of time except for in rainy season

  • temporal isolation

    • organisms reproduce and produce gametes at different times e.g. rainbow trout (spring) + brown trout (autumn)

  • behavioural isolation

    • differing courtship rituals even between individuals that are v close to eachother (sympatric) e.g. birds of paradise

  • morphological isolation

    • e.g. shape of genitalia

post-zygotic isolation

mating may occur but fertile/viable offspring are unable to be formed

  • hybrid stability

    • e.g. ligers/tigons, mules, zedonk, zorse

    • chromosome numbers may not match/chromosomes not homologous so gametes cannot be formed by meiosis

  • hybrid inviability

    • zygote unable to develop (too many mismatched genes/genetically too different)

  • hybrid breakdown

    • F1 generation fertile, 2nd generation not

sympatric speciation by polyploidy

  • speciation can occur when individuals not geographically isolated (sympatric)

  • much more common in plants than animals

  • (post zygotic mechanisms affect animal development)

  • diff species of plant hybridise often, due to pollen released into wind/on insects

  • hybrids often infertile but reproduce asexually initially

  • reproductively isolated as cannot interbreed with parental species

  • chromosome doubling events due to failure of sister chromatids to separate during meiosis (nondisjunction)…due to spindle formation failure results in diploid gametes then polyploidy

  • offspring produces now reproductively isolated from orig. species as chromosomes number now diff.

  • if chromosome no. is odd they will be sterile hybrid but second doubling event could occur to give even no. of chromosomes again.

  • many plants polyploids (possibly 50-70%)

polyploidy in plants

  • Genus Allium incl. onions, garlic + leeks, chives are NAMED EXAMPLE

    • many species polyploids + oft. reproduce asexually

      polyploidy gives allelic diversity + hybrid vigour.

      (better growth) and so may give selective advantage

    • Allium canadense is wild onion from N.America, 2n=14 but variants e.g. 2n=28 and other Alliums e.g. Lithuania: one is 2n=16 and one is tetraplod w/ 32 chromosomes

    • Cultivated potato Solanum tuberosum 2n=48 is polyploid of smaller Solanum brevidens (2n=24)

    • bread wheat is hexaploid

animal polyploids?

  • red viscacha (Tympanoctomys barrerae) that has chromosome no. 102. sperm cells are 2x normal size

  • thought to be tetraploid hybrid of Octomys mimax (2n=52)

  • rodent is from S.America and on red list (extinction risk)