30 June 2016

#142 Species and speciation

Isolating mechanisms can lead to the accumulation of different genetic information in populations, potentially leading to new species.

Species and speciation
species: a group of organisms with
  • similar morphological, physiological, biochemical and behavioural features
  • can interbreed to produce fertile offspring
  • reproductively isolated from other species
speciation: the production of new species

1. Allopatric speciation
- geographical isolation
- population of species split and move to different areas
- each new population experiences different selective pressures --> features change over time, mutations occur
- when the different populations are reintroduced, they can no longer interbreed
--> new species have evolved

2. Sympatric speciation
- ecological and behavioural separation
- sympatric speciation usually occurs through polyploidy

  • polyploidy organism: has more than 2 complete sets of chromosomes
  • happens when meiosis goes wrong when forming gametes
  • tetraploidy: 2+2 = 4; tetraploids are often sterile as 4 sets of chromosomes try to pair up during Meioisis I and get muddled up --> can reproduce asexually; usually happens in plants
  • triploidy: 1+2 = 3; triploidy are always sterile as 3 sets of chromosomes can not be shared evenly between daughter cells
  • the original diploid plant and tetraploid plant can no longer interbreed --> new species formed

Kind of polyploidy
autopolyploid: all sets of chromosomes from the same species
- allopolyploid: different sets of chromosomes from different but related species
Meiosis happens more easily in an allopolyploid than an autopolyploid (e.g.: allotetraploid and autotetraploid) because the chromosomes from each species are not quite identical.
--> allotetraploid can be fertile

Reproductive isolation
- the inability of 2 groups of organisms of the same species to interbreed
- due to geographical separation or behavioural differences

1. Prezygotic isolation

  • individuals not recognizing each other as potential mates
  • animals being physically unable to mate
  • incompatibility of poleen and stigma
  • inability of a male and female gamete fusion

2. Postzygotic isolation
  • failure of cell division in the zygote
  • non-viable offspring
  • viable, but sterile offspring
*postzygotic isolation is more wasteful of energy

  Syllabus 2016-2018

17.3 Evolution

Isolating mechanisms can lead to the accumulation of different genetic information in populations, potentially leading to new species. 

Over prolonged periods of time, some species have remained virtually unchanged, others have changed significantly and many have become extinct.

a) state the general theory of evolution that organisms have changed over time 

b) discuss the molecular evidence that reveals similarities between closely related organisms with reference to mitochondrial DNA and protein sequence data 

c) explain how speciation may occur as a result of geographical separation (allopatric speciation), and ecological and behavioural separation (sympatric speciation) 

d) explain the role of pre-zygotic and post-zygotic isolating mechanisms in the evolution of new species 

e) explain why organisms become extinct, with reference to climate change, competition, habitat loss and killing by humans

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