30 June 2016

#141 Evolution and Extinction

General theory of evolution: organisms have changed over time.

Molecular comparison between species
1. Comparing amino acid sequences of proteins
Number of difference in the nucleotide sequences measure how closely related the species are.

2. Comparing nucleotide sequences of mitochondrial DNA
Human mtDNA:

  • inherited through the female line
  • zygote contains the mitochondria of the ovum
  • mtDNA is circular so can't undergo any form of crossing over, changes in nucleotide sequence can only arise by mutation
Different human populations show differences in mitochondrial DNA sequences. This provides evidence for the origin of different populations --> 'molecular clock' hypothesis:
  • there is a constant rate of mutation over time
  • the greater the number of differences in the sequence of nucleotides, the longer ago those individuals shared a common ancestor
  • 'clock' can be estimated from fossil evidence

Species can become extinct through a variety of mechanisms.
- climate change
- increased competition from better adapted species
- human causes:
  • loss of habitat: draining wetlands, cutting down rainforests, pollution of air, water and soil
  • killing: for sports or for food
- mass extinctions:
  • sudden change in the environment: large asteroid colliding with the Earth

  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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