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Origin of Life & Evolution

  • During his exploration of the Galapagos Islands, Darwin noticed that there were many varieties of finches in the same island.

  • They varied from normal seed eating varieties to those that ate insects.

  • This process of evolution starting from a single point and radiating in different directions is called adaptive radiation.

  • The other example for this is the evolution of the Australian marsupials from a single ancestor. Placental mammals also exhibit similarities to their corresponding marsupial. Example: placental wolf and the Tasmanian wolf

  • When more than one adaptive radiation occurs in an isolated geographical area, the phenomenon is called convergent evolution.

  • The frequency of occurrence of alleles of a gene in a population remains constant through generations unless disturbances such as mutations, non-random mating, etc. are introduced.

  • Genetic equilibrium (gene pool remains constant) is a state which provides a baseline to measure genetic change.

  • Sum total of all allelic frequencies is 1.

  • Individual frequencies are represented as p and q such as in a diploid, where p and q represent the frequency of allele A and a.

The frequency of AA is p2, that of aa is q2, and that of Aa is 2pq.

  • Hence, p2 + 2pq + q2 = 1, which is the expansion of (p + q)2.

  • When the frequency measured is different from that expected, it is indicative of evolutionary change.

  • Hardy-Weinberg equilibrium is affected by

  • gene flow or gene migration
  • genetic drift (changes occurring by chance)
  • mutation
  • genetic recombination
  • natural selection

  • Sometimes, the change in allele frequency is so prominent in the new sample of population that they become a different species and the original drifted population becomes the founder. This effect is called founder effect.

  • The advantageous mutations that help in natural selection over the generations give rise to new phenotypes and result in speciation.

Natural selection causes allele frequencies of a population to change. Depending upon which traits are favoured in a population it can produce three different results.

(1) Stabilizing selection - If both the smallest and largest individuals contribute relatively fewer offspring to the next generation than those closer to average size do, then stabilizing selection is operating. It reduces the variation but does not change mean value.

(2) Directional selection – If individuals at one extreme of the size distribution e.g. (the larger ones) contribute more offspring to the next generation then the other individuals do, then the mean size of individuals in the population will increase. In this case directional population is operating. If directional selection operates for many generations, an evolutionary trend within the population results.

(3) Disruptive selection- When natural selection simultaneously favours individuals at both extremes of the distribution, disruptive selection is operating. As a result we can see two peaks in the distribution of a trait.

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