A) DISTINGUISH BETWEEN HOMOLOGOUS AND ANALOGOUS STRUCTURES. (2M) B) DISCUSS THE VARIOUS EVIDENCES THAT SUPPORT ORGANIC EVOLUTION (18MKS)

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a) Distinguish between Homologous and analogous structures. (2m)
  • The evolution of homologous structures i.e structures of same embryonic origin but have become modified in the course of evolution to exploit a different ecological niches hence become structurally different; e.g. Pentadactyl limb structure in vertebrates;
  • The evolution of analogous structures i.e structures of different embryonic origin but have become modified in the course of evolution to exploit a similar ecological niche hence became structurally similar; 

b) Discuss the various evidences that support organic evolution(18mks)
Palaentology/fossil records;
  • Fossils are past material remains of ancestral organisms accidentally preserved in some naturally occurring materials e.g. sedimentary rocks.
  • They provide a direct evidence of gradual change from one type of organism to another;
  • Their age is determined by radioactive Carbon 14 dating; used to construct evolutionary history of development of a certain – organism; e.g. human skull, horse limbs e.t.c
Comparative anatomy
  • Members of a phylum or group shows similarities in structures or organs performing similar functions;
  • Homologous structure/divergent evolution; same embryonic origin but performing different functions; arose by divergent evolution e.g. pentadactyl limb of vertebrates.
  • Analogous structure/convergent evolution; structures with different embryonic origins but evolved to perform similar functions due to exploitation of different ecological niches; They evoked by convergent evolution e.g. wings of birds and those of insects.
  • Vestigial structures; those which in the the course of time have become less functional; hence reduced in size and adaptation modifications.
Comparative embryology
  • Study of embryos in their early stages of development e.g. those of vertebrates look similar; the more the similarity the more the evolutionary relationship.
Cell biology and biochemistry;
  • Structure and functioning of cells from almost all kinds of organisms are basically similar.
  • All Eukaryotic cells contain same cell organelles; such as mitochondria lysosome; ER e.t.c. indicating a common ancestry.
  • Cells with greater similarity in their various chemical composition indicate a more recent common ancestry.
Geographical distribution
  • Initially there was one land – mass (pangae); split and parts drifted away forming the present continents;
  • After the split members of some species occupying originally similar regions were isolated by barriers and evolved along their own line for instance illamas in S. America, Leopards and Cheetahs in Africa.

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