(a) Define the term organic evolution (b) Outline the contribution of (i) Larmack (ii) Darwin to organic evolution (c) List three evidences of organic evolu...

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(a) Organic evolution

Organic evolution is the gradual change in living organisms from simple earlier forms to more complex and better-adapted forms over a very long period of time; that is, the process by which present-day organisms have descended, with modification, from earlier ancestral forms.

(b) Contributions to organic evolution

• (i) Lamarck: proposed the theory of the inheritance of acquired characteristics. He held that characters an organism develops during its lifetime through use and disuse of parts (for example the long neck of the giraffe from constantly stretching to reach high leaves) are passed on to its offspring, and that in this way organisms change over generations. He was the first to put forward a clear theory of how evolution could occur.
• (ii) Darwin: proposed the theory of evolution by natural selection (survival of the fittest). He observed that organisms produce more offspring than can survive, that there is variation among individuals, and that there is a struggle for existence. Individuals with favourable variations are better adapted, survive and reproduce, passing on their favourable characters, while the less-adapted die out. Over many generations this leads to new, better-adapted species.

(c) Three evidences of organic evolution

• Fossil records (palaeontology), which show a graded series of extinct forms leading to modern ones.
• Comparative anatomy, especially homologous organs (for example the pentadactyl limb of different vertebrates) showing common ancestry.
• Comparative embryology, in which the early embryos of different vertebrates are strikingly similar.
• Vestigial structures (such as the appendix in man) and geographical distribution of organisms. (Any three.)

(d) Role in the formation of new species

• (i) Mutation: a mutation is a sudden heritable change in the genetic material (genes or chromosomes). It produces new inheritable variations in a population. If a mutation is favourable, it is preserved by natural selection and spreads, so that over time the population becomes different from the original and a new species may arise.
• (ii) Isolation: isolation (for example by a geographical barrier such as a mountain, river or sea) separates a population into groups that can no longer interbreed. Each isolated group accumulates its own variations and adapts to its own conditions. In time the groups become so different that even when brought together they can no longer interbreed, and thus new species are formed.