20.

(a) Name and define the three types of natural selection.
(b) State the following:
(i) Hardy Weinberg’s principle
(ii) Theory of recapitulation

a. 

Three types of natural selection:

(i) Directional Selection: Any one phenotype produces a regular change in a gene pool of a population in one direction with respect to one or some specific characteristics-Progressive.

(ii) Disruptive or diversifying selection: The selection breaks up a previously homogenous population into several different adaptive norms. It simultaneously favours individuals at both extremes of the distribution curve/eliminates intermediate types/ population represented by both extreme phenotypes.

(iii) Stabilizing Selection: Favours average or normal phenotypes and eliminates variants or new mutation. It favours homozygosity and mostly intermediate. Acts as a Balancing selection and favours phenotypic features of organism coincide with optimal environmental conditions or one which is best adapted.




b. (i) Hardy Weinberg’s Principle: States that the allele and genotype frequencies in a population will remain constant from generation to generation in the absence of other evolutionary factors or sources/Mathematical Expression (p2 + 2pq + q2 = 1).

(ii) Theory of Recapitulation: Ontogeny (embryonic development of an organism) recapitulates phylogeny of their ancestors in an abbreviated form. / Embryos in their development repeat the evolutionary history of their ancestors.