$(a)$ What is geographical isolation?
$(b)$ Illustrate the formation of a species with the help of an example where individuals become very different from each other and are no longer capable of reproduction among themselves.

(N/A) Geographical isolation refers to the separation of a population by physical barriers such as mountain ranges or large bodies of water,which prevents gene flow between the separated groups.
$(b)$ Consider a large population of red beetles living in a mountain area covered with green bushes. Various sub-populations exist in the neighborhood. Individuals within a sub-population reproduce sexually,and numerous variations accumulate over generations.
If a river forms between these sub-populations,they become geographically isolated:
$(i)$ Variations continue to accumulate independently in each isolated sub-population.
$(ii)$ Over many generations,processes like genetic drift and natural selection cause these sub-populations to diverge significantly.
$(iii)$ Eventually,the genetic differences become so great that individuals from the two groups can no longer interbreed,leading to the formation of a new species.