Question

(i) Which term is given to a sequence of gradual changes over millions of years in which new species are produced?

(ii) Which of the two processes i.e., sexual reproduction or asexual reproduction, brings about marked variations in the offspring?

Answer 1

(i) Definition of evolution:

• Evolution is the process through which new species are formed from pre-existing ones.
• Genetic differences accumulate in the existing species over time which leads to the evolution of new species.
• The process of evolution may take millions of years.
• As this process deals with living organisms, its other name is organic evolution.

(ii) Sexual reproductions show marked variations in the offspring.

Sexual reproduction

• Sexual reproduction is a type of reproduction that involves two parents of the opposite gender.
• The parents produce gametes through meiosis which fuse to form a zygote.

Reasons for the occurrence of marked variations in sexual reproduction are:

• The gametes are formed by meiosis in sexual reproduction. This causes random segregations of the chromosomes of both parents.
• During meiosis, the genetic material is exchanged between the homologous chromosomes.
• These variations accumulate generation after generation due to sexual reproduction.
• Errors in the replication of DNA play a minor role in creating variations in offspring produced by sexual reproduction.

In asexual reproduction, only minor variations caused due to errors during the copying of DNA (deoxyribonucleic acid) can be seen.