Q1.
But how can the same process lead both to similarity (an animal gives birth to similar individual, such as a cow gives birth to a cow) and variation (shows different characteristics like difference in colour and height of individuals of some kind)?
Answer
Because reproduction passes on a complete set of instructions — but not always the same complete set.
Where the similarity comes from. Genes are the instruction manual for building a body, and every offspring receives a full set for its own kind. That is why a calf grows into a cow and a kitten into a cat: the instructions for making blood, bone, muscle and skin in a cow are all there, and they are cow instructions.
Where the variation comes from. Two things:
- In sexual reproduction each gamete carries only half the parent's instructions, and it is a different half each time. When a sperm and an egg join, that particular mixture has never occurred before — which is why one child may inherit a nose like the mother's and another eyes like the father's, and why brothers and sisters look different.
- Small changes can also appear in the instructions as they are passed down.
Why it happens: Similarity and variation are not two processes fighting each other; they are the same copying process seen at two levels. At the level of the kind, the manual is passed on whole — so a cow gives birth to a cow. At the level of the individual, the manual is reshuffled — so no two cows are quite alike. Life needs both: without similarity a kind would not persist, without variation it could never adapt.
Did you know? This also explains the two types of reproduction the chapter goes on to describe. In asexual reproduction there is only one parent, so there is no reshuffling and the young are near-exact copies. In sexual reproduction there are two, so variation is built in.