How many pairs of chromosomes are there in a human cell (in $pairs$)?

In which organisms is a higher degree of variation observed?

Some dinosaurs had feathers although they could not fly,but birds have feathers that help them to fly. In the context of evolution,this means that:

Where are the genes located? What is the chemical nature of a gene?

What are the three ways which can produce variant genotypes in the environment?

New species may be formed if
$(i)$ $DNA$ undergoes significant changes in germ cells
$(ii)$ chromosome number changes in the gamete
$(iii)$ there is no change in the genetic material
$(iv)$ mating does not take place