How has the free ${O_2}$ been released into the earth's atmosphere whereas there was no free ${O_2}$ at the time of origin of life

Goldschmidt has classified evolution into micro, macro, and mega evolution. Which one of the following is referred to as micro-evolution

The branch of biology dealing with the process how to improve the human race by selective breeding is

Darwin's theory of pangenesis says that passage of factors through generation occurs by production in each cell of the body of

Adaptation of a species is its