Speciation:

Process by which populations evolve into different species.

Species:

Group of organisms that can interbreed and produce fertile offspring

Hybrids:

Offspring from cross breeding of 2 different species. They are infertile

Evolution:

Change of heritable characteristics of a species across generations.

Reproductive isolation:

2 populations of the same species can no longer successfully mate together

Post zygotic isolation:

Parents can form a zygote but the zygote is unfit.
ex: no development of embryo, birth always resulting in miscarriages, too big to be born

Pre zygotic isolation:

Parents do not form an zygote together.
This can be due to behavioural changes within the species or due to being separated by huge distances or physical barriers.

Allopatric speciation:

When 1 species diverges into 2 or more due to geographic reasons

It is of 2 types:

Peripatric:

When the populations are separated by barriers

Parapatric:

When the population is spread over a large area

Sympatric speciation:

When 1 species diverges into 2 or more without geographic reasons