From the AS topic Genetic diversity and adaptation you would already know all about natural selection, advantageous versus detrimental traits, etc.
The train of thought leading to natural selection includes these key points:
1. Individuals within a population exhibit variety of phenotypical traits caused by both their alleles and the environment.
Primarily the source of this variation is mutation. Secondarily it is meiosis and the random fertilisation of gametes in the case of sexual reproduction.
2. The balance of survival and reproduction is affected by factors including predation, disease and competition. Some appearances and behaviour can attract more predators while others such as camouflage can avert them.
Disease can impede survival and reproduction, while competition enables hidden traits that might have gone unnoticed or been “neutral” before to come in handy when unforeseen selection pressures arise. If the positive outcome of such competition, such as resources needed for survival, are limited relative to the population seeking them, then competition acts further to select certain traits.
3. Any favourable traits controlled by alelles will end up in more offspring, thereby shifting the alelle frequency and over time, the entire gene pool of a population or species.
Types of selection
We looked at stabilising and directional selection previously in AS.
There is a third type called disruptive selection. Instead of shifting the traits towards an end, or towards a middle ground, disruptive selection splits the pool down the middle, where both extremes of a trait are favourable, but not a middle value.
An example of this is an original population of purple individuals which stand out quite a lot amongst red and blue flowers in a field. They will end up shifting towards either red or blue, but not staying purple as this attracts predators.
Little devil bats. Birds? Anyway.
What is at the heart of new species formation? It all starts with a single population of a species which for whatever reason (off-spec: genetic bottlenecks, founder effect, etc.) ends up being split geographically to the point where no interbreeding occurs for a certain length of time.
Given that the two habitats are different, the individuals in each population will adapt differently to counteract different selection pressures. Say for example the ants in the forest experience a warmer and more nutrient-rich surrounding compared to the emigrated ants on a nearby, although disconnected, beach.
The adaptations acquired by both populations over a long time will get increasingly disparate. When these pass a threshold, the two populations can no longer interbreed, even if the opportunity were given (due to excessive genetic difference). They have now become separate species! This process is called speciation.