Here is an all-time classic example. The most frequent initial moth colour in a population landing on tree trunks was dark, to match that of the tree trunks. Few moths could get away with being light-coloured. Once the tree trunks were painted white, the former moths became very apparent to predators, and so the light-coloured moths evaded predation much better and survived to reproduce. Essentially, the tables had turned!
This resulted in the allele for light colour to spread and become the most frequent compared to that for dark colour. The latter sharply dropped in frequency and became the minority.
This is an example of directional selection. It tends towards an extreme, either the light-coloured or the dark-coloured, depending on scenario. These kind of polymorphic populations that exhibit multiple phenotypes enable the investigation of selection acting upon different individuals.
Selection can also tend towards a “happy medium” and avoid either extreme. This is stabilising selection. If really small lions don’t survive long, but really large lions can’t supply themselves enough food, then the average lions are selected for and achieve the highest frequency.
Directional selection also takes place when antibiotics are used against bacteria. The adaptive pressure favours bacteria that have the antibiotic resistance gene and can survive the hostile environment.
On the other hand, a scenario such as human birth weight showcases stabilising selection. The average weight is large enough to keep the newborn healthy and increasingly able to survive independently, but small enough to enable the actual birth.
Natural selection therefore results in species increasingly and consistently adapted to their environment via anatomical, physiological or behavioural changes.
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