Charles Darwin was the first person to accurately explain how organisms evolve. Unlike Lamarck's theory of use and disuse, Darwin believed that natural selection is the core mechanism of evolution.
How natural selection works?
The population has different traits that can be inherited from their parents and passed on to their offspring. If there are no selective pressures, the probability of these traits in the next generation would be the same as in the parents, and you can consider the population unchanged. However, selective pressures are always present, because organisms tend to overproduce in order to better perpetuate their lineage. Competition for resources is inevitable, as well as diseases, predators, and changes in the environment. Organisms with certain traits have an advantage in coping with these challenges, making them more likely to survive and successfully reproduce offspring with similar traits. The population with increasing advantageous traits will continue the process of natural selection. From generation to generation, favorable traits help individuals survive and reproduce. And these traits are passed on to more and more offspring until the stable traits present in population.
Stabilizing selection, directional selection and disruptive selection
Stabilizing selection: It occurs when individuals with intermediate trait has higher fitness compared to individuals with extreme traits. This type of selection favors the optimal trait and reduces diversity.
Each animal species has a certain range of sizes, neither too large nor too small. For example, the average height of ancient humans ranged from 1.6 to 1.75 meters. Being too short implies a lack of strength, making one vulnerable to death during hunting or warfare. On the other hand, individuals who are too tall require additional food to sustain their metabolism, which could lead to starvation in ancient times when food was scarce. Therefore, individuals with moderate stature were more likely to survive.
Directional selection: The average value of a trait shifts in a particular direction over time. This occurs when individuals with extreme trait have a higher fitness and survival advantage compared to individuals with average or opposite trait. This type of selection is often observed when there is a change in the environment or when a population is adapting to new conditions.
For example, bacteria developing resistance to penicillin is a result of natural selection due to the widespread use of penicillin. Some bacteria possess enzymes that can break down penicillin. The bacteria with higher enzyme activity have a greater chance of survival. Their offspring inherit the same trait, repeating the process of natural selection. Over time, the stronger resistance the bacteria have, the worse penicillin potency become.
Disruptive selection or diversifying selection: In this type of selection, individuals at both ends of the trait spectrum have an advantage, while individuals with intermediate trait values are at a disadvantage and removed by natural selection. Disruptive selection increases genetic diversity within a population.
In the case of Pacific salmon, there is a disruptive selection in their body size. There are two distinct forms: the smaller-bodied and early-maturing "jack" salmon, and the larger-bodied, late-maturing salmon known as "hooknose" salmon due to their hooked nose shape. These two forms employ different strategies to obtain mating rights. Hooknose salmon engage in classic competition, using physical aggression to establish dominance and pass on their genes to the next generation. Jack salmon, on the other hand, surreptitiously release their sperm near the eggs without engaging in direct combat. The intermediate-sized salmon neither have the advantage in physical fights nor can they successfully sneakily fertilize eggs, resulting in their elimination through natural selection.