A characteristic common to all living beings, according to evolutionary theories, is the capacity for evolution.
The evolution of living beings is the process of the disappearance or emergence of new species due to genetic variability. This process is very slow and can take up to thousands of years so it is difficult to keep up with the evolution process.
What is genetic variability?
If we look closely, we will see that, as similar as they may be, the individuals in a population differ somewhat. We call these differences between beings variability.
Let's think about the stickman. This animal is very similar to a tree twig that is often difficult to distinguish from the environment. For this insect, being similar to a twig is an advantage, as it can camouflage itself in the environment and not be noticed by its predators.
Even in the population of stick animals, there are differences between individuals. Those less like tree twigs will be more hunted by predators, so they are less likely to breed. If only the sticklike twigs can reproduce this trait, it will be passed on to the new generation (or the next stick), continuing in the population.
The onset and increase of variability between beings is mainly due to the occurrence of mutations and sexual reproduction.
At mutations - random changes in the genetic material of living beings - cause the appearance of new characteristics. These new features may be advantageous for adapting one's being to the environment or not.
This survival phenomenon of the fittest — that is, the best adapted — beings is what Charles Darwin (1809-1882) called natural selection.
"Fitter" does not mean being "stronger". The fittest in certain environments may be the smallest; what can camouflage itself, what has more puppies; finally, which has characteristics that favor life and reproduction in the environment where he lives.
According to Darwin, the process of natural selection acts constantly. With each change in the environment, it may be possible to have previously adapted individuals who cannot stand the new environmental conditions. For example, a drastic change in the aquatic environment is pollution, so fish that have been adapted to water conditions will survive only if they have "more" to live in the polluted environment. This extra "something" may be the characteristic of supporting toxic metals in water, which previously did not give him an advantage in reproduction, but now it is because it can survive in that environment.
Over time it is still possible for a population to change so much that it is considered a new species.