Speaking from the point of view of evolution, one single individual cannot be considered as its unit. Even if it develops a "beneficial" mutation, that mutation must first appear in a large number of organisms, and only then this group, already different from other groups within its species, can potentially become a different species. Such groups are called populations, and they are considered the basic unit of the evolutionary process.
What is population
Each species lives in its own habitat — a specific zone, beyond which it rarely goes. But even in this zone, there are different climatic conditions and ecological niches. Imagine a large range of forest cats. For many years, cats catch prey in the undergrowth, and if they go beyond their range, then rarely — there is no prey there. But at some point, a cat appears, capable of catching rodents in open spaces, lying in wait for them at their holes. Her offspring will adopt the behavior of their mother, and the group of cats specifying on the open plain will multiply.
Almost all species of living organisms within their habitat live unevenly in this territory: they either break up into groups, choosing different ecological niches (by choosing different types of food), or diverge geographically (for example, someone stays to live on the mainland, and someone gets to the island). These groups are called populations, and the population is considered as a unit of evolution — a group of individuals living separately in a certain habitat. The totality of all populations makes up a species.
Population characteristics
We said in the Species topic, that two species usually cannot live in the same territory with the same ecological niche, but it is sometimes possible for the population. In this case, animals of different populations choose a specialization — for example, different populations of wolves choose different types of prey, such as hares or deer, and with that principle, they can live in the same forest.
The number of populations within a species can vary — sometimes the whole species consists of one single population (this is true for some endangered species, whose representatives are so rare that they cannot form several populations), and sometimes there can be many populations. Most often, populations are somehow isolated from each other. Such isolation can be complete (for example, land animals on different islands cannot physically interact), or incomplete (then random interbreeding occurs from time to time).
Populations usually are capable of self-regulation — births and deaths usually grow cyclically and depend on the density of individuals within the population, the availability of resources, the type of landscape, and so on. Such changes are called population waves. Such waves are easy to trace in life, for example, locusts — their fluctuations occur with a difference of 10-11 years. The same fluctuations are observed in many others, for example, hares.
Another important feature of populations is the accumulation of mutations (read more in the Mutations topic). Mutations can be lethal, or they can slightly "strengthen" an individual, giving it the opportunity to mate with more partners and give birth to more cubs. Populations accumulate mutations, and with evolution, the most "useful" of them remain and become common to all representatives. At the same time, populations undergo stabilizing selection — individuals that differ greatly from the population average (both plus and minus) are usually eliminated.
The population as a unit of evolutionary progress
Consider a group of cats living on the plains. They have cousins that live in the woods and they are still the same kind of cat. But due to the fact that they inhabit different zones, they practically do not meet with each other and at some point, the groups begin to disperse. The new mutations begin to accumulate in a group of plains cats, and the more mutations appear, the more their genome changes. These can be different mutations — weaker claws, stronger jaws, reduced pheromones, and odor. It may even be a mutation in hormones, due to which their breeding period will not be the same as that of forest cats. A new species appears — no longer forest cats: new cats differ from their ancestors both in morphology and physiology.
When an individual in a population has some kind of evolutionally important mutation, it can subsequently be fixed through its descendants and at some point become a common mutation in the genome of all members of the population. This process is called microevolution (it can lead to the evolving of new species and subspecies) and then macroevolution can occur — the emergence of individual genera and more general taxa.
Populations do not always become separate species. This often does not happen, because there is a gene flow — the constant crossing of representatives of different populations among themselves, which ensures a constant relationship between them. For example, if we speak about humans, there are four groups of the blood system (ABC system). But as we can see, all four types can be met everywhere: the frequency of gene A changes from East to West — from low to high, and the frequency of gene B, on the contrary, from high to low. This is because of gene flow — people migrated from one part of the world to another, mixed up.
Sometimes gene drift occurs in a population — a random change in the ratio of alleles. This can most often be observed in closed populations isolated from the rest of their species. Genetic drift can also be seen in human populations, especially in closed or island communities where consanguineous marriages are common. Genetic drift can make a population weaker, but it can also increase the adaptability to the environment and increase the difference between a population from other populations, which can lead to the emergence of a new species. Sometimes the so-called bottleneck population effect can occur — a sharp reduction in the genetic diversity of a population due to a sharp decrease in its numbers, for example, due to a catastrophe (forest fire or epidemic). Despite the randomness of this event, the reduction in genetic diversity leads to a change in the relative and absolute frequencies of alleles of genes.
Conclusion
So, a group of organisms of one species living separately in a certain habitat is called a population. Populations can be isolated from each other, so they can become new species. If the population is isolated, then gene drift can occur in it, when the ratio of alleles suddenly changes randomly. Together with the property of a population to accumulate mutations, this can lead to the fact that genetically the population will no longer be similar to the parent species.