how is it associated with fitness?
The concept of fitness is central to natural selection. In broad terms, individuals that are more "fit" have better potential for survival.
Natural selection
The concept of fitness is central to natural selection. In broad terms, individuals that are more "fit" have better potential for survival, as in the well-known phrase "survival of the fittest", but the precise meaning of the term is much more subtle. 
where does all the natural variation necessary for natural selection come from This occurs partly because random mutations arise in the genome of an individual organism, and their offspring can inherit such mutations. what is this all about? Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. were there any theories based on it? Yes, Darwin's theory. how is it classified? It can be classified in several different ways, such as by its effect on a trait, on genetic diversity, by the life cycle stage where it acts, by the unit of selection, or by the resource being competed for. what can you tell me about its name? The term "natural selection" is most often defined to operate on heritable traits, because these directly participate in evolution.
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Natural selection [SEP] Fitness
The concept of fitness is central to natural selection. In broad terms, individuals that are more "fit" have better potential for survival, as in the well-known phrase "survival of the fittest", but the precise meaning of the term is much more subtle. Modern evolutionary theory defines fitness not by how long an organism lives, but by how successful it is at reproducing. If an organism lives half as long as others of its species, but has twice as many offspring surviving to adulthood, its genes become more common in the adult population of the next generation. Though natural selection acts on individuals, the effects of chance mean that fitness can only really be defined "on average" for the individuals within a population.