Populations evolve, but individuals are selected. Natural selection affects individual organisms.
Variation naturally occurs in populations as new traits arise from random mutations. However, through natural selection only those traits that are beneficial to the organism are passed on to the next generation. Any harmful mutations are naturally weeded out.
When nothing happens to exert strong population pressure on that population, natural selection favors the allele frequency already present. When mutations cause new traits, natural selection weeds these traits out because they're not as efficient as the others.
They both involve the principle of differential reproductive success. Only in one case, the reproductive success is determined by mindless congruence between phenotypic attributes and the environment, and in the other case, humans make the determination what traits should propagate.
Without the heritability of individual traits what difference would it make if the individual was selected. An individual that has a germ line mutation, say, and this mutation could confer survivability and reproductive success on progeny thus passes this mutation to said offspring is selected. Then evolution, the change in allele frequency over time in a population of organisms, could take place. Heritability is all as individuals are selected but populations evolve.
Natural selection (the driving force of evolution) is the selection of genetic variations by how they effect the organism's chances of survival or reproduction. If they diminish it's chances, the organism or it's immediate offspring die and the gene is gone. If the genetic variations increase it's chances, then it survives. Without genetic variations there can be no evolution. Natural selection is the selection (by environmental pressures) of those variations.
Both
Non-random mating is otherwise known as sexual selection. Some see this as distinct from natural selection, but I think that sexual selection is merely a form of, or perhaps more a complication of natural selection. Selection, natural or sexual, is the effect that "guides" evolution, that allows evolution to produce populations suited to their environment.
Differences between individuals may affect differences in their average reproductive success, causing the variant traits of individuals that have greater reproductive success (fitness) to become more prevalent in a given environment than rival traits. As environments change, so may the traits that have a reproductive advantage change. This is natural selection.
Mutation can serve up the raw variation that natural selection selects from, thus alleles are changed over time in populations of organisms; evolution.
Organisms are affected by Natural Selection because Inherited characteristics affected the likelihood of an organism's survival and reproduction.
natural selection is basiclly only the strong survive which means it effects the weak by killing them but bernifits the strong
Variation naturally occurs in populations as new traits arise from random mutations. However, through natural selection only those traits that are beneficial to the organism are passed on to the next generation. Any harmful mutations are naturally weeded out.
Through ongoing natural selection a population adapts to its enviroment
Natural selection is the process which determines the shark's evolution. It is humankind that is threatening the sharks' survival.
species
Evolution by natural selection is currently the only viable theory explaining the diversity of life. However, the mechanism of natural selection is not the only mechanism to affect evolution. There are phenomena such as genetic drift, biased gene conversion, intragenomic conflict, and so on, that aren't exactly the same as natural selection (although they are all intertwined and all affect one another), but do affect the direction of evolution.
Humans directly affect artificial selection. They do this by selecting the specific traits that they prefer which they cannot don in a natural selection.