Selection can either be natural or artificial. Natural selection takes place in a natural environment, and selective forces there can be any number of factors such as drought, high or low temperatures, existence of predators, insect pollinators, and so on. Individuals genetically well-adapted to these conditions, or well-adapted to changing conditions, can survive and proliferate. Genetically less-adapted individuals will have a harder time and may well become extinct. How does selection by natural or artificial means take place? Possibilities for selection of some individuals exist because not all individuals in a population are genetically identical and equally fit. This is because each individual, as we saw, is the product of gene shuffling during meiosis, and therefore different individuals do not possess the same gene combinations.
Selective (artificial) breeding for desired characteristics, along with natural selection imposed by the environment, have produced our domesticated animals, pets, and crops. Successful selective breeding also requires variations within populations. That is, if all the animals or plants had the same characteristics, ones that produced more milk or laid more eggs or had higher oil content—or whatever qualities we wanted to enhance—would not exist. Thus there can be no selection without variation in populations of animals and plants. The ultimate source of this variation is mutations. Let us now consider a few examples of artificial selection.
Humans have produced many successful strains of domesticated animals and crops. These animals and plant have the highest quality of the desirable properties humans look for. For example, if one observes the milk goats at a local county fair, one sees that they all have tremendously large udders, so large that they would have difficulty walking if let out into the mountainous terrain of their ancestors. These goats were bred for the purpose of high milk production and originated from goats that produced more milk than average. Over time, breeders selected progenies from these goats with even higher milk production and bred them again, until the milk production was much higher than in the original goats, and, by consequence, large udder size was obtained. This example shows that artificial selection by breeders can go against what would have been selected for in a natural environment. In nature, goats with inordinately big udders would surely be selected against, as they would not have the agility of small-udder goats and would be at a disadvantage. Genes responsible for large udders would then quickly be diluted in a population of wild goats.
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