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In natural settings, adaptive evolution takes place by natural selection, a process in which differences in traits lead to differences in survival and reproductive success between individuals. This happens because individuals with certain phenotypes have a higher probability of survival and/or produce more surviving offspring than individuals with other phenotypes.
A classic example of this was demonstrated by Rosemary and Peter Grant studying the population of Medium Ground Finches on Daphne Major Island, in the Galapagos archipelago1. In the late 1970’s, the islands experienced a drought during which few plants flowered and produced seeds. The birds were forced to survive on large, thick shelled seeds left over from previous years. Of the population, only the birds with larger beaks could harvest efficiently. As a consequence, birds with small beaks died; the few birds that survived were larger and had thicker, stronger bills. These differences in bill size were genetic, and therefore heritable, so the offspring produced the following year had, on average, larger bill and body sizes than had been present in the population before the drought. Interestingly, 8 years later there was particularly high rainfall, resulting in a resurgence of small seed producing plants. The small beaked finches were observed to be more abundant in subsequent years. Indeed, over the past 5 decades, this finch population has oscillated in accordance with the food type available1,2.
More generally, natural selection leads to evolution of populations, because populations contain genetic variation that results in expression of different phenotypes. In the case of the finches, the birds present in the population before the drought varied in bill size and these differences in turn affected the size of seeds each individual was best able to eat. Without such variation, evolution cannot happen because there is no variation for selection to act upon.
Humans have taken advantage of natural variation to create a wide variety of domesticated plants and animals through artificial selection, also known as selective breeding. In this case, humans cause selection because they select which phenotypes of animal will breed to produce the next generation. Artificial selection has been used for much of human history to produce crops and animals that are more efficient or have desirable traits, such as plants that produce larger fruits and vegetables, or cows that produce more milk.
If selection is applied consistently for many generations, it is possible to produce an organism that hardly resembles the one it originated from. Dogs as diverse as Chihuahuas and Great Danes were derived from gray wolves in a few thousand years. This rate of change tells us that much of the genetic variation necessary to produce these domestic breeds was present in the population of wolves originally descended from. Researchers are studying the genetics of modern breeds of dogs to determine what genetic elements differ between them that might be correlated with their obvious differences in appearance and even behavior. This may then serve as a good model for understanding human genetic diversity3.
Artificial selection in plants can clearly be seen in the diversity of cruciferous vegetable crops derived from Brassica olaracea, a large leafy plant native to coastal southern and Western Europe. It has been bred by human populations in multiple locations, over hundreds of years, to produce an estimated 400 different vegetables, such as turnips, cabbage, kale, broccoli, cauliflower, and Brussels sprouts4.
Researchers, as well as farmers, have taken advantage of another species in the same genus, B. rapus, a subspecies of which has been bred to have an extremely short life cycle of about one month. This variety is known as “Wisconsin Fast Plants,” 5 and is used as a model organism in research and education. B. rapus germinates in a few days, produces flowers after two weeks, and produces viable seeds by the end of four weeks. The plant generally dies at the end of this cycle.