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Widespread variation of phenotypes in natural populations provides the raw material for evolution, which is the change in the inherited traits of populations over successive generations. Natural selection is one of the main mechanisms of evolution and requires variable traits to be heritable and associated with differential survival and/or reproductive success. Phenotypes that correlate with greater success will have more offspring that survive to reproduce in the next generation, and will thus be represented at greater levels in each successive generation.
The reproductive success of an organism relative to others is known as its fitness and any heritable trait that contributes to an organism's fitness is known as an adaptation. Therefore, individuals or phenotypes with higher lifetime reproductive success are considered to be more fit and better adapted to their environment.
An example of differential fitness comes from the snowshoe hare, which is brown in summer but grows a white coat in winter to camouflage in snow. Nonetheless, hares that maintain their brown summer coats throughout the year would be at a fitness disadvantage due to their relatively higher visibility to predators. Therefore, higher predator visibility would decrease the survival rate of the brown coat phenotype relative to the white coat phenotype, consequently reducing its lifetime reproductive success. Climate change, however, threatens organisms that fail to adapt to conditions, thus researchers have been studying how snowshoe hares are coping with warmer temperatures and decreased snow cover in winter. Snowshoe hares that grow a white coat in environments with diminished snow cover become more visible to predators, which reduces their reproductive success. Researchers have found that snowshoe hares did not alter their behavior to adapt to mismatch in their camouflage1 and mismatched hares experienced significant decreases in their survival rates2.
Calculation of fitness relies on the relative survival and reproductive rates of different phenotypes in a population. For a given phenotype, survival rate is the proportion of that phenotype surviving, and reproductive rate is the average offspring produced per individual. The ratio between the products of the survival and reproductive rates for the different phenotypes is known as relative fitness (w). Hence, to calculate w for each phenotype (or trait), first the multiplication product of the survival and reproductive rates are calculated for individual phenotypes, this is then divided by the highest product value amongst them.
w = (SurvivalA x ReproductionA) / (Survivalmax x Reproductionmax)
This equation can be used in turn to calculate the strength of selection on different phenotypes, or the selection coefficient (s), by subtracting w from 1. Therefore, the value of the selection coefficient ranges between 0 and 1; the lower the value, the higher the selection against the specific trait. Thus, a phenotype that has a maximum fitness of 1.0, will be experiencing no selection.
s = 1 – w
There are three main types of selections: directional, stabilizing, and disruptive. Directional selection favors extreme values of a trait in one direction and can be induced by a change in the environment that the population needs to adapt to. One example is the evolution of differential jaw structures of East African cichlid fish in accordance with the diet source3. Stabilizing selection holds a trait at an optimum value; it is generally assumed that any deviation from this optimum is disadvantageous. For example, human fetus size is subject to stabilizing selection; babies born too small are less likely to survive and babies that are too big cannot pass through the birth canal during natural birth4. Disruptive selection favors two different extremes of a phenotype, creating a bimodal distribution. The evolution of West African seedcracker finch beaks resulted in two common phenotypes, large beaks that easily break hard sedge seeds and small beaks for soft seeds. Finches with medium beaks are not particularly good at consuming either seed type, thus are selected against5.
One of the most important ways natural selection impacts the lives of humans is through the rise of antibiotic resistant bacterial strains in hospitals and due to agricultural practices which utilize significant amounts of antibiotics6. The phenomenon of antibiotic resistance is a sort of inadvertent form of natural selection imposed by humans.
A question moving forward is whether natural populations will be able to adapt to the effects of climate change quickly enough to avoid extinction. This is dependent not only on existing variation in populations, but also on the strength of selection being high enough to affect quick change in the population. In this regard, reduced population sizes of many species around the world are of special concern.