Have you ever heard the phrase, survival of the fittest? When most people hear this phrase, they think of physical fitness features such as speed, strength and endurance. But what do biologists really mean by fitness?
It all started when Charles Darwin published The Origin of Species in 1859 which aroused much debate. Darwin's Theory of Natural Selection, which he defined as, ‘…the principle by which each slight variation, if useful, is preserved.’ sparked the interest of a prominent 19th Century thinker, Herbert Spencer. In his book, Principles of Biology, Spencer coined the phrase, ‘survival of the fittest’, to explain Darwin's Natural Selection. This phrase resonated with many including Darwin himself and the term fitness became popular.
However, when biologists think of an organism's fitness, they don't think about its strength, speed or endurance. Instead, they simply consider fitness as the reproductive success of an organism relative to others. Take these Snowshoe Hares for example. Organisms with greater relative fitness have adaptations that allow them to have more offspring that survive and reproduce. These adaptations are called phenotypes defined simply as a visual expression of a trait. For example, Snowshoe Hares are brown in summer, but grow a white coat when the temperature drops. The ability to change fur color allows Snowshoe Hares to camouflage. Since hares with white winter coats are less visible to predators in the snow, survival rates are much higher for hares that turn white in winter. As a result, more color changing hares will survive to reproduce in summer.
To determine whether selection against a phenotype is taking place scientists calculate the fitness of each phenotype known as relative fitness, or ‘w’. This is done by multiplying the survival rate – which is the proportion of that phenotype surviving – by the reproductive rate for each phenotype, which is the average litter size per mating season. Then, the product value of each phenotype is divided by the highest product value among the phenotypes.
Once scientists know the relative fitness, they can calculate the selection coefficient, or ‘s’, which is the strength of selection against a phenotype, by subtracting relative fitness from 1. Higher numbers mean stronger selection against that individual phenotype. Scientists can use survival, relative fitness and selection coefficients to figure out if populations may be changing over time. For example, after the Industrial Revolution, the survival and relative fitness of light- colored peppered moths declined and the selection coefficient against them increased. The effect on the population of melanic moths was exactly the opposite, because conditions were favorable to their survival. After the passage of the Clean Air Act, the soot cleared up and the survival, relative fitness, and selection coefficient values for both phenotypes moved in opposite directions – which shows that selection favored the light moths again.
In general, the majority of biological traits with different potential phenotypes show a normal distribution. Let's take the spots on this group of ladybugs as an example. We can see that they have a varied number of spots from zero up to 12, but the average trait value is in the middle – referred to as a normal distribution as most of the ladybugs in this population have six spots. The height of the curve indicates the number of individuals in the population displaying the given trait. Now imagine that there is selection against the ladybugs with few or no spots because they absorb less heat and don't survive so well in cold temperatures. This would be an example of directional selection which favors extreme values of a trait in one direction.
To understand another type of selection, let's look at robins. If a female robin lays five or more eggs in a single clutch, the risk of malnourishment increases for the chicks. But if the robins lay three or less eggs, they may not have viable offspring. So here, stabilizing selection is holding a trait, egg clutch size, but an optimum value…any deviation from which is disadvantageous. Hence, most robins lay four eggs.
In West Africa, large billed black-bellied seed cracker finches easily break bigger hard seeds of one type of sedge grass while small billed finches effortlessly maneuver and break the smaller seeds of a different type of sedge. Medium billed finches cannot open either seed as efficiently and so are rarely observed. This type of selection is referred to as disruptive – where extremes of a phenotype are optimal which creates a bimodal distribution that deviates from the normal.
In this lab, you will model four population scenarios using pipe cleaners of varying colors and lengths representing different phenotypes and then determine the type of selection occurring in each scenario.
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.
