Natural Selection

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.