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Population Genetics 3: Natural Selection

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You should have a working knowledge of the following terms:

  • balancing selection
  • directional selection
  • diversifying selection
  • fitness
  • frequency-dependent selection
  • heterozygotic advantage
  • sexual dimorphism
  • sexual selection
  • stabilizing selection


Introduction and Goals

The concept of natural selection is sometimes oversimplified as "survival of the fittest." You will learn why this is an oversimplification. In short, there are many different ways that natural selection can influence the composition of a population. By the end of the tutorial you should have a basic understanding of:

  • The three major modes of natural selection
  • Why sexual selection is not a mode of natural selection

Fitness and Selection

Within a typical population, many characters exhibit variation in the form of alternative traits. Some character traits vary continuously, from one extreme to another (e.g., height in a human population), and are referred to as quantitative characters (discussed in tutorial #5, under Polygenic Inheritance). Conversely, some traits vary discretely (e.g., eye color).

Some of these character traits may provide certain individuals with an advantage not possessed by others in the population. If these individuals show differential reproductive success (i.e. they are more fit), then the allele(s) encoding the trait will increase in frequency and natural selection will take place.

Fitness is a term that has a precise biological meaning; namely, reproductive success. Fitness is typically referenced relatively. In simplest terms, an individual who produces four offspring is relatively more fit than one who produces three offspring (providing all offspring themselves survive to reproduce, and hence, are more fit themselves). Thus, the individual producing four fit offspring contribute 25% more of their genetic code to the next generation than the individual producing three fit offspring.

By this definition of fitness, individuals can die young and be fit, providing they produce offspring. Conversely, individuals can be long-lived and unfit if they fail to reproduce.


The Salmon Example

An organism's success has little to do, per se, with its longevity, but rather, its ability to reproduce. Many successful plants (e.g., annual wildflowers) grow for a season, reproduce, and then die. Many successful insects live for only twenty-four hours but still manage to reproduce in order to make their contribution to the gene pool. In some cases, individuals contribute more to the next generation than simply their genes.

Salmon inhabit Pacific waters and spend most of their lives in seawater, but migrate to freshwater streams to breed. The salmon that live in the Pacific Ocean and reproduce in headwater streams (small streams found at higher altitudes where few nutrients exist) have life cycles of approximately 4-5 years. The salmon that survive this perilous journey (approximately 1 in 100) reproduce and then die in the streams where they were born. This doesn't sound like a pleasant existence, however, it actually is a great adaptation to the next generation. By dying immediately after reproduction, adult salmon indirectly provide for their young.

How does this work?


Most Pacific coast headwater streams lack abundant nutrients. The nutrients released from the dead adult salmon enrich the stream and feed algal blooms and invertebrates. In turn, these organisms are a key food source to the hatching offspring.

Figure. Salmon jumping upstream. (Click image to enlarge)


Charles Darwin was 22 years old when he accepted an unpaid position aboard the H.M.S. Beagle. The Beagle sailed on a surveying expedition, primarily to chart the South American coastline. Among other things, Darwin observed many plant and animal species on the Galapagos Islands (off the coast of Ecuador) that exist nowhere else in the world. The species on these islands exhibit great diversity, the most notable being the finches of the Galapagos. The differences in feeding adaptations among these finches led Darwin to describe how such adaptations arise in species. Darwin's observations from the voyage later supported his theory of natural selection in The Origin of Species. In this work, Darwin described two processes. In one process, "descent with modification," Darwin suggested that all life descended from one original form and that diversity was the result of adaptations. Toward the end of The Origin of Species, he used the word evolution. The other process he described was natural selection. We will discuss these processes in the remainder of this tutorial.

At some point, you should consider reading Darwin's book. Although written over 150 years ago, it is still relevant and it is easy to read.


Natural Selection

Natural selection describes how populations, over time, adapt to their environments. Adaptation is based on the generational selection of certain beneficial alleles that will, over time, increase in frequency (and conversely, less advantageous alleles will decrease in frequency). For example, let's say in a certain species of bird there is an allele that leads to curly wing feathers. While these curly feathers may be nice to look at, they may not lead to an increase in fitness. On the contrary, these curly-feathered birds are likely poor flyers and they do not live sufficiently long enough in the wild to reproduce. As a consequence, the curly-wing allele is not very prominent in the population because birds having this allele don't live long enough to breed and contribute the allele to the next generation.

Natural selection can only act on available phenotypic variations within the population. These phenotypic variations are reflections of genotypic variations, which arise from random mutations in the genome, crossing over during meiosis, and sexual reproduction itself.

There are over 160 documented cases of natural selection occurring in wild populations. The pepper moth (Biston betularia) story is often used to illustrate natural selection. There are two forms of the moth: one mottled (light-colored) and one black. In natural conditions, the mottled phenotype is more common because it mimics lichen on trees and because birds prey on the black moth. During the Industrial Revolution, soot from factories darkened the trees and poor air quality destroyed the lichen. This resulted in a shift of the phenotype frequency curve because the mottled phenotype became more visible to predators, and thus became the prey of choice for birds.


Adaptation and the Environment

Natural selection acts upon the available phenotypes in a population to adapt them to their environment by increasing or maintaining favorable genotypes; hence, the better-suited individuals survive. For example, fairy shrimp have adapted to vernal pools (seasonal bodies of water that fill up in the spring and dry up in the summer). The shrimp complete their life cycle in as little as two weeks; hatching, breeding, and then dying. The eggs sit in the sediment until the vernal pool fills up again, and then they hatch and continue their life cycle. In this way, fairy shrimp are well adapted to an environment that certainly would not be suitable to many other aquatic organisms. By passing on their genes to the next generation, the shrimp are exhibiting fitness.

Figure. A fairy shrimp. (Click image to enlarge)

Keep in mind, rarely does a single species live in a particular environment. Life is often a competition between species, and if one species gains an advantage (via a new beneficial allele), then it often is at the expense of other individuals and species living in the same environment. This idea of a continuous race between species to be the best adapted for a particular environment is referred to as the "Red Queen Hypothesis" (from a line uttered by the Red Queen in Alice in Wonderland; i.e., "Here, you see, it takes all the running you do to keep in the same place.")

Types of Selection

An organism has specific traits that are expressed, and these can be influenced by the environment. Recall that the collective expression of traits is referred to as the phenotype. A change in a trait (e.g., flower color) results in an alternative phenotype. It follows then that any change in a population's phenotypic frequency may be a reflection of a changing environment. The pepper moths described earlier represent a good example of this phenomenon. When air quality declined, the darker moths became more common (i.e., the bark on trees became darker with soot and light moths were more visible on the bark, compared to dark moths). Now that air quality has improved, the mottled and lighter phenotype is more common.

At first one might conclude that natural selection always shows a clear increase, over time, of one trait (and a decrease of another). Indeed this occurs (as in the moth example), however, more complex situations give rise to alternative phenotypic distributions. These situations are described by three major modes of natural selection. These modes are used to visualize the microevolution of a population. The three modes will be examined next.


Mode of Selection 1 - Stabilizing Selection

Stabilizing selection favors intermediate variants by acting against extreme phenotypes. Organisms that display stabilizing selection may carry ancestral character traits. Open this animation and click on stabilizing, then the play button to see how stabilizing selection favors an intermediate phenotype.

One condition that favors stabilizing selection is a stable environment. Many terrestrial, aquatic, and marine environments don't change much from year to year. Stabilizing selection can also occur when extremes are a disadvantage. Birth weights in humans are an excellent example. If a baby is born too small, it may not survive. Conversely, if it is born too big, it may not get out of the womb safely or it may endanger the mother (and/or itself).


The horseshoe crab has many stable character traits that display stabilizing selection. Horseshoe crabs represent a group of very ancient arthropods that have inhabited shallow ocean waters. These organisms are very well adapted to scavenging on sandy bottoms. They are a good example of stabilizing selection because many of their features have not changed in the past 250 million years.

Figure. Horseshoe crabs. (Click image to enlarge)

Mode of Selection 2 - Directional Selection

Directional selection favors individuals that are at one end of the phenotypic range, usually during periods of environmental change. This type of selection favors the variants of one extreme by selecting against the other extreme; thus, resulting in a shift of the phenotype frequency curve. Open this animation Animation 1, click on directional, then the play button to see how directional selection favors variants at one extreme of the phenotypic range.

For example, changes in weather patterns can cause directional selection. Let's say that a group of relatively tall plants exists in an area without much wind. The weather patterns change and the plants are now exposed to greater wind forces. We might expect the phenotype frequency curve to shift toward shorter plants that can withstand the environmental pressure. The pepper moth mentioned earlier is another example of directional selection.


Mode of Selection 3 - Diversifying Selection

Diversifying selection (also referred to as disruptive selection) favors individuals at both extremes of the phenotypic range, usually during periods of environmental change. Open this animation, click on diversifying, then the play button to see how diversifying selection selects variants at both extremes of a phenotypic range. Imagine a population in which the main food supply has decreased. Because less preferred food exists, the variants of both extremes may be able to utilize different food supplies, whereas intermediates may not be able to adjust. Selection against intermediates would result, due to the decrease in available preferred food. For example, black-bellied seedcracker finches with small beaks crack soft seeds more easily than those with larger beaks. Finches with large beaks crack hard seeds more easily than those with smaller beaks. Finches with intermediate-sized beaks crack both types of seeds poorly. Therefore, there is a shift in the population to the extremes.


View this animation to review the three modes of selection: 

Balancing Selection

Balancing selection works to maintain a dynamic state of balance between advantageous and disadvantageous alleles. At first this may sound contradictory, however, think about the sickle-cell allele. You learned that individuals that are heterozygous for the allele show an increase in fitness when living in areas of high malaria infestation. Because these individuals are more fit, the allele persists even though the individuals that are homozygous for the allele have a marked disadvantage. At the population level, the sickle-cell allele is advantageous even though individuals with homozygous alleles are at a disadvantage. The heterozygotic advantage keeps the allele in the population via balancing selection.

Another example of balancing selection is frequency-dependent selection. This type of selection is very widespread among traits that affect predator-prey relationships. If a trait provides the prey with an ability to avoid predation, then individuals with this trait can avoid predation and their fitness is increased. However, in many instances predators can adapt to changes in prey characteristics. If the prey allele frequency becomes high for such a trait, then there is a greater chance that more predators will have adapted to this avoidance trait and it will no longer increase fitness of the prey. The allele only endows an increase in fitness in the prey population if it is maintained in the population at a low level.


Sexual Selection

Sexual selection describes the selection of a mate based on a specific heritable trait; therefore, it is not a mode of natural selection. Specifically, sexual selection is based on secondary sexual characteristics (e.g., coloration or behavior). This type of selection usually leads to the enhancement of sexual dimorphism (differences in the secondary sex characteristics) between males and females. For example, think of the peacock and its feathers; female peahens are more likely to mate with males bearing long, brightly colored tail feathers (although these feathers are not directly related to reproduction in any way). Also, the peacock's tail feathers are not a survival advantage; rather, they probably make the peacock more vulnerable to predation and they take a lot of energy to maintain.


Peahen, female pea fowl

Peacock, male pea fowl

(Click image to enlarge)

It is important to point out that sexual selection and natural selection are different in one important aspect. Natural selection increases the adaptation of a population to a specific set of environmental conditions, whereas sexual selection does not. However, both are forms of selection because they act to screen certain alleles from a population.
Other examples of secondary sexual characteristics and sexual dimorphism include the male lion's mane, the bright colors of the male cardinal and male blue jay, and the striking sexual differences in the gigantic beetles Macrodontia cervicornis and Dynastes hercules:


M. cervicornis male

M. cervicornis female

D. hercules male

D. hercules female

(Click images to enlarge).

Another example of sexual selection is observed in fantail darter fish (Etheostoma flabellare). Females show a mating preference toward males that are already guarding eggs. This behavior is presumably a result of males demonstrating their ability to protect eggs to maturity. As an effect of this behavior, some males appear with modified epidermal cells that look like fleshy knobs on their dorsal fins. These fleshy knobs mimic eggs in shape and color. Females have shown mating preference for males displaying these egg mimics.

Figure. Fantail Darter. (Click image to enlarge)


Sexual Selection and Natural Selection

 A recent publication by Johnston et al. (2013) in the journal Nature describes a balanced polymorphism at the RXFP2 locus in Soay sheep (Ovis aries), which occurs via sexual and natural selection.  The RXFP2 explains most of the heritable variation in horn morphology in these sheep.  Two alleles, Ho+ and HoP, where Ho+ confers large horns and HoP confers smaller horns.  Rams with larger horns have higher reproductive success, but rams with shorter horns live longer.  Thus, at the RXFP2 locus, the heterozygous individuals have an advantage over the two homozygotes, Ho+/Ho+ that reproduce more, but live shorter times versus HoP/HoP that reproduce less, but live longer.  Click on the Nature link above to see pictures of the different rams and to view the primary research article.



Natural selection is one of the most important concepts of evolution because it explains how organisms can adapt to a contemporary environment, as well as to another environment at a later time. Keep in mind that natural selection is not a goal-oriented process, in the sense that organisms "know" their evolutionary trajectory. It does not lead to a "perfect organism," but it does explain how the best-adapted organisms in a given situation will propagate their genes with greater likelihood to the next generation.

Natural selection only works if variation exists in the population. Genetic mutation is one source of variation, however, a specific mutation does not arise on demand. Rather, variation is random, but selection is adaptive; beneficial and deleterious alleles arise spontaneously, but only the beneficial alleles will be propagated over successive generations.

Does this mean that every allele is under selection? Not necessarily. Indeed, many alleles may be neither beneficial nor detrimental and so, their frequency may only change due to random events. Do not think that every alternative allele is always under selection; some may be neutral (although if conditions change, a neutral allele may become either beneficial or detrimental and subject to natural selection).

Soon we'll turn our attention from microevolution to macroevolution, the process that governs large-scale changes. But first, we'll take time to consider what makes a group of organisms a species, and some current ideas about how to define species.