Introduction and Learning Objectives
The concept of natural selection is sometimes oversimplified as "survival of the fittest". However, 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 major modes of natural selection
- Why sexual selection is a special case of natural selection, selecting for features than enhance reproductive success but may hinder individual survival
- Summarize the role of natural selection as an adaptive evolutionary force, and describe the results of different types of selection on a population
- Explain stabilizing selection and why it results in a loss of genetic variation in a population.
- Describe directional selection and how it can lead to both genotypic and phenotypic change in a population.
- Diagram diversifying selection and why it can lead to multiple genotypes and phenotypes in a population.
- Compare and contrast intra and inter sexual selection, and the relationship between sexual and natural selection.
- Discuss how frequency dependent selection and heterozygote advantage are both types of balancing selection.
Fitness and Selection
Within a typical population, many characters show 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 #27, under "Polygenic Inheritance"). Other traits vary discretely (e.g., eye color).
Some of these character traits may provide the individuals that have them with an advantage not held by others in the population. If these individuals show greater differential reproductive success (i.e. they are more fit), then the allele(s) encoding the trait will increase in frequency and natural selection will have taken place.
Fitness is a term that has a precise biological meaning - 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 are more fit themselves). The individual producing four fit offspring contributes 25% more of their genes 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 more offspring than other members of their population. Conversely, individuals can be long-lived and unfit if they produce fewer offspring.
The Salmon Example
An organism's success has little to do with its longevity, but rather, its ability to successfully 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, making their contribution to the gene pool. This trait of spawning once in a lifetime, producing many small offspring, is known as semelparity and is characteristic of r-strategists, organisms that invest energy into quick reproduction with high fecundity. On the opposite end of the spectrum lie K-strategists, organisms that produce fewer, larger offspring, and may reproduce many times in their lifetime (iteroparity).
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 (Figure 1). 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 is a great adaptation for 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 for the hatchling offspring.
Figure 1. 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 charting the South American coastline. Among other explorations, 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 tortoises and 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 were used as evidence for his theory of natural selection in The Origin of Species, published in 1859. 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 to describe this process. The other process he described was natural selection.
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 describes how a population, over time, adapts to its environment. Adaptation is based on the increased fitness of individuals carrying advantageous 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 produces 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 may not live sufficiently long enough in the wild to reproduce, so they adversely affect the bird’s fitness. As a consequence, the curly-wing allele would not persist 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 variation within the population. These phenotypic variants are reflections of genotypic variation, which arises from random mutations in the genome, through crossing over during meiosis, and from sexual reproduction itself through random fertilization. Also, natural selection can only adapt organisms to the environmental conditions at that time. If the environment changes, then the selective pressures may change and different alleles and phenotypes may be favored.
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. Under natural conditions, the mottled phenotype is more common because it mimics lichen on trees, so predators (birds) can more easily see and 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, adapting them to their environment by increasing or maintaining favorable genotypes, so the individuals that are better suited to current conditions in the environment survive. For example, fairy shrimp (Figure 2) 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 repeat this life cycle. In this way, fairy shrimp are well adapted to an environment that certainly would not be suitable for many other aquatic organisms.
Rarely does a single species live in a particular environment. Life is often a competition between species, and if one species gains an advantage ( through 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 most well 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.")
Figure 2. A fairy shrimp. (Click image to enlarge)
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 of the organism. 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 when compared to dark moths). Now that air quality has improved, the mottled and lighter phenotype is more common.
Based upon this example, you may conclude that natural selection always shows a clear increase, over time, of one trait (and a decrease of another). Indeed this does occur (as in the moth example), but different selective pressures can give rise to alternative phenotypic distributions. These situations are described by three major modes of natural selection – stabilizing, directional, and diversifying. These modes are used to visualize the microevolution of a population..
Stabilizing selection favors intermediate phenotypic variants by acting against extreme phenotypes. Organisms that display stabilizing selection may show ancestral character traits if the environment has also remained unchanged. View the animation below 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), also reducing survival.
The horseshoe crab (Figure 3) has many stable character traits that display stabilizing selection. Horseshoe crabs represent a group of ancient arthropods that have inhabited shallow ocean waters for a long period of time. These organisms are very well adapted to scavenging on sandy bottoms. They are a good example of stabilizing selection because when living horseshoe crabs are compared to fossils, many of their features have not changed in the past 250 million years.
Figure 3. Horseshoe crabs. (Click image to enlarge)
Directional selection favors individuals with phenotypes that are at one end of the phenotypic range; this type of selection is often seen during periods of environmental change. Directional selection favors the variants at one extreme by selecting against the other extreme; thus, resulting in a shift of the phenotype frequency curve. View the animation below, 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 this change in the environment, a selective pressure. The pepper moth mentioned earlier is another example of directional selection.
Diversifying selection (also referred to as disruptive selection) favors individuals at both extremes of the phenotypic range; this is also seen during periods of environmental change, but is less common than directional selection. 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 of the preferred food is available, the variants of both extremes may be able to utilize different food supplies, whereas intermediates may not be able to use either food source as well. 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 do not crack either type of seed as well. Therefore, there is a shift in the population to both end of the distribution.
View this animation to review the three modes of selection:
Balancing selection maintains 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 who 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 who 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 through balancing selection.
Another example of balancing selection is frequency-dependent selection. This type of selection is common among traits that affect predator-prey relationships. If a trait provides the prey with an ability to avoid predation, then individuals with this trait are less likely to be eaten 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, for example a cryptic coloration, then there is a greater chance that a predator species will eventually adapt to be able to detect that cryptic coloration, and having the trait will no longer increase the fitness of the prey. However, prey with a different coloration may become more fit because the predators do not see that coloration as a potential meal. In this system, a phenotype only leads to increased fitness in the prey population if it is maintained in the population at a low level; once it becomes too common, it loses its effectiveness.
Sexual selection describes the acquisition or selection of a mate based on a specific heritable trait; therefore, it is special case of natural selection because the mate or a rival for mates, not the environment, is the selective agent. 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 (Figure 5); female peahens (Figure 4) are more likely to mate with males bearing long, brightly colored tail feathers with a larger number of eye spots (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. So there is a balance between sexual and natural selection. If female preference pushes a trait further in one direction, natural selection will push back if the exaggerated trait reduces the fitness of the male because he cannot survive to reproduce.
Figure 4. Peahen, female pea fowl
Figure 5. Peacock, male pea fowl
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 change the allele frequencies 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 (Figures 6 and 7) and Dynastes hercules(Figures 8 and 9). Note: the males of both species are over 12 inches long!
Figure 6. M. cervicornis male
Figure 7. M. cervicornis female
Figure 8. D. hercules male
Figure 9. D. hercules female
Another example of sexual selection is observed in fantail darter fish (Etheostoma flabellare; Figure 10). Females show a mating preference toward males that are already guarding eggs. This behavior is presumably a result of the increased fitness of males that are able 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, increasing their fitness.
Figure 10. Fantail Darter. (Click image to enlarge)
Test Your Understanding: Natural Selection and Herbicide Resistance
Paraquat is a broad spectrum herbicide that will kill most plants. It is not selective in its action and is used primarily when a large stand of vegetation needs to be cleared. The herbicide works by interfering with the normal photooxidation process in Photosystem I.
Resistance to this herbicide has been reported in a number of plants, including ferns. Resistance is due to a recessive gene. A power company wants to clear a large stand of ferns that have grown underneath power lines and decides to spray the area with this herbicide. After two weeks a worker returns to the site and notices that about 4% of sporophyte stage ferns survive and appear resistant. A closer inspection reveals that about 20% of the gametophytes have survived.
- Based on what you know about the ploidy-level of the sporophyte and gametophyte stages of the fern life cycle (remember alternation of generations), together with what you know about population genetics, explain why there is a difference in resistance frequencies between the sporophyte and gametophyte stages in the life cycle.
- If the frequency of resistant sporophyte-stage ferns is 16%, use the Hardy-Weinberg equation to predict the frequency of resistant gametophyte-stage ferns. What percentage of the gametophyte-stage ferns are susceptible to paraquat?
- If the power company continues to spray this area with paraquat, predict what will happen to the frequency of the resistance allele in this population. What mode of selection is this an example of?
Natural selection is one of the most important concepts in evolutionary biology 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 most well-adapted organisms in a given environment will be more likely to have more of their alleles represented in 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, neutral and deleterious alleles arise spontaneously, but only the beneficial alleles will increase in frequency over successive generations through selection.
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).
To end the course, we will consider what makes a group of organisms a species, and some current ideas about how to define species.
After reading this tutorial, you should have a working knowledge of the following terms:
Questions? Either send your instructor a message through ANGEL or attend an office hour (the times are posted on ANGEL).