Natural Selection: Videos & Practice Problems

Natural selection is a key mechanism of evolution that alters allele frequencies based on fitness, leading to adaptations. It manifests in three patterns: directional selection shifts traits toward one extreme, stabilizing selection favors average traits, and disruptive selection promotes both extremes. Additionally, balancing selection maintains multiple alleles through frequency-dependent selection and heterozygote advantage, exemplified by sickle cell trait's protection against malaria. Understanding these concepts is crucial for grasping evolutionary dynamics and species adaptation.

Natural Selection

Natural selection is a fundamental mechanism of evolution, primarily responsible for producing adaptations in organisms. It operates by altering the frequency of specific alleles within a population, driven by the fitness of those alleles. This process can be categorized into three main patterns based on which traits exhibit greater fitness: directional selection, stabilizing selection, and disruptive selection.

Directional selection occurs when one extreme of a trait distribution is favored. For instance, in a population of Darwin's finches, if only large seeds are available, birds with larger beaks will have higher fitness. Over time, this leads to a shift in the average phenotype of the population towards larger beaks, illustrating how directional selection can change the characteristics of a population in response to environmental pressures.

In contrast, stabilizing selection favors the average phenotype, reducing variation by selecting against extreme traits. For example, if a stable mix of seed sizes is present, birds with average-sized beaks will thrive, while those with very small or very large beaks may struggle. This type of selection stabilizes the population's traits without changing the average phenotype, as it eliminates the extremes.

Disruptive selection, on the other hand, favors both extremes of a trait distribution while selecting against the average. In a scenario where only small and large seeds are available, birds with either small or large beaks would be more successful than those with average-sized beaks. This can lead to an increase in variation within the population and potentially result in a bimodal distribution of traits. If this process continues, it may even lead to the emergence of two distinct phenotypes or the formation of new species.

Overall, natural selection plays a crucial role in shaping the diversity of life by influencing allele frequencies and promoting adaptations that enhance survival and reproduction in varying environments.

Natural Selection Example 1

In the context of natural selection, different types of selection can lead to varying changes in the average phenotype of a population. Understanding these types is crucial for predicting evolutionary outcomes.

In the first scenario involving a population of ginkgo trees, the fitness of trees is highest at a medium height, as very short trees fail to receive adequate sunlight and very tall trees are prone to wind damage. This situation exemplifies stabilizing selection, where the average phenotype remains stable over time because the extremes are less fit. Therefore, one would expect no change in the average phenotype of this population.

Next, consider the population of oysters. Here, light-colored oysters blend in with shallow rocks, and dark oysters camouflage against dark shadows, while medium-colored oysters are more easily preyed upon. This scenario illustrates disruptive selection, where the average phenotype is disadvantageous, leading to an increase in phenotypic variation. As a result, we would expect to see a rise in the numbers of both light and dark oysters, while the medium-colored oysters would decrease.

Lastly, the evolution of servals, a wildcat species, from normal-sized ears to larger ears optimized for hunting represents directional selection. In this case, the average phenotype shifts towards larger ear size over generations, indicating a clear trend in one direction. Thus, one would predict that the average phenotype will change to reflect larger ears in the population.

These examples highlight how different selection pressures can shape the characteristics of populations, influencing their evolutionary trajectories.

Which of the following statements regarding disruptive selection is true?

• A. The average phenotype has the highest fitness.
• B. It can lead to an increase in genetic variation.
• C. One particular trait becomes more common.
• D. Genotype frequencies remain constant.

Answer: (B)

The graph below shows the birthweights and infant mortality rate as recorded by researchers in London between 1935 and 1946. The bars represent the number of births recorded in 0.5-pound increments. The orange dots represent the mortality rate for each birthweight and are shown on a logarithmic scale. What type of selection does this graph suggest directly affects human birth weight?



Graph adapted from: M.N. Karn and L.S. Penrose, Birth Weight and Gestation Time in Relation to Maternal Age, Parity, and Infant Survival. Annals of Eugenics, January 1951, Vol. 16 No. 1., Page 147-164.

• A. Diversifying selection.
• B. Stabilizing selection.
• C. Artificial selection.
• D. Directional selection.

Answer: (B)

The graph below shows the average tail length of a population of deer mice. The first graph shows the tail length of the mice while living in a prairie ecosystem, and the second shows the tail length many generations after a group of mice moved into a forest ecosystem. Which of the following types of natural selection describes this scenario, and what conclusion could you draw from these data?

• A. Stabilizing selection: shorter-tailed mice had increased fitness in the forest ecosystem.
• B. Stabilizing selection: longer-tailed mice had increased fitness in the forest ecosystem.
• C. Directional selection: shorter-tailed mice had increased fitness in the forest ecosystem.
• D. Directional selection: longer-tailed mice had increased fitness in the forest ecosystem.

Answer: (D)

Balancing Selection

Natural selection plays a crucial role in shaping allele frequencies within populations, favoring alleles that confer high fitness while eliminating those associated with low fitness. However, the presence of multiple alleles for many genes can be explained through a phenomenon known as balancing selection. This type of selection maintains genetic diversity by favoring more than one allele, preventing any single allele from becoming fixed in the population.

Two primary mechanisms of balancing selection are frequency-dependent selection and heterozygote advantage. Frequency-dependent selection occurs when the fitness of a phenotype depends on its frequency relative to other phenotypes. For instance, in a population of snails with varying shell patterns, birds may learn to recognize and prey on the most common shell pattern. As a result, the once common phenotype becomes less favorable, allowing rarer phenotypes to thrive. This dynamic ensures that no single phenotype dominates, thus preserving multiple alleles within the population.

Heterozygote advantage is another critical mechanism, where individuals with two different alleles (heterozygotes) exhibit higher fitness than those with two identical alleles (homozygotes). A classic example of this is seen in the relationship between sickle cell disease and malaria. In regions where malaria is prevalent, individuals with the sickle cell trait (heterozygotes) have a significant survival advantage. They experience mild symptoms from sickle cell but are less susceptible to the severe effects of malaria. In contrast, homozygotes for normal hemoglobin face a medium risk of malaria, while those with sickle cell disease have high mortality risks from both malaria and the disease itself.

To summarize, the interplay of frequency-dependent selection and heterozygote advantage illustrates how balancing selection can sustain genetic diversity within populations. This diversity is essential for the adaptability and resilience of species in changing environments, ensuring that multiple alleles persist over time.

Natural Selection Example 2

In a population exhibiting allele frequencies of p = 0.4 and q = 0.6, the genotype frequencies deviate from those predicted by Hardy-Weinberg equilibrium, indicating the influence of natural selection. To analyze this, we can calculate the expected genotype frequencies under Hardy-Weinberg conditions.

The frequency of the homozygous dominant genotype (RR) is calculated as:

\( p^2 = (0.4)^2 = 0.16 \)

This suggests that we would expect 16% of the population to be homozygous dominant. However, if the observed frequency is lower than this, it indicates that these individuals may not be surviving as well as expected.

Next, the frequency of the heterozygous genotype (Rr) is determined using:

\( 2pq = 2 \times 0.4 \times 0.6 = 0.48 \)

Here, we would expect 48% of the population to be heterozygous. An excess of heterozygotes compared to the Hardy-Weinberg expectation suggests that these individuals are favored by natural selection.

Finally, the frequency of the homozygous recessive genotype (rr) is calculated as:

\( q^2 = (0.6)^2 = 0.36 \)

Similar to the homozygous dominant genotype, if the observed frequency of homozygous recessive individuals is lower than expected, it further supports the idea that these individuals are not surviving as well as the heterozygotes.

In summary, the presence of fewer homozygotes and an excess of heterozygotes indicates a phenomenon known as heterozygote advantage. This occurs when heterozygous individuals have a higher fitness compared to homozygous individuals, leading to an increase in their frequency over generations. Thus, the population is not in Hardy-Weinberg equilibrium due to the selective pressures favoring heterozygotes.

In many species of salmon, there are two mating types in males: large aggressive males and small sneaker males. Large aggressive males fight for territory in which they mate with resident females, while small sneaker males hide and try to quickly enter larger males’ territories to mate with females. “Sneaking” is, on average, a more successful strategy when there are many large males competing for territory, while “Fighting” is, on average, a better strategy when there are few large males present. As described, this scenario describes what two types of selection?

• A. Frequency-dependent selection and stabilizing selection.
• B. Heterozygote advantage and stabilizing selection.
• C. Frequency-dependent selection and disruptive selection.
• D. Heterozygote advantage and disruptive selection.

Answer: (C)

Which of the following statements about frequency-dependent selection are true?
I. In frequency-dependent selection, the rare phenotype is favored.
II. Frequency-dependent selection leads to a loss of genetic variation over time.
III. Frequency-dependent selection is a form of balancing selection.

• A. I & II only.
• B. I & III only.
• C. II & III only.
• D. I, II, & III.

Answer: (B)

Sexual Selection

Sexual selection is a crucial evolutionary process that focuses on traits influencing an organism's ability to obtain mates. While fitness traditionally encompasses survival and reproduction, it is essential to recognize that an organism must also successfully find a mate to reproduce. This concept is particularly relevant when considering the differing investment levels in mating events between sexes.

Typically, females are the high investment sex, producing larger gametes, such as eggs, and often providing more parental care. For instance, female elephants have lengthy gestation periods and nurse their young extensively. In contrast, males usually represent the low investment sex, producing smaller gametes (sperm) and often contributing less to parental care. This disparity in investment leads to sexual selection, which can be observed through sexual dimorphism—distinct differences in secondary sexual characteristics, such as size and coloration, that are not directly related to reproduction.

Sexual selection manifests in two primary forms: intersexual selection and intrasexual selection. Intersexual selection, or mate choice, occurs when the high investment sex (often females) selects mates based on specific traits. This selection process can lead to the development of elaborate features in males, such as the extravagant tails of peacocks, which, while detrimental to survival, enhance mating success. Similarly, bowerbirds construct intricate structures to attract females, showcasing the importance of display in mate selection.

On the other hand, intrasexual selection involves competition within one sex, typically among males, for access to mates or resources that facilitate mating. This competition often revolves around territory control, as seen in elephant seals, where dominant males can secure a significant portion of matings by controlling prime beach territories. The pronounced sexual dimorphism in these seals, with males being substantially larger than females, exemplifies the outcomes of such competition.

To differentiate between the two types of sexual selection, one can remember that intersexual selection involves females evaluating potential mates, while intrasexual selection is characterized by males competing for dominance and access to females. However, it is important to note that these processes often overlap; for example, peacocks may compete for display space while also being subject to female choice, and male elephant seals may engage in competition while females still exercise mate selection.

While sexual dimorphism is evident in many species, it is less pronounced in humans, making it challenging to draw direct parallels between human mating behaviors and those of other organisms. Understanding the complexities of mating systems requires careful consideration of both biological and cultural influences, which can vary significantly across species.

Natural Selection Example 3

Sexual selection is a key concept in understanding mating behaviors and parental investment in various species. In examining three bird populations, we can identify the high investment sex and the type of sexual selection occurring in each case.

In the first example, the red-necked phalarope, males are the high investment sex as they are responsible for paternal care. This means they invest significantly in raising the young, which is a crucial aspect of their reproductive strategy. The females, on the other hand, compete with each other for access to mates, indicating intrasexual selection, where competition occurs within one sex—in this case, among females. This competition can lead to sexual dimorphism, resulting in larger and more aggressive females. It's important to note that while females are typically the high investment sex in many species, this example illustrates a role reversal.

In the second example, the lysine albatrosses present a unique situation where both males and females share parental responsibilities equally. This mutual investment means that neither sex can be classified as the high investment sex, making it a trick question. Additionally, the lack of visible differences between the sexes suggests that neither intersexual nor intrasexual selection is significantly at play, as sexual dimorphism is minimal.

Lastly, in the case of the red-capped mannequin, females are the high investment sex since they are responsible for caring for the young. The males engage in elaborate dances to attract females, which exemplifies intersexual selection. Here, the males display their traits to impress the choosy females, highlighting the dynamics of mate choice where one sex (females) selects mates based on the displays of the other (males).

Overall, these examples illustrate the complexity of sexual selection, where both intersexual and intrasexual selection can occur within the same species, influenced by the specific mating systems and parental investment strategies of the species involved.

Mudskippers, fish that leave the water to mate, perform elaborate dances involving jumping a few inches off the ground to attract mates. This is an example of:

• A. Intersexual selection because it is an example of mate choice.
• B. Intrasexual selection because it is an example of mate choice.
• C. Intersexual selection because it is an example of competition within one sex.
• D. Intrasexual selection because it is an example of competition within one sex.

Answer: (A)

In a 1990 study on caribou by Barrette and Vandal in Behavioral Ecology and Sociobiology, it was observed that in confrontations between male caribou, the males with the larger antlers won 90% of the time. If this is a case of purely intrasexual selection, what else would you expect to be true?

• A. Fighting is a display trait, and females choose males with the larger antlers.
• B. Males with the larger antlers would be expected to provide more paternal care.
• C. Winning a confrontation provides males access to more breeding females.
• D. Winning confrontations would signal to females that the male has higher fitness.

Answer: (C)

Here’s what students ask on this topic:

What is natural selection and how does it affect allele frequencies in a population?

Natural selection is a key mechanism of evolution that alters allele frequencies based on the fitness of those alleles. It leads to adaptations by favoring alleles that enhance survival and reproduction. Over time, alleles with higher fitness become more common, while those with lower fitness are removed. This process can result in directional selection (shifting traits toward one extreme), stabilizing selection (favoring average traits), or disruptive selection (promoting both extremes). Understanding natural selection is crucial for grasping how species adapt and evolve over generations.

What are the three types of natural selection and how do they differ?

The three types of natural selection are directional, stabilizing, and disruptive selection. Directional selection favors one extreme of a trait distribution, causing the average phenotype to shift in that direction. Stabilizing selection favors the average phenotype, reducing variation and maintaining the status quo. Disruptive selection favors both extremes of a trait distribution, increasing variation and potentially leading to the formation of two distinct phenotypes. Each type of selection affects allele frequencies and population traits differently, contributing to the diversity of life forms.

How does balancing selection maintain multiple alleles in a population?

Balancing selection maintains multiple alleles in a population by favoring more than one allele. This can occur through frequency-dependent selection, where the fitness of a phenotype depends on its frequency relative to other phenotypes, or through heterozygote advantage, where individuals with two different alleles have higher fitness than those with two identical alleles. An example of heterozygote advantage is the sickle cell trait, which provides protection against malaria. Balancing selection prevents any single allele from becoming fixed or being completely removed, thus maintaining genetic diversity.

What is sexual selection and how does it differ from natural selection?

Sexual selection is a form of natural selection that focuses on traits affecting an organism's ability to obtain mates. It differs from natural selection, which generally involves traits that enhance survival and overall reproductive success. Sexual selection can lead to the development of secondary sexual characteristics, such as elaborate plumage or mating displays, which may not necessarily improve survival but increase mating success. It can occur through intersexual selection (mate choice) or intrasexual selection (competition within one sex for access to mates).

What is the heterozygote advantage and how does it relate to sickle cell anemia?

Heterozygote advantage occurs when individuals with two different alleles (heterozygotes) have higher fitness than those with two identical alleles (homozygotes). In the case of sickle cell anemia, individuals with one normal hemoglobin allele and one sickle cell allele (heterozygotes) have a higher fitness in malaria-prone regions. They are less likely to die from malaria compared to individuals with two normal alleles and do not suffer from severe sickle cell disease like those with two sickle cell alleles. This advantage maintains both alleles in the population, illustrating balancing selection.