Evidence of Natural Selection: Videos & Practice Problems

Natural selection, a concept introduced by Charles Darwin, explains how populations evolve through the survival and reproduction of individuals with advantageous traits in their environment. This process can be observed in real-time, such as antibiotic resistance in bacteria like MRSA and coat color adaptation in rock pocket mice. In both cases, variation exists within populations, and environmental pressures favor certain traits, leading to evolutionary changes. Understanding these mechanisms is crucial for grasping the dynamics of evolution and the impact of environmental factors on species adaptation.

Natural and Artificial Selection

Understanding natural selection becomes clearer when compared to artificial selection, both of which were concepts introduced by Charles Darwin in his seminal work, On the Origin of Species. Artificial selection refers to the selective breeding of organisms based on traits that are favored by humans. This practice is common in agriculture, where farmers breed crops or animals to enhance desirable characteristics. For instance, pigeon breeding was a popular hobby in the 1800s, leading to the creation of various "fancy" pigeons with unique traits, such as puffed chests or feathered feet. This selective process demonstrates how humans can influence the traits of offspring by choosing specific parent organisms.

Similarly, dogs, which are descendants of gray wolves, showcase the results of artificial selection. By selecting for traits like size, coat type, and temperament, humans have developed a wide variety of dog breeds, from bulldogs to poodles. This selective breeding highlights the power of human choice in shaping the characteristics of a species.

In contrast, natural selection operates through environmental pressures rather than human intervention. It involves the reproduction of individuals with traits that are advantageous in their specific environments. For example, Darwin studied the finches of the Galapagos Islands, noting that different islands had varying food sources that favored distinct beak shapes. Some finches developed larger beaks to crack tough seeds, while others evolved smaller beaks for different diets. This illustrates how environmental factors can lead to the evolution of species over time.

Natural selection can also be observed in big cats, such as lions, tigers, and jaguars, which likely share a common ancestor. Depending on their habitats and the challenges they face, different traits are favored, resulting in the diverse adaptations seen in these species today. Overall, both artificial and natural selection demonstrate the mechanisms through which traits are passed on and how species evolve, albeit through different processes.

Evidence of Natural Selection Example 1

In understanding the concepts of natural selection and artificial selection, it is essential to recognize how both processes influence the traits within a population over time. Both natural and artificial selection lead to changes in trait frequency, indicating that they share a fundamental mechanism of altering genetic characteristics within a population. A key aspect of both natural and artificial selection is that organisms with certain advantageous traits tend to reproduce at higher rates. In natural selection, this advantage arises from environmental pressures, while in artificial selection, human intervention plays a crucial role in determining which traits are favored. This distinction highlights the different driving forces behind reproduction in each context. Artificial selection is characterized by human control over breeding practices, which directly influences the traits that are passed on to future generations. This contrasts with natural selection, where competition for resources and mates in the environment dictates survival and reproductive success. Furthermore, both processes require variation within the population. Without genetic diversity, neither natural nor artificial selection can occur, as there would be no traits to select for or against. In summary, while natural selection and artificial selection share similarities in their outcomes—such as changes in trait frequency and the necessity of variation—they differ fundamentally in their mechanisms. Natural selection is driven by environmental factors, whereas artificial selection is guided by human choices. Understanding these distinctions is crucial for grasping the broader concepts of evolution and genetic change.

Kale, brussels sprouts, cabbage, broccoli, and kohlrabi are all vegetables that come from selecting for different traits in the wild mustard (Brassica oleracea). For example, to produce kale, humans selected plants with the large leaves. Over repeated generations of selection this led to the modern plant kale. This is an example of:

• A. Natural Selection.
• B. Artificial Selection.
• C. Homology.
• D. Use and Disuse.

Answer: (B)

Which statement identifies the main difference between natural selection and artificial selection?

• A. Natural selection is controlled by the environment, while artificial selection is controlled by humans.
• B. Natural selection only influences phenotype, while artificial selection influences genotype.
• C. Natural selection works on existing variation in the community, while artificial selection introduces variation into the community.
• D. Natural selection has the goal of cultivating specific characteristics, while artificial selection does not.

Answer: (A)

Evidence of Natural Selection

Natural selection is a fundamental mechanism through which populations evolve, and it can be observed in real-time through various examples. Two notable instances of natural selection are antibiotic resistance in bacteria, specifically methicillin-resistant Staphylococcus aureus (MRSA), and the coat color variation in rock pocket mice.

In the case of MRSA, the process begins with a population of bacteria that exhibits variation in their resistance to antibiotics. When antibiotics are introduced as a selection pressure, they kill off the susceptible bacteria, leaving behind those that are resistant. This results in a difference in fitness, where the resistant bacteria have a higher chance of survival and reproduction. Over time, as the resistant bacteria continue to reproduce, they become more prevalent in the population. This phenomenon highlights the challenge of antibiotic resistance, which is recognized as a significant public health issue in the 21st century.

The second example involves rock pocket mice that inhabit the desert southwest, where their coat colors vary between brown and black. In areas with black volcanic rock, the selection pressure from predators favors the black mice, as they are better camouflaged against the dark substrate. This leads to a higher fitness for the black mice, allowing them to survive and reproduce more successfully than their tan counterparts. As a result, the frequency of black fur in the population increases over generations. Researchers have extensively studied the genetics of these mice, providing insights into how specific genes contribute to coat color adaptation in response to environmental conditions.

Both examples illustrate the principles of natural selection, emphasizing the importance of variation, selection pressures, and differential fitness in driving evolutionary change. Understanding these concepts is crucial for grasping how populations adapt to their environments over time.

Evidence of Natural Selection Example 2

The peppered moths provide a classic illustration of evolution through natural selection, particularly through the phenomenon known as industrial melanism. In the early 1800s, the majority of peppered moths were light-colored with small dark spots, which allowed them to blend in with the lichen-covered trees. However, during the Industrial Revolution, soot from factories darkened the environment, leading to a significant shift in the moth population. By the early 1900s, most moths had evolved to be fully black, as this coloration provided better camouflage against the soot-covered surfaces.

Natural selection acts on variations within a population, and in this case, the key variation was the color of the moths—light versus dark. In the early 1800s, the light-colored moths had higher fitness because they were better camouflaged against the lichen, allowing them to evade predators and reproduce more effectively. As the environment changed due to industrial pollution, the fitness landscape shifted. By the late 1800s, the black-colored moths became more prevalent, as they were now better suited to survive in the soot-covered environment.

Today, the population of peppered moths has reverted to being predominantly light-colored. This change is attributed to the reduction of soot in urban areas, allowing the lichen to thrive once again. The concept of fitness is dynamic and depends on environmental conditions; as these conditions change, so too does the advantage conferred by specific traits. This ongoing cycle exemplifies the principles of natural selection, where the traits that enhance survival and reproduction in a given environment become more common over time.

Bedbugs are a pest that has been a nuisance throughout human history. In the 1940s, the widespread introduction of the pesticide DDT allowed people to effectively kill bedbugs at home. However, a mutation in the gene that codes for sodium channels in these insects provided resistance to DDT in some bedbugs. DDT was banned in 1974, but most bedbugs today are still resistant to DDT. What could be an explanation for why most bedbugs still have resistance to DDT?

• A. The mutation that provides resistance to DDT also provides resistance to other commonly used pesticides.
• B. Bedbugs have retained resistance in case DDT is used in the future.
• C. The large amount of DDT used before 1974 bred for exceptionally high levels of resistance, meaning resistance is unlikely to wear off for a very long time.
• D. Once a trait has been selected for, a trait will remain in the population going forward.

Answer: (A)

A proposed strategy for combatting the spread of antibiotic resistance is to allow doctors to prescribe only a certain subset of antibiotic drugs and regularly rotate which antibiotics are allowed to be prescribed over several years. Why could this approach limit the spread of antibiotic resistance?

• A. Using different antibiotics at different times will prevent your body from developing too strong a resistance to any one drug.
• B. During times when a specific antibiotic is NOT being used, those bacteria with resistance will not have higher fitness and are more likely to be removed from the population.
• C. Removing the use of certain antibiotics lessens the chance that bacterial genomes will mutate in ways that provide resistance to that specific antibiotic.
• D. This approach would allow bacteria to evolve resistance to less powerful and, therefore, less important drugs while keeping resistance low to more powerful and important antibiotics.

Answer: (B)

Here’s what students ask on this topic:

What is the difference between natural selection and artificial selection?

Natural selection and artificial selection are both processes that result in changes in the traits of populations over time. The key difference lies in the driving force behind the selection. Natural selection is driven by environmental pressures, where individuals with advantageous traits are more likely to survive and reproduce, passing those traits to the next generation. In contrast, artificial selection is driven by human intervention, where individuals with desired traits are selectively bred to produce offspring with those traits. An example of natural selection is the adaptation of finches on the Galapagos Islands, while an example of artificial selection is the breeding of dogs from wolves to create various breeds.

How does antibiotic resistance in bacteria demonstrate natural selection?

Antibiotic resistance in bacteria is a clear example of natural selection. In a population of bacteria, there is variation in resistance to antibiotics. When antibiotics are used, they act as a selection pressure, killing susceptible bacteria while resistant ones survive. The resistant bacteria then reproduce, passing on their resistance genes to the next generation. Over time, the population evolves to become more resistant to the antibiotic. This process demonstrates natural selection as the environment (presence of antibiotics) favors individuals with advantageous traits (antibiotic resistance), leading to evolutionary changes in the population.

What evidence supports the theory of natural selection?

Several lines of evidence support the theory of natural selection. One example is the observation of antibiotic resistance in bacteria, where resistant strains become more common due to the selective pressure of antibiotic use. Another example is the adaptation of the rock pocket mouse, where coat color changes in response to the color of the substrate they live on, providing better camouflage from predators. Additionally, the diversity of finch species on the Galapagos Islands, each adapted to different food sources, supports natural selection. These examples show how environmental pressures can lead to the survival and reproduction of individuals with advantageous traits, driving evolutionary change.

How do the finches on the Galapagos Islands illustrate natural selection?

The finches on the Galapagos Islands are a classic example of natural selection. These finches, which are believed to have a common ancestor, have evolved into different species with varying beak shapes and sizes. The differences in beak morphology are adaptations to the specific food sources available on their respective islands. For instance, finches with large, strong beaks can crack open hard seeds, while those with smaller, pointed beaks are better suited for eating insects. The environmental pressures on each island favored certain beak traits, leading to the survival and reproduction of individuals with those advantageous traits, illustrating the process of natural selection.

What role does variation play in natural selection?

Variation is a crucial component of natural selection. It refers to the differences in traits among individuals within a population. For natural selection to occur, there must be variation in traits that affect an individual's ability to survive and reproduce. These variations can be due to genetic differences, mutations, or recombination during reproduction. When environmental pressures are applied, individuals with advantageous traits are more likely to survive and reproduce, passing those traits to the next generation. Without variation, there would be no differential survival and reproduction, and natural selection could not drive evolutionary change.