Natural Selection: Videos & Practice Problems

Natural selection, a key mechanism of evolution, is based on two observations: variation is inherited and species tend to overproduce offspring. This leads to the inference that certain traits enhance survival and reproduction, resulting in evolutionary changes in populations over time. For example, in snowshoe hares, coat color variation affects camouflage and survival rates. Natural selection acts on existing genetic variation, increasing adaptations to current environments, while fitness is relative and context-dependent. Ultimately, populations evolve, not individuals, as traits become more prevalent through generations.

Natural Selection: Observations and Inferences

Natural selection is a fundamental mechanism of evolution, grounded in two key observations and two logical inferences. These principles, often referred to as Darwin's four postulates, are essential for understanding how natural selection operates within populations.

The first observation is that variation exists within populations. Not all organisms are identical; instead, there is a range of traits, such as coat color in rabbits, which can be inherited from parents to offspring. This genetic variation is crucial as it provides the raw material for natural selection to act upon.

The second observation is overproduction. Many species, including rabbits, tend to produce more offspring than their environment can support. For instance, a female rabbit can have over ten offspring each year, and these young rabbits can begin reproducing at just a few months old. Despite this potential for rapid population growth, not all offspring survive to adulthood due to limited resources and predation, leading to a high mortality rate among young rabbits.

From these observations, we can draw two inferences. The first inference is the concept of selection. Since not all individuals can survive and reproduce, certain traits enhance an organism's chances of survival and reproduction, while others diminish those chances. For example, in a habitat where brown rabbits are better camouflaged than white rabbits, the brown rabbits are more likely to evade predators and reproduce successfully, while the white rabbits may be more easily caught.

The second inference relates to evolution itself. Over time, traits that confer a survival advantage will become more common in the population. If brown rabbits are consistently surviving and reproducing at higher rates, the population will gradually shift to include more brown rabbits and fewer white rabbits. This process exemplifies evolution as a change in the genetic makeup of a population over time.

Understanding these concepts provides a foundation for exploring the complexities of natural selection and its role in shaping biodiversity. As we delve deeper into this topic, we will uncover the nuances and implications of these fundamental principles.

Natural Selection Example 1

The evolution of giraffes' long necks is a classic example of natural selection, illustrating how certain traits can become more common in a population over time. This process begins with variation, where individuals within a species exhibit differences—in this case, neck length. Some ancestral giraffes had shorter necks, while others had longer necks, creating a range of neck lengths within the population.

Next, we consider overproduction, a common characteristic of many organisms, including giraffes. More giraffes are born than can survive to reproduce, leading to competition for resources. In this scenario, the selection pressure favors those with longer necks. These giraffes can reach higher foliage on tall trees, allowing them to access food that shorter-necked individuals cannot. As a result, the longer-necked giraffes are more likely to survive and reproduce, passing on their advantageous trait to the next generation.

Over time, this leads to a shift in the population's average neck length. As shorter-necked individuals reproduce less frequently, their traits become less common, while the traits of longer-necked giraffes become more prevalent. This gradual change in the population exemplifies the process of evolution, demonstrating how natural selection can drive the development of specific traits based on environmental pressures and survival advantages.

Which statement below best restates one of the key observations central to the concept of Natural Selection?

• A. New environments will introduce new variation to a population.
• B. Some variation has a developmental basis or is influenced by the environment.
• C. Some individuals will have more offspring survive and reproduce than others.
• D. Traits become more common in a population when they are used more often.

Answer: (C)

In a population of crickets, all crickets make a “chirp” to attract mates at the same rate. Chirping also attracts predators, however. In one season, more than 50% of the crickets are eaten as prey as a result of chirping. In this simplified example, using only the information given, do you think this population will evolve by natural selection and why?

• A. Yes, because chirping increases reproduction, you can expect chirping to be more frequent in the future.
• B. Yes, because chirping attracts predators, there is a different rate of survivorship among crickets.
• C. No, because chirping attracts both predators and mates, the two effects will cancel each other out.
• D. No, because there is no initial variation in chirping, the population cannot change through natural selection.

Answer: (D)

Important Insights for Understanding Natural Selection

Natural selection is a complex process that involves various factors influencing the survival and reproduction of organisms within a population. A prime example of this is observed in snowshoe hares, which exhibit color variation—some turn white in winter for camouflage, while others remain brown, particularly in warmer regions. This variation is crucial, as natural selection operates on existing differences within a population. If all hares were the same color, there would be no basis for selection, and thus no evolutionary change.

Snowshoe hares can produce a significant number of offspring, with females birthing over 12 young annually. However, not all offspring survive due to predation, especially in years with little snow, where white hares are more easily spotted by predators. Consequently, the population's composition shifts over time, favoring the brown hares that are better adapted to their environment during these conditions. This leads to a decrease in the frequency of white hares in the population.

Key to understanding natural selection is the concept of fitness, which refers to an organism's ability to survive and reproduce relative to others in the same environment. Fitness is not absolute; it is context-dependent. For instance, a hare that produces one offspring may seem successful, but if another produces 82, the latter has a significantly higher fitness. Environmental factors play a critical role in determining which traits are advantageous at any given time.

Survival and reproduction are probabilistic events. In a population where 80% of offspring die, a trait that reduces mortality to 70% can significantly influence the population over time. However, it is essential to note that evolution occurs at the population level, not at the individual level. While individual hares may survive or perish, it is the frequency of traits within the population that changes, leading to evolutionary shifts.

Natural selection enhances the number of adaptations suited to the current environment. As conditions change, the traits that confer higher fitness will also shift, leading to further evolution. This dynamic process illustrates how populations adapt over time, responding to environmental pressures and variations in survival probabilities.

Natural Selection Example 2

The study of medium ground finches on Daphne Major in the Galapagos Islands provides a compelling example of natural selection and evolutionary change in response to environmental pressures, specifically a major drought in 1977. Researchers Rosemary and Peter Grant meticulously cataloged the traits of these birds over many years, allowing them to track changes in beak depth, a critical trait for survival during food shortages.

Before the drought, the average beak depth of the finch population was 9.2 millimeters. After the drought, the survivors that reproduced had an average beak depth of 9.9 millimeters. This indicates a change of 0.7 millimeters, highlighting that even small changes in physical traits can have significant implications for survival and reproduction in a population.

Analysis of the data reveals that birds with deeper beaks had higher fitness during the drought. The distribution of beak sizes shows that a larger percentage of birds with deeper beaks survived and reproduced compared to those with smaller beaks. This suggests that deeper beaks provided a reproductive advantage, allowing these birds to access food resources more effectively during the challenging conditions of the drought.

However, it is important to consider whether this trend is consistent across different years. While deeper beaks were advantageous during the drought, it cannot be assumed that this will always be the case. Previous populations had smaller beaks, indicating that environmental conditions and food availability can influence which traits are favored in a given year. Thus, the relationship between beak depth and survival may vary depending on the specific environmental context.

Looking ahead, the offspring produced in 1977 are likely to resemble the survivors from that year, as beak depth is a heritable trait. The average beak depth of the offspring was approximately 9.7 millimeters, reflecting the traits of the successful parents. This example illustrates how natural selection can lead to evolutionary changes in a population, even if the changes are relatively small, reinforcing the concept that evolution is a gradual process driven by environmental pressures.

Herbicides, chemicals administered by farmers to kill weeds, have been hugely beneficial in increasing the efficiency of modern agriculture. Many of the herbicides used today, however, are less effective than when they were originally introduced because weed populations have evolved resistance. Knowing this, which of the following statements is likely true?

• A. Individual weeds that were able to adapt to the presence of herbicides survived, allowing the species to evolve.
• B. The herbicides must negatively affect both the survival and reproductive ability of the weed species.
• C. Herbicide resistance evolved because some species were able to outcompete other species in the presence of the herbicide.
• D. Species that developed herbicide resistance must have had pre-existing heritable variation as to how likely they were to survive in the presence of an herbicide.

Answer: (D)

Which of the following statements correctly explains a way in which evolution by natural selection and Lamarck's ideas about evolution are different?

• A. Lamarck focused on an organism's ability to reproduce, whereas natural selection focuses on survival.
• B. Natural selection requires variation to already exist in a population, whereas Lamarck said that variation could be created through use and disuse.
• C. Natural selection explains how individuals will adapt to new environments, whereas Lamarck explained how populations change in their current environments.
• D. Lamarckian explanations did not explain how populations become better adapted overtime, whereas natural selection did explain adaptation.

Answer: (B)

Here’s what students ask on this topic:

What are the main observations and inferences that form the basis of natural selection?

Natural selection is based on two main observations and two inferences. The first observation is that variation is inherited; not all organisms in a population are identical, and much of this variation is passed from parent to offspring. The second observation is overproduction; species produce more offspring than the environment can support, leading to competition for resources. From these observations, we infer that certain traits enhance survival and reproduction, making those traits more common in the population over time. This leads to the second inference: evolution, defined as a change in the population over time as advantageous traits accumulate.

How does natural selection lead to evolution?

Natural selection leads to evolution by favoring traits that enhance an organism's survival and reproduction. When a population has genetic variation, certain traits may provide a survival advantage in a given environment. These advantageous traits increase an organism's fitness, or likelihood to reproduce. Over time, these traits become more common in the population as they are passed on to successive generations. This gradual change in the frequency of traits within a population is what we call evolution. For example, in snowshoe hares, coat color affects camouflage and survival rates, leading to more brown hares in areas with less snow.

Why is genetic variation important for natural selection?

Genetic variation is crucial for natural selection because it provides the raw material for evolution. Without variation, all individuals in a population would be identical, and natural selection would have no traits to favor or disfavor. Variation allows some individuals to possess traits that give them a survival or reproductive advantage in a specific environment. These advantageous traits become more common over generations, leading to evolutionary changes. For instance, in snowshoe hares, the presence of both white and brown coat colors allows natural selection to favor the color that provides better camouflage in varying snow conditions.

What is the role of fitness in natural selection?

Fitness in natural selection refers to an organism's ability to survive and reproduce in its environment. It is a relative measure, meaning an organism's fitness is compared to others in the population. Traits that enhance fitness increase an organism's chances of surviving and reproducing, thereby passing those traits to the next generation. For example, in a population of snowshoe hares, brown hares may have higher fitness in areas with less snow because they are better camouflaged from predators. Over time, the frequency of the brown coat trait increases in the population, demonstrating evolution driven by natural selection.

How does the environment influence natural selection?

The environment plays a critical role in natural selection by determining which traits are advantageous. Environmental conditions, such as climate, availability of resources, and presence of predators, influence which traits enhance an organism's survival and reproduction. For example, in snowshoe hares, the amount of snow affects which coat color provides better camouflage. In snowy environments, white hares are less visible to predators, while in areas with less snow, brown hares have a survival advantage. As environmental conditions change, the traits that confer higher fitness may also change, leading to shifts in the population's genetic makeup over time.