Problem 8 - AP Psychology Premium Prep 2021

Question

Refer to the following scenario. A cruise ship with 500 people on board crashes on a deserted island. They like the island so much that they decide to stay. There is a small number of people on board with red hair, an autosomal recessive trait. Assume that red hair provides no increase in fitness. Evolution does not occur when a population is said to be in equilibrium by the Hardy-Weinberg definition. In the situation above, which of the following does NOT support the claim that evolution will occur in this population? (A) The situation describes a small population. (B) Mutations occur in this population. (C) People do not usually mate randomly. (D) Natural selection pressures do not exist on this island.

Step-by-Step Solution

Answer

The correct answer is (D) Natural selection pressures do not exist on this island, as it does not support the claim that evolution will occur in this population. It is one of the conditions required for Hardy-Weinberg equilibrium.

Step 1: Understanding the Hardy-Weinberg Equilibrium Principle

The Hardy-Weinberg equilibrium is a fundamental concept in the field of population genetics. It describes a theoretical situation in which allele frequencies in a population remain constant over generations, meaning no evolution occurs. There are five conditions that must be met for a population to be in Hardy-Weinberg equilibrium: 1. Large population size 2. No mutation 3. Random mating 4. No gene flow (migration) 5. No selection If any of these conditions are not met, it is said that the population is evolving.

Step 2: Analyzing the Statements

Now, let's consider each given statement: (A) The situation describes a small population: A small population size is more susceptible to genetic drift, which can alter allele frequencies over time. Thus, this statement supports the claim that evolution will occur. (B) Mutations occur in this population: Mutations introduce new alleles into the gene pool, and if these mutations affect the individuals' fitness, it can lead to evolution. Therefore, this statement also supports the claim. (C) People do not usually mate randomly: Non-random mating can lead to changes in allele frequencies since certain traits may be either positively or negatively selected for. This statement supports the claim for evolution. (D) Natural selection pressures do not exist on this island: The absence of natural selection pressures means that there is no force actively driving changes in allele frequencies based on individuals' fitness. In this case, it does not support the claim that the population will evolve, as it is one of the conditions required for Hardy-Weinberg equilibrium.

Step 3: Conclusion

Among the given statements, only statement (D) does NOT support the claim that evolution will occur in this population. So, the correct answer is (D) Natural selection pressures do not exist on this island.

Key Concepts

Population Genetics

Population genetics is a branch of biology that studies the genetic composition of biological populations, and the changes in genetic composition that result from the operation of various factors, including natural selection, genetic drift, mutation, and gene flow. It's crucial to understanding how species evolve and adapt over time.

In our scenario of the cruise ship passengers, population genetics would examine how the allele for red hair might spread or diminish as they establish a new population on the island. Factors like the initial number of red-haired individuals, the size of the overall population, and how they choose their mates would all influence the genetic makeup of future generations on the island.

Allele Frequencies

Allele frequencies refer to how common or rare certain alleles are within a population. An allele is a variant form of a gene, and in a given population, the sum of the allele frequencies for a specific gene always adds up to 1.

If we look at the example of red hair on the island population, if the allele for red hair is rare, it will have a low frequency. Understanding allele frequencies is fundamental for predicting how the traits will pass on to future generations. The Hardy-Weinberg principle states that these frequencies should remain constant from one generation to the next unless influenced by specific factors like mutation or non-random mating.

Genetic Drift

Genetic drift is a random process that can cause the allele frequencies in a population to change over time. This effect is particularly strong in small populations where chance events can have a greater impact on the genetic diversity of the population.

In the case of the island population with a small number of people, genetic drift could have a significant influence. For example, if the few red-haired individuals happen to have more offspring, the allele for red hair could become more common in the next generation purely by chance.

Mutations

Mutations are changes to the genetic material of an organism. They can result in new alleles entering a population's gene pool. Most mutations are neutral, having no effect on an organism's fitness, while others can be beneficial or harmful.

For the island inhabitants, a mutation might occur that could affect the frequency of the red hair allele. If a mutation happened in other genes that provides some advantage or disadvantage, this could also lead to evolution as the individuals with the mutation could have different reproductive success.

Non-Random Mating

Non-random mating occurs when individuals choose their partners based on certain traits, rather than pairing by chance. This can lead to an increase in certain alleles and a decrease in others, influencing the genetic structure of the population.

On the island, if individuals preferentially choose mates with or without red hair, this will affect the allele frequencies of the red hair allele. Such mating preferences would disrupt the Hardy-Weinberg equilibrium by not mating randomly, one of the principle's key criteria for a population to not evolve.