Question

The persistence of the sickle allele at high frequency in a population is an example of ________. a. bottlenecking b. inbreeding c. natural selection d. balanced polymorphism

Short answer

d. balanced polymorphism

Step-by-step solution

Understanding the Question

We need to determine which biological concept explains why the sickle allele remains at a high frequency in a population despite being harmful. The given options are different evolutionary concepts.

Exploring Each Option

Consider each option: - Bottlenecking refers to a sharp reduction in population size. - Inbreeding refers to breeding within a small population without much genetic variation. - Natural selection involves differential survival and reproduction based on traits. - Balanced polymorphism refers to maintenance of two or more alleles in a population due to selective advantage.

Matching the Scenario with Concepts

For the sickle allele, it's well known that carriers (heterozygotes) have an advantage in resisting malaria, which maintains both the normal and sickle alleles in the population. This scenario fits with balanced polymorphism because the heterozygote advantage maintains the allele frequency.

Selecting the Correct Concept

Based on the analysis, balanced polymorphism is the correct explanation for the persistence of the sickle allele due to the selective advantage of heterozygote individuals in malarial regions.

Key concepts

Natural Selection

Natural selection is a fundamental mechanism in the process of evolution. It refers to the way in which certain traits become more or less common in a population based on their impact on survival and reproduction. Traits that provide an advantage in a given environment increase the likelihood of an organism surviving to reproduce, thereby passing those advantageous traits to the next generation. This leads to a gradual increase in the frequency of these beneficial traits in the population.

Natural selection operates on the principle of differential survival. It means that individuals with advantageous traits are more likely to survive and reproduce compared to those without these traits. As a result, over many generations, the population evolves. It is important to note that natural selection doesn't "aim for perfection," but rather acts as an editing process based on current environmental conditions.

When discussing the sickle cell allele, natural selection comes into play because the sickle cell trait confers a survival advantage in malaria-prone areas. This advantage increases the likelihood of gene carriers to survive and pass on their genes, thus maintaining the presence of the trait in the gene pool.

Heterozygote Advantage

The concept of heterozygote advantage is a key reason behind the persistence of certain alleles in populations, particularly those that might otherwise be considered detrimental. It occurs when individuals with two different alleles for a specific gene (heterozygotes) have a better chance of survival compared to those with identical alleles (homozygotes).

When discussing the sickle cell allele, heterozygote advantage plays a crucial role. The individuals who are heterozygous, possessing one normal allele and one sickle cell allele, have a degree of protective advantage against malaria. This occurs because the presence of the sickle cell allele leads to conditions that reduce the reproduction and survival of the malaria parasite within the human body.

Thus, although having two copies of the sickle cell allele can result in sickle cell disease, having just one copy provides significant protection against malaria without the full drawbacks of the disease. This advantage ensures that the sickle cell allele remains prevalent in certain populations, particularly those in regions where malaria is common.

Sickle Cell Allele

The sickle cell allele is a genetic variant found in the hemoglobin gene. Hemoglobin is the protein responsible for carrying oxygen in the blood. In individuals with two copies of the sickle cell allele (homozygous), red blood cells can become sickle-shaped, leading to complications such as pain, anemia, and other severe health issues, known as sickle cell disease.

Interestingly, in regions where malaria is endemic, the sickle cell allele offers a counterintuitive advantage. Carriers of one sickle cell allele and one normal hemoglobin allele (heterozygous) do not develop the full symptoms of sickle cell disease. Instead, they exhibit some resistance to malaria, a deadly disease caused by Plasmodium parasites transmitted by mosquitoes.

This resistance arises because the malaria parasite is less effective in infecting and reproducing within red blood cells altered by the sickle cell trait. This results in a selective pressure that favors carriers of the sickle cell allele in malaria-prone areas, contributing to the allele's persistence in the population. Despite its potential harms, the survival advantage it confers against malaria exemplifies the complex interplay between genetics and environment.