Polymorphism and Natural Selection

Polymorphism Overview

  • Definition of Polymorphism: A condition in a population/species with

    • At least 2 alleles of a gene.

    • At least 2 alleles must have a frequency greater than 1%.

  • Causes of Polymorphism:

    • Results from natural selection acting on genetic variation created by mutations.

    • New alleles arise from mutations, which occur at low rates.

    • For a new allele to significantly increase in frequency, it must confer a selective advantage.

Balanced Polymorphism

  • Definition: A specific type of polymorphism where

    • There are at least 2 alleles of a gene.

    • Heterozygotes (individuals with two different alleles) have a higher fitness than homozygotes (individuals with two identical alleles).

  • Genetic Statistics: Approximately 28% of genes are known to be polymorphic.

Example of Balanced Polymorphism: Sickle Cell Trait

  • Structure of Hemoglobin:

    • Composed of 4 polypeptide chains: 2 alpha (α) and 2 beta (β).

    • Each α chain has 141 amino acids; each β chain has 146 amino acids.

  • Sickle Cell Mutation:

    • Caused by a point mutation (a change in one nucleotide) in the β chain, leading to one altered amino acid.

Health Implications of Sickle Cell Disease

  • Health Issues:

    • Anemia

    • Poor growth

    • Circulatory problems affecting the heart, kidneys, and brain

    • Episodes of severe pain

    • Decreased lifespan

  • Management Approaches:

    • Pain medication

    • Bone marrow transplant

    • Gene therapy to promote fetal hemoglobin or replace stem cells with edited cells.

Geographic Distribution of Sickle Cell Allele

  • Polymorphic Nature: In certain regions, the frequency of the sickle cell allele ranges from 5% to 15%.

  • Malaria Association:

    • Malaria, transmitted by mosquitoes, significantly affects populations in Africa and Asia.

Heterozygote Advantage in Malaria

  • Clinical Observations:

    • Heterozygotes for the sickle cell allele show lower rates of malaria infection than homozygotes.

  • Mechanism:

    • Heterozygotes experience selective destruction of red blood cells (RBCs), where infected cells sickle and die, killing the malaria parasites.

    • This confers a dual advantage: oxygen delivery and suppression of parasitic growth.

Environmental Impact on Sickle Cell Trait Evolution

  • Selective Advantage: The advantage of heterozygotes is significant only in malarial environments.

  • Genotype Ratios: Typical offspring genotypes from a population include 1 AA (normal), 2 AS (sickle cell carriers), and 1 SS (sickle cell disease).

  • Evolutionary Context: The spread of farming practices like slash-and-burn agriculture may have influenced the distribution of the sickle cell allele, providing selective benefits in regions affected by malaria.

Other RBC Polymorphisms

  • Thalassemia:

    • Characterized by survival advantages in heterozygotes, homozygous individuals typically experience early mortality.

  • G-6-PD Deficiency:

    • Individuals are often phenotypically normal and also exhibit a survival advantage in malaria-endemic regions.

Conclusion: Polymorphism and Natural Selection

  • Natural selection operates on the variability within populations, ensuring that different RBC mutations (sickle cell, thalassemia, G-6-PD) persist due to their advantages in malaria environments.

  • Populations showcase varied adaptations to environmental stress through a combination of mutation and selective pressures, highlighting evolution’s opportunistic nature.