Neo-Darwinism: Study Notes

For the complete syllabus, check out our PMDC MDCAT Syllabus 2025 article.

Neo-Darwinism

Darwin and Mendel

• Darwin's Theory of Natural Selection emerged in 1859, coinciding with Gregor Mendel's work on the Laws of Inheritance from 1856-1863.
• Mendel's genetic laws remained overlooked for about three decades until their rediscovery in the 20th century.
• Darwin's theory lacked a solid genetic foundation.

Neo-Darwinism: Genetic Basis

• Neo-Darwinism is a modified form of Darwinism that explains species' origins based on genetics.
• Genetic variation is the primary driving force in Neo-Darwinism, distinguishing it from classical Darwinism.
• The main difference lies in the type of variation and selection.

Population Genetics

• Population genetics deals with the origin and inheritance of variations.
• It bridges the gap between evolution and genetics, offering insights into the origin of life and species.
• It starts at the individual level and aggregates at the population level.

Reproductive Isolation and Speciation

• Neo-Darwinism emphasizes reproductive isolation in speciation.
• Reproductive isolation amplifies the fittest genes in a gene pool.
• New species originate through the natural selection of the most advantageous genes.

Hardy-Weinberg Theorem:

• The Hardy-Weinberg Theorem, proposed in 1908 by Godfrey Hardy and Wilhelm Weinberg, is a mathematical principle.
• It demonstrates the gene frequencies of different alleles within a population.
• The theorem asserts that the frequency of dominant alleles will not inherently increase.
• In the absence of disruptive factors, genetic variation in a population remains constant between generations.
• Random mating in large populations with no disruptive influences maintains equilibrium in genotype and allele frequencies.

Hardy-Weinberg Equation: Understanding Genetic Equilibrium

Principle and Equilibrium:

• Hardy and Weinberg formulated an equation to calculate allelic frequencies and genotypes in a population.
• This equation represents genetic equilibrium and is known as the Hardy-Weinberg equilibrium.

Using Allelic Frequencies:

• Example: In Drosophila, the gray body color allele (B) is dominant to the black body allele (b).
• When there are no other alleles for body color, the sum of their frequencies equals one.
• Mathematically, if "p" represents the frequency of allele B and "q" represents the frequency of allele b, then p + q = 1.
• Knowing the frequency of one allele (either p or q) allows determination of the other.

Calculating Offspring Genotypes:

• The Hardy-Weinberg equation can be used to calculate the genotypes of offspring.
• It involves terms like p², 2pq, and q², representing the frequencies of different genotypes.

Problem:

• Given allelic frequencies in a population (A = 0.19, a = 0.81).
• Assume Hardy-Weinberg equilibrium.

Calculations:(a) Percentage of Heterozygous Individuals:

• Heterozygotes (Aa) are represented by the 2pq term in the equation.
• Calculate 2 × 0.19 × 0.81 = 0.31 or 31%.

(b) Percentage of Homozygous Recessives:

• Homozygous recessives (aa) correspond to the q² term in the equation.
• Compute 0.81 × 0.81 = 0.66 or 66%.

Factors Influencing the Hardy-Weinberg Theorem

i) Mutations:

• Mutations introduce new genes into a population.
• They can alter allele frequencies over time.

ii) Selection:

• Natural or artificial selection can favor specific traits.
• Some species may have more opportunities to reproduce, impacting allele frequencies.

iii) Non-Random Mating:

• Mating should not be random within a population.
• Population size must be sufficiently large for the theorem to apply.

iv) Gene Flow:

• Gene flow involves gene exchange between populations of the same species.
• This can occur through migration or gamete transfer.
• When natural selection and genetic drift are absent, gene flow leads to genetic uniformity within a metapopulation.
• Allele frequencies approach equilibrium values equal to the metapopulation's average frequencies.

Reality vs. Idealized State:

• Hardy-Weinberg equilibrium is rarely observed in nature.
• Disruptive forces like mutations, selection, non-random mating, and gene flow commonly occur.
• The theorem represents an idealized state, and natural genetic variations deviate from this equilibrium.

Explore our MDCAT webapp for tests, video lectures, and complete prep resources.