Q.Describe the modern concepts of Natural Selection.

The modern (synthetic) theory of natural selection reinterprets Darwin's original mechanism in the language of population genetics, treating evolution as a change in allele frequencies driven by selection acting on pre-existing heritable variation.

From Darwinism to the Modern Synthesis

• Darwin proposed that individuals with favourable heritable variations survive and reproduce better than others ('survival of the fittest'), but he had no mechanism for how variation arose or was inherited.
• The Modern Synthesis (developed by Fisher, Haldane, Sewall Wright, and others in the 20th century) integrated Darwin's natural selection with Mendelian genetics and chromosome theory, explaining variation as arising from mutation and genetic recombination, and reinterpreting evolution as a change in the frequency of alleles/genotypes in a population's gene pool over generations.

Key features of the modern concept

• Selection acts on phenotypes, but its evolutionary effect is measured as change in underlying allele frequencies.
• Fitness is defined as the relative reproductive success (contribution to the next generation's gene pool) of a given genotype, not simply survival.
• Selection requires heritable variation, differential reproduction, and a consistent selective (environmental) pressure.

Types of natural selection recognised

1. Stabilising selection -- favours individuals with the average/intermediate phenotype and selects against both extremes, reducing variation (e.g., human birth weight clustering around an optimum).
2. Directional selection -- favours one phenotypic extreme over the other, shifting the population's mean value in that direction over time (e.g., increasing antibiotic resistance in bacterial populations under drug pressure). …