Darwin explained natural selection without knowing the physical mechanism of heredity. Gregor Mendel’s experiments showed that inherited factors could remain discrete rather than blending away. In the twentieth century, population genetics united Mendelian inheritance with selection and built the modern synthesis: evolution as change in inherited variation within populations.
Four forces change frequencies
Mutation creates new sequence variants. Natural selection changes their frequencies when variants influence reproductive success. Genetic drift produces chance changes, especially in small populations, and can fix neutral or even mildly harmful variants. Gene flow moves variants among populations. Recombination reshuffles existing variation each generation.
These processes interact. A beneficial mutation can be lost by chance when rare. Gene flow can spread an adaptation or impede local divergence. Population size, mating structure, linkage, and historical bottlenecks leave recognizable signatures.
DNA records kinship, imperfectly
Because descendants inherit modified sequences, genomes contain evidence of common ancestry. Shared disabling mutations, mobile-element insertions, duplicated genes, and chromosome rearrangements can identify branches. Molecular phylogenies are tested against anatomy, fossils, and biogeography.
Individual genes may have different histories because of recombination, incomplete lineage sorting, hybridization, and horizontal transfer. Scientists therefore estimate species relationships from many genomic regions and report uncertainty. A tree is an inference from evidence, not a decorative genealogy.
Novelty through old material
Gene duplication frees one copy to retain an original function while another changes. Regulatory mutations alter when and where genes act. Developmental toolkits shared across animals can produce different bodies through changes in timing, location, and interaction. Large anatomical differences need not require an entirely new inventory of genes.
Not every conserved sequence is an adaptation, and not every visible trait maps to one gene. Many traits are polygenic and environmentally influenced. Neutral theory explains much molecular change without invoking selection, while comparative genomics helps identify sequences constrained by function.
Genomics did not replace fossils or field natural history. It added another archive. The strongest reconstructions are those in which molecules, organisms, geography, and rocks converge on the same branching history.