Modern Synthetic Theory of Evolution

NCERT grounding

NCERT Class XII Biology, Chapter 6 (Evolution), introduces the modern synthesis through two paired sections. §6.6 (Mechanism of Evolution) explicitly states that, although Mendel had already spoken of inheritable factors influencing phenotype, Darwin either ignored Mendel or kept silent on him; in the first decade of the twentieth century Hugo de Vries, working on the evening primrose, proposed mutation — large differences arising suddenly in a population — as the cause of evolution. NCERT clarifies that mutations are random and directionless while Darwinian variations are small and directional, and that de Vries called his mechanism saltation (single-step large mutation). It is studies in population genetics, NCERT notes, that brought clarity by integrating both views.

§6.7 (Hardy-Weinberg Principle) then lists the five factors that disturb genetic equilibrium — gene migration (gene flow), genetic drift, mutation, genetic recombination and natural selection. These are precisely the forces of the modern synthesis. The NIOS supplement (Lesson 1, §1.2.3) makes the name explicit: "Darwin's original theory of Natural Selection was modified [and] termed Neo-Darwinism or Modern Synthetic Theory."

"Mutations are random and directionless while Darwinian variations are small and directional. Evolution for Darwin was gradual while de Vries believed mutation caused speciation and hence called it saltation."

— NCERT Class XII, §6.6

The NIOS lesson lists five postulates of Neo-Darwinism that NEET asks verbatim: (1) the unit of evolution is the population, which carries its own gene pool; (2) heritable genetic changes arise through small gene or chromosomal mutations and their recombinations; (3) natural selection sieves the variants that improve adaptation; (4) the shift in genetic constitution that natural selection drives is called differential reproduction; (5) once a new species has evolved, reproductive isolation keeps it distinct.

The synthesis in detail

Why classical Darwinism was incomplete

Charles Darwin published On the Origin of Species in 1859 and named natural selection as the mechanism of evolution, but he could not explain where heritable variation came from. Gregor Mendel's 1865 paper on inheritance in pea plants — which solved precisely that problem — lay unnoticed in an obscure journal for thirty-five years. When Mendel's work was rediscovered in 1900 by de Vries, Correns and Tschermak, the genetic mechanism of inheritance became visible, but in a way that initially appeared to contradict Darwin: Mendel's factors were discrete, whereas Darwinian variation was continuous. For three decades, geneticists who followed de Vries (the "mutationists") and field naturalists who followed Darwin (the "selectionists") fought over which mechanism really mattered.

The reconciliation began with the mathematical population geneticists R. A. Fisher, J. B. S. Haldane and Sewall Wright in the 1920s, who showed that Mendelian inheritance of small mutational differences, accumulated under natural selection on a population scale, produces the gradual continuous change Darwin had described. Their equations made it clear that mutation and selection were not rival mechanisms but successive stages of one process.

The architects and the synthesis (1930s–1940s)

On the foundation that Fisher, Haldane and Wright laid, five biologists from different disciplines built the modern synthesis. Theodosius Dobzhansky, a geneticist working on natural populations of Drosophila pseudoobscura, brought field genetics together with population mathematics in Genetics and the Origin of Species (1937). Ernst Mayr, an ornithologist and systematist, formulated the biological species concept and the role of geographic isolation in Systematics and the Origin of Species (1942). Julian Huxley, a zoologist, coined the very name in Evolution: The Modern Synthesis (1942). G. Ledyard Stebbins extended the synthesis to plants in Variation and Evolution in Plants (1950). George Gaylord Simpson, a vertebrate palaeontologist, showed in Tempo and Mode in Evolution (1944) that fossil rates of change were fully compatible with the new population genetics.

The population as the unit of evolution

The decisive shift in the modern synthesis is that evolution is no longer described as something that happens to an organism. An individual carries one fixed genotype and cannot itself evolve; it can only reproduce or fail to reproduce. What evolves is the gene pool — the sum of all alleles at all loci in a breeding population — and what is measured is the change in allele frequency from generation to generation. This is the same population that the Hardy-Weinberg principle describes; when its allele frequencies stay constant the population is at genetic equilibrium and is not evolving, and when they shift, the population is evolving.

Five forces of evolution

The modern synthesis identifies five sources of evolutionary change, which are exactly the five factors NCERT names in §6.7 as disturbing Hardy-Weinberg equilibrium. They are not alternatives — every real evolving population is acted on by all five at the same time, although their relative strengths differ between species, time scales and population sizes.

NEET's habit is to fuse three of these forces into one stem. NCERT's own list in §6.7 is precisely the set that can disturb Hardy-Weinberg equilibrium, which is why a "constant gene pool" — the absence of any of these forces — is the only choice that does not disturb equilibrium (NEET 2024, Q.159).

Speciation — the end product

The synthesis treats speciation — the origin of a new species — as the cumulative outcome of these forces operating long enough for two populations to become reproductively isolated. NIOS Lesson 1, §1.2.4 names two modes: allopatric speciation, in which a part of a population becomes geographically separated and diverges under different selection and drift regimes until reproductive isolation evolves; and sympatric speciation, in which a reproductive barrier (typically polyploidy in plants) arises without geographic separation. Both arrive at the same end-state, a population whose gene pool can no longer exchange alleles with its parent. The synthesis also recognises two tempo models — phyletic gradualism (Darwin's slow continuous picture) and punctuated equilibrium (Eldredge and Gould, 1972, in which most morphological change is concentrated at speciation events that are short on geological time scales).

de Vries vs Darwin vs the synthesis

NEET examiners return repeatedly to the contrast between Darwinian variation and de Vriesian mutation. The contrast is a real one, but the modern synthesis is precisely the framework in which it stops being a contradiction. Hugo de Vries (1848–1935), a Dutch botanist and one of the three rediscoverers of Mendel in 1900, worked on the evening primrose, Oenothera lamarckiana. Over years of cultivation he observed plants suddenly throwing off offspring with markedly new traits — what he called mutations. Because the new forms appeared in one step and bred true, he concluded that evolution proceeds by these large discontinuous jumps and not by Darwin's slow accumulation of small variations. He named this single-step mechanism saltation.

The synthesis dissolves the apparent conflict in two moves. First, the molecular events de Vries observed in Oenothera are now known to have been not point mutations but unusual translocation heterozygosity peculiar to that plant — yet his core insight, that genes change spontaneously and produce inheritable novelty, was correct. Second, population genetics shows that mutation supplies the raw material (consistent with de Vries) but it is natural selection acting on small differences across the gene pool that gives evolution its direction and its appearance of gradualism (consistent with Darwin). The classic NEET stem of "random and directionless" (de Vries) versus "small and directional" (Darwin) thus reflects two correct half-truths the synthesis combines.

Worked examples

Common confusion & NEET traps