Modern synthetic theory of evolution
Beyond NCERT In the NEET syllabus; the Class 12 book has no section for it.
Quick answer
Beyond the NCERT text, which does not name it, the modern synthesis connects natural selection with Mendelian genetics and population genetics. Mutation and recombination generate or reorganise variation, while selection, drift and gene flow shape populations. Reproductive barriers can contribute to speciation, but neither a changed trait nor chromosome doubling guarantees a new species.
What is the modern synthetic theory of evolution?
Modern synthetic theory of evolution
The modern synthetic theory of evolution, also called the modern synthesis or neo-Darwinism, joins Darwin’s natural selection with Mendelian genetics and population genetics. Heritable variation from mutation and recombination arises in a population, selection, drift, mutation and gene flow can change allele frequencies, and reproductive isolation lets new species form (beyond the NCERT text; sources below). NCERT ends its account of Darwin and de Vries at the point where this theory begins:
Studies in population genetics, later, brought out some clarity.
NCERT Biology (Class 12), Section 6.6, page 119
In plain words: Darwin knew that offspring vary and that variation is inherited, but not how. Mendel found out how, but his work was not used for decades. In the 1930s and 1940s geneticists, naturalists and palaeontologists put the two together: evolution became a change in the genes of whole populations, shaped by selection and other mechanisms, with reproductive barriers often helping lineages diverge.
The 2026–27 NCERT book does not name this theory, but the NEET (UG) 2026 syllabus lists the “Modern Synthetic theory of Evolution” in Unit 7. NCERT does give several of its parts: Mendel’s inheritable factors, which Darwin, NCERT says, “either ignored these observations or kept silence” about (page 119); mutation, as de Vries proposed (page 119); and the five factors that change allele frequencies (Section 6.7, page 121). Those lines are cited below; the rest is marked as beyond the NCERT text, with its source.
Darwin and Mendel brought together
- NCERT asks what the origin of variation is and how speciation occurs. Even though Mendel had talked of inheritable factors influencing the phenotype, “Darwin either ignored these observations or kept silence” (page 119). Beyond the NCERT text: Darwin was not aware of Mendel’s paper (Wikipedia, “Gregor Mendel”, facts in our words). See Mendel’s experiments in Chapter 4.
- Beyond the NCERT text: between Fisher’s paper of 1918 and Stebbins’ book of 1950, biologists merged Darwin’s theory with Mendel’s laws of heredity, using the mathematics of populations. Selection, Mendel’s genetics and population genetics became one theory, linking changes within populations (microevolution) to diversification over longer timescales (macroevolution) (Wikipedia, “Modern synthesis (20th century)”, facts in our words).
- Genetics later supplied what Darwin lacked, the sources of heritable variation; the revised theory is what NIOS calls neo-Darwinism or the modern synthetic theory (NIOS Senior Secondary Biology, Lesson 1, facts in our words).
The architects of the synthesis
Beyond the NCERT text (Wikipedia, “Modern synthesis (20th century)”, facts in our words):
- R. A. Fisher, The Genetical Theory of Natural Selection (1930): showed that Mendelian genetics fits evolution by natural selection, and that continuous variation can come from many genes (1918).
- J. B. S. Haldane, The Causes of Evolution (1932): with Fisher and Wright, founded theoretical population genetics.
- Sewall Wright, a paper on the adaptive landscape (1932): showed how inbreeding and drift act in small populations.
- Theodosius Dobzhansky, Genetics and the Origin of Species (1937): applied genetics to natural populations and made the population geneticists’ results accessible to field naturalists.
- Ernst Mayr, Systematics and the Origin of Species (1942): the biological species concept; geographic isolation in the origin of species.
- Julian Huxley, Evolution: The Modern Synthesis (1942): gave the synthesis its name.
- G. Ledyard Stebbins, Variation and Evolution in Plants (1950): extended the synthesis to plants, with hybridisation and polyploidy in plant speciation.
Each founder later listed the postulates differently (Mayr 1959, Stebbins 1966, Dobzhansky 1974); all start from mutation as the source of variation, with selection directing change (same source).
Stebbins’ five basic factors
Beyond the NCERT text: G. Ledyard Stebbins numbered five basic factors in Processes of Organic Evolution (1966), as tabulated in Wikipedia, “Modern synthesis (20th century)” (facts in our words). Other founders numbered theirs differently (above), and NCERT’s five factors that affect Hardy–Weinberg equilibrium (Section 6.7, page 121) are a different list. In Stebbins’ order:
- Gene mutation: it supplies new variation but does not decide which way change goes; changes in genes are the raw material of new alleles. See mutations and the genetic code in Chapter 5.
- Genetic recombination: also a source of variation without direction; new combinations of alleles from crossing over and independent assortment in meiosis and from fertilisation.
- Changes in chromosome structure and number (chromosomal organisation): deletions, duplications and other chromosomal aberrations (Chapter 4), and polyploidy; they alter which genes are inherited together, and so how the gene pool’s variation is packaged.
- Natural selection: it is the factor that steers how the gene pool changes; the better adapted leave more progeny, so allele frequencies change.
- Reproductive isolation: barriers that stop changed populations from interbreeding, so that they become separate species; it narrows the routes along which selection can steer a population.
The first three supply variation without direction; selection and isolation shape it into adapted populations and new species, though none of them by itself guarantees a new species (our reasoning). Stebbins’ 1950 preface already saw evolution at three levels: mutation and recombination make individuals differ, selection changes populations, and selection with isolating mechanisms separates species (Stebbins, Variation and Evolution in Plants, 1950, preface; facts in our words). The book has chapters on polyploidy and the karyotype (Wikipedia, “Variation and Evolution in Plants”).
Other central processes also matter:
- Gene flow brings in alleles from other populations; genetic drift changes allele frequencies by chance and can lose alleles. NCERT names both among its five factors (Section 6.7, page 121); Wright showed that drift acts most strongly in small, isolated populations (Wikipedia, “Modern synthesis (20th century)”, facts in our words).
- Hybridisation between differentiated populations or closely related species adds variation; Stebbins gave it a chapter of his book (as above).
The main ideas of the theory
Beyond the NCERT text (NIOS Senior Secondary Biology, Lesson 1, facts in our words):
- The unit that evolves is the population, with its gene pool.
- Mutation and recombination give individuals new heritable differences.
- Natural selection lets better-suited variants leave more offspring (differential reproduction), so allele frequencies shift.
- In time a new species can form, kept apart by reproductive isolation.
This fits NCERT’s own account: a change in the frequency of alleles in a population is interpreted as evolution (Section 6.7, page 121).
Reproductive isolation and speciation
- Species. Beyond the NCERT text: in Ernst Mayr’s biological species concept, a species is a set of populations whose members breed, or could breed, with one another and which are cut off reproductively from all other populations (Wikipedia, “Modern synthesis (20th century)”, facts in our words). NIOS gives the working test: members of one species can breed together and have fertile offspring (NIOS, Lesson 1). NCERT Class 11 groups individuals with fundamental similarities, told apart by distinct morphological differences, as a species (Chapter 1, page 6).
- Isolating mechanisms. Mayr sorted them into two groups: pre-zygotic barriers act before fertilisation, and post-zygotic barriers act after it (Wikipedia, “Reproductive isolation”, facts in our words). The examples below follow NIOS, Lesson 1.
| Barrier | When it acts | How it works |
|---|---|---|
| Habitat (ecological) | pre-zygotic | they live in different habitats of one region, such as water and land, so rarely meet |
| Seasonal (temporal) | pre-zygotic | they breed at different times of the year |
| Behavioural (ethological) | pre-zygotic | courtship signals such as songs or colours differ, so mates are not recognised across species |
| Mechanical | pre-zygotic | the mating organs do not fit together |
| Physiological (gametic) | pre-zygotic | the gametes fail: sperm may not survive in the other species’ reproductive tract |
| Hybrid inviability | post-zygotic | the hybrid embryo does not survive |
| Hybrid sterility | post-zygotic | the hybrid lives but cannot breed, as the mule, from a female horse and a male donkey |
| F2 breakdown | post-zygotic | hybrid descendants, often in F or backcross generations, have reduced viability or fertility |
- Allopatric speciation: part of a population is separated geographically, for example by a mountain range; the two groups meet different conditions, so different variants are favoured in each, until the two populations can no longer interbreed (NIOS, Lesson 1; Wikipedia, “Speciation”, facts in our words).
- Sympatric speciation: one species gives rise to two or more without any geographic separation, all living in one area (Wikipedia, “Speciation”, facts in our words). In plants it often comes from polyploidy: offspring whose chromosome number has doubled (diploid to tetraploid) cannot interbreed with the parent population from the first generation, so a new species can arise quickly (Wikipedia, “Speciation”, facts in our words; NIOS, Lesson 1); whether the new line then establishes itself is a separate question (our reasoning).
- NCERT’s own example of a population that may become a new species is the drifted sample of the founder effect (Section 6.7, page 121).
Solved examples
Example 1: What the synthesis added to Darwin
Darwin’s theory had natural selection. What did the modern synthetic theory add?
Answer: the sources of heritable variation, which Darwin did not know: gene mutation, changes in chromosomes and genetic recombination, inherited by Mendel’s rules. It also made the population and its gene pool the unit of evolution, measured evolution as a change in allele frequencies, and added reproductive isolation to explain how new species form (beyond the NCERT text; NIOS, Lesson 1; NCERT page 121 for allele frequencies).
Example 2: Naming the barrier
Name the isolating mechanism and say whether it is pre-zygotic or post-zygotic: (a) two frog populations breed in different months; (b) a mule is sterile; (c) two bird species have different courtship songs.
Answer: (a) seasonal (temporal) isolation, pre-zygotic; (b) hybrid sterility, post-zygotic; (c) behavioural (ethological) isolation, pre-zygotic (beyond the NCERT text; NIOS, Lesson 1).
Example 3: Defining a species (NCERT Exercise 3)
Attempt a clear definition of the term species.
Answer: NCERT Class 11 calls a group of individuals with fundamental similarities a species, distinguished from closely related species by distinct morphological differences (Chapter 1, page 6). For evolution a sharper test is reproduction: in Mayr’s biological species concept, a species is a set of populations whose members breed, or could breed, with one another and which are cut off reproductively from all others (beyond the NCERT text). So a horse and a donkey are different species: they can mate, but their hybrid, the mule, is usually sterile. The test cannot be used for organisms that reproduce only asexually, or for fossils, which are still told apart by form (our reasoning).
Common mistakes to avoid
Common mistake
Thinking the synthesis replaced natural selection
The modern synthesis keeps natural selection at its centre and adds genetics: selection acts on heritable variation from mutation and recombination (beyond the NCERT text).
Common mistake
Equating the modern theory with de Vries’ mutation theory
De Vries held that single large mutations make new species (saltation; NCERT page 119). The synthetic theory sees mutations, mostly small, as raw material that selection sorts; NIOS calls de Vries’ theory of historical importance only.
Common mistake
Calling the mule a species
A mule is a horse–donkey hybrid, usually sterile, rather than a self-sustaining species: with an odd number of chromosomes (63), it seldom forms working gametes. Rare fertile female mules do not erase the broader reproductive barrier (Wikipedia, “Mule”, facts in our words).
Quick revision summary
- Modern synthetic theory (neo-Darwinism): natural selection + Mendelian genetics + population genetics; the population and its gene pool are the unit of evolution. Beyond NCERT; listed in NEET (UG) 2026, Unit 7.
- Architects: Fisher, Haldane and Wright (population genetics), Dobzhansky, Mayr, Huxley (named it, 1942) and Stebbins (plants).
- Stebbins’ five factors (1966): gene mutation; genetic recombination; changes in chromosome structure and number; natural selection; reproductive isolation. Gene flow and hybridisation add variation; genetic drift changes allele frequencies by chance.
- Isolation: pre-zygotic (habitat, seasonal, behavioural, mechanical, gametic) and post-zygotic (hybrid inviability, hybrid sterility, F2 breakdown).
- Speciation: allopatric (geographic separation) and sympatric (same area, often by polyploidy in plants).
Formula sheet
- synthesis: natural selection + Mendelian genetics
- 5 factors: mutation, recombination, chromosome changes, selection, isolation
- gene flow and drift also shape populations; hybridisation may add variation
- isolation: pre-zygotic or post-zygotic
- allopatric: separated; sympatric: same area
Questions students ask
Why is reproductive isolation necessary for speciation?
Reproductive isolation is necessary for speciation because, by our reasoning from NCERT’s account of gene flow, two populations that still interbreed keep exchanging alleles, which tends to even out their differences: NCERT notes that migrants add alleles to the population they join and take them from the one they leave (page 121). Beyond the NCERT text, biological species are themselves defined by reproductive isolation (Westram and colleagues 2022, Journal of Evolutionary Biology, CC BY). Once a barrier stops successful interbreeding, by our reasoning, mutation, selection and drift can change each gene pool on its own.
What is the difference between geographic isolation and reproductive isolation?
Beyond the NCERT text, geographic isolation keeps populations apart by place and reproductive isolation by their own heritable differences: in the first, few migrants pass between them because mountains, rivers or sheer distance lie in the way; in the second, genetic differences make matings between them rarer or their hybrids less fit, which cuts the flow of genes even where the two meet (Westram and colleagues 2022, Journal of Evolutionary Biology, CC BY). So, by our reading, geographic isolation lasts only as long as the barrier, while reproductive isolation, once strong enough, can keep two populations apart even when they live side by side again.
What causes reproductive isolation to evolve?
Beyond the NCERT text, reproductive isolation can evolve as a side effect of populations adapting to different surroundings: in an experiment on the fungus Neurospora, lines grown for about 1500 generations, nearly all asexual, in high salt or in cold mated less successfully with lines adapted to the other condition than with lines adapted to the same one (Dettman and colleagues 2008, BMC Evolutionary Biology, CC BY). Pooling 34 such studies on arthropods, yeast and vertebrates, a 2025 meta-analysis found isolation stronger between populations selected in different environments than between populations kept in similar ones (Jarrett and colleagues, Nature Ecology & Evolution, CC BY).
Practice questions
Try each question first, then open the answer to check your working.
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Q1. Name Stebbins’ five basic factors of evolution.
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Gene mutation, genetic recombination, changes in chromosome structure and number, natural selection and reproductive isolation, in Stebbins’ order (beyond the NCERT text).
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Q2. Who named the modern synthesis, and in which book?
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Julian Huxley, in Evolution: The Modern Synthesis (1942) (beyond the NCERT text).
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Q3. Which kind of speciation often follows polyploidy in plants?
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Sympatric speciation: polyploid plants cannot interbreed with the diploid population they came from, even though they live in the same area; doubling alone does not guarantee that the new line establishes itself (beyond the NCERT text).