Mechanism of evolution
NCERT Class 12 Biology Chapter 6 Section 6.6
Quick answer
Darwin emphasised selection on heritable variation, whereas de Vries proposed large sudden mutations and saltational speciation. Modern genetics shows that mutations have varied effects and are not directed toward future needs. Mutation supplies new alleles, while selection and other population processes influence their fate.
With your textbook
NCERT Biology (Class 12), Chapter 6 “Evolution”, Section 6.6 “Mechanism of Evolution”, page 119.
Open the official NCERT chapter (PDF, 12.7 MB, ncert.nic.in) (opens in a new tab)
Checked against the 2026-27 reprint on .
What is the mechanism of evolution?
Mechanism of evolution
- Two questions remained after Darwin: what is the origin of variation, and how does speciation occur (page 119)?
- 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, so “ignored” should not be read as a deliberate choice (Wikipedia, “Gregor Mendel”, facts in our words).
Mutations are random and directionless while Darwinian variations are small and directional.
NCERT Biology (Class 12), Section 6.6, page 119
Beyond the NCERT text, a modern qualification: the quoted contrast is historical. Mutations can have small, large or no detectable phenotypic effects; they are not generated to meet an organism’s needs. Selection can produce directional change, but variation itself is not foresighted (Wikipedia, “Mutation” and “Natural selection”, facts in our words).
In plain words: Darwin explained how nature picks among variants but not where the variants come from. In the early twentieth century, de Vries suggested that new species arise from sudden large changes, mutations, rather than from many small differences added up over time.
Hugo de Vries and mutation
- In the first decade of the twentieth century, Hugo de Vries, from his work on the evening primrose, brought forth the idea of mutations: large differences arising suddenly in a population (page 119).
- He believed that it is mutation that causes evolution, and not the minor heritable variations that Darwin talked about (page 119).
Darwinian variation and de Vries’ mutation compared
| Feature | Darwinian variation | de Vries’ mutation |
|---|---|---|
| Size | small | large, arising suddenly |
| Direction in NCERT’s historical comparison | described as directional | random, directionless |
| Evolution | gradual | in a single step: saltation |
(Historical textbook comparison, NCERT page 119; not modern definitions of all mutations.)
Saltation
- Evolution for Darwin was gradual, while de Vries believed that mutation caused speciation, and so called it saltation, a single-step large mutation (page 119).
- Later studies in population genetics brought out some clarity (page 119). The kinds of mutation are described in Chapter 4.
Solved examples
Example 1: de Vries against Darwin
How did de Vries’ view of the cause of evolution differ from Darwin’s?
Answer: Darwin held that evolution is gradual, through small, heritable, directional variations selected by nature. De Vries, from his work on the evening primrose, held that evolution is caused by mutations, large differences that arise suddenly and are random and directionless, and that a single-step large mutation could cause speciation: saltation (NCERT page 119). Beyond the NCERT text: it is selection, not the variation itself, that gives change a direction (Wikipedia, “Natural selection”, facts in our words).
Example 2: What is saltation?
Define saltation and name the scientist who proposed it.
Answer: saltation is speciation by a single-step large mutation; it was proposed by Hugo de Vries (NCERT page 119).
Common mistakes to avoid
Common mistake
Calling mutations directional
NCERT: mutations are random and directionless, while Darwinian variations are small and directional (NCERT page 119). Beyond the NCERT text: mutation is not directed toward future needs; it is selection that can favour one direction (Wikipedia, “Natural selection”, facts in our words).
Common mistake
Crediting Darwin with Mendel’s factors
Mendel talked of inheritable factors; NCERT says Darwin either ignored these observations or kept silence (NCERT page 119). Beyond the NCERT text: Darwin was not aware of Mendel’s paper, so he did not use Mendel’s factors (Wikipedia, “Gregor Mendel”, facts in our words).
Quick revision summary
- After Darwin: what causes variation, and how does speciation occur? Darwin did not use Mendel’s factors.
- Hugo de Vries (evening primrose, early twentieth century): mutations, large differences arising suddenly, cause evolution.
- Mutations are random and directionless; Darwinian variations are small and directional (NCERT). Darwin: gradual evolution; de Vries: saltation, a single-step large mutation causing speciation.
- Beyond the NCERT text: mutations vary in the size of their effect and are not directed toward an organism’s needs; selection acts on heritable variation and can give change a direction (Wikipedia, “Mutation” and “Natural selection”).
- Population genetics later brought some clarity.
Formula sheet
- de Vries: evening primrose, mutation
- saltation: single-step large mutation
- mutation: random, directionless
- Darwinian variation (NCERT): small, directional
Questions students ask
Why was de Vries’ mutation theory later questioned?
Beyond the NCERT text, de Vries’ mutation theory was questioned because the plant he studied turned out to have an unusual chromosome make-up, so most of the new forms he had counted as mutants came from rearranged chromosomes rather than from changes in single genes (Nei and Nozawa 2011, Genome Biology and Evolution). The same review notes that de Vries called any heritable change in a character a mutation, whereas from the 1910s, as Thomas Morgan’s group found many single-gene mutations, many geneticists came to keep the word for those. NCERT Chapter 4 ties mutation to “alteration of DNA sequences” (page 72).
Has saltation ever produced a new species?
Beyond the NCERT text, a new species can arise in one step when its chromosome sets multiply, which by our reading fits NCERT’s saltation: one form that de Vries named as a new species proved to have four chromosome sets and persisted as a separate species (Nei and Nozawa 2011). NCERT Chapter 4 calls an increase in a whole set of chromosomes polyploidy, often seen in plants (page 75), and the review cites studies finding it in the origin of 20–40 per cent of flowering-plant species. Across living things, though, it judges such speciation rare, most new species arising from gene changes that build up reproductive isolation.
Is saltation the same as punctuated equilibrium?
No, saltation and punctuated equilibrium are different ideas: saltation, as NCERT gives it, is speciation by one large mutation (page 119), while punctuated equilibrium, beyond the NCERT text, is a pattern described by Eldredge and Gould in 1972 in which species show little net change in form for long periods, called stasis, and most change is tied to speciation (Jablonski 2017, Evolutionary Biology, CC BY). Jablonski adds that large-scale change does not call for a return to the older models of macromutation and saltation, though events such as genome duplication may be modern cases of such jumps.
Practice questions
Try each question first, then open the answer to check your working.
-
Q1. On which plant did Hugo de Vries base his idea of mutation?
Show answerHide answer
The evening primrose (NCERT page 119).
-
Q2. According to de Vries, what causes evolution?
Show answerHide answer
Mutation, large differences arising suddenly in a population, rather than the minor heritable variations Darwin described (NCERT page 119).
-
Q3. Which kind of evolution did Darwin describe, gradual or sudden?
Show answerHide answer
Gradual (NCERT page 119).