The Question Darwin Left Open
Darwin had shown that natural selection acts on the variation within a population, but this raised a question he could not answer: where does that variation come from in the first place, and how exactly does one species give rise to another? Around the same time Mendel had spoken of inheritable 'factors' that shape the characters of an organism, yet Darwin either ignored these observations or kept silence about them (NCERT's phrasing).
So the mechanism behind the raw material of evolution — the origin of variation and the process of speciation — was left for others to work out.
Hugo de Vries and the Evening Primrose
In the first decade of the twentieth century, Hugo de Vries approached this question through his work on the evening primrose (Oenothera lamarckiana). He noticed large differences appearing suddenly in the plants — differences that were not there in the parents and did not arise by any gradual step. He called such sudden, large changes mutations.
From this de Vries drew a bold conclusion. He held that it was mutation that caused evolution, not the minor, heritable variations that Darwin had emphasised. In his view, the important changes were the big ones that appeared all at once.
Mutation Versus Darwinian Variation
The two kinds of change de Vries and Darwin spoke of are quite different in character. Mutations, as de Vries described them, are random and directionless — they appear without any tendency toward a particular outcome. Darwinian variations, by contrast, are small and, in the way natural selection shapes them over generations, directional.
This difference in the scale and nature of variation lay at the heart of the disagreement between the two views, and it set the stage for a deeper argument about how fast evolution actually proceeds.
Gradual Change or a Single Leap?
For Darwin, evolution was gradual — a slow accumulation of small heritable variations over many generations, gently reshaping a population until new forms appeared. De Vries saw it differently. He believed that mutation, a single large change, could bring about speciation in one step. He gave this idea the name saltation, meaning a single-step large mutation.
So the same phenomenon — the origin of a new species — was pictured in two very different ways: Darwin's slow, step-by-step change against de Vries' sudden jump. Later work in population genetics would bring some clarity to the debate, weaving together variation, inheritance and selection into a fuller account of how evolution works.
Where the Argument Settled
Neither picture, taken alone, was the last word. Studies in population genetics later showed how mutation supplies fresh variation, how that variation is reshuffled through inheritance, and how natural selection then acts on it within populations. In this fuller view, mutation is one source of the raw material, and selection is the process that shapes it — the two are partners rather than rivals.
That is why we can hold on to Darwin's insight about gradual change by natural selection while also giving mutation, as de Vries urged, its proper place as a source of new variation. The mechanism of evolution, then, is neither pure gradualism nor pure saltation, but variation and selection working together over time.
Quick Recap
- Darwin's theory left open where variation comes from; though Mendel had spoken of inheritable 'factors', Darwin ignored or kept silent on them.
- In the early 1900s, Hugo de Vries, working on the evening primrose, proposed mutations — large differences arising suddenly.
- He argued that mutation, not Darwin's minor variations, drives evolution.
- Mutations are random and directionless; Darwinian variations are small and directional.
- Darwin saw evolution as gradual; de Vries thought mutation caused speciation in a single large step — saltation.
- Later work in population genetics brought clarity, combining mutation, inheritance and selection.
Solved Examples — Section 8
Q1. What important question about evolution did Darwin's theory leave unanswered?
Answer: It did not explain where the variation that natural selection acts on comes from, nor exactly how speciation occurs.
Q2. What did Darwin do with Mendel's idea of inheritable 'factors'?
Answer: He either ignored these observations or kept silence about them, so the source of variation stayed unexplained in his account.
Q3. On which plant did Hugo de Vries base his idea of mutations, and what did he observe?
Answer: He worked on the evening primrose, where he saw large differences appearing suddenly in the plants — changes he called mutations.
Q4. How do mutations differ from Darwinian variations in character?
Answer: Mutations are random and directionless, whereas Darwinian variations are small and directional.
Q5. What is saltation, and how does it differ from Darwin's view of evolution?
Answer: Saltation is a single-step large mutation that de Vries thought could cause speciation at once. Darwin, by contrast, saw evolution as gradual, built from many small heritable variations.
Q6. What eventually brought clarity to the debate between gradual variation and mutation?
Answer: Studies in population genetics, which showed how mutation supplies variation and how inheritance and natural selection then act on it within populations.