âOur ignorance of the laws of variation is profound. Not in one case out of a hundred can we pretend to assign any reason why this or that part differs, more or less, from the same part in the parents.â â Charles Darwin
Natural selection can only work on variations that already exist. But what causes variation in the first place? Darwin admits this is one of the most difficult questions in biology. Writing before the discovery of genes and DNA, he explores the patterns he can observeâeven when the underlying mechanisms remain mysterious.
Darwin is remarkably honest about the limits of his knowledge. Despite this uncertainty, Darwin identifies several patterns and principles that seem to govern how variation occurs. Some of these insights proved remarkably prescient; others were later abandoned as genetics developed.
Darwin observes that organisms placed in new environments often become more variable. The conditions of life seem to somehow affect the reproductive system, causing offspring to differ more from their parents.
Wild species brought into domestication show increased variation within a few generations. Changed food, climate, and care seem to âunsettleâ the organismâs hereditary constitution, producing more raw material for selection.
Darwin doesnât know the mechanism, but modern genetics vindicates his observation: stress and environmental change can indeed affect mutation rates and gene expression, producing more variation.
Darwin, like most of his contemporaries, believed that the use or disuse of organs could affect inheritanceâa view now called Lamarckism:
While the inheritance of acquired characteristics is now rejected, Darwinâs examples often have valid explanations: natural selection can reduce or eliminate unused structures because maintaining them is costly. The pattern Darwin observed was real; his explanation was incomplete.
One of Darwinâs most important insights: the organism is an integrated whole, and changing one part often affects others.
When selection acts on one trait, other traits may change as a byproductâeither because the same genes affect multiple traits (pleiotropy) or because developmental processes link different structures together.
This principle matters because selection may produce unexpected side effects. A trait may spread not because itâs directly selected, but because itâs correlated with something that is.
Darwin notices that organisms seem to have a âbudgetâ for growthâdeveloping one part heavily often means other parts are reduced:
Resources devoted to one structure are unavailable for others. A plant putting energy into enormous flowers may have smaller leaves. An animal with massive horns may have a smaller body. Natural selection can exploit this trade-off by redirecting resources to what matters most.
This anticipates modern life-history theory, which studies how organisms allocate limited resources among growth, maintenance, and reproduction.
Darwin observes that variations tend to appear at the same age in offspring as they first appeared in parents. This has important consequences:
If a variation appears early in development, it may affect the entire life course. If it appears late, earlier stages remain unchanged. This explains why embryos often resemble each other more than adults doâlate-appearing variations have diverged the adults while leaving embryonic stages similar.
This insight will become crucial in Chapter 13, where Darwin uses embryological similarity as evidence for common ancestry.
Sometimes offspring display traits not seen in their immediate parents but found in more remote ancestors:
The reappearance of ancestral traits after generations of absence. Domestic pigeons sometimes produce offspring with the blue color and wing bars of the wild rock dove. Horse foals occasionally have leg stripes reminiscent of zebras.
Darwin interprets this as evidence that ancestral traits remain latent, capable of reappearing when conditions permit. Modern genetics explains this through recessive alleles that can be hidden for generations before emerging when both parents carry them.