The Origin of Species · Chapter
Individual Differences explained
Darwin introduces the concept of individual differences—the slight variations among offspring of the same parents—and argues that these differences are often inherited, providing raw material for natural selection.
What happens
Darwin introduces the concept of individual differences—the slight variations among offspring of the same parents—and argues that these differences are often inherited, providing raw material for natural selection. He discusses the variability of both unimportant and important parts, the perplexing case of polymorphic genera, and examples of dimorphism and trimorphism in animals and plants, citing observations by Sir John Lubbock, Mr. Wallace, and Fritz Müller.
Themes in this chapter
Variation and Inheritance
Individual differences are inherited and provide the materials for natural selection to act upon.
Natural Selection and the Struggle for Existence
Variations that are of no service or disservice are not seized on by natural selection, as seen in polymorphic genera.
Characters to notice
- Sir John Lubbock
Showed variability in main nerves of Coccus and muscles in insect larvae.
- Mr. Wallace
Described dimorphic female butterflies in the Malayan Archipelago and varieties connected by intermediate links.
- Fritz Müller
Described dimorphic males of Brazilian Crustaceans, such as Tanais.
Key passages
“These individual differences are of the highest importance for us, for they are often inherited, as must be familiar to everyone; and they thus afford materials for natural selection to act on and accumulate, in the same manner as man accumulates in any given direction individual differences in his domesticated productions.”
Individual differences are crucial because they are frequently inherited, giving natural selection something to work with, just as breeders accumulate small variations in domesticated animals.
Darwin emphasizes the parallel between natural and artificial selection.
“I am inclined to suspect that we see, at least in some of these polymorphic genera, variations which are of no service or disservice to the species, and which consequently have not been seized on and rendered definite by natural selection, as hereafter to be explained.”
In some highly variable genera, the variations may be neutral—neither helpful nor harmful—so natural selection has not fixed them into distinct forms.
Darwin introduces the idea of neutral variation not acted upon by selection.