Chapter 4 of Darwin’s Origin of Species is a crucial chapter, outlining his views
on Natural Selection, but it is also 51 pages long.
In this video, I will give a breakdown of the 8 different sections of this chapter,
and say a few words about what you should focus on in reading each section.
The chapter begins with 8 pages comparing natural selection to artificial selection.
This is pages 80-87 in the First Edition. In the first paragraph, Darwin defines Natural
Selection as “the preservation of favourable variations and the rejection of injurious
variations.” Key points of this section include: that natural
selection is capable of even more powerful and more profound transformations than artificial
selection by selective breeding, that natural selection is capable of producing an immense
diversity of organisms, and the survival of only the best-adapted ones, that natural selection
adaptations are gradual, almost too gradual to be observed, and that natural selection
can only select for traits that increase the survivability of the individual possessing
them—altruistic or self-sacrificial traits are going to be a problem for this theory.
The second section is 3 pages of comments on what Darwin calls “Sexual Selection.”
The editor, James Costa, notes that here Darwin recognizes that “reproduction is the ultimate
evolutionary currency.” Main points include: that within each species,
males compete against other males to mate & reproduce, and females compete against other
females. Both these selection processes work to improve and transform the species from
generation to generation. The third section is 5½ pages of illustrations
of Natural Selection in action. Darwin describes at length some observed cases that show his
theory and how it works. These cases include hunting wolves and the pollination of flowers
by bees. The fourth section is 6 pages long, dealing
with intercrossing, or out-breeding. It was known in Darwin’s time that repeated
inbreeding caused weak and sickly offspring, and that occasionally crossing varieties with
more-distant varieties led to increased health and vigor. Darwin gives several examples illustrating
and supporting this conclusion. The fifth section is 7½ pages on circumstances
favorable to Natural Selection, which Darwin calls “an extremely intricate subject.”
He discusses the effects of various factors on natural selection. These include: a large
number of heritable variations, intercrossing, physical isolation, and largeness of area
in which the species dwells. The sixth section is a 2 page treatment of
extinction. Darwin considered the factors and mechanisms of species extinction. This
short section is worth a careful read-through. The seventh and longest section is 16 pages,
and includes the only illustration in the book, a chart. It deals with “divergence
of character,” the way competition between similar species eventually produces separate
distinct species. Major points include: the mechanism of divergence,
with examples from nature and from artificial selection, and the benefits of divergence
and species diversification, again with many examples.
Darwin then explores the way divergence of character combines with natural selection
and extinction, and this is where he uses his chart. The remainder of this section,
pages 116-126, along with the included chart, to which it frequently refers, is worth your
careful attention. The final section is 3½ pages long, and it
is a summary of the entire chapter. The summary provides a good overview of Darwin’s argument
so far, and I urge you to read it carefully, both before and after you read the entire
chapter. I hope you have found this breakdown of the
sections of Chapter 4 helpful. Thanks for watching today; goodbye.