Happy 150th Anniversary On the Origin of Species!

November 24, 1859, 150 years ago today, saw the publication of Charles Darwin’s On the Origin of Species, By Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life. All 1,250 copies of the initial print run of the book were scooped up by readers eager to see the British naturalist going rogue with his radical new theory of evolution. Every since then much has been written about what, exactly, evolution is.

In his 1988 book, Toward a New Philosophy of Biology, the late Harvard biologist Ernst Mayr, arguably the greatest evolutionary theorist since Darwin, employed technical definition: “Evolution is change in the adaptation and in the diversity of populations of organisms.” Adaptation and diversity are evolution’s dual nature, “the ‘vertical’ phenomenon of adaptive change and the ‘horizontal’ phenomenon of populations, incipient species, and new species.” And I’ll never forget Mayr’s definition of a species, because I had to memorize it in my first course on evolutionary biology: “A species is a group of actually or potentially interbreeding natural populations reproductively isolated from other such populations.” In his classic 1982 book, The Growth of Biological Thought, Mayr outlines five general tenets of evolution, followed by five specific points about how natural selection works.

1. Evolution: Organisms change through time. Both the fossil record of life’s history, and nature today document and reveal this change.

2. Descent with modification: Evolution proceeds via branching through common descent. Offspring are similar to but not exact replicas of their parents. This produces the necessary variation to allow for adaptation to an ever-changing environment.

3. Gradualism: Change is slow, steady, and stately. Given enough time, evolution accounts for species change.

4. Multiplication of speciation: Evolution does not just produce new species; it produces an increasing number of new species.

5. Natural selection: The process of evolutionary change, co-discovered by Darwin and Alfred Russel Wallace, operates in the following manner:

A. Populations tend to increase indefinitely in a geometric ratio: 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024…

B. In a natural environment, however, population numbers stabilize at a certain level.

C. Therefore, there must be a “struggle for existence” because not all of the organisms produced can survive.

D. There is variation in every species.

E. In the struggle for existence, those individuals with variations that are better adapted to the environment leave behind more offspring than individuals that are less well adapted. This is known as differential reproductive success.

This process of natural selection, when carried out over countless generations, gradually leads varieties of species to develop into new species. Within the natural selection paradigm, points A, B, and D are observations, C and E are inferences. C follows from A and B, and E follows from all three observations. In Darwin’s own words in On the Origin of Species, here are his observations that led to his inference:

A struggle for existence inevitably follows from the high rate at which all organic beings tend to increase. Every being, which during its natural lifetime produces several eggs or seeds, must suffer destruction during some period of its life, and during some season or occasional year, otherwise, on the principle of geometrical increase, its numbers would quickly become so inordinately great that no country could support the product. Hence, as more individuals are produced than can possibly survive, there must in every case be a struggle for existence, either one individual with another of the same species, or with the individuals of distinct species, or with the physical conditions of life.

And here is his inference:

It may be said that natural selection is daily and hourly scrutinising, throughout the world, every variation, even the slightest; rejecting that which is bad, preserving and adding up all that is good; silently and insensibly working, whenever and wherever opportunity offers, at the improvement of each organic being in relation to its organic and inorganic conditions of life. We see nothing of these slow changes in progress, until the hand of time has marked the long lapses of ages, and then so imperfect is our view into long past geological ages, that we only see that the forms of life are now different from what they formerly were.”

Natural selection operates primarily at the local level. It is the process of organisms struggling to survive and reproduce in order to propagate their genes into the next generation. In his 1976 book, The Selfish Gene, the Oxford evolutionary biologist Richard Dawkins succinctly defined evolution as “random mutation plus non-random cumulative selection.” Dawkins especially emphasizes the nonrandom element in the process in order to counter the myth that evolution is completely random, as in the creationists’ specious argument that evolution is the equivalent of a warehouse full of parts randomly assorting themselves into a jumbo jet. If evolution were truly random there would be no biological jumbo jets. Genetic mutations and the mixing of parental genes in offspring may be random, but the selection of genes through the survival of their hosts is anything but random. Out of this process of self-organized directional selection emerges complexity and diversity.

That is the power and beauty of natural selection as the driving process behind evolution, and so let us today celebrate 150 years of this glorious book.