In the early 1809 the Napoleonic wars raged in Europe. Nothing new there, they had been raging for some years but still a matter of some consequence to those in the vicinity of the combatants. On February 11 Fulton patented the steamboat, on February 3 Illinois first organized as a territory. Later that month the supreme court ruled that the federal government’s power was greater than that of any state. In March Madison would be sworn in as president (the first to do so in American-made clothes apparently). This was the world that greeted both Abe Lincoln and Charles Darwin as they entered it 200 years ago today.
This is by way of a happy birthday to both and a celebration of Darwin and the importance of variation in his work - variety is the spice of life after all. If you don't want historical stuff just scroll down to part II
I. Darwin by the Numbers - A Brief rundown on Darwin’s life. He was a child of the upper class and had an early passion for natural history but was otherwise undirected. At first he planned to train to be a doctor but had not taste for surgery. So instead he was educated to join the clergy. After graduation his family got him the post of captain’s companion on HMS Beagle. For five years in the early 1830s the Beagle roamed the seas (actually it mostly charted the coast of South America with a bit of roaming mixed in here and there). Darwin experienced the richness of the biological world in that time. This was a common experience for European scientists of the era. The European view of the natural world was based on studying the depauperate fauna and flora of Europe, especially northern Europe. In the 18th and 19th Centuries a flood of specimens from around the world revealed a world much richer in life than previously imagined.
After returning to England Darwin married his first cousin Emma Wedgewood (of the Wedgewood China family). The marriage made him very wealthy (as opposed to moderately wealthy) and he settled down to the life of a gentleman scholar. In many senses the rest of his life was quite uneventful. He lived and worked at his home in Kent, raised a family, and corresponded with other scholars. He suffered from a mysterious illness and did not venture out into public to any significant extent. He became an expert on barnacles and wrote works on the formation of coral reefs and the modification of the soil by earthworms. A respectable and thoroughly unexciting Victorian scholar.
However, early during this time Darwin formulated what noted scientists such as E.O. Wilson and James Watson have described as the most important idea (really a set of ideas) in history. And here we get to what I really want to discuss in this diary – the formulation of Darwin’s theory of evolution by natural selection.
The history of Darwin’s work is marked by several interesting aspects: the long period in which he delayed publication and accumulated evidence, the shock of Wallace’s independent discovery, the rush to publication, and the attempts over the last two decades of his life to answer his critics.
Darwin’s ideas appear to have matured in the late 1830s when he was still a young man and only a few years back from his trip. By the early 1840s he had written a summary of his theory which he eventually showed to some of his colleagues. What was this idea that he hoped would make him the Newton of biology but that he rightly dreaded would make him the center of social controversy.
Although no aspect of Darwin’s work is particularly complex it is hard to summarize because it is an amalgam of several different concepts. Some were pre-existing while others were original to Darwin. Ernst Mayr, in his book, ‘One Long Argument’ discusses Darwin’s theory as really being five theories.
- Life on earth is not constant, new forms of life arise and old ones go extinct. This was a fairly commonly held point of view among learned people of the time. Darwin just made the case more forcefully and tied it into a larger framework.
- All life on earth has a common ancestor – there wasn’t much direct evidence to support this at the time – little information about cell structure or DNA that would tie all of life together. Darwin did hedge a bit, indicating that multiple origins of life were possible but clearly he felt that was unlikely.
- The diversification of life occurred through a populations separating and become different from one another until eventually they became different species. All living things can be linked through a common genealogy (now called a phylogeny).
- Evolution has proceeded through gradual changes accumulated over very long periods of time. This has been famously criticized by Niles Eldridge and Stephen Jay Gould.
- Natural selection is the primary mechanism driving evolutionary change. Natural selection required two supporting ideas – that a struggle for resources was a fairly constant condition throughout the natural world and that traits are passed from parents to offspring.
Who knows if Darwin would have published in his lifetime if Wallace hadn’t contacted him with the same idea. Alfred Russell Wallace was a lesser scientist but led a more interesting life. Raised in genteel quasi-poverty Wallace made himself a career as a specimen collector for natural history museums. He mounted expeditions to the Amazon and to southeast Asia in which he traveled essentially on his own for years at a time. It was while he was in Malaysia that he came up with the idea of natural selection while suffering from malaria.
Wallace and Darwin co-authored a paper which was read to the Linnean society in 1858 – the first publication of the idea of natural selection. ‘The Origin of Species’ followed the next year. Darwin considered it an ‘abstract’ of what he intended to publish.
Wallace graciously deferred to Darwin and throughout his long life always gave Darwin full credit for the idea. Wallace was a prominent proponent of progressive ideas in the late 19th Century – he strongly supported women’s rights and socialism. He was also an avid proponent of spiritualism.
Darwin published 6 editions of the ‘Origin’ in his lifetime. He also published a second major book on evolution ‘ The Descent of Man and Selection in Relation to Sex’ in 1871. The latter explicitly laid out the case for humans as evolved creatures that are part of the same evolutionary lineage as other beings. All of this later writing incorporates attempts to answer criticisms of his work. Which leads us to...
II Darwin’s Theory and the Problem of Variation
The Theory of Evolution by Natural Selection is an idea fundamentally different in nature from other scientific ideas that had come before it. Natural selection is based on the concept of variation. Instead of being statistical noise obscuring the truth, variation is the core, the heart of the mechanism. It is no accident that several early statistical techniques were developed by scientists working on evolutionary questions (Karl Pearson, Sewall Wright, and, most famously, R.A. Fisher).
Darwin struggled greatly with the problem of variation. He knew nothing of genetics and accepted the idea of blending inheritance that was commonly held at the time. Under this idea offspring were a blend of their parents much as if you mixed two colors of paint. Because of the blending the two original colors are lost in the offspring. If blending inheritance was true then variation should rapidly vanish from any population.
The rediscovery of Mendelian genetics in 1900 did away with the problem of inheritance but the issue of variation persists. How does genetic variation persist in populations in the face of natural selection? There are four major evolutionary forces that cause genetic changes in populations: selection, drift (random changes), migration (movement of individuals between populations), and mutation. Selection usually acts to reduce variation (if having a longer beak is better then short-beaked individuals tend to become less abundant) but there are situations in which it can maintain variation. The simplest example is overdominance or heterozygote advantage. In this situation individuals with two different alleles (versions of a gene) are better off then individuals with two copies of the same allele. There are few examples known of this – the most famous being the sickle cell allele. Individuals with one copy of the sickle cell allele and one copy of the ‘normal’ (non-sickle) allele are resistant to malaria. So the sickle cell allele persists in large numbers in some African populations despite the fact that individuals with two copies of the sickle cell allele have a debilitating disease.
A more cheerful example is guppies, small tropical fish found in Trinidad and northern South America. Male guppies are brightly colored and females use the colors to select mates. Unlike most species in which mate choice is based on ornamentation, male guppies are highly variable in color pattern. Some have lots of orange, some lots of blue, some lots of black. And the pattern of the colors varies greatly as well. It turns out that females appear to prefer males with unfamiliar color patterns. They like a bit of variety. This may be a mechanism to avoid mating with relatives as they often live in small pools in streams and most fish close to you are likely to be kin.
I have emphasized variation because to quote his final paragraph – ‘there is grandeur in this view of life’ and a big part of that grand vision is variety. ‘Endless forms most beautiful’ living in a ‘tangled bank’. A world that is incredibly rich in variety, beauty, terror, and almost unimaginable strangeness. A world of orchids and peregrine falcons but also a world of bacteria that eat rocks, deep sea animals that live their entire lives on whale carcasses that have drifted to the bottom, fish that change sex in response to social conditions, parasites that make fish jump out of the water so they get eaten by birds. This is a world that I want to keep living in and a big part of the reason I come here.