I'll highlight two of them here. The first was a surprise to me: Darwin did not think of populations, a crucial entity in population genetics. Except, as Padian explains, he did talk about "groups of individuals in the same species", which I think sounds pretty god-damned close.
5. Population thinkingSecondly, the myth that Darwin thought evolutionary change must always happen in tiny, incremental steps. Gradualism meant something else to Darwin than it does to most people today. Personally I am not sure how large steps he considered plausible. Read this and let's hear what you think.
It is often maintained that Darwin was the first biologist to think in modern populational terms (e.g., Mayr 1982). There is no evidence for this view. For Darwin, natural selection operated on individuals. He did not recognize population structure within species as we do today. For one reason, he didn't recognize species as real. He made no distinction among species, races, varieties, and subspecies. More than any other biologist of his generation, he thought of them as stages along a continuum of evolutionary diversification and separation of lineages. The word “population” does not appear in On the Origin of Species, even though Malthus's Essay on Population was a principal stimulus to his idea of natural selection. The closest Darwin comes is in his discussions in Origin, The Descent of Man, and elsewhere that suggest how groups of individuals in the same species could diversify structurally and ecologically under different selective pressures in different geographical regions. Remember too that mathematical modeling, the basis of modern populational thinking, was not one of his strengths. He did not have a developed sense of the quantitative flow of inherited traits within and among populations—this was developed only in the early decades of the 20th century, and then by mathematicians (Fisher 1958, Provine 1971).
7. Gradual change is slow and steadyAgain, again, again, we celebrate Darwin's 200th birthday and The Origin's 150 anniversary this year. Next year is bound to be mundane.
When in Chile during the voyage of the Beagle, Darwin experienced a huge earthquake that leveled Concepción and injured and killed many people. On the ship the next day he looked down the coast and saw that the cliffs had been raised several meters, and that this was simply the latest instance of such cataclysms. He referred to the event in his diary as a “gradual change.” It seems strange to our ears to think of the effects of earthquakes as “gradual,” but the etymology of the word comes from the Latin gradus, meaning “step.” In Darwin's day, “gradual” often meant steplike (the Oxford English Dictionary uses the example from Addison and Steele's Spectator of rows in an auditorium). Consider the discrete markings on a graduated cylinder, and that students all graduate on the same day, as opposed to all through the year. Yes, the gradual steps were small. But it is unlikely that Darwin would have endorsed the classic gradualism of the Modern Synthesis to the exclusion of punctuated equilibria (Eldredge and Gould 1972). On the other hand, he was opposed to any kind of large, sudden change, which is why he rejected Huxley's entreaty that he abjure the doctrine that “natura non facit saltum” (nature makes no leap).