We properly honor Darwin on the 150th anniversary of his Origin of Species, though more proper would be to have honored both Darwin and Wallace last year, when their ideas were jointly presented to the Linnaean Society. Indeed, their ideas actually rest on the cell theory, which was presented by Virchow in the same year (1858).
At the meeting Ken attended in Brazil last week, he got involved in a discussion with the distinguished ecologist Doug Futuyma of SUNY/Stony Brook. Ken had asserted that despite the title of his book, Darwin had not, in fact, solved the 'species' problem. First, beyond individuals, species are the nearest we have to objective categories in nature. Usually, we define species as populations that cannot interbreed to produce fertile offspring. But even there our definitions are often vague or imprecise.
Variation, even genomewide variation, can exist without speciation (it does among individuals within every species!). Widespread adaptive variation can exist without leading to speciation (humans are variable worldwide for presumably adaptive reasons--e.g., skin color, yet we're one species). And mating barriers can arise without adaptation in the usual sense (e.g., hybrid sterility genes).
In that sense Darwin did not solve the species problem he named his book after. Doug Futuyma suggested, however, that Darwin's main objective was not speciation per se, but the process that leads to it. Indeed, Darwin wanted 'natural selection' in the title of his book, because that was the process he was invoking as an extension of artificial selection by breeders, to explain long-term biological change and the origin of adaptive structures.
But was 'species' an incidental interest or a primary one? We think the answer is that species was indeed a central objective, and yet it is not separable today, nor in Darwin's mind, from the ultimate result of the process which is speciation. This seems clear in the way Darwin's book was written, in the materials presented to the Linnaean Society, and also in letters he wrote around the time of the book and earlier, around 1844, when he drafted a private sketch of his ideas.
The process was an extension of agricultural and hobby breeding, that clearly led to variation. But Darwin was also determined to show that species--natural 'types'--were not the result of specific acts of creation. The nature of 'transmutation' as it was often called at the time, was hotly debated and of course then, as now, centrally involved religious explanations of the world. Darwin was convinced that 'varieties' and 'species' arose gradually through natural processes.
So, while he did not solve the species problem per se (which is not a 'neat' problem in any case), he provided brilliant insight as to the nature of the processes that, in various ways, are involved in natural divergence that leads to the origin of species.