Clerestory

Darwin and r/K selection

November 11, 2020

Today I’m thinking about Darwin and r/K selection theory.

Wikipedia had led me to believe that this was an idea of Robert MacArthur and E. O. Wilson in 1967’s The Theory of Island Biogeography which had been on my list for a while.

But last week I started reading The Origin of Species and Darwin seems to be pushing in that direction already in 1859. Here he’s talking about artificial selection (“Variation under Domestication”, p35 of my edition):

I will now say a few words on the circumstances, favourable or the reverse, to man’s power of selection. A high degree of variability is obviously favourable, as freely giving the materials for selection to work on; not that mere individual differences are not amply sufficient, with extreme care, to allow of the accumulation of a large amount of modification in almost any desired direction. But as variations manifestly useful or pleasing to man appear only occasionally, the chance of their appearance will be much increased by a large number of individuals being kept. Hence number is of the highest importance for success.

All else being equal, in other words, a larger number of individuals should mean more chances for variation. Larger populations should therefore be more variable.

I’m pretty sure he predicts “r-species” under natural selection, too. If that is of interest, let me know and I’ll try to hunt down the passage.

Darwin continues:

On this principle Marshall formerly remarked, with respect to the sheep of part of Yorkshire, “As they generally belong to poor people, and are mostly in small lots, they never can be improved.” On the other hand, nurserymen, from keeping large stocks of the same plant, are generally far more successful than amateurs in raising new and valuable varieties. A large number of individuals of an animal or plant can be reared only where the conditions for its propagation are favourable. When the individuals are scanty all will be allowed to breed, whatever their quality may be, and this will effectually prevent selection. But probably the most important element is that the animal or plant should be so highly valued by man, that the closest attention is paid to even the slightest deviations in its qualities or structure. Unless such attention be paid nothing can be effected.

So one problem with small lots is that they usually can’t afford to sacrifice individuals. This seems to relate to economies of scale, somehow; typically I’d think of economies of scale in terms of factories, which would have high overheads in terms of switching what they’re producing. But it’s an interesting idea that if the product you’re dealing with is natural, you might actually have more variability if you have a larger stock, not less.

Darwin’s anecdote about people assuming the wrong directionality of strawberry variability is quite funny:

I have seen it gravely remarked, that it was most fortunate that the strawberry began to vary just when gardeners began to attend to this plant. No doubt the strawberry had always varied since it was cultivated, but the slight varieties had been neglected. As soon, however, as gardeners picked out individual plants with slightly larger, earlier, or better fruit, and raised seedlings from them, and again picked out the best seedlings and bred from them, then (with some aid by crossing distinct species) those many admirable varieties of the strawberry were raised which have appeared during the last half-century.

He also makes me very curious about all these allegedly “admirable varieties of the strawberry.” I want to try these strawberries! Have we lost these varieties or are there still many?

Next, I continue thinking about Darwin and punctuated equilibrum.


I'm Bryan Kam. I'm thinking about complexity and selfhood. Please sign up to my newsletter or see more here.