Darwin and Dewberries

This essay accompanies a video on the Darwin channel.

All over Europe, blackberry bushes (Rubus fruticosus) form thick hedges (ronces) out in the sun that are nearly impenetrable by humans or other large animals. The vinelike branches are thick, strong, and covered with nasty thorns. At first it might seem strange that the vines would be so well protected against animals, since they produce delicious blackberries that natural selection has designed for animal dispersal. Animals eat them, the seeds pass through the gut, and come out in a new location—exactly what the plant “wants” to happen. But apparently the ronces “want” birds, not mammals, to disperse the seeds.

The blackberry bushes have numerous adaptations to life in bright sunlight. The branches are thick with sapwood that conducts water to the thick leaves, which are white on the underside, an adaptation to occasionally hot, dry conditions. Because the shrubs grow so fast, they can produce an abundance of fruits, which are first green, then red, then black when fully ripe. The flowers begin to open in spring; then, starting in July, you and other animals can feast on the berries, that is, when you can reach them. 

Meanwhile, in the shade, another species of Rubus grows: Rubus caesius, the dewberry. They are adapted to life in the shade: narrower stems that conduct less water, and they have thinner leaves, which are not white on the underside. The flowers do not open until August, and the fruits are not ripe until September. Because they grow in the shade, they grow more slowly, and are smaller. For every one dewberry, a bird may find a hundred blackberries.

They also have fewer thorns. These differences are not just due to sun vs. shade conditions; even when both of the species grow together in the shade, one can distinguish them, for example by the number of leaflets (often five in blackberries, only three in dewberries).

Clearly, blackberries are the dominant species. Dewberries hide in the shade. There appears to be competition between the two species. Blackberries rule the summer sunshine; dewberries could not possibly win in competition against them. But dewberries find a refuge from competition by blooming later. Natural selection has favored dewberries that reproduce a little bit in the late summer shade, over dewberries that might uselessly attempt to reproduce in the same place and same time as blackberries. This is evolution (a kind called temporal niche displacement) acting upon the dewberries. It spells the difference between barely surviving, and not surviving at all.

Temporal displacement keeps the species from interbreeding. I have no idea whether they could interbreed; no one knows exactly how closely related the two species are. But they don’t interbreed because they flower at different times.

And they don’t live in exactly the same places. When a bird looks for a juicy berry, a dewberry is just as good as a blackberry, but a bird is very unlikely to eat both of them. As a result, a poop-load of blackberry seeds is likely to end up in a separate location from a poop-load of dewberry seeds.

It is conceivable that natural selection also acts upon the blackberries. Right through August and September, blackberries produce big red berries that ought to turn black. But when I look for ripe blackberries in August, I find that almost all of them have withered before I, or any birds, have a chance to eat them. I find this puzzling. The weather remains good for ripe blackberries, but something makes them turn into little dried black blobs.

Thus the fact that dewberry flowers and fruits are delayed, and also the fact that the late blackberry fruits wither, both keep the two species from blending together. Evolution at work. A human parallel would be an ethnic minority that could blend in with the majority, and lose its distinctness, remains distinct and survives by living in little, separate communities.