|Blackberries grow on prickly vines or brambles,
and are members of the Rose Family (Rosaceae).
|Mulberries grow on trees, and are
members of the Mulberry Family
In terms of the teaching of botany and evolution, however, the similarities and differences between the two berries tell a powerful story. Though they function the same way in natural fruit dispersal, they are not related at all. Blackberries are members of the the very fruitful family, Rosaceae, which includes raspberries, strawberries, plums, cherries, peaches, apricots, apples, pears, rose hips and many more. Mulberries, on the other hand are members of the Moraceae, which includes figs, breadfruit, and rubber trees.
The structures of the two fruits are quite different. Blackberries are aggregate fruits, which means that the cluster of drupelets derive from a cone of separate carpels belonging to a single flower. Mulberries on the other hand are technically infructescences, or compound fruits, similar to pineapples. Each drupelet forms from its own tiny flower. So there are fundamental developmental differences that lead to the similar looking fruits.
This might seem like a geeky bit of botanical trivia that would quickly make dinner guests fall asleep, but in the classroom, however, it illustrates some of the most central phenomena of evolution: adaptation, adaptive radiation, and convergent evolution.
These fruits, first of all, are adapted for dispersal by animals, primarily birds, though I have had to keep an eye out for hungry black bears as well while picking berries along roadsides in Washington State. Both go through green and red phases before turning black at ripening. This primes the animals for the coming feast. The berries are sweet, juicy, and flavorful. The animals gobble down the fruits, and the digestive process strips away the juicy tissues, leaving the tiny seeds to pass through the alimentary canal. The animals tend to move about after feeding, leaving seeds in their feces. (See also "What is an Adaptation?)
The different kinds of fruit to be found in the Rose Family are an example of adaptive radiation - the evolution of a variety of descendant species from a common ancestor. As the descendants of the common ancestor began spreading into new habitats and new geographic areas, they adapted to local conditions, including local fruit dispersers.
As other families went through their own adaptive radiations, some descendants encountered the same opportunity for dispersal, and developed similar physical characteristics, but with tell-tale differences in underlying structure. This is convergent evolution - the development of very similar adaptations from unrelated ancestors. The ancestors of the Rose Family happened to have flowers with multiple separate carpels, and so easily evolved into aggregate fruits, while the ancestors of the Mulberry Family had tiny flowers with just one carpel in each, so a similar fruit was most easily developed by grouping the fruits of many flowers together. I have posted earlier about evolution of cactus-like members in unrelated families, along with numerous examples of convergent evolution in animals. (See "Of cacti and humans – are certain life forms inevitable?"