How could fruits and flowers evolve?

The problem

Some Creationists have argued that fruits and flowers seem to be there for the benefit of Man. As one put it,

Why most edible fruits on earth are within the reach of man (or at least when they are ripe), why they taste perfect and their size fits perfectly into the hands of mankind. Why don't those fruits fit all animals?

How they could evolve

Science's theory is that fruits are not a gift, but a bribe. Or, to put it another way, the plants have a deal going with the animals that consume the fruit, or that consume the nectar in the flower.

The deal is that the animals will carry some of the plant's seeds away to new places. In the case of flowers, the animal carries pollen to other flowers of the same species. They are paid for this service in food.

It's easy to see how this deal could evolve gradually. Some plant happened to be more edible than usual, and therefore its seeds were spread more than usual. (And, each seed was deposited inside a nice mound of, um, fertilizer.) The offspring inherited this slight tendency to be edible. And, among the offspring, some happened to be even more edible. And around and around it went. The occasional mutation would help, of course, but most of the changes didn't need mutation. If the tallest people had all the kids, pretty soon everyone would look like a basketball player.

Evidence they are for animals

Insects can see ultraviolet light. Put another way, insects can see higher frequencies of light than humans can. So, if flowers are for insects, we would predict that flowers have patterns on them that only insects can see. And, now that humans have ultraviolet photography, that turns out to be the truth.

There are nuts which are only eaten by animals. For example, squirrels eat acorns, but humans find them bitter. Similarly, Colorado squirrels live by eating Ponderosa pine cones, which humans don't eat.

The loud colors and loud smells of fruits and flowers are advertising: hey, over here! The advertising is sometimes clearly aimed at animals. For example, there is a tropical flower which attracts insects with a smell just like rotting meat.

The seed inside a fruit must have a tough enough rind to survive travellling through an animal's gut. Many tropical species have tough seeds, which survive being eaten by the toughest fruit-eating animal in their neighborhood. The passage abrades the rind, so that the sprouting seed will eventually be able to break out. There is one tree species which is currently in danger of extinction, because the animal it counted on has gone extinct, and the other local animals don't abrade the rinds enough to let the seeds sprout.

The seeds inside a fruit are sometimes bitter tasting or even poisonous, thus encouraging animals to eat only the fruit. Peach pits are a good example. If fruits were here by benevolence, surely they wouldn't contain poison.

Fruits are generally not at a convenient height for humans. Strawberries and melons are down near the ground. Small animals and four-footed animals find this convenient, but humans don't. And trees tend to have their fruit well above where humans can reach. In jungles, fruits are usually up in the canopy, far above humans, but easily accessible to monkeys, birds, insects and fruit bats.

Last modified: 9 November 2001

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