Barriers that Keep Different Species from Interbreeding

What keeps members of two related species from mating with each other? There are many different reproductive barriers. Species might not interbreed simply because their mating or flowering seasons don’t overlap. Some corals, for example, reproduce only one night a year, spewing out masses of eggs and sperm into the sea over a several-hour period. Closely related species living in the same area remain distinct because their peak spawning periods are several hours apart, preventing eggs of on...

 Theory of Evolution Predicts the Existence of a Species

Both Darwin and his co-discoverer of natural selection, Wallace, called attention to an amazing orchid from Madagascar, Angraecum sesquipedale (see colour page 4), and both men made the same remarkable prediction, which was later triumphantly vindicated. This orchid has tubular nectaries that reach down more than 11 inches by Darwin's own ruler. That's nearly 30 centimetres. A related species, Angraecum longicalcar, has nectar-bearing spurs that are even longer, up to 40 centimetres (more tha...

 Hummingbirds are Proof of the Cost of Sex

If sex had no cost, hummingbirds would not exist. Hummingbirds eat nectar, which is produced by flowers to lure pollinating insects and birds. Nectar is a pure gift by the plant of its hard-won sugar to the hummingbird, a gift given only because the hummingbird will then carry pollen to another plant. To have sex with another plant, the first plant must bribe the pollen carrier with nectar. Nectar is therefore a pure, unadulterated cost incurred by the plant in its quest for sex. If sex had n...