First the Bees, then the Birds

Botanists have an advantage over zoologists—their subjects sit still, more or less. They can’t hide, and they can’t run either. Similarly, gardeners can rogue out unwanted seedlings a bit more heartlessly than, say, dog breeders can snuff runt puppies. (Well, most can. I myself have a sad, sentimental plant infirmary at home.) This, combined with some other differences between plants and animals, sometimes gives us a chance to peer closely at evolution.
On a couple of recent projects, biologists used good old garden techniques combined with hotshot bench science to get a sense of just what it takes to make a species. Plant speciation has already taken some interesting turns here in California: for example, there’s a monkeyflower species, Mimulus cupriphilus, that seems to be only about 50 years old. It grows solely on copper-mine tailings, for which there’s no natural equivalent, and that’s how long we’ve had copper mines in its range. (See Terrain, Spring 2002, The Wild West.) Makes one wonder just how big an internal shift it takes to put a population into enough reproductive isolation to make it a new species.
Evidently it doesn’t take much: a change in a single allele—one half of a gene pair on a chromosome—can result in an external change big enough to change a plant’s “client”—the species that prefers its nectar, and that pollinates it. A group of such client species is called a pollinator guild: bees or gnats or hummingbirds, for example. A switch in a plant’s set of pollinators, all by itself, can be enough of a reproductive isolation to set the stage for speciation.
Reproductive isolation itself provides a common working definition of species—“groups that can breed among themselves, producing fertile offspring.” Ernst Mayr, a generation or two ago, fingered geographic isolation, or “island effects,” as one way to kickstart the founding of a species. To simplify: a plant seedhead gets blown to an isolated place and sprouts, and its descendants pollinate each other with no outside genetic input. Any genetic oddity—and we all have them—occurs more frequently in an inbred population, and those that work well make plants that reproduce more. Eventually, given enough accumulated changes, you end up with something like the Hawai’ian silversword that its mainland tarweed ancestors wouldn’t recognize, and couldn’t breed with: a new species.
A group of our own monkeyflowers turns out to be ideal for studying this interaction of inside and outside. By crossbreeding two closely related species, Mimulus cardinalis and M. lewisii, and  recrossing the offspring, a couple of scientists changed only the flower color of each—one allele change—which resulted in each species moving from bee pollination to hummingbird pollination or vice versa. Other scientists have studied flower shapes, stamen and pistil lengths, and nectar production. Bird-pollinated (ornithophilous) plants have lots more nectar in each flower than bee-pollinated (melittophilous) ones, for example, and bees seem to ignore red flowers, leaving the field clear for birds. Characters other than color all differ between bird- and insect-pollinated monkeyflowers, but evidently all it takes is one change to make that big switch; maybe the rest follow as tendencies that accumulate gradually. Here’s another clue about how Nature does her gardening.
On a much larger scale, students of nature are tracking geography and time and the movements of populations. We have many species of monkeyflower in North America, and a lot of them are neighbors to each other. How did they get isolated enough to speciate?
There are at least seven genera of Western plants that have one species each  pollinated by birds, and five genera with two such species each; other smaller genera that are completely bird-pollinated are close relatives of and probably descendants of genera that are insect-pollinated. This pattern suggests that being bird-pollinated is a secondary, or derived, condition, and that such plants descended from insect-pollinated ancestors.
It’s not surprising that hummingbird-pollinated species exist only in hummingbirds’ range, but you have to wonder how they arranged themselves. Without birds, how did the plants reproduce, and in an area where their favorites are scarce, why would the hummers be there?
Part of the answer is that the preferences aren’t exclusive: a hungry hummer will try any flower, even if it’s a bee-pollinated species with less nectar. The birds don’t much care what color the flower is, either. It’s not so much that hummers like red more as that bees like it—or notice it—less than yellow. In the postglacial shuffle, imagine a plant population bereft of its bee pollinators. The individuals that appeal most to local hummers leave more descendants. Eventually—especially given that a single gene change is enough to create a major operational difference—this dance of desperation leads to flowers that bees  ignore and that get pollinated only with each other, by hummingbirds: a reproductive isolate. Even if the glacier retreats farther or the valley is exalted, even if the new species rejoins its cousins, the change in pollinators will tend to maintain its integrity as a new species.

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