Roses tell the same story as dogs, but with one difference, which is relevant to our softening-up strategy. The flower of the rose, even before human eyes and noses embarked on their work of genetic chiselling, owed its very existence to millions of years of very similar sculpting by insect eyes and noses (well, antennae, which is what insects smell with). And the same is true of all the flowers that beautify our gardens.
The sunflower, Helianthus annuus, is a North American plant whose wild form looks like an aster or large daisy. Cultivated sunflowers today have been domesticated to the point where their flowers are the size of a dinner plate.* 'Mammoth' sunflowers, originally bred in Russia, are 12 to 17 feet high, the head diameter is close to one foot, which is more than ten times the size of a wild sunflower's disc, and there is normally only one head per plant, instead of the many, much smaller, flowers of the wild plant. The Russians started breeding this American flower, by the way, for religious reasons. During Lent and Advent, the use of oil in cooking was banned by the Orthodox Church. Conveniently, and for a reason that I - untutored in the profundities of theology - shall not presume to fathom, sunflower seed oil was deemed to be exempt from this prohibition.! This provided one of the economic pressures that drove the recent selective breeding of the sunflower. Long before the modern era, however, native Americans had been cultivating these nutritious and spectacular flowers for food, for dyes and for decoration, and they achieved results intermediate between the wild sunflower and the extravagant extremes of modern cultivars. But before that again, sunflowers, like all brightly coloured flowers, owed their very existence to selective breeding by insects.
The same is true of most of the flowers we are aware of - probably all the flowers that are coloured anything other than green and whose smell is anything more than just vaguely plant-like. Not all the work was done by insects - for some flowers the pollinators that did the initial selective breeding were hummingbirds, bats, even frogs - but the principle is the same. Garden flowers have been further enhanced by us, but the wild flowers with which we started only caught our attention in the first place because insects and other selective agents had been there before us. Generations of ancestral flowers were chosen by generations of ancestral insects or hummingbirds or other natural pollinators. It is a perfectly good example of selective breeding, with the minor difference that the breeders were insects and hummingbirds, not humans. At least, I think the difference is minor. You may not, in which case I still have some softening up to do.
What might tempt us to think it a major difference? For one thing, humans consciously set out to breed, say, the darkest, most blackish purple rose they can, and they do it to satisfy an aesthetic whim, or because they think other people will pay money for it. Insects do it not for aesthetic reasons but for reasons of . . . well, here we need to back up and look at the whole matter of flowers and their relationship with their pollinators. Here's the background. For reasons I won't go into now, it is of the essence of sexual reproduction that you shouldn't fertilize yourself. If you did that, after all, there'd be little point in bothering with sexual reproduction in the first place. Pollen must somehow be transported from one plant to another. Hermaphroditic plants that have male and female parts within one flower often go to elaborate lengths to stop the male half from fertilizing the female half. Darwin himself studied the ingenious way this is achieved in primroses.
Taking the need for cross-fertilization as a given, how do flowers achieve the feat of moving pollen across the physical gap that separates them from other flowers of the same species? The obvious way is by the wind, and plenty of plants use it. Pollen is a fine, light powder. If you release enough of it on a breezy day, one or two grains may have the luck to land on the right spot in a flower of the right species. But wind pollination is wasteful. A huge surplus of pollen needs to be manufactured, as hay fever sufferers know. The vast majority of pollen grains land somewhere other than where they should, and all that energy and costly materiel is wasted. There is a more directed way for pollen to be targeted.
Why don't plants choose the animal option, and walk around looking for another plant of the same species, then copulate with it? That's a harder question to deal with than you might think. It's circular simply to assert that plants don't walk, but I'm afraid that will have to do for now.* The fact is, plants don't walk. But animals walk. And animals fly, and they have nervous systems capable of directing them towards particular targets, with sought-for shapes and colours. So if only there were some way to persuade an animal to dust itself with pollen and then walk or preferably fly to another plant of the right species . . .
Well, the answer's no secret: that's exactly what happens. The story is in some cases highly complex and in all cases fascinating. Many flowers use a bribe of food, usually nectar. Maybe bribe is too loaded a word. Would you prefer 'payment for services rendered'? I'm happy with both, so long as we don't misunderstand them in a human way. Nectar is sugary syrup, and it is manufactured by plants specifically and only for paying, and fuelling, bees, butterflies, hummingbirds, bats and other hired transport. It is costly to make, funnelling off a proportion of the sunshine energy trapped by the leaves, the solar panels of the plant. From the point of view of the bees and hummingbirds, it is high-energy aviation fuel. The energy locked up in the sugars of nectar could have been used elsewhere in the economy of the plant, perhaps to make roots, or to fill the underground storage magazines that we call tubers, bulbs and corms, or even to make huge quantities of pollen for broadcasting to the four winds. Evidently, for a large number of plant species, the trade-off works out in favour of paying insects and birds for their wings, and fuelling their flight muscles with sugar. It's not a totally overwhelming advantage, however, because some plants do use wind pollination, presumably because details of their economic circumstances tip their balance that way. Plants have an energy economy and, as with any economy, trade-offs may favour different options under different circumstances. That's an important lesson in evolution, by the way. Different species do things in different ways, and we often won't understand the differences until we have examined the whole economy of the species.
If wind pollination is at one end of a continuum of cross-fertilization techniques - shall we call it the profligate end? - what is at the other end, the 'magic bullet' end? Very few insects can be relied upon to fly like a magic bullet straight from the flower where they have picked up pollen to another flower of exactly the right species. Some just go to any old flower, or possibly any flower of the right colour, and it is still a matter of luck whether it happens to be the same species as the flower that has just paid it in nectar. Nevertheless, there are some lovely examples of flowers that lie far out towards the magic bullet end of the continuum. High on the list are orchids, and it's no wonder that Darwin devoted a whole book to them.
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 than 15 inches). Darwin, purely on the strength of A. sesquipedale s existence in Madagascar, predicted in his orchid book of 1862 that there must be 'moths capable of extension to a length of between ten and eleven inches'. Wallace, five years later (it isn't clear whether he had read Darwin's book) mentioned several moths whose probosces were nearly long enough to meet the case.
I have carefully measured the proboscis of a specimen of Macrosila cluentius from South America in the collection of the British Museum, and find it to be nine inches and a quarter long! One from tropical Africa (Macrosila morganii) is seven inches and a half. A species having a proboscis two or three inches longer could reach the nectar in the largest flowers of Angr&cum sesquipedale, whose nectaries vary in length from ten to fourteen inches. That such a moth exists in Madagascar may be safely predicted; and naturalists who visit that island should search for it with as much confidence as astronomers searched for the planet Neptune, - and they will be equally successful!
In 1903, after Darwin's death but well within Wallace's long lifetime, a hitherto unknown moth was discovered which turned out to fulfil the Darwin/Wallace prediction, and was duly honoured with the sub-specific name praedicta. But even Xanthopan morganipraedicta, 'Darwin's hawk moth', is not sufficiently well endowed to pollinate A. longicalcar, and the existence of this flower encourages us to suspect the existence of an even longer-tongued moth, with the same confidence as Wallace invoked the predicted discovery of the planet Neptune. By the way, this little example gives the lie, yet again, to the allegation that evolutionary science cannot be predictive because it concerns past history. The Darwin/ Wallace prediction was still a perfectly valid one, even though the praedicta moth must already have existed before they made it. They were predicting that, at some time in the future, somebody would discover a moth with a tongue long enough to reach the nectar in A. sesquipedale.
Insects have good colour vision, but their whole spectrum is shifted towards the ultraviolet and away from the red. Like us, they see yellow, green, blue and violet. Unlike us, however, they also see well into the ultraviolet range; and they don't see red, at 'our' end of the spectrum. If you have a red tubular flower in your garden it is a good bet, though not a certain prediction, that in the wild it is pollinated not by insects but by birds, who see well at the red end of the spectrum - perhaps hummingbirds if it is a New World plant, or sunbirds if an Old World plant. Flowers that look plain to us may actually be lavishly decorated with spots or stripes for the benefit of insects, ornamentation that we can't see because we are blind to ultraviolet. Many flowers guide bees in to land by little runway markings, painted on the flower in ultraviolet pigments, which the human eye can't see.
The evening primrose (Oenothera) looks yellow to us. But a photograph taken through an ultraviolet filter shows a pattern for the benefit of bees, which we can't see with normal vision (see colour page 5). In the photograph it appears as red, but that is a 'false colour': an arbitrary choice by the photographic process. It doesn't mean that bees would see it as red. Nobody knows how ultraviolet (or yellow or any other colour) looks to a bee (I don't even know how red looks to you - an old philosophical chestnut).
A meadow full of flowers is nature's Times Square, nature's Piccadilly Circus. A slow-motion neon sign, it changes from week to week as different flowers come into season, carefully prompted by cues from, for example, the changing length of days to synchronize with others of their own species. This floral extravaganza, splashed across the green canvas of a meadow, has been shaped and coloured, magnified and titivated by the past choices made by animal eyes: bee eyes, butterfly eyes, hoverfly eyes. In New World forests we'd have to add hummingbird or in African forests sunbird eyes to the list.
Hummingbirds and sunbirds are not particularly closely related, by the way. They look and behave like each other because they have converged upon the same way of life, largely revolving around flowers and nectar (although they eat insects as well as nectar). They have long beaks for probing nectaries, extended by even longer tongues. Sunbirds are less accomplished hoverers than hummingbirds, who can even go backwards like a helicopter. Also convergent, although from a far distant vantage point in the animal kingdom, are the hummingbird hawk moths, again consummate hoverers with spectacularly long tongues (all three types of nectar junkie are illustrated on colour page 5).
We shall return to convergent evolution later in the book, after properly understanding natural selection.
Here, in this chapter, flowers are seducing us, drawing us in, step by step, lining our path to that understanding. Hummingbird eyes, hawk-moth eyes, butterfly eyes, hoverfly eyes, bee eyes are critically cast over wild flowers, generation after generation, shaping them, colouring them, swelling them, patterning and stippling them, in almost exactly the same way as human eyes later did with our garden varieties; and with dogs, cows, cabbages and corn.
For the flower, insect pollination represents a huge advance in economy over the wasteful scattergun of wind pollination. Even if a bee visits flowers indiscriminately, lurching promiscuously from buttercup to cornflower, from poppy to celandine, a pollen grain clinging to its hairy abdomen has a much greater chance of hitting the right target - a second flower of the same species - than it would have if scattered on the wind. Slightly better would be a bee with a preference for a particular colour, say blue. Or a bee that, while not having any long-term colour preference, tends to form colour habits, so that it chooses colours in runs. Better still would be an insect that visits flowers of only one species. And there are flowers, like the Madagascar orchid that inspired the Darwin/Wallace prediction, whose nectar is available only to certain insects that specialize in that kind of flower and benefit from their monopoly over it. Those Madagascar moths are the ultimate magic bullets.
From a moth's point of view, flowers that reliably provide nectar are like docile, productive milch cows. From the flowers' point of view, moths that reliably transport their pollen to other flowers of the same species are like a well-paid Federal Express service, or like well-trained homing pigeons. Each side could be said to have domesticated the other, selectively breeding them to do a better job than they previously did. Human breeders of prize roses have had almost exactly the same kinds of effects on flowers as insects have - just exaggerated them a bit. Insects bred flowers to be bright and showy. Gardeners made them brighter and showier still. Insects made roses pleasantly fragrant. We came along and made them even more so. Incidentally, it is a fortunate coincidence that the fragrances that bees and butterflies prefer happen to appeal to us too. Flowers such as 'stinking Benjamin' (Trillium erectum) or the 'corpse flower' (Amorphophallus titanum), which use flesh flies or carrion beetles as pollinators, often nauseate us, because they mimic the smell of decaying meat. Such flowers have not, I presume, had their scents enhanced by human domesticators.
Of course, the relationship between insects and flowers is a two-way street, and we mustn't neglect to look in both directions. Insects may 'breed' flowers to be more beautiful, but not because they enjoy the beauty.* Rather, the flowers benefit from being perceived as attractive by insects. The insects, by choosing the most attractive flowers to visit, inadvertently 'breed for' floral beauty. At the same time, the flowers are breeding the insects for pollination ability. Then again, I have implied that insects breed flowers for high nectar yield, like dairymen breeding massively uddered Friesians. But it is in the flowers' interests to ration their nectar. Satiate an insect and it has no incentive to go on and look for a second flower - bad news for the first flower, for which the second visit, the pollinating visit, is the whole point of the exercise. From the flowers' point of view, a delicate balance must be struck between providing too much nectar (no visit to a second flower) and too little (no incentive to visit the first flower).
Insects have milked flowers for their nectar, and bred them for increased yield - probably encountering resistance from the flowers, as we have just seen. Have beekeepers (or horticulturalists with the interests of beekeepers in mind) bred flowers to be even more productive of nectar, just as dairy farmers bred Friesian and Jersey cows? I'd be intrigued to know the answer. Meanwhile, there's no doubt of the close parallel between horticulturalists as breeders of pretty and fragrant flowers, and bees and butterflies, hummingbirds and sunbirds doing the same thing.
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