Coevolution of Bees and Flowers

Nature Vol. 278 19 April l979 689

FLOWERS provide nectar for bees, older). This arrangement of flowers and in return a bee acts as a kind Coevolution of ensures that a visiting bee arrives at of flying penis, carrying pollen from the female flowers near the base one plant to fertilise the ovaries of bees and flowers from John R. Krebs having just come from the male another. Plants and their pollinators flowers near the top ·of the previous are a classic example of mutualism: flowers first. As it moves up the in plant. In this way the chance of out they have coevolved through evolu florescence the bee experiences gradu cmssing is increased, and it is known tionary time in a reciprocal beneficial ally diminishing returns from the that in Delphinium at least outcross relationship. However this superfi successively smaller flowers, until ing results in a higher percentage cially harmonious view may conceal there comes a point at which more seed set (Price & Wascr Nature 277, the true nature of the interaction. It nectar per second could be obtained 294; 1979). The spiral arrangement is perhaps more accurate to think of by travelling to the bottom of the of flowers around the stem is also bees having evolved towards efficient next inflorescence. Pyke's measure important to the plant. A bee docs exploitation of plants as a food source, ments suggested that bees roughly fol not usually carry enough pollen to while plants have evolved adaptations low this optimal departure rule, and fertilise all the female flowers on a to minimise the price they pay for this accounts for the fact that a bee plant, so if it visited every flower it getting bees to transfer pollen. G. H. usually leaves before reaching the top would extract some nectar without Pyke ( Oecologia 36, 281; 1978) has of an inflorescence. At first sight one providing pollen in return. The plants' used this line of argument to develop might think that it would pay the bee evolutionary answer seems to be to a new insight into the structure of to visit every single flower on its way arrange its inflorescence in such a certain inflorescences and the forag up the inflorescence, rather than mis way that an efficient bee actually does ing behaviour of bees. He studied sing nectar-rich flowers near the better by missing some of the female three plant genera Delphinium, Acon bottom. However, the spiral arrange flowers. itum, and Epilobium, all of which ment of flowers reduces the benefit Given the design of plants, bumble are cross-fertilised by bumblebees of this movement rule. The extra bees seem to approximate an optimal (Bombus spp.), and have their flowers energetic costs to the bee involved in set of movement rules for maximis arranged in a vertical spike. manoeuvering sideways to follow a ing net rate of harvesting nectar. At Bumblebees feeding on these plants spiral path more than offset the bene the same time the plants have evolved follow a characteristic movement pat fit of visiting all the flowers. The a design which makes highly efficient tern. They almost invariably start at rule of moving straight upwards gives (perhaps optimal, but not enough is the bottom of an inflorescence, move a higher net energy gain. known about benefits to plants) use up the spike, and leave to fly to the Bumblebees seem to have evolved of bees as pollinators. It might ap next plant before reaching the top. a set of rules for efficient exploitation pear that the coevolutionary struggle Within the inflorescence, a bee typi of nectar, but the plants are equally has reached a stable point where bees cally moves vertically upwards from under an evolutionary pressure to ex and plants are doing as well as they one flower to the nearest one above. ploit bees, and Pyke suggests that the can, but there is another aspect to The flowers of all three plant genera flower arrangement can be interpreted the problem. Different species of are arranged in a spiral around the in this way. The larger, older, nectar plants (as well as different individuals vertical spike, so that by moving ver rich flowers at the bottom of an in of the same species) compete to at tically upwards, a bee misses out florescence are female, while the tract pollinators, and a complete ac some of the flowers. The bees' move younger flowers at the top are male count of the coevolution of bees and ment rules are related to nectar (the plants are protandrous herma plants must he ahle to explain why availability. Flowers at the bottom of phrodites, meaning that each flower different plants living in the same area an inflorescence are older, bigger and turns from male to female as it gets are designed to offer nectar in differ contain more nectar than those at the ent ways. The answer to this sort of John R. Krebs is in the Edward Grey top; hence by starting at the bottom Institute of Field Ornithology, University question may emerge from approaches the bee visits the most profitable of Oxford. such as Pyke's.

for comparison to determine the (Fig. I). This function can be com and an unoccupied antibonding H-Pd changes caused by the adsorbate. pared with a difference photoemission surface hand at high energies (not A convenient way to explore the spectrum (Demuth Surface Sci. 65, shown in Fig. I)_ Other surface struc effects of the adsorbate is to calculate 369; I977) to determine which geometry ture is found and analysed. the surface density of states with and fits the measured results. The features of the H-Pd bonding without the adsorbate and to examine One prominent feature common to state can be studied using charge the difference in these two functions all the 6N(E) curves is the dip near density contour maps. These illustrate A.N(E). Since the density-of-s1a,tes func the metal Fermi energy (0 eV). This the details of the mixing between the tion determines the number of elec dip reflects the adsorbate correlated hydrogen and Pd orbitals. Louie has tronic states in each energy interval, movement of states away from this also calculated the total electronic surface or adsorbate states show up as region to lower energy. The peaks at charge distribution and finds that some peaks in this function in the energy lower energies arise from the new charge spills out into the vacuum above interval corresponding to the binding hydrogen induced states formed from the surface. The values for the number energy of the state. The difference hydrogen Is and Pd d-states. Com of valence electrons per surface unit function 6N(E) demonstrates how the parison of the 6N(E) spectra rules cell are 0.2 in the vacuum above the states near the surface are affected by out site A and favours B and C which surface and 9.7, 10.1, 10.0, 10.0 for the the adsorbate. differ little in their environment. Louie first, second, third and fourth layers The site where the hydrogen atom concludes that the hydrogen atoms into the bulk (10 is the hulk value). is adsorbed on the Pd (Ill) surface prefer a threefold hollow over the top The calculated work function is in ex isn't known, but three different sites site. This geometry yields an occupied cellent agreement with experiment are likely. Louie's calculation models H-Pd bonding surface band centred Although Louie's calculation only all thrP-e sites (A, B and C) and com a' 6.5 eV below the Fermi energy (Fig. considers a specific adsorbate on a putes the 6N(E) function for each case I) in good agreement with experiment specific face of one transition metal,