(perpendicular to the c-axis) of other crys- tals. In fact, it has already been shown that growing SiC crystals on surfaces with a different crystal orientation — the a-faces — fully suppresses the formation of micropipes. However, crystals made in this way are affec- ted by basal-plane stacking faults. Neverthe- less, Nakamura et al. started with a SiC single crystal that had been grown on an a-face, inheriting a high density of dislocations from its seed crystal. Then, taking a section of the crystal along the a-axis of that face, they allowed the crystal to develop on the other a-face (Fig. 2), afterwards continuing with classic growth on the c-face. Nakamura et al. call this the repeated a-face (RAF) growth process. It is the repetition of the a-face step that ensures that stacking faults are elimi- nated and dislocations suppressed. The ingots of SiC produced are, say the authors, “virtually dislocation-free”. These results are spectacular: the RAF process is a major innovation in materials science. Silicon carbide has become, at last, a contender for silicon’s crown. ■ Roland Madar, of CNRS, is in the Department of Physics, Institut National Polytechnique de Grenoble, Grenoble 38402, France. e-mail: roland.madar@inpg.fr 1. Nakamura, D. et al. Nature 430, 1009–1012 (2004). 2. Tairov, Yu. M. & Tsvetkov, V. F. J. Cryst. Growth 43, 209–212 (1978). 3. Peters, D., Friedrichs, P., Schörner, R. & Stephani, D. Mater. Sci. Forum 389–393, 1125–1128 (2002). 4. Lendenmann, H., Dahlquist, F., Bergman, J. P., Bleichner, H. & Hallin, C. Mater. Sci. Forum 389–393, 1259–1264 (2002). news and views NATURE | VOL 430 | 26 AUGUST 2004 | www.nature.com/nature 975 strains that lack the same system 9 . Perhaps the most common form of microbial coop- eration is the secretion of substances outside the cell to do various kinds of work, such as digestion. The work provides a public good, in which the benefits are available not just to the secretor,but to anyone in the neighbour- hood. This puts individuals into a social bind.Why work to help others when you can be lazy and freeload off your neighbours? The obvious answer is kin selection. If your effort helps others with the same genes, it is not wasted. Once it is recognized that microbes have interesting social behaviour, two clear advantages can be exploited. First, many microbes are well understood biochemically and genetically. Second, it is easy to manipu- late entire populations of microbes over space and to select them over time. Griffin et al. 5 made full use of these advantages in their study of a freeloading mutant of the bacteri- um Pseudomonas aeruginosa. This ‘cheater’ mutant is deficient in production of a public benefit called siderophores. Siderophores are synthesized and excreted in response to iron deficiency. They then bind iron, making it available for uptake — either by the secretor or by its neighbours. The mutant saves on the cost of producing siderophores but can still obtain iron if it has secretor neighbours. Griffin et al. selected populations, start- ing with equal numbers of cheaters and secretors, over six cycles of group inter- action, each lasting about seven generations (Fig. 1, overleaf). In natural populations with limited dispersal, neighbours tend to be both relatives and competitors, but Griffin et al. were able to vary the relatedness and scale of competition independently. High or low relatedness was imposed by allowing bacteria to grow and interact in groups derived from a single bacterium (a cheater or a secretor) or from two bacteria (initially in a ratio of one cheater to one secretor), respectively. The scale of competi- tion was manipulated during the transfers to form each next cycle of groups. Global com- petition was allowed by mixing the groups and then choosing random individuals to start the next cycle. The more productive groups thus contributed more to the next cycle. Local competition was imposed by taking an equal number of individuals from each group, without mixing between the cycles. Crossing the relatedness and compe- tition treatments yields a simple two-by-two design that permits the study of both factors and their interaction. As expected, at the end of the experiment the high-relatedness treatments yielded high- er frequencies of the cooperative secretors than the low-relatedness treatments, demon- strating the importance of kinship in the evo- lution of cooperation. But the greater novelty of the study comes from another theory- hexagonal tube-like cavities, with diameters in the sub-micrometre to few-micrometre range, that develop parallel to the c-axis of the hexagonal structure (which is the com- mon direction of crystal growth). They are the most harmful defects in SiC. Intensive work on various aspects of the PVT growth technique — reactor design, growth conditions, seed orientation and sur- face preparation — has led to a considerable reduction in the number of these defects in SiC ingots and wafers. The best commercial- ly available 3-inch-diameter wafers (of the 4H polytype) have micropipe densities of 10–100 cm ǁ2 and dislocation densities of 10 3 –10 4 cm ǁ2 . This quality of crystal is good enough for use in some commercial SiC devices, such as Schottky diodes 3 , but not for others. For instance, in high-power bipolar devices, there is a degradation in the materi- al’s electrical properties that seems to be related to the development of extended stacking faults, originating from in-plane dislocations in the SiC (ref. 4). So reducing the density of dislocations in an SiC wafer,and in the epitaxial layer on top of it that forms the active part of the device, is an absolute necessity for the development of high-power SiC technology. Nakamura et al. 1 have succeeded: their new process pro- duces SiC crystals with a dramatically lower dislocation density — although, perversely, the first step of the process actually creates a high density of stacking faults. Most SiC crystals are grown on the c-faces Social evolution Kinship is relative David C. Queller Kinship fosters the evolution of cooperation. However, a once-heretical theory and an unconventional social organism show that the cooperation-enhancing effect of kinship is sometimes negated. A s evolutionary biologist W. D. Hamil- ton showed more than 40 years ago, selfish genes can lead to cooperation and altruism 1 . A gene can spread by helping other individuals that carry copies of the gene. This extension of individual selection, called kin selection, works best with the recognition of close relatives, but Hamilton also thought that if the dispersal of individ- uals is limited, this might build up enough local genetic similarity to favour a less tar- geted kind of altruism towards neighbours in general. Because local dispersal is very common, this mechanism might greatly expand the range of cooperation in nature. However, later models 2–4 suggest that this is not necessarily so: when limited dispersal makes neighbours close relatives, it also makes them close competitors, and this can negate the effect of kinship. On page 1024 of this issue 5 , Griffin and colleagues provide the first experimental test of this effect, confirming that both kinship and the scale of competition matter. The study is part of a growing trend towards using microorganisms to study social evolution 6 . Sociobiologists have been slow to recognize that microorganisms have social interactions worthy of study.However, any reader of John Bonner’s work on cellular slime moulds 7 will realize that these single- celled amoebae interact in interesting ways; some give up their lives to form a stalk that furthers the dispersal of others. Analogous behaviour occurs in Myxobacteria 8 . And interesting behaviour is not limited to the exotica of the microbial world; the old laboratory workhorse Escherichia coli has strains that aid their own type by wielding poison–antidote systems to destroy other ©2004 Nature Publishing Group