Breaking taboos in the tropics: incest promotes colonization by wood-boring beetles

1 Inbreeding and parthenogenesis are especially frequent in colonizing spec ies of plants and animals, and inbreeding in wood-boring species in the wee vil families Scolytinae and Platypodidae is especially common on small isla nds. In order to study the relationship between colonization success, islan d attributes and mating system in these beetles, we analysed the relative p roportions of inbreeders and outbreeders for 45 Pacific and Old World tropi cal islands plus two adjacent mainland sites, and scored islands for size, distance from nearest source population, and maximum altitude. 2 The numbers of wood-borer species decreased with decreasing island size, as expected; the degree of isolation and maximum island altitude had neglig ible effects on total species numbers. 3 Numbers of outbreeding species decreased more rapidly with island size th an did those of inbreeders. Comparing species with similar ecology (e.g. am brosia beetles) showed that this difference was best explained by different ial success in colonization, rather than by differences in resource utiliza tion or sampling biases. This conclusion was further supported by analyses of data from small islands, which suggested that outbreeding species have a higher degree of endemism and that inbreeding species are generally more w idespread. 4 Recently established small populations necessarily go through a period of severe inbreeding, which should affect inbreeding species much less than o utbreeding ones. In addition, non-genetic ecological and behavioural ('Alle e') effects are also expected to reduce the success of outbreeding colonist s much more than that of inbreeders: compared with inbreeders, outbreeders are expected to have slower growth rates, have greater difficulties with ma te-location and be vulnerable to random extinction over a longer period.