Diversity and Relatedness Enhance Survival in Colour Polymorphic Grasshoppers

Abstract

Evolutionary theory predicts that different resource utilization and behaviour by alternative phenotypes may reduce competition and enhance productivity and individual performance in polymorphic, as compared with monomorphic, groups of individuals. However, firm evidence that members of more heterogeneous groups benefit from enhanced survival has been scarce or lacking. Furthermore, benefits associated with phenotypic diversity may be counterbalanced by costs mediated by reduced relatedness, since closely related individuals typically are more similar. Pygmy grasshoppers (Tetrix subulata) are characterized by extensive polymorphism in colour pattern, morphology, behaviour and physiology. We studied experimental groups founded by different numbers of mothers and found that survival was higher in low than in high density, that survival peaked at intermediate colour morph diversity in high density, and that survival was independent of diversity in low density where competition was less intense. We further demonstrate that survival was enhanced by relatedness, as expected if antagonistic and competitive interactions are discriminately directed towards non-siblings. We therefore also performed behavioural observations and staged encounters which confirmed that individuals recognized and responded differently to siblings than to non-siblings. We conclude that negative effects associated with competition are less manifest in diverse groups, that there is conflicting selection for and against genetic diversity occurring simultaneously, and that diversity and relatedness may facilitate the productivity and ecological success of groups of interacting individuals.

Variation in founder groups promotes establishment success in the wild

“…Superior success of more variable groups is therefore expected also if greater generalism (at the level of the group when summed across different individuals) with regard to thermal requirements, dietary preferences or life-history characteristics promotes establishment [4]. Finally, there is experimental evidence that, at high density, survival of captive pygmy grasshopper individuals is enhanced by high phenotypic diversity, indicating that resource utilization is more efficient and competition less manifest in polymorphic groups [40].…”

Go High or Go Low? Adaptive Evolution of High and Low Relatedness Societies in Social Hymenoptera

“…Clearly, drifting also increases the genetic diversity in the workforce population and this can through social heterosis positively affect a variety of group-level characters Fewell, 2007, 2008). In grasshoppers, groups of mixed relatedness survive better than groups composed of only sibs (Caesar et al, 2010). In harvester ants, intracolonial relatedness and colony growth are significantly negatively correlated which indicates that multiple mating by queens produces fitness advantages (Wiernasz et al, 2004).…”

Higher establishment success in more diverse groups of pygmy grasshoppers under seminatural conditions

“…Establishment success may increase also because diverse founder groups are more likely to include already pre-adapted individuals that can survive the environmental conditions at the new site (i.e., a sampling effect; Gamfeldt and Kallstrom 2007), and because standing genetic variance increases the likelihood of evolutionary rescue (Bell and Gonzales 2009). Under limited resources, groups that consist of differently specialized morphs may also benefit from decreased intraspecific competition (Forsman et al 2008, Hughes et al 2008, Caesar et al 2010.…”

“…T. subulata exhibits a high degree of color polymorphism. Morphs range from light gray via different shades of brown to black, with some morphs being uniform and others mottled or patterned with stripes or speckles (see Appendix A and illustrations available in Caesar et al 2010. Results from dual-choice experiments indicate that there is no mate choice based on color morph and that females are polyandrous ).…”