Evolutionary ecology of seed predation and seed dispersal in a polymorphic acacia (Australia)

Fruit color polymorphisms are widespread in nature, but their evolutionary dynamics remain poorly understood. While frugivores have been identified as the most likely agents of selection, field evidence is inconclusive; furthermore, alternative agents of selection have rarely been investigated. Here, I examine patterns and agents of natural selection on Acacia ligulata, a shrub of the Australian arid zone that exhibits a red/orange/yellow aril color polymorphism.; Chapter One investigates the genetics, chemistry and geographic distribution of the polymorphism, as well as selection on early life history stages. Aril color in A. ligulata appears to have a heritable basis, and color morph frequencies show clinal variation. While I found no evidence of selection during seedling emergence and survival, differences in the chemical composition of arils strongly suggest the possibility of selection by animal consumers.; Chapter Two explores spatiotemporal patterns of seed production and insect seed predation in four populations over three years. Red and yellow morphs each had the highest seed production in different sites, suggesting that spatially variable selection may be acting to maintain the polymorphism. Heteropteran insects contributed to these patterns via differential ovule predation among morphs. Because aril color is not visible to the insects, differential predation is evidently a response to pleiotropic effects of fruit color alleles. Chapter Three assesses the role of ant and bird consumers in selection on the color polymorphism. While ants did not interact differentially with the color morphs, birds expressed biases that translated into differences in the relative fitnesses of morphs. Bird seed predators acted to decrease fitness of the yellow morph, at least in one year, while bird seed dispersers likely had spatially variable effects.; Finally, Chapter Four digresses from consideration of fruit-color polymorphism in A. ligulata to focus on the plant's relationship with a subsocial spider, Phryganoporus candidus. Experiments demonstrated that spiders protect seeds from predatory insects and increase seed production. In turn, both prey capture rates and spider persistence on A. ligulata exceeded that on an alternate host, suggesting that A. ligulata provides benefits to spiders. This study is the first to document mutual benefits in a plant-spider association.