1. Biotic resistance is the ability of communities to inhibit the establishment, spread or impact of novel species. However, the interactions that underlie biotic resistance depend heavily on the contexts in which species interact. Consequently, studies of biotic resistance that consider single processes, patches, species or life-history stages may provide an incomplete picture of the capacity for communities to resist invasion. 2. Many organisms have multiphasic life cycles, where individuals can occupy distinct niches at different stages of the life-history. Generally, studies of biotic resistance focus on interactions within a single life-history stage, and interactions at other life-history stages are overlooked. Here, we demonstrate that different mechanisms of biotic resistance occur across the life history and together limit the invasion success of an introduced marine invertebrate (Ciona intestinalis) in northern California. 3. We tested the role of interactions (competition and predation) with the resident community in limiting the abundance of Ciona through experiments conducted on fertilization, larval survival, settlement, early post-settlement survival, and the survival of juveniles and adults. 4. Under some circumstances, Ciona became abundant in mid-successional stages and showed more rapid growth rates than a morphologically similar native species, Ascidia ceratodes. However, predators reduced Ciona abundance much more than that of Ascidia at several life stages. 5. Furthermore, Ciona appeared to be a weaker competitor at the adult stage. Early life-history interactions with other sessile species at the fertilization, larval and recruit stages had modest to no effects on Ciona abundance. 6. The presence of biotic resistance mechanisms acting at multiple life stages, and potentially under different conditions, suggests that different components of biotic resistance interact to enhance the resident community's resistance to invasion.

Phenotypes tend to remain relatively constant in natural populations, suggesting a limit to trait evolution. Although stationary phenotypes suggest stabilizing selection, directional selection is more commonly reported. However, selection on phenotypes will have no evolutionary consequence if the traits do not genetically covary with fitness, a covariance known as the Robertson–Price Identity. The nature of this genetic covariance determines if phenotypes will evolve directionally or whether they reside at an evolutionary optimum. Here, we show how a set of traits can be shown to be under net stabilizing selection through an application of the multivariate Robertson–Price Identity. We characterize how a suite of male sexual displays genetically covaries with fitness in a population of Drosophila serrata. Despite strong directional sexual selection on these phenotypes directly and significant genetic variance in them, little genetic covariance was detected with overall fitness. Instead, genetic analysis of trait deviations showed substantial stabilizing selection on the genetic variance of these traits with respect to overall fitness, indicating that they reside at an evolutionary optimum. In the presence of widespread pleiotropy, stabilizing selection on focal traits will arise through the net effects of selection on other, often unmeasured, traits and will tend to be stronger on trait combinations than single traits. Such selection may be difficult to detect in phenotypic analyses if the environmental covariance between the traits and fitness obscures the underlying genetic associations. The genetic analysis of trait deviations provides a way of detecting the missing stabilizing selection inferred by recent metaanalyses.

An enduring hypothesis for the proximal benefits of sex is that recombination increases the genetic variation among offspring and that this genetic variation increases offspring performance. A corollary of this hypothesis is that mothers that mate multiply increase genetic variation within a clutch and gain benefits due to genetic diversity alone. Many studies have demonstrated that multiple mating can increase offspring performance, but most attribute this increase to sexual selection and the role of genetic variation has received less attention. Here, we used a breeding design to generate populations of full-siblings, half-siblings and unrelated individuals of the solitary ascidian Ciona intestinalis. Importantly, we preclude the potentially confounding influences of maternal effects and sexual selection. We found that individuals in populations with greater genetic diversity had greater performance (metamorphic success, post-metamorphic survival and post-metamorphic size) than individuals in populations with lower genetic diversity. Furthermore, we show that by mating with multiple males and thereby increasing genetic variation within a single clutch of offspring, females gain indirect fitness benefits in the absence of mate-choice. Our results show that when siblings are likely to interact, genetic variation among individuals can decrease competition for resources and generate substantial fitness benefits within a single generation