Trade-offs, geography, and limits to thermal adaptation in a tide pool copepod

Antagonistic correlations among traits may slow the rate of adaptation to a changing environment. The tide pool copepod Tigriopus californicus is locally adapted to temperature, but within populations, the response to selection for increased heat tolerance plateaus rapidly, suggesting either limited variation within populations or costs of increased tolerance. To measure possible costs of thermal tolerance, we selected for increased upper lethal limits for 10 generations in 22 lines of T. californicus from six populations. Then, for each line, we measured six fitness-related traits. Selected lines showed an overall increase in male and female body sizes, fecundity, and starvation resistance, suggesting a small benefit from (rather than costs of) increased tolerance. The effect of selection on correlated traits also varied significantly by population for five traits, indicating that the genetic basis for the selection response differed among populations. Our results suggest that adaptation was limited by the presence of variation within isolated populations rather than by costs of increased tolerance.

Exclusive male care despite extreme female promiscuity and low paternity in a marine snail

Males exhibit striking variation in the degree to which they invest in offspring, from merely provisioning females with sperm, to providing exclusive post-zygotic care. Paternity assurance is often invoked to explain this variation: the greater a male's confidence of paternity, the more he should be willing to provide care. Here, we report a striking exception to expectations based on paternity assurance: despite high levels of female promiscuity, males of a marine snail provide exclusive, and costly, care of offspring. Remarkably, genetic paternity analyses reveal cuckoldry in all broods, with fewer than 25% of offspring being sired by the caring male, although caring males sired proportionally more offspring in a given clutch than any other fathers did individually. This system presents the most extreme example of the coexistence of high levels of female promiscuity, low paternity, and costly male care, and emphasises the still unresolved roles of natural and sexual selection in the evolution of male parental care.

Extraordinarily rapid life-history divergence between Cryptasterina sea star species

Life history plays a critical role in governing microevolutionary processes such as gene flow and adaptation, as well as macroevolutionary processes such speciation. Here, we use multilocus phylogeographic analyses to examine a speciation event involving spectacular life-history differences between sister species of sea stars. Cryptasterina hystera has evolved a suite of derived life-history traits (including internal self-fertilization and brood protection) that differ from its sister species Cryptasterina pentagona, a gonochoric broadcast spawner. We show that these species have only been reproductively isolated for approximately 6000 years (95% highest posterior density of 905-22 628), and that this life-history change may be responsible for dramatic genetic consequences, including low nucleotide diversity, zero heterozygosity and no gene flow. The rapid divergence of these species rules out some mechanisms of isolation such as adaptation to microhabitats in sympatry, or slow divergence by genetic drift during prolonged isolation. We hypothesize that the large phenotypic differences between species relative to the short divergence time suggests that the life-history differences observed may be direct responses to disruptive selection between populations. We speculate that local environmental or demographic differences at the southern range margin are possible mechanisms of selection driving one of the fastest known marine speciation events.

Limited potential for adaptation to climate change in a broadly distributed marine crustacean

The extent to which acclimation and genetic adaptation might buffer natural populations against climate change is largely unknown. Most models predicting biological responses to environmental change assume that species' climatic envelopes are homogeneous both in space and time. Although recent discussions have questioned this assumption, few empirical studies have characterized intraspecific patterns of genetic variation in traits directly related to environmental tolerance limits. We test the extent of such variation in the broadly distributed tidepool copepod Tigriopus californicus using laboratory rearing and selection experiments to quantify thermal tolerance and scope for adaptation in eight populations spanning more than 17° of latitude. Tigriopus californicus exhibit striking local adaptation to temperature, with less than 1 per cent of the total quantitative variance for thermal tolerance partitioned within populations. Moreover, heat-tolerant phenotypes observed in low-latitude populations cannot be achieved in high-latitude populations, either through acclimation or 10 generations of strong selection. Finally, in four populations there was no increase in thermal tolerance between generations 5 and 10 of selection, suggesting that standing variation had already been depleted. Thus, plasticity and adaptation appear to have limited capacity to buffer these isolated populations against further increases in temperature. Our results suggest that models assuming a uniform climatic envelope may greatly underestimate extinction risk in species with strong local adaptation.

Post-glacial redistribution and shifts in productivity of giant kelp forests

Quaternary glacial-interglacial cycles create lasting biogeographic, demographic and genetic effects on ecosystems, yet the ecological effects of ice ages on benthic marine communities are unknown. We analysed long-term datasets to develop a niche-based model of southern Californian giant kelp (Macrocystis pyrifera) forest distribution as a function of oceanography and geomorphology, and synthesized palaeo-oceanographic records to show that late Quaternary climate change probably drove high millennial variability in the distribution and productivity of this foundation species. Our predictions suggest that kelp forest biomass increased up to threefold from the glacial maximum to the mid-Holocene, then rapidly declined by 40-70 per cent to present levels. The peak in kelp forest productivity would have coincided with the earliest coastal archaeological sites in the New World. Similar late Quaternary changes in kelp forest distribution and productivity probably occurred in coastal upwelling systems along active continental margins worldwide, which would have resulted in complex shifts in the relative productivity of terrestrial and marine components of coastal ecosystems.

Discordant distribution of populations and genetic variation in a sea star with high dispersal potential

Patiria miniata, a broadcast-spawning sea star species with high dispersal potential, has a geographic range in the intertidal zone of the northeast Pacific Ocean from Alaska to California that is characterized by a large range gap in Washington and Oregon. We analyzed spatial genetic variation across the P. miniata range using multilocus sequence data (mtDNA, nuclear introns) and multilocus genotype data (microsatellites). We found a strong phylogeographic break at Queen Charlotte Sound in British Columbia that was not in the location predicted by the geographical distribution of the populations. However, this population genetic discontinuity does correspond to previously described phylogeographic breaks in other species. Northern populations from Alaska and Haida Gwaii were strongly differentiated from all southern populations from Vancouver Island and California. Populations from Vancouver Island and California were undifferentiated with evidence of high gene flow or very recent separation across the range disjunction between them. The surprising and discordant spatial distribution of populations and alleles suggests that historical vicariance (possibly caused by glaciations) and contemporary dispersal barriers (possibly caused by oceanographic conditions) both shape population genetic structure in this species.

Contrasting demographic history and phylogeographical patterns in two Indo-Pacific gastropods

Marine species with ranges that span the Indo-Australian Archipelago (IAA) exhibit a range of phylogeographical patterns, most of which are interpreted in the context of vicariance between Indian and Pacific Ocean populations during Pliocene and Pleistocene low sea-level stands. However, patterns often vary among ecologically similar taxa, sometimes even within genera. This study compares phylogeographical patterns in two species of highly dispersive neritid gastropod, Nerita albicilla and Nerita plicata, with nearly sympatric ranges that span the Indo-Pacific. Mitochondrial COI sequences from >1000 individuals from 97 sites reveal similar phylogenies in both species (two divergent clades differing by 3.2% and 2.3%, for N. albicilla and N. plicata, respectively). However, despite ecological similarity and congeneric status, the two species exhibit phylogeographical discordance. N. albicilla has maintained reciprocal monophyly of Indian and Pacific Ocean populations, while N. plicata is panmictic between oceans, but displays a genetic cline in the Central Pacific. Although this difference might be explained by qualitatively different demographic histories, parameter estimates from three coalescent models indicate that both species have high levels of gene flow between demes (2Nem>75), and share a common history of population expansion that is likely associated with cyclical flooding of continental shelves and island lagoons following low sea-level stands. Results indicate that ecologically similar, codistributed species may respond very differently to shared environmental processes, suggesting that relatively minor differences in traits such as pelagic larval duration or microhabitat association may profoundly impact phylogeographical structure.

Ecological and evolutionary insights from species invasions

Species invasions provide numerous unplanned and frequently, but imperfectly, replicated experiments that can be used to better understand the natural world. Classic studies by Darwin, Grinnell, Elton and others on these species-invasion experiments provided invaluable insights for ecology and evolutionary biology. Recent studies of invasions have resulted in additional insights, six of which we discuss here; these insights highlight the utility of using exotic species as 'model organisms'. We also discuss a nascent hypothesis that might provide a more general, predictive understanding of invasions and community assembly. Finally, we emphasize how the study of invasions can help to inform our understanding of applied problems, such as extinction, ecosystem function and the response of species to climate change.

Color polymorphism and genetic structure in the sea star Pisaster ochraceus

The sea star Pisaster ochraceus is one of the more striking species on the rocky shores of the Northeast Pacific, in part due to the dramatic color polymorphism of the adults. Along the open Pacific coast, Pisaster populations are 6%-28% orange, with a small percentage of brilliant purple stars and a large percentage of reddish-brown to dull purple ones. However, populations in the San Juan Island Archipelago (Washington, USA) and the southern Strait of Georgia (British Columbia, Canada) are almost entirely brilliant purple. The factors that maintain the color polymorphism, and those that contribute to among-site variation in color frequencies, remain unknown. We examined the relationships between color frequencies and several ecological and morphological variables, and conducted a large-scale phylogeographic survey of Pisaster populations. We found very low population genetic structure, suggesting that gene flow is high and geographic variation in color frequencies is not a vestige of Pleistocene glacial refugia. Color frequencies are also unrelated to adult size and to the frequency of injury within a population. However, there are suggestive relationships between color frequency and diet, and with areas of potentially low salinity. We propose that, although the color polymorphism may have an underlying genetic component, the regional-scale variation in color frequency is ecologically controlled.

Strong genetic clines and geographical variation in gene flow in the rocky intertidal barnacle Balanus glandula

A long-standing issue in marine biology is identifying spatial scales at which populations of sessile adults are connected by planktonic offspring. We examined the genetic continuity of the acorn barnacle Balanus glandula, an abundant member of rocky intertidal communities of the northeastern Pacific Ocean, and compared these genetic patterns to the nearshore oceanography described by trajectories of surface drifters. Consistent with its broad dispersal potential, barnacle populations are genetically similar at both mitochondrial (cytochrome oxidase I) and nuclear (elongation factor 1-alpha) loci across broad swaths of the species' range. In central California, however, there is a striking genetic cline across 475 km of coastline between northern and southern populations. These patterns indicate that gene flow within central California is far more restricted spatially than among other populations. Possible reasons for the steep cline include the slow secondary introgression of historically separated populations, a balance between diversifying selection and dispersal, or some mix of both. Geographic trajectories of oceanic drifters closely parallel geographical patterns of gene flow. Drifters placed to the north (Oregon; approximately 44 degrees N) and south (Santa Barbara, California; approximately 34 degrees N) of the cline disperse hundreds of kilometers within 40 days, yet over the long-term their trajectories never overlapped. The lack of communication between waters originating in Oregon and southern California probably helps to maintain strong genetic differentiation between these regions. More broadly, the geographical variation in gene flow implies that focusing on species-level averages of gene flow can mask biologically important variance within species which reflects local environmental conditions and historical events.

The distribution and evolutionary history ofWolbachiainfection in native and introduced populations of the invasive argentine ant (Linepithema humile)

Wolbachia pipientis is a maternally transmitted bacterium that often alters the life history of its insect host to maximize transmission to subsequent generations. Here we report on the frequency and distribution of Wolbachia infection in a widespread invasive species, the Argentine ant (Linepithema humile). We screened 1175 individual Argentine ants from 89 nests on five continents and several islands, including numerous locations in both the native (South American) and introduced ranges. We detected Wolbachia in four of 11 native populations, but only one of 21 introduced populations was infected. In the Argentine ant's native range, the distribution of Wolbachia supergroups A and B was nonoverlapping. By coupling infection frequency data with behaviourally defined colony boundaries, we show that infected and uninfected colonies are often adjacent to one another, supporting the proposition that little female-mediated gene flow occurs among Argentine ant colonies. We also conduct a phylogenetic analysis, and show that the Wolbachia infecting both native and introduced populations of Argentine ants belong to two lineages that appear to be specialized on infecting New World ants. One other lineage of Wolbachia has undergone frequent, recent episodes of horizontal transmission between distantly related, introduced insect hosts.

Genetic diversity, asymmetrical aggression, and recognition in a widespread invasive species

The evolution and persistence of cooperative social units depends on the ability to distinguish group members from nonmembers. The precision of discrimination, in turn, relies on variation in the labels that individuals use to recognize group members. However, this same variation can be selected against if individuals that are rejected as nonmembers incur a high cost. Here we provide evidence that selection against individuals from genetically diverse groups has contributed to the formation of the unicolonial colony structure that characterizes introduced populations of the invasive Argentine ant (Linepithema humile). Studies in both the laboratory and the field showed that individuals from less genetically diverse colonies attack individuals from more diverse colonies and that attackers survived agonistic encounters more than six times as often as recipients of aggression. This selection, in concert with reductions in genetic diversity after a founder event, likely creates a barrier to the establishment of new, genetically diverse introductions from the native range and may reduce genetic diversity within established populations in the introduced range.

Mate selection and the evolution of highly polymorphic self/nonself recognition genes

Multicellular organisms use the products of highly polymorphic genes to distinguish self from conspecific nonself cells or tissues. These allorecognition polymorphisms may regulate somatic interactions between hosts and pathogens or between competitors (to avoid various forms of parasitism), as well as reproductive interactions between mates or between gametes (to avoid inbreeding). In both cases, rare alleles may be advantageous, but it remains unclear which mechanism maintains the genetic polymorphism for specificity in self/nonself recognition. Contrary to earlier reports, we show that mate selection cannot be a strong force maintaining allorecognition polymorphism in two colonial marine invertebrates. Instead, the regulation of intraspecific competitive interactions appears to promote the evolution of polymorphisms in these species.

KIN INTERACTIONS IN A COLONIAL HYDROZOAN (HYDRACTINIA SYMBIOLONGICARPUS): POPULATION STRUCTURE ON A MOBILE LANDSCAPE

Many sessile colonial organisms intensively compete with conspecifics for growing space. This competition can result in either cooperative fusion or aggressive rejection between colonies, and some species have evolved highly polymorphic genetic systems that mediate the outcome of these interactions. Here we demonstrate the potential for interactions among close kin as the basis for the evolutionary maintenance of a genetically polymorphic allorecognition system in the colonial hydroid Hydractinia symbiolongicarpus, which lives on gastropod shells occupied by hermit crabs. Fusion between hydroids in the laboratory is restricted mainly to encounters between full siblings, whereas other encounters result in aggressive rejection. Natural selection acting on the costs or benefits of fusion between colonies could be responsible for the present maintenance of such a highly specific behavioral response, but only if encounters between fusible colonies still occur in contemporary populations. The large size of these hydroid populations and the mobility of the crabs should limit the potential for interactions among closely related hydroids on the same shell. However, RAPD polymorphisms among a large sample of hydroids from a population off the coast of Massachusetts indicate that genetically similar colonies are often found together on the same shell. Some genetic distances between colonies on the same shell were low relative to genetic distances between colonies on different shells or genetic distances between known full siblings from laboratory matings. We conservatively estimate that 2-18% of co-occurring colonies may be full sibling pairs. These observations suggest that encounters between genetically similar hydroids are common, despite the mobile nature of their habitat, and these encounters may provide frequent opportunities for natural selection to influence the evolution of cooperative and agonistic behaviors and their polymorphic genetic basis.