Jon Mark Scriber

Polymorphisms for melanic form of insects may provide various selective advantages. However, melanic alleles may have significant/subtle pleiotrophic “costs.” Several potential pleiotrophic effects of the W (=Y)‐linked melanism gene in Papilio glaucus L. (Lepidoptera) showed no costs for melanic versus yellow in adult size, oviposition preferences, fecundity, egg viability, larval survival/growth rates, cold stress tolerance, or postdiapause emergence times. Sexual selection (males choosing yellow rather than mimetic dark females) had been suggested to provide a balanced polymorphism in P. glaucus , but spermatophore counts in wild females and direct field tethering studies of size‐matched pairs of virgin females (dark and yellow), show that male preferences are random or frequency‐dependent from Florida to Michigan, providing no yellow counter‐advantages. Recent frequency declines of dark (melanic/mimetic) females in P. glaucus populations are shown in several major populations from Florida (27.3°N latitude) to Ohio (38.5° N). Summer temperatures have increased significantly at all these locations during this time (1999–2018), but whether dark morphs may be more vulnerable (in any stage) to such climate warming remains to be determined. Additional potential reasons for the frequency declines in mimetic females are discussed: (i) genetic introgression of Z‐linked melanism suppressor genes from P. canadensis (R & J) and the hybrid species, P. appalachiensis (Pavulaan & Wright), (ii) differential developmental incompatibilities, or Haldane effects, known to occur in hybrids, (iii) selection against intermediately melanic (“dusty”) females (with the W‐linked melanic gene, b+) which higher temperatures can cause.

Adaptations to “thermal time” (=Degree-day) constraints on developmental rates and voltinism for North American tiger swallowtail butterflies involve most life stages, and at higher latitudes include: smaller pupae/adults; larger eggs; oviposition on most nutritious larval host plants; earlier spring adult emergences; faster larval growth and shorter molting durations at lower temperatures. Here we report on forewing sizes through 30 years for both the northern univoltine P. canadensis (with obligate diapause) from the Great Lakes historical hybrid zone northward to central Alaska (65° N latitude), and the multivoltine, P. glaucus from this hybrid zone southward to central Florida (27° N latitude). Despite recent climate warming, no increases in mean forewing lengths of P. glaucus were observed at any major collection location (FL to MI) from the 1980s to 2013 across this long latitudinal transect (which reflects the “converse of Bergmann’s size Rule”, with smaller females at higher latitudes). Unlike lower latitudes, the Alaska, Ontonogon, and Chippewa/Mackinac locations (for P. canadensis) showed no significant increases in D-day accumulations, which could explain lack of size change in these northernmost locations. As a result of 3–4 decades of empirical data from major collection sites across these latitudinal clines of North America, a general “voltinism/size/D-day” model is presented, which more closely predicts female size based on D-day accumulations, than does latitude. However, local “climatic cold pockets” in northern Michigan and Wisconsin historically appeared to exert especially strong size constraints on female forewing lengths, but forewing lengths quickly increased with local summer warming during the recent decade, especially near the warming edges of the cold pockets. Results of fine-scale analyses of these “cold pockets” are in contrast to non-significant changes for other Papilio populations seen across the latitudinal transect for P. glaucus and P. canadensis in general, highlighting the importance of scale in adaptations to climate change. Furthermore, we also show that rapid size increases in cold pocket P. canadensis females with recent summer warming are more likely to result from phenotypic plasticity than genotypic introgression from P. glaucus, which does increase size in late-flight hybrids and P. appalachiensis.

Comprising 50%–75% of the world’s fauna, insects are a prominent part of biodiversity in communities and ecosystems globally. Biodiversity across all levels of biological classifications is fundamentally based on genetic diversity. However, the integration of genomics and phylogenetics into conservation management may not be as rapid as climate change. The genetics of hybrid introgression as a source of novel variation for ecological divergence and evolutionary speciation (and resilience) may generate adaptive potential and diversity fast enough to respond to locally-altered environmental conditions. Major plant and herbivore hybrid zones with associated communities deserve conservation consideration. This review addresses functional genetics across multi-trophic-level interactions including “invasive species” in various ecosystems as they may become disrupted in different ways by rapid climate change. “Invasive genes” (into new species and populations) need to be recognized for their positive creative potential and addressed in conservation programs. “Genetic rescue” via hybrid translocations may provide needed adaptive flexibility for rapid adaptation to environmental change. While concerns persist for some conservationists, this review emphasizes the positive aspects of hybrids and hybridization. Specific implications of natural genetic introgression are addressed with a few examples from butterflies, including transgressive phenotypes and climate-driven homoploid recombinant hybrid speciation. Some specific examples illustrate these points using the swallowtail butterflies (Papilionidae) with their long-term historical data base (phylogeographical diversity changes) and recent (3-decade) climate-driven temporal and genetic divergence in recombinant homoploid hybrids and relatively recent hybrid speciation of Papilio appalachiensis in North America. Climate-induced “reshuffling” (recombinations) of species composition, genotypes, and genomes may become increasingly ecologically and evolutionarily predictable, but future conservation management programs are more likely to remain constrained by human behavior than by lack of academic knowledge.

Abstract  It is generally thought that insects inhabiting lower latitudes are more severely impacted by changes in their thermal environment than are high latitude species. This is attributed to the wider range of temperatures to which high‐latitude species are exposed. By contrast, low‐latitude species have typically evolved in more thermally stable environments with a narrower range of temperature variation. However, deviation from this pattern can occur and here we report that under variable winter conditions a higher latitude species may be more sensitive to thermal variation than its lower latitude sister species. Using split broods, we examined the survival and adult emergence success of diapausing pupae of Papilio canadensis and P. glaucus, as well as a unique, recombinant hybrid population (“late‐flight”) to short periods of mid‐winter cold and heat stress. Our results indicate that the higher latitude, univoltine populations (P. canadensis and late‐flights) exhibit lower pupal survival than the lower latitude, facultative diapauser (P. glaucus) for all mid‐winter thermal stress treatments, both high and low. Size differences alone do not appear to account for the observed differences in survival or metabolic costs in these three phenotypes, as late‐flight individuals are similar in size to P. glaucus. We attribute the observed differences in survival and weight loss to potential metabolic differences and variation in the intensity of diapause, in addition to divergent adaptation to winter precipitation levels (e.g. snow cover) and the influences this may have on microhabitat temperature moderation.

Abstract The search for pattern in the ecology and evolutionary biology of insect–plant associations has fascinated biologists for centuries. High levels of tropical (low‐latitude) plant and insect diversity relative to poleward latitudes and the disproportionate abundance of host‐specialized insect herbivores have been noted. This review addresses several aspects of local insect specialization, host use abilities (and loss of these abilities with specialization), host‐associated evolutionary divergence, and ecological (including “hybrid”) speciation, with special reference to the generation of biodiversity and the geographic and taxonomic identification of “species borders” for swallowtail butterflies (Papilionidae). From ancient phytochemically defined angiosperm affiliations that trace back millions of years to recent and very local specialized populations, the Papilionidae (swallowtail butterflies) have provided a model for enhanced understanding of localized ecological patterns and genetically based evolutionary processes. They have served as a useful group for evaluating the feeding specialization/physiological efficiency hypothesis. They have shown how the abiotic (thermal) environment interacts with host nutrirional suitability to generate “voltinism/suitability” gradients in specialization or preference latitudinally, and geographical mosaics locally. Several studies reviewed here suggest strongly that the oscillation hypothesis for speciation does have considerable merit, but at the same time, some species‐level host specializations may lead to evolutionary dead‐ends, especially with rapid environmental/habitat changes involving their host plants. Latitudinal gradients in species richness and degree of herbivore feeding specialization have been impacted by recent developments in ecological genetics and evolutionary ecology. Localized insect–plant associations that span the biospectrum from polyphenisms, polymorphisms, biotypes, demes, host races, to cryptic species, remain academically contentious, with simple definitions still debated. However, molecular analyses combined with ecological, ethological and physiological studies, have already begun to unveil some answers for many important ecological/evolutionary questions.