Kenneth Petren

An Experimental Demonstration of Exploitation Competition in an Ongoing Invasion

A native asexual gecko, Lepidodactylus lugubris, declines numerically when the sexual gecko Hemidactylus frenatus invades urban/suburban habitats throughout the Pacific. Previous studies showed that the competitive displacement occurs rapidly and is facilitated by clumped insect resources. Five lines of evidence suggest that the mechanism of displacement is primarily due to differences in the ability of each species to exploit insect resources. (1) These species show nearly complete diet overlap. (2) Insects are a limiting resource for both geckos as evidenced by positive demographic effects with in- creased insect abundance. (3) Hemidactylus frenatus depletes insect resources to lower levels than L. lugubris, which results in reduced rates of resource acquisition in L. lugubris. (4) This reduced resource acquisition translates into significant reductions in the body condition, fecundity, and survivorship of L. lugubris individuals. (5) Evidence for inter- ference (and other) mechanisms does not account for these negative demographic effects on L. lugubris. Interspecific competition is stronger than intraspecific competition for L. lugubris, with increasing L. lugubris density having negligible effect on H. frenatus, mirroring the asym- metry of the large-scale displacement. The superior harvesting ability of H. frenatus is most pronounced when insects are clumped spatially and temporally, and is attributable to a variety of species-specific traits such as their larger body size, faster running speed, and reduced intraspecific interference while foraging. We conclude that clumped resources can increase interspecific exploitation competition, and this mechanism may contribute to spe- cies turnover when human environmental alterations redistribute resources.

ENVIRONMENTAL CONDITIONS AFFECT THE MAGNITUDE OF INBREEDING DEPRESSION IN SURVIVAL OF DARWIN'S FINCHES

Abstract Understanding the fitness consequences of inbreeding (inbreeding depression) is of importance to evolutionary and conservation biology. There is ample evidence for inbreeding depression in captivity, and data from wild populations are accumulating. However, we still lack a good quantitative understanding of inbreeding depression and what influences its magnitude in natural populations. Specifically, the relationship between the magnitude of inbreeding depression and environmental severity is unclear. We quantified inbreeding depression in survival and reproduction in populations of cactus finches (Geospiza scandens) and medium ground finches (Geospiza fortis) living on Isla Daphne Major in the Galapagos Archipelago. Our analyses showed that inbreeding strongly reduced the recruitment probability (probability of breeding given that an adult is alive) in both species. Additionally, in G. scandens, first‐year survival of an offspring withf= 0.25 was reduced by 21% and adults withf= 0.25 experienced a 45% reduction in their annual probability of survival. The magnitude of inbreeding depression in both adult and juvenile survival of this species was strongly modified by two environmental conditions, food availability and number of competitors. In juveniles, inbreeding depression was only present in years with low food availability, and in adults inbreeding depression was five times more severe in years with low food availability and large population sizes. The combination of relatively severe inbreeding depression in survival and the reduced recruitment probability led to the fact that very few inbred G. scandens ever succeeded in breeding. Other than recruitment probability, no other trait showed evidence of inbreeding depression in G. fortis, probably for two reasons: a relatively high rate of extrapair paternity (20%), which may lead to an underestimate of the apparent inbreeding depression, and low sample sizes of highly inbred G. fortis, which leads to low statistical power. Using data from juvenile survival, we estimated the number of lethal equivalents carried by G. scandens, G. fortis, and another congener, G. magnirostris. These results suggest that substantial inbreeding depression can exist in insular populations of birds, and that the magnitude of the inbreeding depression is a function of environmental conditions.

A PHYLOGENY OF DARWIN'S FINCHES BASED ON MICROSATELLITE DNA LENGTH VARIATION

Allele length variation at 16 microsatellite loci was used to estimate the phylogeny of 13 out of the 14 species of Darwins finches. The resulting topology was similar to previous phylogenies based on morphological and allozyme variation. An unexpected result was that genetic divergence among Galápagos Island populations of the warbler finch (Certhidea olivacea) predates the radiation of all other Darwins finches. This deep split is surprising in view of the relatively weak morphological differentiation among Certhidea populations and supports the hypothesis that the ancestor of all Darwins finches was phenotypically similar to Certhidea. The results also resolve a biogeographical problem: the Cocos Island finch evolved after the Galápagos finch radiation was under way, supporting the hypothesis that this distant island was colonized from the Galápagos Islands. Monophyletic relationships are supported for both major groups, the ground finches (Geospiza) and the tree finches (Camarhynchus and Cactospiza), although the vegetarian finch (Platyspiza crassirostris) appears to have diverged prior to the separation of ground and tree finches. These results demonstrate the use of microsatellites for reconstructing phylogenies of closely related species and interpreting their evolutionary and biogeographic histories.

Convergent evolution of Darwin's finches caused by introgressive hybridization and selection.

Abstract Between 1973 and 2003 mean morphological features of the cactus finch, Geospiza scandens, and the medium ground finch, G. fortis, populations on the Galápagos island of Daphne Major were subject to fluctuating directional selection. An increase in bluntness or robustness in the beak of G. scandens after 1990 can only partly be explained by selection. We use 16 microsatellite loci to test predictions of the previously proposed hypothesis that introgressive hybridization contributed to the trend, resulting in genes flowing predominantly from G. fortis to G. scandens. To identify F1 hybrids and backcrosses we use pedigrees where known, supplemented by the results of assignment tests based on 14 autosomal loci when parents were not known. We analyze changes in morphology and allelic composition in the two populations over a period of 15–20 years. With samples that included F1 hybrids and backcrosses, the G. scandens population became more similar to the G. fortis population both genetically and morphologically. Gene flow between species was estimated to be three times greater from G. fortis to G. scandens than in the opposite direction, resulting in a 20% reduction in the genetic difference between the species. Nevertheless, removing identified F1 hybrids and backcrosses from the total sample and reanalyzing the traits did not eliminate the convergence. The two species also converged in beak shape by 22.2% and in body size by 45.5%. A combination of introgressive hybridization and selection jointly provide the best explanation of convergence in morphology and genetic constitution under the changed ecological conditions following a major El Niño event in 1983. The study illustrates how species without postmating barriers to gene exchange can alternate between convergence and divergence when environmental conditions oscillate.

EFFECTS OF EL NIñO EVENTS ON DARWIN'S FINCH PRODUCTIVITY

We studied the effects of heavy and prolonged rainfall associated with four El Nino events on the reproduction of Darwins finches on the Galapagos island of Daphne Major. Rainfall varied in the El Nino years from 195 mm to 1359 mm, exceeding the maximum in the other years by 40% to 1000%. Two species were studied: Geospiza fortis, Medium Ground Finch, and G. scandens, Cactus Finch. Almost all eggs, nestlings, and fledglings produced by banded females were recorded in the El Nino years of 1983, 1987, 1991, and 1998. Finch production in these years was compared with production in 10 other years of breeding in the period 1976-1990; there was no breeding in three drought years, 1985, 1988, and 1989. Breeding differed in the two sets of years in several ways. More broods were produced in El Nino years than in non-El Nino years (maximum 10 clutches per female vs. five clutches), the period of breeding was longer (maximum eight months vs. four), average clutch sizes (range 2-6 eggs) were distinctly larger (four vs. three), and average egg and fledgling production per female was larger by a factor of four. The two species differed in some features of breeding, but differences were minor in comparison with the marked seasonal and annual variation. Finch production varied among El Nino years, being greatest in the year of most rain over the longest period (1983), and least in the next wettest year (1998). The surprisingly low production in 1998 is attributed in part to interactions with other finches, and in part to exceptionally high temperatures. Temperature, although postively correlated with rainfall, had an independent negative effect upon hatching and overall breeding success of G. fortis. Breeding by both species in the year of birth (hatch) occurred in two El Nino years with the most extended wet seasons: 1983 and 1987. Young breeders had lower clutch sizes and breeding success than did contemporaneously breeding older birds. Observations in different El Nino years show that finch population responses to major climatic perturbations such as elevated rainfall vary for two major reasons: the perturbations themselves vary in strength and duration, and responses to them are determined, in part, by preceding conditions. Those preceding conditions, in turn, are determined by whether drought or normal conditions precede the perturbation, and on the interval since the previous El Nino event. Thus, perturbations of natural systems can be fully understood only in a broad temporal context.

Comparative landscape genetics and the adaptive radiation of Darwin's finches: the role of peripheral isolation.

We use genetic divergence at 16 microsatellite loci to investigate how geographical features of the Galápagos landscape structure island populations of Darwins finches. We compare the three most genetically divergent groups of Darwins finches comprising morphologically and ecologically similar allopatric populations: the cactus finches (Geospiza scandens and Geospiza conirostris), the sharp‐beaked ground finches (Geospiza difficilis) and the warbler finches (Certhidea olivacea and Certhidea fusca). Evidence of reduced genetic diversity due to drift was limited to warbler finches on small, peripheral islands. Evidence of low levels of recent interisland migration was widespread throughout all three groups. The hypothesis of distance‐limited dispersal received the strongest support in cactus and sharp‐beaked ground finches as evidenced by patterns of isolation by distance, while warbler finches showed a weaker relationship. Support for the hypothesis that gene flow constrains morphological divergence was only found in one of eight comparisons within these groups. Among warbler finches, genetic divergence was relatively high while phenotypic divergence was low, implicating stabilizing selection rather than constraint due to gene flow. We conclude that the adaptive radiation of Darwins finches has occurred in the presence of ongoing but low levels of gene flow caused by distance‐dependent interisland dispersal. Gene flow does not constrain phenotypic divergence, but may augment genetic variation and facilitate evolution due to natural selection. Both microsatellites and mtDNA agree in that subsets of peripheral populations of two older groups are genetically more similar to other species that underwent dramatic morphological change. The apparent decoupling of morphological and molecular evolution may be accounted for by a modification of Lacks two‐stage model of speciation: relative ecological stasis in allopatry followed by secondary contact, ecological interactions and asymmetric phenotypic divergence.

Hybridization in the Recent Past

The question we address in this article is how hybridization in the recent past can be detected in recently evolved species. Such species may not have evolved genetic incompatibilities and may hybridize with little or no fitness loss. Hybridization can be recognized by relatively small genetic differences between sympatric populations because sympatric populations have the opportunity to interbreed whereas allopatric populations do not. Using microsatellite DNA data from Darwin’s finches in the Galápagos archipelago, we compare sympatric and allopatric genetic distances in pairs of Geospiza and Camarhynchus species. In agreement with the hybridization hypothesis, we found a statistically strong tendency for a species to be more similar genetically to a sympatric relative than to allopatric populations of that relative. Hybridization has been studied directly on two islands, but it is evidently more widespread in the archipelago. We argue that introgressive hybridization may have been a persistent feature of the adaptive radiation through most of its history, facilitating evolutionary diversification and occasionally affecting both the speed and direction of evolution.

Colonization and diversification of Galápagos terrestrial fauna: a phylogenetic and biogeographical synthesis

Remote oceanic islands have long been recognized as natural models for the study of evolutionary processes involved in diversification. Their remoteness provides opportunities for isolation and divergence of populations, which make islands remarkable settings for the study of diversification. Groups of islands may share a relatively similar geological history and comparable climate, but their inhabitants experience subtly different environments and have distinct evolutionary histories, offering the potential for comparative studies. A range of organisms have colonized the Galápagos Islands, and various lineages have radiated throughout the archipelago to form unique assemblages. This review pays particular attention to molecular phylogenetic studies of Galápagos terrestrial fauna. We find that most of the Galápagos terrestrial fauna have diversified in parallel to the geological formation of the islands. Lineages have occasionally diversified within islands, and the clearest cases occur in taxa with very low vagility and on large islands with diverse habitats. Ecology and habitat specialization appear to be critical in speciation both within and between islands. Although the number of phylogenetic studies is continuously increasing, studies of natural history, ecology, evolution and behaviour are essential to completely reveal how diversification proceeded on these islands.

Microsatellite primers from Geospiza fortis and cross‐species amplification in Darwin's finches

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INVASIONS AND COMPETITIVE DISPLACEMENT AMONG HOUSE GECKOS IN THE TROPICAL PACIFIC

Since the 1 930s the common house gecko, Hemidactylus frenatus, a sexual species, has been inadvertently introduced to many tropical Pacific islands. Using mark- recapture censuses and visual gecko searches we found that Lepidodactylus lugubris, an asexual gecko previously common on these islands, is nearly 800% more abundant on buildings in the urban/suburban environment on islands that lack H. frenatus than it is on islands where H. frenatus is present. On buildings in Hawaii and Fiji that have been surveyed in different years, the proportion of H. frenatus relative to L. lugubris has sig- nificantly increased over time. The degree of numerical dominance is also related to climate and habitat: L. lugubris is relatively more common on the more mesic, cooler, windward sides of large islands compared to the more arid sides of islands and in general H. frenatus does not penetrate forest habitats, where L. lugubris remains one of the most common geckos. Overall, buildings with external electric lights have more geckos than unlit buildings. Where the two species occur on the same buildings H. frenatus is closer, on average, to the prime feeding sites near lights that attract insects. In the absence of H. frenatus, L. lugubris is found closer to the lights. We hypothesize that the previously demonstrated agonistic dominance of H. frenatus over L. lugubris (Bolger and Case 1992) is likely to lead to competitive superiority when insect prey are concentrated into patches that are structurally simple (like flat building walls with lights), allowing easy detection of prey and intruders.