Darwin, Herschel, and the role of analogy in Darwin’s origin

In what follows, I consider the role of analogy in the first edition of Darwin’s Origin. I argue that Darwin follows Herschel’s methodology and hence exploits an analogy between artificial and natural selection that allows him generalize selection as a cause of evolutionary change. This argument strategy is not equivalent to an argument from analogy. Reading Darwin’s argument as conforming to Herschel’s two-step methodology of causal analysis followed by generalization allows us to understand the role and placement of Darwin’s discussion of artificial selection in the Origin, without making the mistake of portraying Darwin’s argument for the existence and character of natural selection as an analogical argument.     

Evolution in caves: Darwin’s ‘wrecks of ancient life’ in the molecular era

Cave animals have historically attracted the attention of evolutionary biologists because of their bizarre ‘regressive’ characters and convergent evolution. However, understanding of their biogeographic and evolutionary history, including mechanisms of speciation, has remained elusive. In the last decade, molecular data have been obtained for subterranean taxa and their surface relatives, which have allowed some of the classical debates on the evolution of cave fauna to be revisited. Here, we review some of the major studies, focusing on the contribution of phylogeography in the following areas: biogeographic history and the relative roles of dispersal and vicariance, colonization history, cryptic species diversity and modes of speciation of cave animals. We further consider the limitations of current research and prospects for the future. Phylogeographic studies have confirmed that cave species are often cryptic, with highly restricted distributions, but have also shown that their divergence and potential speciation may occur despite the presence of gene flow from surface populations. Significantly, phylogeographic studies have provided evidence for speciation and adaptive evolution within the confines of cave environments, questioning the assumption that cave species evolved directly from surface ancestors. Recent technical developments involving ‘next generation’ DNA sequencing and theoretical developments in coalescent and population modelling are likely to revolutionize the field further, particularly in the study of speciation and the genetic basis of adaptation and convergent evolution within subterranean habitats. In summary, phylogeographic studies have provided an unprecedented insight into the evolution of these unique fauna, and the future of the field should be inspiring and data rich.     

Foraging Behavior in Two Species of Ectatomma (Formicidae: Ponerinae): Individual Learning of Orientation and Timing

The foraging behavior of marked individuals of Ectatomma ruidum and E. tuberculatum (Formicidae: Ponerinae) was observed on Barro Colorado Island, Panama, in order to determine whether learning of orientation and timing is involved when foraging on patchy resources on the forest floor. Foraging experiments under seminatural conditions were designed to control for the spatiotemporal distribution of resources at one or two preying sites. Using both single- and multi–event past experience, individual foragers of E. ruidum and (although less significantly) E. tuberculatum made directional and temporal adjustments to their behavior in response to previous differential foraging success. In spite of considerable individual variation in foraging efficiency, it is hypothesized that the observed differences in cognitive abilities can be invoked to explain ecological differences among both species.     

Foraging Behavior and Coexistence of Two Sunbird Species in a Kenyan Woodland

We investigated the mechanism of coexistence of the rare Amani Sunbird (Hedydipna pallidigastra) and the widespread Collared Sunbird (H. collaris), within Brachystegia woodland in the Arabuko‐Sokoke Forest, Kenya. We compared how prey abundance and search strategies affect resource exploitation by the two species. We used foraging theory to direct our measures of feeding activities as influenced by sunbird species, tree species and foraging height. We evaluated invertebrate abundance among tree species at different heights within trees. The Collared Sunbird primarily used the understory, and the Amani Sunbird primarily used the upper‐canopy. Overall, the rate of prey attacks per flight of the Amani Sunbird was 2.8 times greater than that of the Collared Sunbird. The Amani Sunbird, however, used increased search and attack rates in the understory compared with the mid‐ and upper‐canopies, but the Collared Sunbird foraged similarly throughout all strata. We hypothesize that the increased foraging rate of the Amani in the understory reflects increased foraging costs due to interference from the Collared Sunbird in that stratum. Furthermore, the Collared Sunbird exploits rich patches by moving frequently from place to place. The Amani Sunbird forages slowly, with reduced travel rates, and with a greater number of prey captures within a patch. Arthropod density did not differ among the vegetative strata, but was higher in Brachystegia spiciformis and Hymenaea verrucosa than in six other tree species. We hypothesize that the Amani Sunbird appears dependent upon continued tall B. spiciformis trees within the canopy of the Arabuko‐Sokoke Forest.     

Comparing Reversal-Learning Abilities,Sucrose Responsiveness,and Foraging Experience Between Scout and Non-Scout Honey bee (Apis mellifera) Foragers

Honey bee (Apis mellifera) colonies divide foraging activities between scouts, who search for new sources of food, and non-scouts, who rely on information from waggle dances to find food sources. Molecular analyses of scouts and non-scouts have revealed differences in the expression of numerous genes, including several related to neurotransmitter signaling. Despite this progress, we know almost nothing about cognitive, sensory, or behavioral differences that underlie scouting. We tested three hypotheses related to differences between scouts and non-scouts. First, we tested whether scouts and non-scouts differ in their reversal-learning abilities and found that scouts showed a significantly faster reversal in their response to an odor that was punished and then rewarded. The results also suggested an interaction between the effects of foraging role (scout or non-scout) and seasonal effects on reversal-learning abilities. Second, we tested whether scouts and non-scouts show differences in responsiveness to sucrose rewards and found no difference. Third, we hypothesized that scouts have more foraging experience than do non-scouts. We tested this by comparing wing damage and found that non-scouts showed greater wing damage in the early summer but not the late summer. Together, these three results contribute to our understanding of the underpinnings of scouting behavior.     

2种织圆网蜘蛛个体大小与蛛网粘丝区面积间关系

在不惊扰蜘蛛的情况下对悦目金蛛Argiope amoena和圆尾肖蛸Tetragnatha vermiformis的个体大小与蛛网粘丝区面积问的相关性进行了野外观察。主要测量了蜘蛛体长、蛛网粘丝区直径、蛛网框丝固着点间最大距离、网中枢到地面或水面距离、网平面与水平线的夹角等参数;另外还对蛛网框丝固着点间最大距离作为圆网蛛网址选择行为量化指标的可行性进行了探讨。结果表明,2种蜘蛛体长与蛛网粘丝区面积以及蜘蛛体长与蛛网框丝固着点间最大距离均呈现明显的正相关。悦目金蛛体长与蛛网粘丝区面积的相关性高于圜尾肖蛸,这可能与悦目金蛛在其生境中易于找到框丝固着点、而圆尾肖蛸在其生境中较难找到框丝同着点有关;     

Evidence for rapid downward fecundity selection in an ectoparasite (Philornis downsi) with earlier host mortality in Darwin’s finches

Fecundity selection is a critical component of fitness and a major driver of adaptive evolution. Trade‐offs between parasite mortality and host resources are likely to impose a selection pressure on parasite fecundity, but this is little studied in natural systems. The ‘fecundity advantage hypothesis’ predicts female‐biased sexual size dimorphism whereby larger females produce more offspring. Parasitic insects are useful for exploring the interplay between host resource availability and parasite fecundity, because female body size is a reliable proxy for fecundity in insects. Here we explore temporal changes in body size in the myiasis‐causing parasite Philornis downsi (Diptera: Muscidae) on the Galápagos Islands under conditions of earlier in‐nest host mortality. We aim to investigate the effects of decreasing host resources on parasite body size and fecundity. Across a 12‐year period, we observed a mean of c. 17% P. downsi mortality in host nests with 55 ± 6.2% host mortality and a trend of c. 66% higher host mortality throughout the study period. Using specimens from 116 Darwin's finch nests (Passeriformes: Thraupidae) and 114 traps, we found that over time, P. downsi pupae mass decreased by c. 32%, and male (c. 6%) and female adult size (c. 11%) decreased. Notably, females had c. 26% smaller abdomens in later years, and female abdomen size was correlated with number of eggs. Our findings imply natural selection for faster P. downsi pupation and consequently smaller body size and lower parasite fecundity in this newly evolving host–parasite system.     

Phenotypic Correlates of Scrounging Behavior in Zebra Finches: Role of Foraging Efficiency and Dominance

In flocks, individuals can search for their own food using the producer tactic or exploit the discoveries of companions using the scrounger tactic. Models of the producer–scrounger game usually assume that tactic payoffs are independent of individual phenotypic traits. However, factors such as dominance status or foraging efficiency may constrain the use of tactics and lead to asymmetric tactic use among individuals. For instance, in flocks composed of foragers with unequal foraging efficiency, foragers that are less efficient at obtaining food are expected to rely on the scrounger tactic to a greater extent. I examined the role of foraging efficiency and dominance status as potential correlates of scrounging behavior in small aviary flocks of zebra finches (Taenopygia guttata). Individual foraging efficiency was documented in each flock in a treatment that prevented scrounging. In a subsequent treatment that allowed scrounging, higher levels of scrounging occurred as predicted in foragers with lower foraging efficiency. Dominance status was a poor predictor of tactic choice. Birds that arrived later on the foraging grid foraged less efficiently when scrounging was prevented and used scrounging to a greater extent when allowed, suggesting a link between boldness, foraging efficiency and the choice of foraging tactics in small flocks of zebra finches.     

The Onset of Song Learning and Song Tutor Selection in Fledgling Zebra Finches

We investigated song development in the pre‐independent zebra finch (aged 15–35 d), a period when neural pathways for song learning and production are forming and social influences outside the family are limited. Expt 1 investigated the onset and the minimum duration of tutoring needed for song learning in fledglings. We found most begin to learn song from 25 d of age and need about 10 d contact with the father tutor to make accurate copies. This onset corresponds with major developments in the formation of the neural circuitry implicated in song acquisition. Subsong also begins on day 25 suggesting that the sensory and motor phases of song learning fully overlap in the zebra finch. Our findings support the hypothesis that the song circuitry is fully functional by 35 d of age and the sensitive phase for zebra finches extends therefore from about days 25–65. However, only the first 10 d of this period are necessary to learn a tutor's song with fair accuracy. Expt 2 investigated the role of the paternal bond, spatial proximity and mating status in a fledgling's choice of song tutor where the father was the sole parent. Young chose the father over single unrelated males (expt 2a) or unrelated males in company with their female partners (expt 2b). Given the close spatial proximity of both potential tutors to the fledglings it is probably the filial bond, established via paternal care that is the cause of this preference. Zebra finches sing the same song phrase in two contrasting contexts: female‐directed song during pre‐copulatory courtship and undirected song where no female or display is involved. In expt 3 we tested the song learning preference of pre‐independent young for two categories of non‐paternal tutors: those singing predominantly female‐directed song and those singing exclusively undirected song. There was a small, but significant, preference for fledgling zebra finches to copy songs from males that sang female‐directed song. This preference is consistent with the hypothesis that young males not only learn the acoustic features of their tutor's song but also the visual and dynamic movements that constitute the courtship display.     

Species recognition in Darwin's finches (Geospiza, Gould). III. Male responses to playback of different song types,dialects and heterospecific songs

The songs of the six different species of Darwin's ground finches (Geospiza) on the Galápagos Islands are difficult to distinguish unambiguously because of high levels of intraspecific variation and interspecific similarity in some cases. We recorded the responses of males on five islands to playback of (a) the two main conspecific song types, A and B, (b) local conspecific and heterospecific song, and (c) local and foreign dialects. Males reacted equally strongly to different conspecific song types (A and B), but responded significantly more strongly to local conspecific song than to either heterospecific song or foreign dialect. These results are inconsistent with earlier suggestions that song types subdivide Geospiza populations and that Geospiza song lacks species-distinctness because of loss-of-contrast or character convergence. The apparent paradox of low song specificity and well-developed acoustic discrimination is discussed in the light of other data showing that close-range species recognition also depends on visual cues.     

为何要信达尔文的演化论——论《物种起源》的二十五重简约美

科学理论的本质较量是解释力之简约美的较量, 其使得科学革命成为可能。本文通过整理与剖析达尔文的《物种起源》的论述, 展示了生命科学史上最伟大革命的内在理性, 即, 演化论相对于神创论的25重简约性优势, 分别体现在解释: (1)驯化品种的产生、(2)驯化品种在内外性状层面的多样性的差异性、(3) (杂交的子代的)遗传性状的变化、(4)杂种与混血在健康度与生殖力方面的差异、(5)杂种与混血在性状变化上的差异、(6)生物形态分布的簇状格局、(7)生物形态分化在不同分类阶元内部的不均等性、(8)不同性状分化的不均等性、(9)物种间性状差别的渐变性、(10)伴随环境变化的形态变化、(11)相对环境变化的形态惯性、(12)生物间竞争强度分布的不均等性、(13)不同物种间的形态同似性、(14)个体间的形态差异随着个体发育而变大的现象、(15)生物的痕迹构造、(16)生物地理分布的系统差异性(贯穿诸生物分类阶元的稳定偏向的差异性)、(17)物种的“等级性”与其地理分布范围的关系、(18)生物地理分布的“岛屿”现象、(19)岛屿物种的总丰度低而土著物种丰度高的特征、(20)岛屿生物的类别构成特征、(21)岛屿生物与相邻大陆的生物间的相似度关系、(22)物种迁移与物种灭绝的地域差异性、(23)物种灭绝的渐变模式、(24)古生物与现存生物在形态上的关系、(25)胚胎形态与古生物形态间的相似关系。正是这25重简约美, 铸造了《物种起源》作为最伟大的科学著作的底色, 并奠定了现代生命科学研究的大方向与总方法——“范式”。这一剖析亦有助于我们理解科学发展的真精神以及把握当代科学研究的进展。