Natural selection favors larger body mass and size under specific ecological pressures.
the verdict
SUPPORTED
the evidence backs this
refutedsupported
the weight of evidence
8 sources for · 0 against
Multiple scientific studies and evolutionary biology references confirm that natural selection and directional selection frequently favor larger body mass and size when driven by specific ecological pressures such as hunting large prey or habitat characteristics.
A complete understanding of life history evolution requires an appreciation for the influence of natural selection and genetic variation on intrapopulation phenotypic variation in key life history traits. Experimental manipulation provides particularly useful insight into the biological importance of this variation. Consequently, I undertook a field experiment in which I manipulated body size and evaluated selective predation in order to investigate the ecological and evolutionary significance of phenotypic variation in hatchling turtles. I incubated eggs from 17 clutches of common snapping turtles (Chelydra serpentina) on wet and dry substrates, producing "large" and "small" hatchlings, respectively. Hatchlings were individually marked, measured, and then released simultaneously at a typical nest site in a National Wildlife Refuge area, Whiteside County, Illinois from which the eggs collected originally. Subsequently, hatchlings were recaptured in drift fences erected along the edge of the Mississippi River. Sixty—six of 112 hatchling C. serpentina were recaptured. Survivorship was not related to clutch, incubation conditions, or locomotor performance, but was significantly size dependent. Natural selection favored larger hatchlings. The strength of selection acting directly on three size traits, as measured by standardized selection gradients (°), was 0.327 (PCI [principal component 1] °overall size and mass), 0.282 (PC3 = plastron length), and 0.162 (PC4 = trade—off between hatchling mass and width). The opportunity for selection was moderate (I =0.703), but did not constrain selection. Full—sib heritabilities of body size of hatchling turtles were variable (h 2 w e t = 0.72 ° 0.34 and h 2 d r y = 0.17 @+ 0.28), but were not significantly different between the two moisture treatments. These results provide support for the "bigger is better" hypothesis. Larger hatchling Chelydra serpentina exhibited significantly greater survivorship than smaller individuals during movement from the nest site to water. However, larger body size of hatchling turtles may not evolve rapidly because the strength of selection was moderate in magnitude and the heritability was relatively low. These results emphasize the need for more detailed ecological investigations of the early life history of these organisms. Furthermore, this study highlights the utility of experimental manipulations of phenotypic variation for evaluating the potential impact of natural selection on life history evolution.
Abstract
1. Body size in parasitic insects can be subjected to contrasting selective pressures, especially if they complete their development within a single host. On the one hand, a larger body size is associated with a higher fitness. On the other hand, the host offers a discrete amount of resources, thus constraining the evolution of a disproportionate body size.
2. The present study used the weevil
Curculio elephas
as a study model. Larvae develop within a single acorn, feeding on its cotyledons, and larval body size is strongly related to individual fitness.
3. The relationship between larval and acorn size was negatively exponential. Larval growth was constrained in small acorns, which did not provide enough food for the weevils to attain their potential size. Larval size increased and levelled off in acorns over a certain size (inflexion point), in which cotyledons were rarely depleted. When there were more than one larva per acorn, a larger acorn was necessary to avoid food depletion.
4. The results show that
C. elephas
larvae are sometimes endoparasitic, living on the edge of host holding capacity. If they were smaller they could avoid food depletion more easily, but the fitness benefits linked to a larger size have probably promoted body size increase. The strong negative effects of conspecific competition may have possibly influenced female strategy of laying a single egg per seed.
5. Being larger and fitter, but always within the limits of the available host sizes, may be one main evolutionary dilemma in endoparasites.
Abstract Freshwater amphipods are used as a research model for studies of female mating preferences, the nature of benefits that drive female choice, and the influence of sexual conflict on female preference. Direct selection on female mating preferences is hypothesized to occur during the contact pairing phase, because costs incurred or benefits gained by females during pairing are likely to depend on traits of guarding males. Comparative studies of ecomorphs in the genus Hyalella shed light on the evolution of female preference and preferred male traits. In species from habitats where large body size is favored by ecological processes females prefer larger males. In species subject to intense fish predation, however, mortality selection favors small body size and female preference for larger males is weak. These divergent mating biases are consistent with direct and indirect selection on female preference acting under the disparate regimes of natural selection faced by the ecomorphs.
cheetah, and the puma, exhibit evidence of directional selection favoring larger body mass. These larger body sizes are likely linked to hunting large prey
In population genetics, directional selection is a mode of natural selection in which individuals with a trait (for example, beak size) at one extreme of a phenotypic distribution have better fitness than individuals with intermediate or opposite extreme phenotypes. Over time, the allele frequencies, and consequently the population mean for the trait, shift consistently in the direction of the ext
D…
This study examines the role of lineage-specific directional selection on body size evolution in felids, revealing that several species, including those in the Panthera genus (lions, tigers, leopards, jaguars, snow leopards), the cheetah, and the puma, exhibit evidence of directional selection favoring larger body mass. These larger body sizes are likely linked to hunting large prey and solitary hunting strategies, which favor physical strength and size. Conversely, the clouded leopard did not show evidence of directional selection for body size, suggesting different ecological pressures, and the jaguarundi showed no clear selection for smaller size despite being smaller than its relatives. These findings highlight that body size evolution in felids is not uniform and is strongly influenced by ecological factors such as prey size and hunting behavior. The study concludes that directional selection for increased body size is likely associated with the need for larger predators to capture large prey, and solitary hunting may accelerate this selection, although the evolutionary paths for different felid lineages can vary considerably.
Despite the large amount of work on frog mating systems, the potential role of predators as an agent of selection on breeding adults has received very little study. Here, I use data from multiple populations of Pacific Treefrogs (Pseudacris regilla) to demonstrate that sexual selection from mating success favors larger males, but natural selection from predation by giant water bugs favors smaller males. Additionally, I found no relationship between male body condition and mating success or predation risk. This result demonstrates that predation is a potentially important agent of selection that counteracts sexual selection in anurans.
We experimentally manipulated the strength of selection in the field on red-winged blackbirds (Agelaius phoeniceus) to test hypotheses about contrasting selective forces that favor either large or small males in sexually size dimorphic birds. Selander (1972) argued that sexual selection favors larger males, while survival selection eventually stabilizes male size because larger males do not survive as well as smaller males during harsh winters. Searcy (1979a) proposed instead that sexual selection may be self limiting: male size might be stabilized not by overwinter mortality, but by breeding-season sexual selection that favors smaller males. Under conditions of energetic stress, smaller males should be able to display more and thus achieve higher reproductive success. Using feeders that provisioned males or females but not both, we produced conditions that mimicked the extremes of natural conditions. We found experimental support for the hypothesis that when food is abundant, sexual selection favors larger males. But even under conditions of severe energetic stress, smaller males did not gain larger harems, as the self-limiting hypothesis predicted. Larger males were more energetically stressed than smaller males, but in ways that affected their future reproductive output rather than their current reproductive performance. Stressed males that returned had smaller wings and tails than those that did not return; among returning stressed males, relative harem sizes were inverse
acquiring flower nectar, and supporting the energy demands of their larger body size. Directional selection thus favors the larger hummingbirds in terms
Hummingbirds are birds native to the Americas and comprise the biological family Trochilidae. With approximately 375 species and 113 genera, they occur from Alaska to Tierra del Fuego, but most species are found in Central and South America. As of 2026, 21 hummingbird species are listed as endangered or critically endangered, with about 255 species declining in population.
Hummingbirds have varied
Hummingbirds exhibit sexual size dimorphism according to Rensch's rule, in which males are smaller than females in small-bodied species, and males are larger than females in large-bodied species. The extent of this sexual size difference varies among clades of hummingbirds. For example, the Mellisugini clade (bees) exhibits a large size dimorphism, with females being larger than males. Conversely, the Lesbiini clade (coquettes) displays very little size dimorphism; males and females are similar in size. Sexual dimorphisms in bill size and shape are also present between male and female hummingbirds, where in many clades, females have longer, more curved bills favored for accessing nectar from tall flowers. For males and females of the same size, females tend to have larger bills.
Sexual size and bill differences likely evolved due to constraints imposed by courtship, because mating displays of male hummingbirds require complex aerial maneuvers. Males tend to be smaller than females, allowing conservation of energy to forage…
Abstract In many animal species, larger body size is positively correlated with male mating success and female fecundity. However, in the case of insects, in high seasonality environments, natural selection favors a faster maturation that decreases the risk of pre‐reproductive death. However, this advantageous adaptation comes at a tradeoff, resulting in a reduction in body size. Maturation time is influenced by environmental factors, such as temperature, season length, and food availability during the rains. The geographic variation in these parameters provides an opportunity to study their impact on the adaptive evolution of body size in Pyrgomorphidae grasshoppers. These grasshoppers exhibit remarkable variation in body size and wing development and can be found in diverse plant communities across Africa, Asia, Australia, and tropical America. In this study, we utilized a phylogenetic approach to examine the evolution of body size, considering climatic factors, and the influence of sexual selection on size differences between males and females. We found a positive correlation between mean annual temperature and sexual size dimorphism (SSD). Remarkably, species exhibiting a strong bias toward larger females were found to be adapted to regions with higher temperatures. In the Pyrgomorphidae family, an intermediate body size was identified as the ancestral trait. Additionally, winged male and female grasshoppers were observed to be larger than their wingless counterparts. Des
Everything we examined (8) — 7 independent sources
This check searched the claim as stated. It did not run a separate search for evidence against it.