Temporally varying disruptive selection in the medium ground finch (Geospiza fortis)

Abstract

Disruptive natural selection within populations exploiting different resources is considered to be a major driver of adaptive radiation and the production of biodiversity. Fitness functions, which describe the relationships between trait variation and fitness, can help to illuminate how this disruptive selection leads to population differentiation. However, a single fitness function represents only a particular selection regime over a single specified time period (often a single season or a year), and therefore might not capture longer-term dynamics. Here, we build a series of annual fitness functions that quantify the relationships between phenotype and apparent survival. These functions are based on a 9-year mark–recapture dataset of over 600 medium ground finches ( Geospiza fortis ) within a population bimodal for beak size. We then relate changes in the shape of these functions to climate variables. We find that disruptive selection between small and large beak morphotypes, as reported previously for 2 years, is present throughout the study period, but that the intensity of this selection varies in association with the harshness of environment. In particular, we find that disruptive selection was strongest when precipitation was high during the dry season of the previous year. Our results shed light on climatic factors associated with disruptive selection in Darwin's finches, and highlight the role of temporally varying fitness functions in modulating the extent of population differentiation.

Shape-shifting: changing animal morphologies as a response to climatic warming

“…While this is simpler than the original analysis [51], we have found that temperature may affect both survival and bill size. The opposing trends of rainfall and temperature that we find in bill surface area could indicate that the morphs are using different thermoregulatory strategies [56] big morphs can use their beaks for effective radiative thermoregulation even when temperatures are high, whereas small morphs cannot, thus leading to higher survival of the big morphs in hot years.…”

“…Additionally, climatic warming is not always discussed even when temperature is the focus: Studies looking to verify Allen's rule can, while verifying the link to temperature, leave out the temporal trend associated with climate change, despite data spanning meaningful time frames of several decades (e.g., [16,44,50]). Some studies in recent years have explicitly tested for morphological changes over time, but they have been more focussed on the role of different proximal factors, such as diet, or other aspects of anthropogenic disturbance and climate change [51,52] (Box 1).…”

“…Temperature and rainfall effects on Galapagos finches Galapagos finches are a classic study animal in evolutionary biology and provide textbook examples for the effect of environment on morphology. A recent study by Beausoleil et al [51] looked at disruptive selection in the medium ground finch (Geospiza fortis), which has two morphs distinguishable on the basis of bill size and is known to effectively thermoregulate with their bills (Figure 1; [29]). They found that disruptive selection was stronger when rainfall was high during the previous dry period, and they postulate that this was due to decreased competition within the two morphs during periods of plenty (as opposed to dry periods, when there would be increased competition within morphs due to lower resources) [51].…”

“…A recent study by Beausoleil et al [51] looked at disruptive selection in the medium ground finch (Geospiza fortis), which has two morphs distinguishable on the basis of bill size and is known to effectively thermoregulate with their bills (Figure 1; [29]). They found that disruptive selection was stronger when rainfall was high during the previous dry period, and they postulate that this was due to decreased competition within the two morphs during periods of plenty (as opposed to dry periods, when there would be increased competition within morphs due to lower resources) [51]. In order to see if temperature could be contributing to this pattern and to overall bill size in this species, we reanalysed their data with temperature as a covariate (all data were provided in their supplementary material).…”

Hybridization alters the shape of the genotypic fitness landscape, increasing access to novel fitness peaks during adaptive radiation

“…Simpson (7) later described the phenotypic evolution of populations through time on a rugged landscape, in which isolated clusters of fitness peaks represent 'adaptive zones' to which populations evolve from adjacent regions of low fitness (8). Lande and Arnold formalized the analysis of selection and estimation of phenotypic fitness landscapes (9)(10)(11), leading to empirical studies of fitness landscapes in numerous empirical systems (12)(13)(14)(15)(16)(17)(18). Fitness surfaces also provide a central component of speciation models and theory (19)(20)(21).…”