Numerous variables can affect an organism’s survival, including its age and sex, the demographics of the population in which it resides, and environmental conditions like climate, and habitat. However, the relative importance of these factors is poorly understood. Dan Warner described his investigation into factors affecting natural selection in wild populations of anoles in his talk titled, “Spatial and temporal variation in phenotypic selection in the lizard Anolis sagrei.”
Warner measured directional selection on A. sagrei on six islands in the Matanzas National Estuarine Reserve in Florida. These islands were intentionally founded with populations having unequal adult sex ratios. Half of the islands were founded with more males than females (male-biased), and the other islands received more females than males (female-biased). This manipulation was done to strengthen the effects of male-male competition on the male-biased islands. Warner measured survival selection on adult and juvenile body size by marking and recapturing individuals over the last three years.
Warner found a lot of variation in the strength of directional selection on adult and juvenile body size both across islands and within each island in different years. However, there was no relationship between the strength of selection on each island and either habitat structure (represented by canopy openness) or island size. Thus, the probability of survival at a particular body size does not seem to depend on environment.
However, population demographics did seem to affect survival at different body sizes. There was a negative correlation between the strength of selection on body size and the density of adult lizards, indicating that smaller body sizes are favored at high population densities (and vice versa). This trend was observed in both adults and juveniles, but was more pronounced in juveniles. Warner hypothesized that it was the density of adult males in particular, rather than the total density of adults, that was driving the observed trend. To test this idea, he tested for a correlation between the strength of selection on juvenile body size and the adult sex ratio. He found a negative correlation, indicating that large juveniles are favored in more female-biased populations while small juveniles do better in male-biased populations. One possible explanation is that on islands with male-biased sex ratios, large juveniles are more likely to come into contact with territorial adult males, are more likely to be perceived by these males as a possible competitor, and are therefore more likely to be harassed by these males. The presence of adult males might even reduce recruitment, as evidenced by slower population growth rates on male-biased versus female-biased islands.
These results suggest that patterns of natural selection on individuals can depend on characteristics of the population. Only with long-term field studies such as this one can we begin to unravel the many factors affecting selection in wild populations.