Selection may not act alone to determine variation in life history traits across depth gradients

Researchers Rolanda Lange, Keyne Monro and Dustin Marshall have been asking the question “why do some organisms grow, reproduce and die more quickly than others?” They are interested in understanding how  variation in these basic life history traits is sustained over small spatial scales.

They chose to focus this research question on a well known colonial bryozoan and investigate differences in a number of life history traits, predicted to influence reproductive success, across a three metre depth gradient.

They were interested in whether differences in selection along a strong environmental gradient, such as depth, can maintain the variation in life history traits.

The life history traits they measured included the onset of senescence, which reflects the beginning of colony deterioration, the growing edge width, which indicates the potential for future growth and module size which reflects the investment in each module within a colony.

Rolanda, Keyne and Dustin then looked at the relationship between these traits and colony lifetime reproductive success for 173 individual Watersipora colonies over a 22-week period.

Traits measured in Watersipora colonies

There was variation in most of the traits they measured but the patterns of selection were not always conducive to maintaining these differences. For example, late module size was larger in shallow habitats while the analysis suggested that selection should actually reduce module size in the shallow habitat (reproductive output was less where late modules were larger).

It seems likely given the small spatial scale and the limited dispersal of Watersipora that observed differences in life history traits are a result of phenotypic plasticity, but this plasticity is not always adaptive.

Understanding spatial variation in selection is the first step to understanding how environmental complexity can shape evolutionary adaptation.

This research was published in the journal Evolution.