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.


Limited evolutionary responses to harvesting regime in the intensive production of algae

Authors: Rebecca J Lawton, Nicholas A Paul, Dustin J Marshall, and Keyne Monro

Published in: Journal of Applied Phycology, January 2017


Plastic changes in the growth and productivity of algae in response to environment and stocking density are well established. In contrast, the capacity for such changes to persist once environmental differences cease, potentially signalling an evolutionary response, have rarely been tested for algae in intensive production systems.

We tested whether continuous differences in harvesting regime (a high stocking density/low-yield regime versus low stocking density/high-yield regime) generated changes in biomass productivity and other growth metrics within several strains of the clonal macroalga Oedogonium (Chlorophyta, Oedogoniales) and whether such changes persisted once differential harvesting yields ceased.

We found considerable plasticity in growth rate and biomass productivity over a 12-week period of active selection (i.e. repeated high-yield and low-yield harvesting of clonal lineages within strains) and that strains responded differently to this selection pressure over time.

While small, but significant, differences in growth rates of clonal lineages exposed to high-yield vs low-yield harvesting regimes were maintained after prolonged culture under a common selection regime (i.e. medium-yield harvesting), differences in biomass productivity were not. There was no evidence for positive or negative effects of maintaining multiple strains in polyculture on growth and biomass productivity.

Overall, we detected limited potential for evolutionary responses to harvesting regime in the main commercial trait of interest — biomass productivity. This outcome is important for commercial cultivation in intensive production systems, since it identifies a low risk that harvesting practices will impact negatively on biomass productivity in the longer term.

Lawton RJ, Paul NA, Marshall DJ, Monro K (2017) Limited evolutionary responses to harvesting regime in the intensive production of algae. Journal of Applied Phycology, PDF DOI

Environment-dependent variation in selection on life history across small spatial scales

Authors: Rolanda Lange, Keyne Monro, and Dustin J Marshall

Published in: Evolution, volume 70, issue 10 (October 2016)


Variation in life-history traits is ubiquitous, even though genetic variation is thought to be depleted by selection.

One potential mechanism for the maintenance of trait variation is spatially variable selection.

We explored spatial variation in selection in the field for a colonial marine invertebrate that shows phenotypic differences across a depth gradient of only 3 m. Our analysis included life-history traits relating to module size, colony growth, and phenology.

Directional selection on colony growth varied in strength across depths, while module size was under directional selection at one depth but not the other. Differences in selection may explain some of the observed phenotypic differentiation among depths for one trait but not another: instead, selection should actually erode the differences observed for this trait.

Our results suggest selection is not acting alone to maintain trait variation within and across environments in this system.

Lange R, Monro K, Marshall DJ (2016) Environment-dependent variation in selection on life history across small spatial scales, Evolution, PDF DOI