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.


Larger mothers, larger offspring

Larger larvae from the colonial bryozoan species Bugula neritina had higher survival and growth relative to smaller larvae, but when amongst siblings, smaller larvae were positively advantaged and grew as large (or even larger) than their bigger counterparts.

There may be an adaptive explanation for these findings. Larger mothers may produce larger offspring to facilitate their dispersal to habitats where they perform best (that is, in isolation from siblings) and smaller mothers may produce smaller offspring that disperse less and therefore will end up in habitats with siblings where they perform best.

The results from this study by Hayley Cameron and colleagues from the Centre for Geometric Biology and the National Oceanic and Atmospheric Administration in the USA, contrast with classical theories that predict that larger offspring are produced by larger, more fecund, mothers to offset the competitive effects of more siblings.

Bryozoan colonies growing on plates at different densities. Survival and growth was recorded after four weeks.
Bryozoan colonies growing on plates at different densities. Survival and growth was recorded after four weeks.

Identifying the mechanisms for these underlying correlations between maternal size and offspring size has important ecological implications. Average body size of individuals has been reduced in many systems, and this research suggests – for some species at least – the smaller offspring that will result from smaller mothers may still be able to perform well in the maternal habitats, but that dispersal to new habitats may be constrained.

This is one of the few field studies testing these theories. Hayley and her colleagues were able to experimentally manipulate both sibling density and offspring size of the arborescent bryozoan, Bugula neritina, and monitor the survival and growth of different-sized individual larvae following deployment in the field. Further studies will be needed to increase our understanding of why offspring size co-varies with maternal size across a range of taxa.

This research was published in the journal Ecology.

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