The adaptive significance of population differentiation in offspring size of the least killifish, Heterandria formosa.

Jeff Leips,Joseph Travis,F Helen Rodd

doi:10.1002/ece3.509

Abstract

We tested the hypothesis that density-dependent competition influences the evolution of offspring size. We studied two populations of the least killifish (Heterandria formosa) that differ dramatically in population density; these populations are genetically differentiated for offspring size, and females from both populations produce larger offspring when they experience higher social densities. To look at the influences of population of origin and relative body size on competitive ability, we held females from the high-density population at two different densities to create large and small offspring with the same genetic background. We measured the competitive ability of those offspring in mesocosms that contained either pure or mixed population treatments at either high or low density. High density increased competition, which was most evident in greatly reduced individual growth rates. Larger offspring from the high-density population significantly delayed the onset of maturity of fish from the low-density population. From our results, we infer that competitive conditions in nature have contributed to the evolution of genetically based interpopulation differences in offspring size as well as plasticity in offspring size in response to conspecific density.

Highlights

Propagule size is a key life-history trait that directly influences both offspring and maternal fitness
Quadrupling the density of offspring significantly reduced offspring growth rate by about 50% (F1,16 = 7.41, P = 0.009, Fig S2) and this effect was similar for both populations
We did not detect an effect of the identity of the competitor on growth rate (TP vs. Wacissa River (WR)(H) fish, F1,18 = 0.24, P = 0.63) nor did we find that the effect of density depended on the competitor type (TP vs. WR(H)), F1,18 = 0.66, P = 0.42)

Summary

Introduction

Propagule size is a key life-history trait that directly influences both offspring and maternal fitness. High variation in offspring size among conspecific populations and related species (Reznick and Endler 1982; Amat et al 2001; Marshall and Keough 2007; Reed et al 2009; Riesch et al 2009; Collin and Salazar 2010) raises the questions of how much of this variation is directly adaptive, under which circumstances might it be adaptive, and for whom, parent or offspring, does the bulk of any benefit accrue. Population density is one ecological factor that theory predicts will play an important role in selecting for variation in offspring size and number (Brockelman 1975; Parker and Begon 1986).

Methods

Results

Conclusion