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Estimating individual contributions to population growth: evolutionary fitness in ecological time - PubMed

  • ️Sun Jan 01 2006

Estimating individual contributions to population growth: evolutionary fitness in ecological time

T Coulson et al. Proc Biol Sci. 2006.

Abstract

Ecological and evolutionary change is generated by variation in individual performance. Biologists have consequently long been interested in decomposing change measured at the population level into contributions from individuals, the traits they express and the alleles they carry. We present a novel method of estimating individual contributions to population growth and changes in distributions of quantitative traits and alleles. An individual's contribution to population growth is an individual's realized annual fitness. We demonstrate how the quantities we develop can be used to address a range of empirical questions, and provide an application to a detailed dataset of Soay sheep. The approach provides results that are consistent with those obtained using lifetime estimates of individual performance, yet is substantially more powerful as it allows lifetime performance to be decomposed into annual survival and fecundity contributions.

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Figures

Figure 1
Figure 1

Individual contributions to population growth summed across individuals in the same phenotypic states for the Soay sheep. (a) Age-specific values of pt(i) for the Soay sheep averaged across years 1986–2003, (b) time series of age-specific contributions to population growth. The solid lines represent lambs (negative values of pt(i), and four and five year olds (positive values). There is substantial inter-annual variation in the contribution of different age-classes to population growth and (c) the contribution of different cohorts to population growth. Note that the foot and mouth outbreak means data are not available for 2001.

Figure 2
Figure 2

Using pt(i) to estimate selection in the Soay sheep. (a) An example of a selection gradient calculated using birth weights and pt(i) values for all Soay sheep (males and females) living in the population in 1993. Each dot represents an individual. Males and females of known ages are included in the plot. Data are transformed to have a mean of zero and a standard deviation of unity prior to analyses. (b) A time series of fluctuations in selection pressure on birth weight calculated using pt(i). There is evidence that the strength of selection on birth weight has declined during the course of the study. The horizontal dotted line shows the estimate of selection calculated using lifetime reproductive success. This is close to the mean of the estimates of fluctuating selection calculated using pt(i) (solid circles) (c) scatter plot showing the lack of correlation between annual estimates of selection calculated using pt(i) and cohort based estimates calculated using LRS.

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