The variance effective population size (NeV\documentclass[12pt]{minimal}
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\begin{document}$$N_{eV}$$\end{document}) is frequently used to quantify the expected rate at which a population’s allele frequencies change over time. The purpose of this paper is to find expressions for the global NeV\documentclass[12pt]{minimal}
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\begin{document}$$N_{eV}$$\end{document} of a spatially structured population that are of interest for conservation of species. Since NeV\documentclass[12pt]{minimal}
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\begin{document}$$N_{eV}$$\end{document} depends on allele frequency change, we start by dividing the cause of allele frequency change into genetic drift within subpopulations (I) and a second component mainly due to migration between subpopulations (II). We investigate in detail how these two components depend on the way in which subpopulations are weighted as well as their dependence on parameters of the model such a migration rates, and local effective and census sizes. It is shown that under certain conditions the impact of II is eliminated, and NeV\documentclass[12pt]{minimal}
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\begin{document}$$N_{eV}$$\end{document} of the metapopulation is maximized, when subpopulations are weighted proportionally to their long term reproductive contributions. This maximal NeV\documentclass[12pt]{minimal}
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\begin{document}$$N_{eV}$$\end{document} is the sought for global effective size, since it approximates the gene diversity effective size NeGD\documentclass[12pt]{minimal}
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\begin{document}$$N_{eGD}$$\end{document}, a quantifier of the rate of loss of genetic diversity that is relevant for conservation of species and populations. We also propose two novel versions of NeV\documentclass[12pt]{minimal}
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\begin{document}$$N_{eV}$$\end{document}, one of which (the backward version of NeV\documentclass[12pt]{minimal}
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\begin{document}$$N_{eV}$$\end{document}) is most stable, exists for most populations, and is closer to NeGD\documentclass[12pt]{minimal}
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\begin{document}$$N_{eGD}$$\end{document} than the classical notion of NeV\documentclass[12pt]{minimal}
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\begin{document}$$N_{eV}$$\end{document}. Expressions for the optimal length of the time interval for measuring genetic change are developed, that make it possible to estimate any version of NeV\documentclass[12pt]{minimal}
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\begin{document}$$N_{eV}$$\end{document} with maximal accuracy.
