Studies on the phylogeographic structure of species (strains) have yielded insights into their geographic distribution but tell less about strain's capacity to invade novel environments. Extensive age-specific life table data on two strains of the invasive Palearctic alfalfa weevil, Hypera postica (Gyllenhal) (i.e., Ebro Valley, Spain (S) and Hamadan, Iran (I) strains) having disparate vital rates, are used to develop weather-driven physiologically based demographic models (PBDMs) of their biology. The PBDMs are used to explore prospectively their invasive potential across much of the Holarctic. Using N.E. Gilbert's theory that fecundity is selected to the level appropriate for the population in its environment, we explore the interacting effects of multiples of observed fecundity, temperature-dependent mortality and density-dependent population growth constraints on stabilizing adaptation in strains S and I to weather in their native area. Aestivating adults ((Formula presented.)) bridge the critical winter period, and their annual production is used as a metric of adaptation. Maximization of (Formula presented.) in the native environments supports Gilbert's supposition. The S and I strains have wide invasive potential, and we posit stabilizing selection for maximizing (Formula presented.) would occur after successful invasion of novel environments. We propose that the evolving adaptation of an invasive strain to extant and climate change weather can be examined by periodic updates of the biodemographic biology in the weather-driven PBDMs. © 2025 The Author(s). Agricultural and Forest Entomology published by John Wiley & Sons Ltd on behalf of Royal Entomological Society.

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