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Lack's hypothesis

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Ingrid Makus

 Lack’s hypothesis predicts that parents will attempt to rear that number of young that maximizes the number of surviving offspring. Data indicate that parents often rear fewer offspring. Efforts to identify which of Lack’s assumptions are violated have led to the discovery of additional trade-offs and improved estimates of lifetime fitness. Lack’s Hypothesis Applied to Parasitoid Wasps  Lack’s hypothesis was used to explore the evolution of clutch size in parasitoid wasps o Parasitoid wasps use a stingerlike ovipositor to inject their eggs into the eggs or body cavity of a host insect o When the larval parasitoids hatch, they eat the host alive from the inside o The larvae then pupate inside the empty cuticle of the host, finally emerging as adults to mate and repeat the life cycle o For a parasitoid, a host is analogous to a nest o A female parasitoid can lay a clutch of one or more eggs in a single host o The larvae compete among themselves for food, so there is a trade-off between clutch size and teh survivial of individual larvae o An added twist with insects is that adult size is highly flexible o Competition for food may result in larvae simply becoming smaller adults o The maternal fitness associated with a given clutch size must therefore be calculated as the product of the clutch size, the probability of survival of individual larvae, and the expected lifetime egg production by offspring of the size that will emerge  Lack’s hypothesis is a useful null model for other organisms in addition to birds  Why do female wasps typically lay clutches smaller than the predicted sizes? o Larger clutch sizes may reduce offspring fitness in ways that the researchers did not include in their calculations; and there may be trade-offs between a female’s investment in a particular clutch and her own future survival or reproductive performance o Unlike birds, female parasitoid wasps may produce more than one clutch in rapid succession  Soon after she has laid one clutch, a female wasp may begin looking for another host to parasitize  The appropriate measure of a wasp’s fitness with regard to clutch size may not be the discrete fitness she gains from a single clutch  Instead, it may be the rate at which her fitness rises as she searches f
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