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chapter 15.2.docx

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Biology
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Biology 4218A
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How Many Offspring Should an Individual Produce in a Given Year?  The more offspring a parent (or pair of parents) attempts to raise at once, the less time and energy the parent can devote to caring for each one Clutch Size in Birds  Selection will favour the clutch size that produces the most surviving offspring  Any individual offspring will survive decreases with increasing clutch size  The ability of the parents to feed any individual offspring declines as the number of offspring increases  In fig 13.19a, we assume that the decline in offspring survival is a linear function of clutch size, but the model depends only on survival being a decreasing function o The number of surviving offspring reaches a maximum at an intermediate clutch size  In an experiment, the mean clutch size was 8.53 o The average number of surviving offspring from clutches of each size was also determined  This number was highest for clutches of 12 eggs  When researchers added 3 eggs to each of a large number of clutches, the most productive clutch size was still 12  In other words, birds that produced smaller clutches apparently could have increased their reproductive success for the year by laying 12 eggs  This indicates that natural selection favours larger clutches than the birds in the population actually produce o Because the average clutch size was less than the most productive clutch size, these results are not consistent with Lack’s hypothesis  Lack’s hypothesis: selection will favour the clutch size that produces the most surviving offspring  Another experiment showed that: The majority of studies have shown that birds lay smaller clutches than predicted  Lack’s hypothesis assumes that there is no trade-off between a parent’s reproductive effort in one year and its survival or reproductive performance in future years o When reproduction is costly and selection favours withholding some reproductive effort for the future, the optimal clutch size may be less than the most productive clutch size  Second, Lack’s hypothesis assumes that the only effect of clutch size on offspring is in determining whether the offspring survive o Being part of a large clutch may, impose other costs on individual offspring than just reducing their probability of survival  Clutch size affects not only offspring survival, but also offspring reproductive performance o This suggest that there is a trade-off between the quality and quantity of offspring produced o When larger clutches entail lower offspring reproductive success, the optimal clutch size will be smaller than the most numerically productive clutch size  Thrid, the discrepancy between Lack’s hypothesis and the behaviour of individual birds may sometimes be more apparent than real o In fig. 13.17, the birds that laid fewer than 12 eggs did so because they had lower reproductive capacities-and that each bird was producing a clutch size that would optimize its own reproductive success  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
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