A third reason for the existence of an, often negative, correlation between the trait under selection and environmental sensitivity has not much to do with locations on the genome or with multiple functions of individual genes. It has more to do with the fact that the animal has ‘to make choices’ about what to spend its resources on. First of all, these are not conscious choices. We call them choices because resources spent on one process or trait cannot be spend on something else. The mechanism behind the ‘decision making’ what to spend the resources on is not very clear yet, but most likely consist of a combination of genetics, and factors such as life stage of the animal, condition, health, and a number of other factors. Whether the resources that are required for the various processes are available will depend on the availability, and quality of the available resources, but also on the intake capacity of the animal. Think about feed intake capacity in dairy cattle. Some cows cannot ingest enough feed to maintain body condition. They spend all resources on milk production, but have restricted feed intake capacity so they have to use some of their resource reserves (body condition) as well.
Thus:
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Genetic Thus: genetic correlations can exist for several reasons:
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Animals have some (but not complete!) flexibility in shifting resources to various processes. It seems that some animals are better capable of doing that than others, and there are indications that this is heritable. For sure there is a heritable component to feed intake capacity. So if we would compose a very simple model, we can say that resource intake needs to be divided across anything related to survival on the one hand, and reproduction on the other. This is illustrated in figure at 6A. We now put the animal in a more demanding environment. In order to survive it will need to obtain more resource for survival. If possible it will increase its feed intake. However, if that already was at its maximum, it will have to shift resources that were allocated to reproduction to survival (figure at 6B). In this environment there is a negative correlation between survival and reproduction. Better survival is achieved at the expense of reproduction.
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