The environment can also be improved so that less resources need to be allocated to survival and more can be spend on reproduction, resulting in a shift towards reproduction (figure 6C). Animals that have the highest proportion of their resources allocated to reproduction will have the largest share in the next generation. So selection pressure automatically will be on the large proportion of resources allocated to reproduction. You can imagine that after a number of generations the animals that allocate more to survival have been reduced in number. In this environment that is not a problem, because only limited resources are required for survival. However, if you put those animals in a poorer quality environment, they will not manage anymore. A negative correlation has developed between reproduction and survival. This is another type of negative correlation than with the milk production and reproduction, even though the reasons for development are similar. In this case the animals are very reproductive, but need a good environment for survival. In the previous case the animals had very good ‘survivability’ (i.e. milk production), but at the expense of reproduction.
In our modern farm animal species both types of negative correlations occur. We have selected the animals for very high performance and, simultaneously, we tried to optimise their environment so that they could show their potential. By doing so, we have created animals that perform very well under optimised conditions. However, they also have become more sensitive to a decrease in environmental quality compared to what they were selected for. For example, broilers have become quite sensitive to fluctuations in ambient temperature. The range at which they feel comfortable is much decreased as compared to that of, for example, laying hens. Of course there is more than just resources involved, but the principle of not having to deal with a demanding environment so that all effort can be put in the selection criterion (growth) is applicable.