A much agreed-on theory on what causes evolutionary tradeoffs is that due to resource limitations (e.g. energy, habitat/space, time) the simultaneous optimization of two traits cannot be achieved.
[3] Researchers have made different mathematical expansions to the 'Y model' in order to gain insights about evolutionary tradeoffs.
An important point that many authors make when discussing the concept of how tradeoffs affect evolutionary change is the ambiguous use of the word 'constraint'.
For example, through experimental manipulation in the lab researchers were able to see that an increase in reproductive activity is correlated with a decrease in longevity in the male fruit fly (Drosophila melanogaster).
[8] More evidence of the tradeoff between reproduction and survival comes from a study done on pinnipeds, where both genital length and testes mass are negatively associated with investment in precopulatory weaponry.
In a study of pregnant adolescents, researchers observed that less energy was allocated to fetuses of women still growing than those who had completed their growth.
This can be linked back to the fact that ovarian steroids act as both bone trophic hormones and mitotic stimulants in breast tissue.