Natural selection is a greatly overused explanation for almost any trait in living organisms.
However, Darwinian expectations seldom match facts. A paper published in PLOS BIOLOGY begins with the sentence: “In natural populations, quantitative trait dynamics often do not appear to follow evolutionary predictions.”
That’s right. The research is on the size of snow voles (Chionomys nivalis). Timothée Bonnet and his colleagues at the University of Zurich observed the tiny rodents for ten years and found out that while bigger voles had more offspring, this did not lead to an increase in body size in the next generation.