Research on chromosome evolution accumulated in the past three decades that seems to validate the hypothesis postulated that chromosome changes that increase or decrease of chromosome number are adaptive partly because they increase or decrease in variation via effects on recombination and segregation in meiosis. The paper reviews some of the new data relevant to this question and especially focus on studies on interspecific and intraspecific chromosome variation in placental mammals. We find data in support of that hypothesis coming from many areas and for the first time suggest applications in some domesticated and invasive species of mammals. This hypothesis does not explains all chromosome number variation in mammals but it does advance our understanding and opens future avenues of research into mammalian variation and adaptability.