Why do you think type 2 diabetes occurs so much more often than other inheritable diseases? Propose a hypothesis for why this allele has not been eliminated by natural selection. Which type of species best describes a mouse: opportunistic or equilibrium? Explain why a mouse or rat population would continuously grow while other mammals (cheetahs) are nearly extinct.

This allele has not been eliminated by natural selection because it may confer an adaptive advantage for particular environmental conditions  

Mice and rats are opportunistic species. These species can rapidly adapt to different environmental conditions, while cheetahs are unable to adapt to changing environments.  

Explanation:

An allele is a gene variant that may be beneficial, neutral, or deleterious for a particular environment. Different alleles are not eliminated by natural selection against other alleles (even when a priori seem to be deleterious) because they confer a selective advantage in a given environment. For example, the allele for sickle cell anemia is caused by a point mutation which is lethal in homo-zygous individuals, but heterozygous individuals are less likely to die of malaria, thereby this allele has been positively selected in geographic areas where malaria is an endemic disease. Moreover, mice and rats are opportunistic species, i.e., they are species that rapidly adapt to varied living conditions. These species are characterized to have high growth rates and produce many offspring. On the other hand, the cheetah is a highly specialized species that is unable to adapt to changing environments. In consequence, cheetah populations are more prone to become extinct than rat and mice populations.