This happens because of a process known as random genetic drift, says Dr Wilson, whereby some genetic variants become more common and others are lost.

威尔逊博士说， 这种情况的发生是因为一种被称为随机遗传漂变的过程，在这一过程中， 一些遗传变异变得更加普遍， 而另一些则消失了。

And though the proportions of different blood types vary between human populations, scientists aren't sure why they evolved; perhaps as protection against blood born diseases, or due to random genetic drift.

虽然不同血型的比例在人群中大不相同，科学家也不能确定为什么它们会这样发展；也许是为了防止血液传播疾病，或者只是任意的基因漂流。