 The careful reader may have noted that the previous paragraph invoked probabilistic language: what tends to happen, what could happen, what is likely to happen. Indeed, mathematical evolutionary models today (see the entry on  population genetics)  are typically statistical models. This fact about evolutionary models has given rise to a debate in the philosophy of evolution over whether natural selection and genetic drift should be be understood as causes of evolution, as most biologists conceive them, or as mere statistical summaries of lower-level causes: births, deaths, etc. (The  natural selection  and  genetic drift  entries give more information about this debate). It is for this reason that this entry uses the more neutral phrase “modes of evolution” so as not to beg any questions under dispute between the causalist and the statisticalist.