Well, it's the universal view in evolutionary biology, in everything I've read and people I've talked to. It's also clearly stated in the opening paragraphs of the wikipedia entry for example [1].
Or were you asking why biologists are so sure? I'm not a biologist, but my understanding is that the evidence for natural selection is simply overwhelming on all levels (archeological, experimental, theoretical, simulations, etc.). Other evolutionary processes have interesting work behind them, but the burden of proof to show they matter as much as natural selection has just not been hit yet.
From what I gather, the core concept of the evolution which is speciation has not been demonstrated to be caused by natural selection. Everything I've read about so far is merely demonstrating adaptation of the species as opposed to a new species emerging.
There isn't much debate on this anymore in biology - practically every important evolutionary biologist accepts that speciation is driven by natural selection. Of course other factors matter too, like goegraphic separation, and so forth; they can provide an opportunity for natural selection to drive changes between groups, that when they grow large enough become a difference in species. But it is natural selection that creates those differences.
what makes you so sure of that?
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