Speciation is driven by the interplay between deterministic (e.g., environmental pressure driving natural selection) and non-deterministic factors (e.g. stochastic demographic processes) that promote reproductive isolation between populations. How deterministic and non-deterministic factors interplay in speciation is vastly unknown. However, two extreme models of speciation are possible to identify: speciation via niche divergence (ND) when deterministic factors are the main component, and speciation via niche conservatism (NC) when non-deterministic factors are the main component. How common are the speciation modes via ND or ND in nature? That remains unknown. Here, by using phylogenetic data, niche modeling, and geographic information systems tools, I studied the ferns from the American continent to estimate the commonness of ND and NC between sister species pairs. The results show that ecological speciation via ND is similarly common at all latitudes, suggesting an important role of disparate local adaptation at diverging species regardless of their latitudes. But surprisingly, non-ecological speciation via NC is more common in the tropics, suggesting that geographic isolation, rather than ecological isolation, is predominant in the diversification of tropical ferns. The novelty of this research: this is the first empirical evidence that the frequency of the two modes of speciation varies with latitude.