Plants that are able to self-fertilize may have an advantage when colonizing new habitats as they don't need to find a mate to reproduce. Baker's law describes this tendency and predicts an over-representation of self-compatible species among remote island colonizers. The hypothesis has been tested numerous times since it was put forward in 1955, however, the results have been very mixed.
Assessing 100+ such studies, researchers identified a number of weaknesses potentially responsible for the ambiguity. The majority of studies focused only on island species, disregarding mainland populations as a comparison, and the scope of most studies was also limited to a few species and islands.
To overcome this, the authors of this paper compared worldwide frequency of breeding systems on mainland versus island for three plant families of more than 1,500 species combined. Using GBIF-mediated occurrences combined with other sources to determine the distributions of each species, they found that 66 per cent of island species were self-compatible, whereas only 41 per cent of mainland species were able to self fertilize.
This significant difference, consistent across all three families, provides strong support for Baker's hypothesis.