Allopatric speciation occurs as a result of genetic divergence caused by geographical isolation: over time one species becomes two due to adaptations to different environments. Populations can also become genetically isolated even without geographic isolation and this is known as sympatric speciation. Two species of Howea (H. belmoreana and H. forsteriana) occur on Lord Howe Island, a small subtropical island of less than 12 km2, situated 580 km off the eastern coast of Australia. The island was formed by volcanic activity about 6.9 million years ago. Data on flowering time show that these two species are reproductively isolated with H. forsteriana flowering before H. belmoreana. Flowering-time differences seem to be directly influenced by substrate-induced physiological changes, where the former species prefers calcarenite, a recent basic sedimentary rock formation that dominates low-lying parts of the island.
Next: Monkeyflowers (Mimulus spp.)
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