The Howea palms of Lord Howe Island are one of the best-supported examples of sympatric speciation.Lord Howe Island is a small, isolated volcanic island in the Tasman Sea, about 600 km east of Australia. Two palm species in the genus Howea are endemic to it:1. Howea forsteriana (the kentia palm, a popular houseplant).2. Howea belmoreana (the curly palm).DNA evidence shows the two species are each other's closest relatives and evolved from a common ancestor on the island itself. The island is far too small for a geographical barrier to have separated the populations, so the speciation must have been sympatric.The explanation is based on soil type and temporal isolation:1. Much of the island has acidic volcanic soil, but some areas have alkaline calcareous soil (formed from coral sand).2. Some ancestral palms grew on the calcareous soil. The change in soil affected their physiology and shifted their flowering time.3. H. forsteriana, which grows mainly on calcareous soil, now flowers about six weeks earlier than H. belmoreana, which grows on volcanic soil.4. Because pollen from one species is not available when the other species' flowers are receptive, the two groups rarely cross-pollinate. Gene flow is reduced, so the gene pools diverge.
Natural selection then adapted each species to its soil and conditions, and the differences in flowering time were reinforced. This shows how temporal isolation can separate two species that grow side by side.
Natural selection then adapted each species to its soil and conditions, and the differences in flowering time were reinforced. This shows how temporal isolation can separate two species that grow side by side.