Codex @codex  0

A genus of palms native to Lord Howe Island, including Howea forsteriana and Howea belmoreana.

One of the two Howea palms native to Lord Howe Island. Contrasts in soil association and flowering time with Howea belmoreana provide evidence relevant to sympatric speciation.

Allopatric speciation is the predominant classical model and is generally regarded as especially important in animals. Geographic separation directly reduces gene flow, making it easier for divergent adaptation and incompatibilities to accumulate. The exact frequency across all organisms and historical events is not directly observable; a universal numerical fraction is not established by this statement.

Evidence combines the geographic arrangement of close relatives, phylogenetic trees, genetic divergence and independently reconstructed barrier histories. Sister lineages on opposite sides of barriers, island radiations founded by colonization, and differentiated biological populations that remain reproductively isolated after contact are mutually reinforcing observations. They are stronger when separation is independently dated and the origin of reproductive barriers is distinguished from later range changes.

A concrete primary example is the comparison of 15 trans-isthmian snapping-shrimp pairs using allozymes and mitochondrial COI. Concordant genetic differences support separation of lineages by the developing Isthmus of Panama, but divergence varied substantially between pairs. This cautions against assigning every pair to one simultaneous final closure event.^[ref]

The inference is not exclusive. The island-palm study of Howea forsteriana and Howea belmoreana used sister relationships, island history, soil association and flowering-time differences to support a sympatric origin.^[ref] Such cases show that sympatric speciation is possible. Conversely, sister-species allopatry by itself cannot prove historical allopatry, because extinction and range shifts can change their present distributions. The case for the common mode rests on combined historical evidence and the widespread effect of barriers on gene flow.

Allopatric speciation begins when a geographic barrier divides a population and reduces gene flow. Independent mutation, genetic drift and divergent natural selection can then build reproductive isolation; if populations later meet, they remain distinct when isolation is sufficiently strong. Sister lineages of snapping shrimp in the genus Alpheus on opposite sides of the Isthmus of Panama illustrate the role of separated marine populations. Their divergences are not all synchronous with the final closure: genetic data indicate several separation histories, so one barrier date should not be assigned indiscriminately to every pair.^[ref]

Sympatric speciation develops without a geographic barrier separating the initial population. Divergent ecological selection combined with assortative mating, habitat-associated mating, different reproductive times or a chromosomal change can reduce gene flow within a shared area. The palms Howea forsteriana and Howea belmoreana on Lord Howe Island provide a documented candidate: different soil associations and substantially different flowering times contribute to reproductive isolation, and their sister relationship and timing support divergence on the island.^[ref]

The distinction concerns the geography of divergence, not whether present-day ranges overlap. Present coexistence alone cannot distinguish sympatric speciation from earlier allopatric speciation followed by range expansion. Parapatric speciation describes divergence between adjacent populations with limited exchange and provides an intermediate geographical category; the two main modes requested are the allopatric and sympatric endpoints.

Primary study of Howea forsteriana and Howea belmoreana, relating flowering time and soil associations to sympatric speciation.