Speciation
Speciation occurs when different populations of a single species become reproductively isolated and evolve independently, leading to the formation of new species. Repeated speciation events are responsible for the extensive diversity of life on earth today.
Use this page to revise the following concepts within speciation:
Speciation can happen through two primary mechanisms: allopatric speciation , where the reproductive isolation is due to a physical barrier, and sympatric speciation , where there is no physical barrier.
Allopatric Speciation
Allopatric speciation occurs when different populations of a species become geographically isolated from one another, typically by natural barriers such as mountains, rivers, or oceans. This physical separation prevents gene flow between the populations. Over time, each group experiences different environmental pressures and accumulates genetic differences through mutation, natural selection , and genetic drift . As these differences increase, the populations may eventually become so distinct that they can no longer interbreed, resulting in the formation of new species.
Galapagos finches as an example of allopatric speciation
Charles Darwin’s study of finches on the Galapagos Islands is a classic example of allopatric speciation. A single species of finch likely arrived on the islands and then became isolated on different islands. Over time, each population adapted to its specific environment, with distinct beak shapes and sizes suited to the type of food available on each island. These adaptations eventually led to the different species of galapagos finches studies by Darwin.
Sympatric Speciation
Sympatric speciation occurs when new species evolve from a single ancestral species within the same geographic region, without physical barriers to separate populations. Instead, reproductive isolation arises through the accumulation of ecological, behavioral, or genetic differences. Unlike allopatric speciation, where geographic barriers prevent gene flow, sympatric speciation occurs despite close proximity, relying instead on shifts in local habitat preference, changes in mating behavior, or genetic mutations that reduce interbreeding.
These barriers to reproduction in sympatric speciation can be divided into prezygotic and postzygotic barriers. Prezygotic barriers prevent mating or fertilisation (the creation of a zygote) from occurring while postzygotic barriers prevent successful interbreeding despite fertilisation occuring.
Howea Palms on Lord Howe Island as an example of sympatric speciation
The Howea palms on Lord Howe Island, located off the coast of Australia, provide an example of sympatric speciation. Despite inhabiting the same small geographic area, two distinct species, Howea forsteriana and Howea belmoreana, have evolved in response to subtle ecological differences, particularly variations in soil type.
Lord Howe Island features a range of soils, from nutrient-rich volcanic soils to more acidic, less fertile calcareous soils. H. forsteriana predominantly grows in the fertile volcanic soils found in the lower regions of the island while H. belmoreana can grow in both the fertile volcanic soil and the more acidic and less fertile calcareous soils of the island. In the richer volcanic soils, however, H. forsteriana often outcompetes H. belmoreana. Over time, this separation by habitat preference facilitated reproductive isolation despite the lack of physical barriers.
The mechanism of the prezygotic isolation between the two palm species is temporal isolation. The flowering times of Howea forsteriana and Howea belmoreana differ slightly, with H. forsteriana flowering earlier in the year and H. belmoreana flowering later. Thus, even though the species grow in close proximity, their reproductive events do not overlap. As a result, cross-pollination is limited, preventing gene flow between the two populations. Over time, this reproductive isolation has contributed to the accumulation of genetic differences, ultimately leading to the divergence of the palms into two distinct species.
