Interpretation of patterns of genetic variation in endemic plant species of oceanic islands

Abstract

Oceanic islands offer special opportunities for understanding the patterns and processes of evolution. The availability of molecular markers in recent decades has enhanced these opportunities, facilitating the use of population genetics to reveal divergence and speciation in island systems. A common pattern seen in taxa on oceanic islands is a decreased level of genetic variation within and among populations, and the founder effect has often been invoked to explain this observation. Founder effects have a major impact on immigrant populations, but, over millions of years, the original genetic signature will normally be erased as a result of mutation, recombination, drift and selection. Therefore, the types and degrees of genetic modifications that occur must often be caused by other factors, which should be considered when explaining the patterns of genetic variation. The age of the island is extremely important because oceanic islands subside on their submarine plates over time. Erosion caused by wind, rain and wave action combine to grind down soft volcanic substrates. These geomorphological events can have a dramatic impact on population number and size, and hence levels of genetic diversity. The mode of speciation is also of significance. With anagenesis, genetic variation accumulates through time, whereas, with cladogenenesis, the gene pool splits into populations of adaptively radiating species. Breeding systems, population sizes and generation times are also important, as is hybridization between closely related taxa. Human disturbance has affected plant population number and size through the harvesting of forests and the introduction of invasive plants and animals. Therefore, the explanation of the observed levels of genetic variation in species of oceanic islands requires the consideration of many interconnected physical, biological and anthropomorphic factors.

Do island plant populations really have lower genetic variation than mainland populations? Effects of selection and distribution range on genetic diversity estimates

“…The widespread distribution range of Periploca in the Canaries and our genetic results are consistent with the idea that successful colonization across and within islands translates into large numbers of demographically stable populations (Table ) that tend to accumulate neutral genetic variation by mutation, immigration and recombination (Frankham ; Stuessy et al . ). Accordingly, it could be hypothesized that different distribution ranges in island endemics may indicate different potential for colonization and population dynamics, which, in most cases, appears to have a reflection in GD levels.…”

“…; Stuessy et al . ). Thus, some studies suggest that increased distance to the mainland causes levels of within‐population GD to decrease, as a consequence of founder events coupled with limited dispersal from source areas (e.g.…”

“…These features, along with other ecological conditions common to most islands (temperate climates, reduced levels of herbivory and competitors), can favour population establishment, persistence and, ultimately, large effective sizes, thus increasing genetic variability within populations (Frankham ; Stuessy et al . ). Because plant species have been poorly represented in earlier meta‐analyses, and many examples of island taxa showing little GD were narrowly distributed endemics (e.g.…”

Geographical distance and barriers explain population genetic patterns in an endangered island perennial

“…Therefore, L. watsoniana is not likely to be a neopolyploid, although formation of new cytotypes and their demographic establishment have been recorded for ocean islands (Crawford et al . 2009; Stuessy et al . 2014).…”

O gênero Peperomia da Ilha da Trindade : um caso de isolamento?

“…ser mais próxima filogeneticamente de P. glabella nos permite inferir que P. sp. originou-se a partir de Peperomia glabella, num evento conhecido como anagênese, onde uma população original chega a uma ilha podendo se ampliar e espalhar, acumulando modificações morfológicas e genéticas, sendo considerada taxonomicamente e biologicamente uma nova espécie (Stuessy et al 2006(Stuessy et al , 2014. Com o passar do tempo e devido ao isolamento geográfico essas mutações genéticas e morfológicas foram se acumulando de forma que a população fundadora de Peperomia glabella oriunda do continente não é mais P. glabella e sim Peperomia sp.…”