Comparative phylogeography of oceanic archipelagos: Hotspots for inferences of evolutionary process

Abstract

Remote island archipelagos offer superb opportunities to study the evolution of community assembly because of their relatively young and simple communities where speciation contributes to the origin and evolution of community structure. There is great potential for common phylogeographic patterns among remote archipelagos that originate through hotspot volcanism, particularly when the islands formed are spatially isolated and linearly arranged. The progression rule is characterized by a phylogeographic concordance between island age and lineage age in a species radiation. Progression is most likely to arise when a species radiation begins on an older island before the emergence of younger islands of a hotspot archipelago. In the simplest form of progression, colonization of younger islands as they emerge and offer appropriate habitat, is coincident with cladogenesis. In this paper, we review recent discoveries of the progression rule on seven hotspot archipelagos. We then discuss advantages that progression offers to the study of community assembly, and insights that community dynamics may offer toward understanding the evolution of progression. We describe results from two compelling cases of progression where the mosaic genome may offer insights into contrasting demographic histories that shed light on mechanisms of speciation and progression on remote archipelagos.

Keywords: community assembly; priority effect; progression rule; radiation zone; speciation.

Fig. 1.

Hypothetical progression patterns. (A) A simple progression pattern with cladogenesis coincident with interisland migration. Hatch marks show interisland migrations. (B) A complex progression showing clades, grades, and back migration. Hatch marks show interisland migrations. Colors correspond to islands: green, Kauai; orange, Oahu; purple, Molokai; blue, Maui; and red, Hawaii.

Fig. 2.

Map of seven archipelagoes that show a variable geological age chronology. (Inset) Relationship between island age and distance from the hotspot for each archipelago (estimated in the case of the Azores) (106). This reflects the match of the geographical arrangement of islands to their age, with tighter relationships in archipelagoes such as that of Hawaii, in which islands are produced in a conveyor belt fashion. The figure includes only islands of area >10 km², as smaller islands tend to be atolls with communities limited to coastal strand. Trend lines are shown (except for the Azores in which there is no clear trend line). Background image reproduced from the GEBCO world map 2014, www.gebco.net.

Fig. 3.

Progression and the mosaic genome. (A) Area cladogram derived from the nuclear AFLP phylogeny of Laupala (36), a genus of flightless crickets endemic to Hawaii. Two species groups are shown (green discs overlaying resolved nodes). An overall progression is evident, with two progression subpatterns in each species group. Minimum interisland migrations are marked by K (Kauai), O (Oahu), Mo (Molokai), Ma (Mauai), and H (Hawaii). Additional abbreviations include OG (outgroup), EMa (East Maui), and WMa (West Maui). A neighbor joining tree based on Nei-Li distances is shown. Terminal taxa are species represented by 2–10 individuals color coded to the island to which they are endemic. Bootstrap values are shown for 1,000 neighbor joining replicates below branches. (B) An area cladogram derived from the mtDNA phylogeny of Laupala (101). Shown is the maximum parsimony tree with the highest likelihood score; terminal taxa are unique sequences from concatenated 12s, 16s, and tRNA^val regions. The mtDNA topology is not congruent with the AFLP tree, and species groups are not evident in the mtDNA tree. An overall progression is evident, with two back migrations marked by blue discs. One possible minimum interisland migration scenario is shown, marked by K (Kauai), O (Oahu), Mo (Molokai), Ma (Maui), and H (Hawaii). Bootstrap values are shown for 1,000 parsimony replicates below branches.