UC Berkeley

The diversity of the biota of any place and time is often the product of a long and complex history of ecological and evolutionary assembly through the fundamental processes of immigration, speciation and extinction. This biotic assembly typically occurs amid a backdrop of constant abiotic and biotic change at multiple spatiotemporal scales. Given this potentially complex abiotic and biotic history, investigating the core processes that underlie the assembly of regional biotas, as well as the abiotic, biotic and historical factors that influence them, is a major challenge at the intersection of biogeography, macroevolution and ecology.

Oceanic islands provide ideal opportunities to investigate the processes underlying the assembly of regional biotas. Given their volcanic origins, their landscapes represent initial blank slates that are then populated by successful long-distance dispersal events by various organisms, and the subsequent diversification of some of these colonist lineages. Furthermore, their relatively predictable geologic histories and lifespans (i.e., ontogenies) make the study of how the diversification of lineages are shaped over geologic time-scales especially tractable, compared to the more complex abiotic and biotic histories that often shape continental, mainland biotas. Remote hotspot archipelagoes such as that of Hawaii are especially exciting case studies, as they are composed of islands formed in sequence through the movement of the Pacific tectonic plate over a relatively stationary mantle hotspot, and so that each island captures a record of diversification at different stages. The multiple “snapshots” of diversity provided by the differently-aged islands of the Hawaiian archipelago, as well as their well-defined and quantifiable ontogenies, allow the diversities observed on each of the main islands to be leveraged to understand how diversity on islands change through the lifespan of an island, as if each island were a different point on a time series. In Chapter 1, I investigate whether, and how, the changing geologic backdrop (in this case, island area as each island grows and later subsides) has shaped the diversification (speciation, immigration and extinction) of Hawaiian plant and animal lineages on each of the islands. I used a geologically informed diversity-dependent model of species richness change that incorporates estimates of how island area has changed through time, while also taking into account island-specific ontogenetic differences among the four current main islands / island complexes of the Hawaiian archipelago, and tested my models against alternative models that do not account for island ontogenetic changes. I find that, at the broadest temporal scales, the geologic dynamics of the Hawaiian Islands have had a profound influence on the macroevolutionary history of most of the 14 endemic lineages examined, with all lineages undergoing radiations as islands grow, and most of them now undergoing long-term evolutionary declines as the islands undergo subsidence and erosion with age.

As mentioned above, the isolation and de novo formation of the Hawaiian Islands imposes a strong filter, limiting successful colonization of the islands to a subset of mainland plant and animal groups that have the traits necessary for long-distance dispersal. As a result, it has often been assumed that the Hawaiian Islands often represent a biogeographic sink for many lineages, with colonization of the remote islands representing “dead ends” in the history of dispersal of lineages that prove successful. In some cases, this is certainly true, with some lineages evolving characteristics that reduce dispersal ability relatively quickly upon successful colonization. However, molecular phylogenetic data are slowly challenging this narrative, and it is increasingly demonstrated that the Hawaiian archipelago may instead also play a fundamental role as a biogeographic source for other parts of the Pacific and even mainland areas. In Chapter 2, testing this more nuanced view of the biogeographic history of Hawaiian lineages in the context of the greater Pacific, I constructed a molecular phylogeny of one of the most species-rich angiosperm radiations in the Pacific, Peperomia (Piperaceae), using full plastome sequences generated via a high-throughput shotgun sequencing approach. The final tree consisted of about half the total number of species described in the Pacific. Incredibly, I found that the Pacific has played host to four separate and distinct colonist lineages that originated from the Neotropics, although whether these colonization events occurred through direct dispersal from South and Central America, or via indirect routes of dispersal through the Afro and Paleotropics remain unclear. Nonetheless, in support of the newer view of the role of the Hawaiian Islands in Pacific biogeography, my molecular phylogeny paints a picture of frequent dispersal within the south Pacific, with two independent radiations of Peperomia on Hawaii, and subsequent southwards dispersal of some individuals derived from one of these radiations to the Marquesas Islands. My results emphasize the importance of expanding the biogeographic lens through which many island lineages are typically viewed, and future global-level sampling will shed more light on the role of the islands of the Pacific in generating the diversity of many globally important plant lineages.

Lastly, while my first chapter focuses on the role of landscape dynamism on the tempo of diversification on Hawaii, and my second chapter focuses on the role of the Hawaiian Islands in the biogeographic history of lineages in the greater Pacific, the third chapter of this dissertation instead takes a closer look at whether the climatic niches of lineages may influence the modes of speciation among eight large radiations of endemic plant groups. The central hypothesis is that the climatic niche of lineages may play a role in shaping the inter-island dispersal of species, and hence the geographic isolation of populations on different islands. For instance, a wet forest species on one island may be expected to more easily colonize wet forests on other islands (i.e., climatic niche conservatism). If climatic niche conservatism were a dominant force in shaping the mode of diversification, one may expect cladogenetic events in allopatry to be associated with lower rates of climatic niche evolution relative to cladogenetic events in sympatry (here defined as sister lineages co-occurring on at least one island). To test this idea, I reconstructed the phylogenetic relationships within eight species-rich plant radiations on Hawaii, and inferred the geographic context of all cladogenetic events in each plant group using probabilistic biogeographic models. I then looked at whether cladogenetic events in sympatry (i.e., sister lineages that overlap in biogeographic range) versus cladogenetic events in allopatry (diverged sister lineages that occur on different islands) were associated with higher or lower rates of climatic niche evolution. I found, in a combined analysis of all groups examined, that rates of climatic niche evolution do not appear to be explained by differences in speciation mode. However, two of the plant groups examined (Schiedea and Kadua) show significant but opposing relationships between speciation mode, with lineages of Kadua showing lower rates of niche evolution in allopatry and lineages of Schiedea diverging in allopatry associated with higher rates of niche evolution. Overall, my results suggest that lineage-specific differences likely have a strong influence on rates of climatic niche evolution and their interaction with dispersal and speciation.

Overall, oceanic islands are, and will likely remain, powerful microcosms by which strategies for understanding how mainland biotas are developed. The three chapters of this dissertation highlight the need for a multi-faceted approach in studying the ecological and evolutionary assembly of regional biotas: from its response to geologic changes over long-time scales (Chapter 1), the larger biogeographic context from which biotas are assembled (Chapter 2), to the often context-specific role of climatic niche evolution in shaping the dispersal and speciation of lineages (Chapter 3).