Biogeography and Evolution in New Zealand

Michael Heads

• Publisher: CRC Press
• Year: 2016
• Print ISBN: 9780367658380
• eBook ISBN: 9781315351216
• Topic: Biological Sciences
• Subtopic: Ecology
• Index: Biological Sciences

Biogeography and Evolution in New Zealand provides the first in-depth treatment of the biogeography of New Zealand, a region that has been a place of long-enduring interest to ecologists, evolutionary scientists, geographers, geologists, and scientists in related disciplines. It serves as a key addition to the contemporary discussion on regionalization—how is New Zealand different from the rest of the world? With what other areas does it share its geology, history, and biota? Do new molecular phylogenies show that New Zealand may be seen as a biological 'parallel universe' within global evolution?

1

The Spatial Basis of Biogeography

To do science is to search for repeated patterns, not simply to accumulate facts, and to do the science of geographical ecology is to search for patterns of plant and animal life that can be put on a map.

MacArthur, 1972: 1

Until recently, phylogeography has not taken its “geography” base seriously enough… particularly in light of rampant, blind application of molecular clock methodologies to complex geographic and historical questions…. The result is a field that seriously needs to broaden its vision…. A fundamental concern is that phylogeographic approaches have focused largely on the “phylo” part and less on the “geography.”

Peterson and Lieberman, 2012: 1

This book discusses many distribution patterns seen in and around New Zealand, and explanations are suggested for most of these. The Earth and its living layer exist together and interact in complex ways that are studied by ecologists. In addition, the Earth and its life evolve together. This concept was controversial when it was first proposed (Croizat, 1964) and is still disputed (McDowall, 2010a,b), but it has gained wide acceptance and is now often quoted as an anonymous maxim (e.g., Carroll, 2010: 126). Many aspects of distribution—elevational and latitudinal ranges, for example—display obvious correlations with aspects of the environment. Yet many distributions are not explained by current ecological variables. These probably result from changes over evolutionary time, and they have been accounted for either by Earth’s history, as in Croizat’s (1964) model, or by chance dispersal, a process that does not depend on geological change.

The Spatial Component of Evolution: Basic Concepts and Methods of Analysis

Biological groups, especially close relatives, are often allopatric; that is, they occupy different areas. Allopatric groups may be separated by a gap (they are “disjunct”), but their distributions often meet without any obvious physical barrier between them. In other groups, the distributions overlap—they show varying degrees of sympatry. Biogeography needs to explain both allopatry and sympatry, but allopatry is a distinctive, fundamental phenomenon seen in most groups, and many authors have accepted its primary significance in evolution. Among the New Zealand gentians, for example, “allopatry is much more frequent than sympatry” (Glenny, 2004).

Species Concepts

The concept of species is controversial, and the particular concept that is used in a biological analysis is often regarded as critical. This book uses the species concept outlined by Darwin (1859: 32): “I look at the term species, as one arbitrarily given for the sake of convenience to a set of individuals closely resembling each other, and that it does not essentially differ from the term variety.” This is very different from the neodarwinian concept, in which species have a fundamental importance. In Darwin’s model, species are not regarded as special compared with groups at other ranks, above or below the species level (Heads, 2014). As Dennett (1996: 95) wrote, discussing the definition of species: “Where should we draw the line? Darwin shows that we don’t need to draw the line in an essentialist way to get on with our science… I am inclined to interpret the persisting debates [about species definitions] as more a matter of Aristotelan tidiness than a useful disciplinary trait.” Dobzhansky (1937: 312) argued in a similar vein: “Species is a stage in a process, not a static unit.” (In contrast, the neodarwinian view stressed that “a species is not a stage of a process, but the result of a process” [Mayr, 1942: 119].)

In the Darwinian view, accepted here, the key problem in evolution and biogeography is not speciation, but the process of differentiation in general, whether at, above, or below the level of species. In cases of clinal variation, there are no distinct groups at any rank, although clines are just as significant for biogeography as distinct phylogenetic breaks and often occur at the same locality.

Dispersal and Vicariance

In the traditional model, all groups attain their distribution by dispersal, radiating out from a localized center of origin (Matthew, 1915). In a vicariance model, two or more groups instead attain their individual distributions, not by expanding outward from a center of origin but by the division (vicariance) of a widespread ancestor. (Of course, all groups originate in a particular area or center, even in a vicariance event. But the term “center of origin,” as used in the literature and in this book, refers to the restricted area inferred in the first model.)

For example, consider a genus comprising one species in the east of a continent and one in the west, with the two meeting in the middle. In dispersal theory, each species originated at an independent point and has spread out by dispersal from there, eventually meeting the other species. In a vicariance interpretation, an ancestor that was widespread in both the east and west of the continent has split down the middle, and the mutual boundary of the species represents this fracture. Each species has evolved over a broad front and neither has attained its distribution by spread from a “center of origin.”

On an even larger scale, a global group may divide into, say, northern hemisphere and southern hemisphere descendants, neither of which has migrated to its current position. The two main phylogenetic groups, or clades, of the cypress family, Cupressaceae, provide a good example. The subfamily Cupressoideae is regarded as Laurasian, while the subfamily Callitroideae is Gondwanan, and the break between the two is attributed to vicariance (Mao et al., 2012; Yang et al., 2012). Vicariance has been defined as “allopatric (geographical) speciation caused by the origination of a barrier within the range of the ancestral species” (Crisp et al., 2011; italics added); it can also lead to differentiation above (or below) species level.

Darwin (1859: 352) established modern dispersal theory in the first edition of the Origin of Species. The idea proposes that if a group is present in more than one area, it must have moved to all these areas (except for one—the center of origin). In the fifth edition of his book, Darwin (1869: 467) emphasized this view, arguing that “not only all the individuals of the same species, but that allied species, although now inhabiting the most distant points, have proceeded from a single area—the birthplace of their early progenitor.” This view denies the possibility of vicariance, in other words, that the group has been present in more than one area ever since its origin, and that it evolved by diverging from another group that occupied another area. In both the Darwinian and the neodarwinian syntheses, allopatric differentiation is caused by chance dispersal from a center of origin, while the overlap of clades is also caused by dispersal. In vicariance biogeography, allopatric differentiation is caused by vicariance, while overlap is caused by dispersal.

The Modern Synthesis (Huxley, 1942; Mayr, 1942) has provided the evolutionary paradigm for most studies over the last 70 years, and its approach to evolution in space and time is based on Darwin’s ideas as presented in the work of Matthew (1915). Allopatry is explained by dispersal, and vicariance is regarded as very rare. Mayr (1965) concluded: “Quite obviously, except for a few extreme [i.e., local] endemics, every species is a colonizer because it would not have the range it has, if it had not spread there by range expansion, by ‘colonization’, from some original place of origin.” New clades did not inherit their range from their ancestor.

Most biologists and also leading philosophers have followed Mayr. In a discussion on the New Zealand flora, Winkworth et al. (1999: 1324) wrote that the “extraordinary evolutionary importance of recent long-distance dispersal” is “clearly evident from molecular data”; it is “obvious.” McGlone et al. (2001: 209) argued that “long-distance dispersal has played a key role in the development of the New Zealand flora, and… is important in explaining patterns of geographic distribution and endemism within the archipelago.”

Dennett (1996: 99) wrote that in many cases, “perhaps almost all,” speciation occurs by a process “in which a small group—maybe a single mating pair—wander off and start a lineage that becomes reproductively isolated.” In dispersal theory, the groups of a region are there because they have dispersed there, not because they evolved there.

Distribution patterns often display striking repetition even among groups with different means of dispersal and ecology. These could all be the result of chance dispersal and adaptation, as theory suggests. Yet the principle of multiple working hypotheses (Chamberlin, 1965; cf. Mill, 1869) indicates that it would be desirable to have an alternative explanation—any explanation—that accounts for the evidence and does not break the rules of logic. In practice, a vicariance model provides an alternative. This was rejected by Mayr (1982), but views have changed since the topic began to be discussed in the 1970s. The importance of vicariance is now acknowledged in an increasing number of studies (more than 2000 in 2014; Google Scholar).

Until the 1970s, biogeographic studies assumed that a center of origin and chance dispersal (along with extinction) explained the distribution of most groups. In contrast, most models now use both chance dispersal and vicariance. The approach favored here relies less on particular events in single groups and instead proposes alternating, community-wide phases of vicariance and range expansion using normal means of dispersal.

“Dispersal biogeography” is a research program that explains the geographic distribution of organisms by chance dispersal, while “vicariance biogeography” explains distribution with reference to geological events (Gillespie et al., 2012). Ree and Smith (2008) observed that “historical biogeography is grounded in the notion that Earth history has profoundly influenced the geographic ranges of species,” but this in fact only applies to vicariance biogeography, where both vicariance and overlap (range expansion) are attributed to geological events. In dispersal biogeography, the same pattern in different groups is attributed to individual dispersal events at different times, not to single causes such as geological events.

Authors sometimes suggest that “vicariance… can occur only after the range of a species has already expanded via dispersal [range expansion]” (Crisp et al., 2011). This will be true in some cases, but in many cases, the direct ancestor will itself have attained its wide distribution not by range expansion, but by vicariance, splitting off from a group with an even wider range.

An Example of Vicariance and Subsequent Dispersal: Psilotaceae

The fern family Psilotaceae comprises two genera:

Tmesipteris: Throughout mainland New Zealand, the Auckland Islands, New Caledonia, and the eastern seaboard of Australia, north to New Guinea and the Philippines, and east to Fiji, Samoa, the Society Islands, and the Marquesas Islands.

Psilotum: Pantropical, south to the central North Island.

Tmesipteris occurs on its own in the southern parts of its range (Tasmania, southern North Island, South Island, Stewart Island, Auckland Islands), while Psilotum occurs on its own through most of the tropics. The two genera overlap in an area extending from the Philippines south to the central North Island and east to the central Pacific, but despite this, the two genera display widespread allopatry.

One dispersal account of the group suggested that Tmesipteris had a restricted center of origin, from which it “probably spread through much of Australia and what is now New Zealand and New Caledonia prior to the breakup of Gondwanaland” (Brownsey and Lovis, 1987: 445). Still, the authors admitted that its absence from South America is then “not readily explained.” A dispersalist study of Psilotum based on phylogeny would also find a localized center of origin for that genus.

In contrast with the center-of-origin approach, a vicariance model accepts that the absence of Tmesipteris from South America, for example, is only relative; the genus as such is absent, but it is represented there by its closest ally, or sister group, Psilotum. The two genera show a high level of allopatry, and this is consistent with their origin by vicariance. Psilotum is pantropical, suggesting that the initial break with Tmesipteris occurred somewhere in their overlap zone, Philippines–New Zealand–central Pacific, a region that has undergone extreme tectonic disturbance through most of the Phanerozoic. Following their allopatric differentiation, subsequent overlap of the two genera can be explained by range expansion (dispersal) in one or both genera, but there is no need to postulate any range expansion of either genus outside this region. How the Psilotum + Tmesipteris common ancestor attained its distribution is another question, which, as usual, would need to consider the group’s sister.

Two Different Concepts of “Dispersal”

The term “dispersal” has many different meanings, including daily migration, annual migration, colonization of areas within the group’s range, range expansion, chance dispersal involving speciation, and other processes. Because of this ambiguity, it is not surprising that discussions and debates about “dispersal” can be confusing. Two of the processes termed dispersal are of special relevance here.

“Normal” Dispersal: An Observed Ecological Process

“Normal” dispersal—the physical movement of organisms—is observed every day. Normal dispersal can take place over short distances, as in small, flightless invertebrates, or very long distances, as in seabirds, but this dispersal in itself does not lead to speciation. In a typical example, weeds may invade a garden, but they do not speciate there. The details of “normal dispersal” tend to be discussed in the ecology literature. Ecologists use the term “long-distance dispersal” to mean the normal, long-distance movement of individuals, without implying that there has been any speciation.

Chance Dispersal: An Inferred Mode of Speciation

In contrast with normal, observed dispersal, the inferred process of “chance dispersal” often refers to a mode of speciation, and the term is used in this way in texts on biogeography, systematics, and evolution. Crisp et al. (2011) defined “long-distance dispersal and establishment” as “allopatric (geographical) speciation caused by an exceptional dispersal event….” In this process, an individual disperses by chance over a feature that otherwise acts as a barrier for the group. The concept is not the same as the normal long-distance dispersal observed in, say, albatrosses, which occurs every day and does not lead to speciation or to any taxonomic differentiation.

Long-distance dispersal leading to speciation has not been observed. Authors often infer long-distance dispersal, calculated using phylogenies and center-of-origin programs, as “demonstrated” or “revealed” (e.g., Voelker et al., 2014; Müller et al., 2015), but this is true only in a theoretical framework, unlike observations on normal, ecological dispersal. A phylogeny indicates relationships and spatial patterns, not the mode of differentiation or speciation.

In many cases, “long-distance dispersal” is not an accurate description of speciation by chance dispersal, as the distance thought to have been crossed is short. Long-distance dispersal is often proposed to account for trans-oceanic affinities, but it is also proposed for differentiation across rivers (e.g., in the primates of the Amazon basin) and even smaller barriers. The key concept in speciation by long-distance dispersal is not the long distance, but the exceptional, unpredictable, or chance nature of the process. Only one or a few founders disperse across the barrier, the rest of the group does not. Thus, the process of long-distance dispersal is sometimes described as “chance dispersal” or “jump dispersal.”

Whatever term is used, the process of chance dispersal has often been proposed for New Zealand plants and animals. Landis et al. (2008) (reiterating the conclusions of Campbell and Landis, 2001, and Waters and Craw, 2006) considered that “the entire New Zealand terrestrial biota actually became established by acc...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Dedication
• Table of Contents
• Preface
• Acknowledgments
• Abbreviations
• 1. The Spatial Basis of Biogeography
• 2. Analyzing the Timeline of Evolution
• 3. New Zealand Geology
• 4. Introduction to the New Zealand Biota and Its Geography
• 5. Biogeography of Northern New Zealand: The Offshore Islands, the Northland–East Coast Allochthon, and the Taupo Volcanic Zone
• 6. Biogeography of New Zealand’s Subantarctic Islands and the Chatham Islands
• 7. Biogeography of Stewart Island and the Southern South Island: Mesozoic–Paleogene Geology
• 8. Biogeography of the Northern South Island, and Its Mesozoic–Paleogene Geology
• 9. Biogeography and Neogene Geology of Mainland New Zealand: Alpine Fault Strike-Slip, Kaikoura Orogeny, and Pleistocene Glaciation
• 10. Case Studies of New Zealand Plants
• 11. Some More Case Studies of New Zealand Plants
• 12. Case Studies of New Zealand Animals
• 13. Structural Evolution and Ecology
• 14. Case Studies of a Trend: Morphological Reduction and Fusion Series
• 15. Biogeography and Evolution in New Zealand Birds
• 16. Biogeography and Evolution in New Zealand Bats
• 17. Conclusions
• Glossary of Geological Terms
• References
• Index