John C. Briggs

COINCIDENT BIOGEOGRAPHIC PATTERNS : INDO-WEST PACIFIC OCEAN

The majority of tropical marine families demonstrate their greatest concentration of species within the relatively small East Indies Triangle. In every direction, the species diversity decreases with distance from the East Indies. Other patterns suggest that the East Indies is where the average generic age is youngest, where some historical routes of dispersal originate, where the most apomorphic species occur, where genetic diversity is the greatest, and where extinctions are likely to originate. These coincident patterns provide support for the hypothesis that the East Indies has been operating as a center of evolutionary radiation. The driving force for this dynamic system is apparently the predominance of successful speciation involving relatively large populations with higher genetic diversity. This mechanism fits the centrifugal speciation model that was proposed more than 50 years ago.

Fishes of Worldwide (Circumtropical) Distribution

N these days of concentration on fishes of relatively restricted areas, by means of regional checklists, circumscribed faunal works, and geographically limited revisions, one is apt to forget that many fishes are relatively unrestricted in distribution, and therefore cannot be said to belong to any one zoogeographic area. Although they may tend to interfere with attempts to give precise definition to the various zoogeographic regions, such species must not be overlooked, as they form an undeniable and interesting part of the general pattern of the dispersal of marine fishes.

Fishes and Birds: Gondwana Life Rafts Reconsidered

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The Historic Biogeography of India: Isolation or Contact?

Geophysical maps depicting continental movement have consistently shown India, as it moved northward, to be located far out in the Tethys Sea. India split off from the African east coast about 148 m.y.a. From that time onward, according to almost all geophysical accounts, India was isolated from all of other continents until the early Miocene when it made contact with Eurasia. But the biological data, both fossil and Recent, indicate that this concept cannot be correct. If India had really existed as an isolated, oceanic continent for about 100 m.y., it should have developed a peculiar biota with many endemic genera and families in its terrestrial and shallow marine habitats. But there are virtually no remains of organisms indicating that India was isolated for any substantial time (millions of years). Instead, we find that almost all Indian taxa were possessed in common with other continents. As time went on, the northern relationships became stronger and the southern ones weaker. Most of the recent geophysical accounts show India not making contact with Eurasia until the early Miocene, but fossil materials show that this event must have taken place by the early Eocene. It has been postulated that, as India moved northward, it created a biogeographic barrier that separated marine fish populations and resulted in the east-west provinces that are now apparent in the Indian Ocean. At the same time, the barrier effect was supposed to have resulted in the formation of sister species that are now located far apart. Information currently available indicates that most living, tropical marine species are probably not over 3 m.y. old. Consequently, the northward movement of India, which took place primarily between 148 and 50 m.y.a., could have no bearing on the relationships of modern species.

Comparative phylogeography of the ocean planet.

Understanding how geography, oceanography, and climate have ultimately shaped marine biodiversity requires aligning the distributions of genetic diversity across multiple taxa. Here, we examine phylogeographic partitions in the sea against a backdrop of biogeographic provinces defined by taxonomy, endemism, and species composition. The taxonomic identities used to define biogeographic provinces are routinely accompanied by diagnostic genetic differences between sister species, indicating interspecific concordance between biogeography and phylogeography. In cases where individual species are distributed across two or more biogeographic provinces, shifts in genotype frequencies often align with biogeographic boundaries, providing intraspecific concordance between biogeography and phylogeography. Here, we provide examples of comparative phylogeography from (i) tropical seas that host the highest marine biodiversity, (ii) temperate seas with high productivity but volatile coastlines, (iii) migratory marine fauna, and (iv) plankton that are the most abundant eukaryotes on earth. Tropical and temperate zones both show impacts of glacial cycles, the former primarily through changing sea levels, and the latter through coastal habitat disruption. The general concordance between biogeography and phylogeography indicates that the population-level genetic divergences observed between provinces are a starting point for macroevolutionary divergences between species. However, isolation between provinces does not account for all marine biodiversity; the remainder arises through alternative pathways, such as ecological speciation and parapatric (semiisolated) divergences within provinces and biodiversity hotspots.

A Cretaceous-Tertiary Mass Extinction?Were most of Earth's species killed off?

For the past decade, the scientific and popular press have carried frequent articles about a catastrophic mass extinction that supposedly destroyed the majority of the earths species, including the dinosaurs, approximately 65 million years ago. Since 1980, more than 2000 papers and books have dealt with some aspect of a mass extinction at the Cretaceous-Tertiary (K/T) boundary. One authoritative estimate of the severity of the extinctions is that 60-80% of all the living species became extinct at this boundary (Raup 1988). There appears to be a general acceptance of the fact that such a great catastrophe did occur. Most of the argument among scientists now is devoted to the determination of the cause. In this article, I argue that the species changes at the K/T boundary were neither sudden nor catastrophic. They were most likely caused by a regression of sea level that led to a decrease in primary production.

The marine East Indies: centre of origin?

Much of the argument about the existence or nonexistence of centres of origin has been focused on the marine East Indies. During the past 10 years, considerable new information has become available. Patterns suggesting historic dispersals from the East Indies have been described. It is now apparent that there is, in several animal groups, an increasing trend in average generic age from the East Indies outward. Paleontological studies have revealed significant onshore to offshore evolutionary trends during much of the Phanerozoic. Antitropical and other disjunct patterns in the Indo-West Pacific suggest that the ex- tinction process often begins in the East Indies and spreads outward. Widespread Indo-West Pacific species, that possibly originated in the East Indies, oc- casionally surmount biogeographic barriers to colo- nize adjacent regions. The biogeographic patterns described are compat- ible with, and provide support for, the centre of origin hypothesis. Successful evolution, which results in the formation of new phyletic lines, takes place in the nearshore waters of centres of origin. From such centres, new species, which may eventually give rise to new higher taxa, spread out both vertically and hor- izontally. As they spread vertically, they replace older forms from the mid- and outer shelf and eventually from the slope. As they spread horizontally, they replace their more generalized, peripheral relatives, leaving as evidence the historic dispersal and age pat- terns which have been noted.

The North Atlantic Ocean: Need for Proactive Management

Abstract Among the major oceans of the world, the North Atlantic presents a unique problem in regard to the management of its commercial and recreational fisheries. Almost all of its populations of large-sized, predatory fishes have collapsed to the extent that they no longer play an effective role in the ecosystem. There is a fundamental difference in the biodiversity and present state of the fisheries between the North Atlantic and the North Pacific. Much of the difference is attributable to their contrasting geological and climatic histories. The two oceans had been effectively separated until about 3.5+ million years ago, when the Bering Strait was formed permitting the Great Trans-Arctic Biotic Interchange. During the interchange, hundreds of Pacific species migrated to the Atlantic but only a few in the opposite direction. Although the Atlantic benefitted from the interchange, its species diversity remained relatively low. Today, the North Atlantic, in comparison to the North Pacific, possesses only...

The Sea-Level Panama Canal: Potential Biological Catastrophe

While the possibility of a sea-level This land barrier is also effective for Atlantic. While the Indo-West Pacific canal somewhere in the vicinity of the marine invertebrates. Haig (1956, undoubtedly serves as the primary Isthmus of Panama has been discussed 1960) studied the crab family Porcelevolutionary and distributional center for many years, its feasibility as an lanidae in both the Western Atlantic (Briggs, 1966), the Western Atlantic engineering project has become enand Eastern Pacific and found that only Region may be said to rank second in hanced as the result of recent experiabout 7% of the species were common importance. Its geographic area is mental work with nuclear devices that to the two areas; de Laubenfels (1936) larger (Fig. 1), its habitat diversity can be used for excavation. It appears found a similar distribution in about greater, and its fauna considerably now that the undertaking of this project 11% of the sponges he studied; and richer than for each of the remaining will be strongly supported as soon as Ekman (1953), about 2.5% for the two regions. Since the Western Atlanthe current economic crisis in the echinoderms. It seems, therefore, that tic species are the products of a richer United States is over. Until recently, only a very small proportion of the and therefore more stable ecosystem, the only facet of the plan that had species in the major groups of marine we may expect that they would prove drawn the attention of many biologists animals are found on both sides of the to be competitively superior to those was the possibility of radiation damage. Isthmus of Panama. The present Panspecies that are endemic to the EastHowever, Rubinoff (1968) finally ama Canal has not notably altered this ern Pacific or Eastern Atlantic. pointed out that there would be other relationship since, for most of its length, An examination of the faunal relaimportant biological effects and gave it is a freshwater passage forming an tionships between the Western Atlantic examples of disastrous invasions that effective barrier for all but a few and the Eastern Atlantic does provide have occurred in other places as the euryhaline species. good circumstantial evidence that results of human interference. With regard to the tropical waters on species from the former are competitiveThe New World Land Barrier each side of the isthmus, there is no ly dominant. An impressive number reason to suspect that each area is not have managed to traverse the open The IsthmusNew World Lanamd Barrier, with supporting its optimum number of waters of the central Atlantic (The narrowthe Isthmus of Panama complete block toing its species. Studies of terrestrial biotas Mid-Atlantic Barrier) and to establish the movement of tropical marine species have indicated that most continental themselves on the eastern side. For between the Western I Atlantic and habitats are ecologically saturated (Elexample, in the shore fishes there are

Thinking of Biology Biotic replacements—extinction or clade interaction?

Have the changes which lead us from one geological state to another been, on a lang average, uniform in their intensity, or have they consisted of epochs of paroxysmal and catastrophic action, interposed hetween periods of comparative tranquillity? These two opioions will probahly for some time divide the geological world ioto two seets, which may perhaps he designated as the Uniformitarians and the Catastrophists (WheweIl1832).