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1 Why Mockingbird Speciation in the Galapagos

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1 Why Mockingbird Speciation in the Galapagos 1 WHY MOCKINGBIRD SPECIATION IN THE GALAPAGOS ARCHIPELAGO IS LIMITED TO FOUR SPECIES ERIKA HARRELL DARWIN, EVOLUTION, AND GALAPAGOS DURHAM OCTOBER 2008 2 Introduction The central question of this study is why mockingbird speciation in the Galapagos is limited to only four species. The answer to this question is significant becauses it pertains to the broader question of what characters of a species or environment make for less selective pressures and more limited speciation and what characters of a species/environment selectively pressure for greater speciation. It also addresses the question of how specific characters of a species and an environment can interact to selectively pressure for broad or limited speciation. I chose to study these questions by observing the mockingbirds of Galapagos because these birds not only exhibit limited speciation, but can conveniently be compared to the finches of the Galapagos, which have speciated significantly more than the mockingbirds. Advantageously, both sets of species are confined to the Galapagos archipelago and experience much habitat overlap. Therefore, by comparing them, it is easy to determine what characters of each and of their environment pressured for or against extreme speciation. My primary hypothesis is that mockingbirds are resourceful generalists and omnivores, so there was no selective pressure on these birds to split into specialists with regard to food consumption. Finches, on the other hand, have a diet that is restricted by the fact that they are chiefly herbivores. Such a restriction is a signicant selective pressure that the mockingbirds simply did not have. To test my hypothesis, I considered experimentation and data gathered by accomplished researchers, and observed the different species of both mockingbirds and finches in the Galapagos wild. 3 Background There are four species of mockingbird on the Galapagos archipelago. All four species are cooperative breeders, meaning young, non-breeding “helpers” aid in feeding and protecting fledglings. (Curry & Grant 1989:441) Each species also remains in its natal territory and, thus, exhibits territorial behavior. All species are enthusiastic omnivores with long, curved “multipurpose” breaks, prefer “hopping” on land to flying, and inhabit the island dry zones. (Curry & Grant 1989: 443) Hood Mockingbird using multi-use beak to resourcefully dig through sand for food on the island of Espanola. (Harrell 2008) 4 Hood Mockingbird pecking at sleeping marine iguana. Probably in search of food. (Harrell 2008) Typical Galapagos Arid/Dry Zone. (Harrell 2008) 5 The most widespread, Nesomimus parvulus, is commonly known as the Galapagos Mockingbird. N. parvulus is found on all islands in the northwestern portion of the archipelago except for Pinzon, where it was most likely outcompeted by feral rats. It is not found on any island which contains another mockingbird species (Floreana, Espanola, San Cristobal). There are eight subspecies of N. parvulus, each of which is found on its own island or cluster of islands. (Curry & Grant 1989: 445) Galapagos Mockingbird on the island of Santa Cruz. (Harrell 2008) Nesomimus macdonaldi, the Hood Mockingbird, is found only on Espanola. It is the largest and most aggressive of the mockingbirds of Galapagos. N. macdonaldi is also the most opportunistic feeder and the most reluctant to fly. (Nelson 1968: 31) 6 Hood Mockingbird, Espanola. (Hedlin 2008) Hood Mockingbird aggression. (Easton 2008) Nesomimus trifasciatus, the Charles Mockingbird, was once found only on Floreana but has since been confined to Floreana’s satellite islands Champion and Gardner due to the introduction of feral predators. There are less than one hundred of these rare birds left in Galapagos. (Harris 1982: 121) 7 Charles Mockingbird (Harrell 2008) Lastly, Nesomimus melanotis, the Chatham mockingbird, can be found on San Cristobal. Its plumage is intermediate between that of the Galapagos and Hood Mockingbirds and it is the shyest species. N. melanotis exhibits less cooperative breeding than the other species and has its nests highest due to predation. (Castro & Phillips 1996: 120) 8 Chatham Mockingbird (Schwarz 2006) There are thirteen species of finch on the Galapagos islands. The founding finch population is thought to have been herbivorous, and according to Darwin’s proposed “principle of divergence” competition for food pressured the finches to their current specialized diets of grubs, ticks, blood, cactus pulp, nectar, leaves, buds, and a wide variety of seeds. (Kricher 2006: 136) The resulting speciation/adapative radiation was much more extreme than that of the mockers. The finches inhabit the same arid zones of each island that the mockingbirds do. (Kricher 2006: 138) Hypotheses The main hypothesis that I considered while examining my research question is that, due to their omnivorous tendencies, there was no selective pressure on the mockingbirds of Galapagos to specialize their diets and, therefore, a low degree of mockingbird speciation occurred. The mockingbirds are a truly hearty set of species and, while researching in Galapagos, I received a 9 firsthand account from a tour guide of Hood Mockingbirds commonly consuming sea lion placenta. One alternate hypothesis is that not enough time has passed since the mockingbirds first reached Galapagos for the birds to have speciated significantly. However, this hypothesis can be dispelled by the fact that mainland mockingbirds demonstrate limited speciation in much the same way that the Galapagos mockingbirds do. Because mockingbirds have narrow speciation globally, time cannot be considered a speciation constraint for the birds. Methods The first measures taken were to research the ancestry of the mockingbirds. After general ancestry was verified, the confluence of wind-driven currents at the archipelago was observed in order to determine how mockingbird colonization of the Galapagos most likely occurred. Next, research was conducted to ascertain the cause of greater mockingbird speciation in the southern islands than in the northern islands. Throughout this series of research, the diets of the finches and the mockingbirds were also verified and compared, as well as the vegetation found in the arid zones of each island in Galapagos. Findings A team composed of B. Arbogast, S. Drovetski, R. Curry, P. Boag, G. Seutin, P. Grant, B. R. Grant, and D. Anderson conducted DNA testing on the mockingbirds revealing that the Galapagos Mockingbird is most closely related to a mainland species known as the Tropical Mockingbird while the Hood, Charles, and Chatham Mockingbirds are most closely related to 10 the Chilean Mockingbird. DNA testing also revealed that the Galapagos Mockingbird was not genetically similar to the Chilean Mockingbird. The Tropical Mockingbird is found in Central America and the Caribeean while the Chilean Mockingbird inhabits the west coast of South America. Further research indicated that the Humboldt Current, Southern Equatorial Current, and the Panama Current intersect at the Galapagos, and that the islands to the south are upwind of the northern islands. (Arbogast 2006: 4) The mockingbird diet was verified to include everything from seeds, fruits, and cactus pulp to insects, spiders, baby birds, eggs, sea lion placentas, unattended tourist food and whatever else the birds found available to them at any given time. (Kricher 2006: 134) Each finch species had its own specialized diet and had adapted traits, particularly with regard to beak size and shape, that are advantageous in the acquisition and consumption of each specific food/foods in its diet. (Kricher 2006: 139) For example, the Large Ground Finch has a very large beak relative to its body size in order to facilitate the crushing of larger seeds. Large Ground Finch on Espanola. (Harrell 2008) According to a study conducted by Robert Bowman, each island of Galapagos has distinct vegetation variation. Bowman’s studies determined that this variation even occurs from island to island within the same ecological zones. (Kricher 2006: 144) 11 Conclusions and Recommendations The Galapagos Mockingbird is not genetically similar to the ancestor of the Charles, Chatham, and Hood Mockingbirds, therefore, it probably descended from a different species than the latter three, which were all genetically similar to their likely ancestor the Chilean Mockingbird. (Arbogast 2006: 4) Dotted line separates the two clusters of islands affected by each separate mockingbird colonization event. (Arbogast 2006: 2) In conclusion, there were probably two mockingbird colonization events. The winds that drive the Humboldt Current most likely distributed the Chilean Mockingbird across the southernmost islands of the archipelago and the winds driving the Panama Current probably distributed the Tropical Mockingbird across the northwestern islands of the archipelago. Because the islands to the south are upwind of the islands to the north, it follows that the Tropical Mockingbird, now the Galapagos Mockingbird, would keep to the north rather than flying upwind and colonizing 12 the islands to the south. The southernmost islands are also considerably isolated from the northern islands and from eachother. Therefore, because mockers remain in their natal territories and spend more time on land than in flight, and because the southern islands are somewhat cut off from the rest of the archipelago and from eachother, the Chilean mockingbird did not attempt to colonize the northern islands. Nor was there contact between the populations
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