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Topic choice: The adaptive radiation of the Hawaiian honeycreeper system.

The Hawaiian Islands have been slowly forming for millions of years which makes them a very unique setting to study evolutionary biology. The islands are rich with endemic species which are considered to be unique to a specific location. An example of a large, highly diverse endemic species of the Hawaiian Islands would be the Hawaiian Honeycreepers. These birds have been part of a vast adaptive radiation going on for millions of years. It is fascinating how this species was able to achieve high phenotypic diversity while being constrained to the Hawaiian Islands. The Hawaiian Honeycreepers are morphologically and behaviorally very similar, but each island hosts unique and different subspecies representative of the Honeycreeper system. The Honeycreeper adaptive radiation was shaped by island formation and natural selection. As each new island formed, the land was barren and lacking any taxa. Slowly, the new island would become colonized and niches established, resulting in new community structure and new selection pressures. A form of allopatric speciation has occurred via the peripheral isolate model. As the honeycreeper taxa dispersed to the newly formed islands, they were isolated from their previous community and new behavioral traits would begin to emerge. When considering the new community structure and emerging traits in new environments, the honeycreepers would have been susceptible to strong selection pressures and genetic drift. With constant colonization of newly formed islands, the Hawaiian Honeycreeper rate and pattern of radiation was significantly affected. The Hawaiian Islands formed linearly, in order from north-west to south east, throughout time. A study was conducted to determine the approximate ages of the Hawaiian Islands using K—Ar –based dating (Fig. 1). Kauai was determined to be about 5.1 million years old, Oahu about 3.7 million years old and the youngest island of Hawaii about .43 million years old (Fleischer and McIntosh, 1998). By determining the maximum age of the islands, they were able infer the maximum age possible of organisms inhabiting the island which could be used to pinpoint divergence times of the Hawaiian Honeycreeper system. Previous studies suggest that they Hawaiian Honeycreepers diverged from a mainland avian species, cardueline finch, approximately five million years ago (Eggert et al., 2009). The estimated dates of island formations were used alongside a rate calibration for the cytochrome b gene to determine key dates of the Hawaiian Honeycreeper radiation. The major spilt between two large clades of Hawaiian honeycreepers occurred about 4.5 million years ago (Fleischer and McIntosh, 1998).

By being able to estimate dates of island formation, it makes it possible to track the speciation events that occurred throughout the Hawaiian Honeycreeper system. It has been said that speciation is most often coupled with island formation. With this information, it was possible for a study to be done in which the rate of evolution of the Hawaiian Honeycreepers was estimated. They sampled the mitochondrial genome of the Hawaiian Honeycreeper system and split them into functional regions for analysis. It was found that the average rate of sequence divergence was about 1.8 percent per million years and the rate of the divergence for the cytochrome b gene was about 2.1 percent per million years (Lerner et al., 2011).

The microsatellite loci of genes are very useful genetic markers when studying the evolution of a system. Previous studies have been able to find significant positive correlations when comparing mutation rate and repeat lengths of microsatellite loci within a gene pool. The adaptive radiation of the Hawaiian Honeycreepers was studied using eight microsatellite loci of ten honeycreeper species. The eight loci were also studied in one species of a cardueline finch. The study of the microsatellite divergence was coupled with the divergence of mitochondrial DNA and the results showed to be highly correlated in distance. They found that, based off of microsatellite genetic distances, the Hawaiian honeycreepers showed a divergence time of up to 4.7 million years ago (Eggert et al., 2009). Although they recovered high positive correlations, creating phylogenetic trees based off of these genetic distances was only useful to provide insight about very closely related species. One problem that these authors faced was the high extinction rate of the Hawaiian Honeycreepers. The system has been drastically affected by human interaction through habitat loss and the introduction of new predators. Twenty-eight of the fifty-three known species of honeycreepers are extinct, and the remaining honeycreepers are greatly endangered (Jarvi et al., 2004). With more than half of the Hawaiian Honeycreeper population being extinct, sampling and testing phylogenies becomes much more challenging.

A new and increasingly abundant problem of the Hawaiian Honeycreeper system is the introduction of avian malaria. The Hawaiian strains of malaria cause disease in honeycreepers which is considered to be a major limiting factor of their abundance. Like with any disease, there are varying levels of susceptibility throughout the population, which makes the honeycreeper species quite interesting to study. The major histocompatibility complex (Mhc) of the honeycreeper’s were studied in order to observe the balancing selection that takes place between pathogens and host genes. They were specifically studying the Mhc antigens of peptides produced from the pathogens (Jarvi et al., 2004). Two individual subspecies of the Hawaiian Honeycreepers are very closely related and yet their susceptibility to avian malaria is drastically different. They were interested in find how the pathogen is driving selection on host genes and slowly changing the population as a whole with regards to avian malaria. Their study results suggest that one subspecies experience a more recent coalescence time for their sequences causing the increased susceptibility of the subspecies (Jarvi et al., 2004). By having this increased susceptibility to avian malaria, this subspecies will be able to continue to reproduce and possibly may diverge into another subspecies.

Natural selection has played a major role in allowing the Hawaiian Honeycreepers to adapt in ways that have allowed them to exist for millions of years. As new islands slowly formed, the honeycreepers would travel to these new, unpopulated islands and began to colonize allowing another subspecies to emerge through adaptations to their new environment. With the honeycreeper population slowly diminishing, it is important that we determine key adaptations that would allow to Hawaiian Honeycreepers to continue to diverge and populate new islands as they are formed without causing more extinction to occur.

FINAL DRAFT STARTS HERE

The Adaptive Radiation of the Hawaiian Honeycreepers

The Hawaiian Islands have been slowly forming for millions of years, making them a very unique setting to study evolutionary biology. The islands are rich with endemic species which are considered to be unique to a specific location. An example of a large, highly diverse endemic species of the Hawaiian Islands would be the Hawaiian Honeycreepers. These birds have been part of a vast adaptive radiation going on for millions of years. An adaptive radiation is characterized by a species diversification that is caused by the need to exploit different resources from different environments (Schluter, 1996). It is fascinating how this species was able to achieve high phenotypic diversity while being constrained to the Hawaiian Islands. The Hawaiian Honeycreepers are morphologically and behaviorally very similar, but each island hosts unique subspecies representative of the Honeycreeper system. The Honeycreeper adaptive radiation was shaped by island formation and natural selection. A form of allopatric speciation has occurred via the peripheral isolate model. As each new island formed, the land was barren and lacking any taxa. Slowly, the species would disperse to the new island and it too would become colonized while new niches were established. As the honeycreeper taxa dispersed to the newly formed islands, they were isolated from their previous community and new behavioral traits would begin to emerge. Each time a dispersal event occurred, it would result in new community structure, which would present new selection pressures on the species. When considering the new community structure and emerging traits in new environments, the honeycreepers would have been susceptible to strong selection pressures and genetic drift. With constant colonization of newly formed islands, the Hawaiian Honeycreeper rate and pattern of radiation were significantly affected. Throughout time, the Hawaiian Islands formed linearly from north-west to south east. A study was conducted to determine the approximate ages of the Hawaiian Islands using K—Ar –based dating of the oldest found igneous rocks from each island. Kauai was determined to be about 5.1 million years old, Oahu about 3.7 million years old and the youngest island of Hawaii about 0.43 million years old (Fleischer and McIntosh, 1998). By determining the maximum age of the islands, inferences could be made about the maximum age possible of organisms inhabiting the island. These ages were then used to pinpoint divergence times of the Hawaiian Honeycreeper system. Previous studies suggest that the Hawaiian Honeycreepers diverged from a mainland species, the cardueline finch, approximately five million years ago once Kauai was formed (Eggert et al., 2009). After the first initial dispersal of the cardueline finch, a rapid speciation would occur to allow new adaptations to thrive in a new environment.

Due to the effects of erosion on the islands, the fossil records of the Hawaiian Honeycreepers are very limited. The times of ancestor divergence can only be inferred using phylogenetic data. Fleischer and colleagues studied the molecular divergence of many honeycreeper taxa in order to estimate the times of divergence that accommodated their adaptive radiation. They chose to use a molecular rate calibration of the mitochondrial DNA cytochrome b gene from honeycreeper taxa of each island. The estimated dates of island formations were used alongside a rate calibration for the cytochrome b gene to determine key dates of the Hawaiian Honeycreeper radiation (Fig. 1). They discovered a major spilt between two large clades of the Hawaiian Honeycreepers, occurring about 4.5 million years ago when the species dispersed from Kauai to Oahu (Fleischer and McIntosh, 1998).

By being able to estimate dates of island formation, it makes it possible to track the speciation events that occurred throughout the Hawaiian Honeycreeper system. It has been said that speciation is most often coupled with island formation. With this information, it was possible for a study to be done in which the rate of evolution of the Hawaiian Honeycreepers was estimated. Lerner and colleagues sampled the mitochondrial genome of the Hawaiian Honeycreeper system and split the genome into its respective functional regions for analysis. Mitochondrial genomes are preferred when studying phylogenetic history, because a large portion of the mtDNA is not functional (Lerner et al., 2011). The non-functional portions of the genome are not expressed in phenotypes, therefore they would not have been affected by the adaptations that shaped the distinct species. By analyzing the noncoding sections of the genome, they were able to focus strictly on the divergence from ancestors. They discovered a distinct divergence that occurred between 5.8 and 2.4 mya, which parallels the formations of Oahu and Maui Nui.

The newly formed islands were able to accommodate growing populations while the new environments were causing high rates of new adaptations. The timing of this divergence was able to facilitate the rapid speciation and adaptive radiation. Lerner and his colleagues were then able to estimate evolutionary rates using mitochondrial and nuclear information. The rate of the divergence for the mtDNA cytochrome b gene was found to be about 2.1 percent per million years (Lerner et al., 2011). Using whole mitochondrial DNA sequences, it was found that the average rate of sequence divergence was about 1.8 percent per million years (Lerner et al., 2011).

The microsatellite loci of genes are very useful genetic markers when studying the evolution of a system. Previous studies have been able to find significant positive correlations when comparing mutation rate and repeat lengths of microsatellite loci within a gene pool. The adaptive radiation of the Hawaiian Honeycreepers was studied using eight microsatellite loci of ten honeycreeper species. The eight loci were also studied in one species of cardueline finch. The study of the microsatellite divergence was coupled with the divergence of mitochondrial DNA and the results showed to be highly correlated in distance. They found that, based off of microsatellite genetic distances, the Hawaiian honeycreepers showed a divergence time of up to 4.7 million years ago (Eggert et al., 2009). Although they recovered high positive correlations, creating phylogenetic trees based off of these genetic distances was only useful to provide insight about very closely related species. One problem that the study faced was the high extinction rate of the Hawaiian Honeycreepers. The system has been drastically affected by human interaction through habitat loss and the introduction of new predators. Twenty-eight of the fifty-three known species of honeycreepers are extinct, and the remaining honeycreepers are greatly endangered (Jarvi et al., 2004). With more than half of the Hawaiian Honeycreeper population being extinct, sampling and testing phylogenies becomes much more challenging.

Natural selection plays a large role in the adaptive radiations of species, but determining how much diversity natural selection provides is not easily distinguishable. Diversity can be caused through pathways of divergence, species hybridization, or through convergence. When two species are similar, it can be because they diverged from a common ancestor, or because their environments were similar and natural selection caused them to converge onto the same adaptations. The most diversification of the species is seen in each subspecies method of feed and the bill shape (James, 2003). Reding and colleagues conducted an analysis of honeycreeper taxa to determine causation of their extensive adaptive radiation. They were specifically interested in the honeycreepers of the most distant island from each other, Kauai and Hawaii. With the islands being far away from each other, chances of species hybridization would be low, but the species inhabiting both islands are very similar to one another. Honeycreepers from Kauai and Hawaii are placed within the same genus. Reding was interested in determining whether these birds were an example of convergent evolution within the adaptive radiation. Morphological and behavioral characteristics were compared with a phylogeny produced from mictochondrial DNA, and it was determined that the similar features of the Kauai and Hawaii ‘creepers’ was not due to divergence from a common ancestor. The similarities in morphology and behavior were due to the similar environment and selective pressures of Kauai and Hawaii (Reding, 2009).

A new and increasingly abundant problem of the Hawaiian Honeycreeper system is the introduction of avian malaria. The Hawaiian strains of malaria cause disease in honeycreepers which is considered to be a major limiting factor of their abundance (Atkinson, 2009). Like with any disease, there are varying levels of susceptibility throughout the population, which makes the honeycreeper species quite interesting to study. The major histocompatibility complex (Mhc) of the honeycreeper’s were studied in order to observe the balancing selection that takes place between pathogens and host genes. They were specifically studying the Mhc antigens of peptides produced from the pathogens (Jarvi et al., 2004).

Two individual subspecies of the Hawaiian Honeycreepers are very closely related and yet their susceptibility to avian malaria is drastically different. They were interested in find how the pathogen is driving selection on host genes and slowly changing the population as a whole with regards to avian malaria. Their results suggest that one subspecies has experienced a more recent coalescence time for their sequences, causing the increased susceptibility of the subspecies (Jarvi et al., 2004). The recent coalescence time indicates that the subspecies diverged from their common ancestor much more recently than other subspecies of the Hawaiian Honeycreepers, which means that they have had less time to gain advantageous adaptations. This would give them a disadvantage in fighting the infection caused by avian malaria. By having this increased susceptibility to avian malaria, this subspecies will very likely be unable to continue to reproduce or diverge into another subspecies.

Natural selection has played a major role in allowing the Hawaiian Honeycreepers to adapt in ways that have allowed them to exist for millions of years. As new islands slowly formed, the honeycreepers would travel to these new, unpopulated islands and begin to colonize, allowing another subspecies to emerge through adaptations to their new environment. With the honeycreeper population slowly diminishing, it is important to determine key adaptations that would allow the Hawaiian Honeycreepers to continue to diverge and populate new islands as they are formed, without causing more extinction.

'''Atkinson CT, LaPointe DA. 2009. Introduced avian diseases, climate change, and the future of the Hawaiian Honeycreepers. J Avian Med Surg 23(1): 53-63.'''

'''Eggert LS, Beadell JS, McClung A, McIntosh CE, Fleischer RC. 2009. Evolution of microsatellite loci in the adaptive radiation of Hawaiian honeycreepers. J Hered 00(2): 137-147.'''

'''Fleischer RC, McIntosh CE, Tarr CL. 1998. Evolution on a volcanic conveyor belt: using phylogeographic reconstructions and K – Ar- based ages of the Hawaiian islands to estimate molecular evolutionary rates. Mol Ecol 7: 533-545.'''

'''James, HF. 2004. The osteology and phylogeny of the Hawaiian finch radiation (Fringillidae: Drepanidini), including extinct taxt. Zool J Linn Soc-Lond 141: 207-255.'''

'''Jarvi, SI, Tarr CL, McIntosh CE, Atkinson CT, Fleischer RC. 2004. Natural selection of the major histocompatibility complex (Mhc) in Hawaiian honeycreepers (Drepanidinae). Mol Ecol 13: 2157-2168.'''

'''Lerner HLR, Meyer M, James HF, Hofreiter M, Fleischer RC. 2011. Multilocus resolution of phylogeny and timescale in the extant adaptive radiation of Hawaiian honeycreepers. Curr Biol 21: 1838-1844.'''

'''Reding DM, Foster JT, James HF, Pratt HD, Fleisher RC. 2009. Convergent evolution of ‘creepers’ in the Hawaiian honeycreeper radiation. Biol Lett 5: 221-224.'''

'''Schluter, D. 1996. Ecological causes of adaptive radiation. Am Nat 148: S40-S64.'''

https://en.wikipedia.org/wiki/Adaptive_radiation

Edit

An exceptional example of adaptive radiation would be the avian species of the Hawaiian honeycreepers. Via natural selection, these birds adapted rapidly and converged based on the different environments of the Hawaiian islands.

Hawaiian Honeycreepers
Another fascinating evolutionary example of an adaptive radiation would be an endemic species of the Hawaiian Islands. The Hawaiian Honeycreepers are a large, highly diverse species which have been part of a vast adaptive radiation, that began as the Hawaiian Islands started to form. The Honeycreeper species was shaped by island formation and natural selection. The mechanism by which this adaptive radiation occurred, can be described as allopatric speciation via the peripheral isolate model. Each time a new island formed, a dispersal event would occur which would result in new community structures on each island. New selection pressures forced the adaptive radiation of the Hawaiian Honeycreepers, as they needed to exploit new resources from the different environments of each island.[11] It has been determined that many of the similar morphologies and behaviors of the Hawaiian Honeycreepers, located on distant islands, are due to convergence of analogous traits caused by similar environments.[12]

[11] Schluter, D. 1996. Ecological causes of adaptive radiation. Am Nat 148: S40-S64.

[12] Reding, DM; Foster, JT; James, HF; Pratt, D; Fleischer, RC (2009). "Convergent evolution of 'creepers' in the Hawaiian honeycreeper radiation". Biology letters 5: 221–224.

Suggestions

Hawaiian honeycreepers example


 * Darwin's finches are a model species when studying avian island radiation, but the Hawaiian honeycreepers are an impressive example as well. It would be interesting to add a section about the rapid honeycreeper radiation throughout the Hawaiian islands. It is a well studied radiation with numerous different approaches in comparing phylogeny and determining how diverse the population became through adaptive radiation.

Phylogenetic tree


 * It would be beneficial for readers to see a phylogenetic tree of a population that underwent adaptive radiation. Readers would be able to visualize how much diversity can emerge from a single ancestor.

https://en.wikipedia.org/wiki/Endemism_in_the_Hawaiian_Islands

Suggestion

Island formations


 * I believe it would be beneficial to add estimated dates of island formations. Island formation dates could be a start to deeper discussions and correlations between time and species divergence.


 * Fleischer, RC; McIntosh, CE; Tarr, CL (1998). "Evolution on a volcanic conveyor belt: using phylogeographic reconstructions and K-Ar - based ages of the Hawaiian Islands to estimate molecular evolutionary rates". Molecular ecology 7: 533–545.

Edit

Island formation
Throughout time, the Hawaiian Islands formed linearly from north-west to the south-east. A study was conducted to determine the approximate ages of the Hawaiian Islands using K--Ar-based dating of the oldest found igneous rocks from each island. Kauai was determined to be about 5.1 million years old, Oahu about 3.7 million years old and the youngest island of Hawaii about 0.43 million years old.[1] By determining the maximum age of the islands, inferences could be made about the maximum age possible of organisms inhabiting the island. The newly formed islands were able to accommodate growing populations, while the new environments were causing high rates of new adaptations.

[1] Fleischer RC, McIntosh CE, Tarr CL. 1998. Evolution on a volcanic conveyor belt: using phylogeographic reconstructions and K--Ar-based ages of the Hawaiian islands to estimate molecular evolutionary rates. Mol Ecol 7: 533-545.