ScienceDaily (Mar. 25, 2008) — The rate at which new species are formed in a group of closely related animals decreases as the total number of different species in that group goes up, according to new research.
The research team believes these findings suggest that new species appear less and less as the number of species in a region approaches the maximum number that it can support.
In order for new species to thrive, they need to evolve to occupy their own niche in the ecosystem, relying on certain foods and habitats for survival that are sufficiently different from those of other closely related species.
Competition between closely related species for food and habitat becomes more intense the more species there are, and researchers believe this could be the reason for the drop-off in the appearance of new species over time.
Dr. Albert Phillimore, from Imperial College London's NERC Centre for Population Biology, lead author on the paper, explains: "The number of niches in any given region is finite, and our research supports the idea that the rate of speciation slows down as the number of niches begins to run out.
"In essence, it seems like increased competition between species could place limits on the number of species that evolve."
The new study used detailed analysis of the family trees, or phylogenies, of 45 different bird families. By examining the rate at which new species have arisen in each of these trees over a period of millions of years, scientists saw that the rate of appearance of new species seemed to be much higher in the early stages of the family tree, compared to more recent lower rates.
For example, when the researchers examined the phylogeny of tit birds they found that some 10 million years ago, species formed rapidly but this rate has slowed over time to perhaps a quarter of the initial rate.
Journal reference: Phillimore AB, Price TD (2008) Density-dependent cladogenesis in birds. PLoS Biol 6(3): e71. doi:10.1371/journal.pbio.0060071
Tuesday, March 25, 2008
Study Suggests Humans Can Speed Evolution
ScienceDaily (Aug. 5, 2004) — Atlanta (August 4,2004) -- It’s no secret that life in the 21st century moves at a rapid pace. Human inventions such as the Internet, mobile phones and fiber optic cable have increased the speed of communication, making it possible for someone to be virtually in two places at once. But can humans speed up the rate of one of nature’s most basic and slowest processes, evolution? A study by J. Todd Streelman, new assistant professor of biology at the Georgia Institute of Technology suggests that humans may have sped up the evolutionary clock for one species of fish.
Cichlid fish are well known to biologists for their rapid rate of evolution. While it takes many animals thousands of years to form new species, the cichlids of Africa’s Lake Malawi are estimated to have formed 1,000 new species in only 500,000 years, lightning speed in evolutionary terms. In the 1960s a fish exporter may have unwittingly set the stage for an evolutionary explosion when he introduced individuals of the species Cynotilapia afra to Mitande Point on the lake’s Thumbi West Island. As of 1983, the species hadn’t budged from Mitande Point. But when Streelman, then at the University of New Hampshire, Durham, and colleagues went to the island in 2001, they found the fish had evolved into two genetically distinct varieties in less than 20 years. The study appears in the August 13 edition of Molecular Ecology.
“This is a great example of human-induced evolution in action,” said Streelman. “It adds to a growing list of cases, including introduced salmon, flies and plants, where human disturbance has set the stage for contemporary evolution on scales we’ve not witnessed before.”
The fish have evolved into two genetically distinct and differently colored populations, one on the north side of the island, the other on the south, said Streelman. Cichlid color patterns are important in mate selection, so these distinct markings may promote the evolution of new species.
Whether or not that happens and how long it will take is a question to which Streelman is eager to find the answer. “It could be that we'll have new species in another 20 years, although this depends on a number of factors. Either way, we have a wonderful opportunity to follow the evolutionary trajectory of these populations over time. We plan to return to the island next July to do further study,” he said. “Thumbi West will be a valuable place to work for years to come.”
Cichlid fish are well known to biologists for their rapid rate of evolution. While it takes many animals thousands of years to form new species, the cichlids of Africa’s Lake Malawi are estimated to have formed 1,000 new species in only 500,000 years, lightning speed in evolutionary terms. In the 1960s a fish exporter may have unwittingly set the stage for an evolutionary explosion when he introduced individuals of the species Cynotilapia afra to Mitande Point on the lake’s Thumbi West Island. As of 1983, the species hadn’t budged from Mitande Point. But when Streelman, then at the University of New Hampshire, Durham, and colleagues went to the island in 2001, they found the fish had evolved into two genetically distinct varieties in less than 20 years. The study appears in the August 13 edition of Molecular Ecology.
“This is a great example of human-induced evolution in action,” said Streelman. “It adds to a growing list of cases, including introduced salmon, flies and plants, where human disturbance has set the stage for contemporary evolution on scales we’ve not witnessed before.”
The fish have evolved into two genetically distinct and differently colored populations, one on the north side of the island, the other on the south, said Streelman. Cichlid color patterns are important in mate selection, so these distinct markings may promote the evolution of new species.
Whether or not that happens and how long it will take is a question to which Streelman is eager to find the answer. “It could be that we'll have new species in another 20 years, although this depends on a number of factors. Either way, we have a wonderful opportunity to follow the evolutionary trajectory of these populations over time. We plan to return to the island next July to do further study,” he said. “Thumbi West will be a valuable place to work for years to come.”
Choosy Females Make Colourful Males
ScienceDaily (May 12, 2006) — Female fish prefer brightly coloured males because they are easier to see and are in better shape concludes Dutch researcher Martine Maan following her study of fish speciation in the East African Lakes. Environmental variation subsequently leads to differences in preference and eventually to speciation.
Evolutionary theory predicts that species can diverge if different females choose different characteristics in males. Yet females often pay attention to traits that reveal something about the quality of a male. As a result, females are likely to share the same preferences. In Lake Victoria cichlid fish, Martine Maan found a solution for this paradox: in different species, different traits reveal male quality.
She examined two closely related species, one with blue males and the other with red males. Females prefer males of the right colour, blue or red, and within those categories they choose the most brightly coloured males. They do so for good reasons: brightly coloured males from both species carry fewer parasites and are thus in better condition. Moreover, both species are adapted to different infection risks, which are associated with a difference in water depth and food choice. It is therefore in the females' interest to mate with their own males.
Red and blue light
Yet how did these differences evolve? The red species occurs in deeper water than the blue species and therefore experiences different light conditions. Behavioural experiments showed that both species have adapted to this: the red species is more sensitive to red light and the blue species is more sensitive to blue light. For females of the red species, red males are therefore more conspicuous than blue ones, and vice versa. Males of other colours are inconspicuous and unattractive, and therefore produce few offspring. Eventually only the bright red and bright blue fish remain, and two separate species can arise.
Due to the introduction of the Nile perch, deforestation and population growth, water transparency in Lake Victoria is declining. In turbid water, cichlid females are less choosy and males are less brightly coloured. This research therefore underlines the importance of measures to counteract the ongoing eutrophication of the lake.
Evolutionary theory predicts that species can diverge if different females choose different characteristics in males. Yet females often pay attention to traits that reveal something about the quality of a male. As a result, females are likely to share the same preferences. In Lake Victoria cichlid fish, Martine Maan found a solution for this paradox: in different species, different traits reveal male quality.
She examined two closely related species, one with blue males and the other with red males. Females prefer males of the right colour, blue or red, and within those categories they choose the most brightly coloured males. They do so for good reasons: brightly coloured males from both species carry fewer parasites and are thus in better condition. Moreover, both species are adapted to different infection risks, which are associated with a difference in water depth and food choice. It is therefore in the females' interest to mate with their own males.
Red and blue light
Yet how did these differences evolve? The red species occurs in deeper water than the blue species and therefore experiences different light conditions. Behavioural experiments showed that both species have adapted to this: the red species is more sensitive to red light and the blue species is more sensitive to blue light. For females of the red species, red males are therefore more conspicuous than blue ones, and vice versa. Males of other colours are inconspicuous and unattractive, and therefore produce few offspring. Eventually only the bright red and bright blue fish remain, and two separate species can arise.
Due to the introduction of the Nile perch, deforestation and population growth, water transparency in Lake Victoria is declining. In turbid water, cichlid females are less choosy and males are less brightly coloured. This research therefore underlines the importance of measures to counteract the ongoing eutrophication of the lake.
Multiple Genes Permit Closely Related Fish Species To Mix And Match Their Color Vision
ScienceDaily (Oct. 16, 2005) — Vision, like other biological attributes, is shaped by evolution through environmental pressures and demands, and even closely-related species that are in other ways very similar might respond to their particular environments by interpreting the visual world slightly differently, using photoreceptors that are attuned to particular wavelengths of light. By studying a special group of closely-related fish species inhabiting the Great Lakes of Africa, researchers have uncovered clues to understanding how the components of color vision can undergo change over a relatively short period of evolutionary time.
The work is reported by James K. Bowmaker of University College London, Karen L. Carleton of the University of New Hampshire, and their colleagues.
Cichlid fish of the East African Rift Lakes are renowned for their diversity: Owing to migrations of ancestor species out of Lake Tanganyika and into other lakes, such as Lake Malawi, it has been estimated that hundreds of new cichlid species have arisen in these lakes in the last 100,000 years. Thanks to the relatively recent colonization by these fish of different ecological niches, as well as the prominent role of nuptual coloring in the mating preferences of these species, the cichlids offer a unique opportunity to study how color vision can undergo change in rapidly evolving species. For example, because color plays a significant role in mate choice, differences in color vision could greatly influence and even drive cichlid speciation.
In the new work, the researchers performed physiological and molecular genetic analyses of color vision in cichlid fish from Lake Malawi and demonstrated that differences in color vision between closely related species arise from individual species' using different subsets of distinct visual pigments. The scientists showed that although an unexpectedly large group of these visual pigments are available to all the species, each expresses the pigments selectively, and in an individual way, resulting in differences in how the visual world is sensed.
The researchers identified a total of seven "cone" (color-sensing) visual pigments underlying color vision in these cichlids. They have measured the sensitivities of the cones to different wavelengths of light and isolated the seven genes that give rise to the pigment proteins. The seven cone types have maximum sensitivities ranging from the red end of the spectrum right through to the ultraviolet--light outside the range of human sensitivity. The researchers showed that in order to tune its color vision, each cichlid species primarily expresses three of the seven cone pigment genes encoded by their genomes.
It is not clear why such closely related cichlid species have evolved such different visual sensitivities, but the sensitivities most likely relate to such selective forces as foraging specializations and subtle differences in the underwater light environment. Evolutionary comparison of pigment genes suggests that other groups of fish may use a similar strategy for shaping their color vision.
###
The researchers included Juliet W.L. Parry, Aba Carboo, David M. Hunt, and James K. Bowmaker of University College in London, United Kingdom; Karen L. Carleton and Tyrone Spady of the University of New Hampshire, Durham, New Hampshire. This work was supported by the Leverhulme Trust and by the National Science Foundation.
Parry et al.: "Mix and match colour vision: tuning spectral sensitivity by differential opsin gene expression in Lake Malawi cichlids." Publishing in Current Biology, Vol. 15, pages 1734-1739, October 11, 2005. DOI 10.1016/j.cub.2005.08.010 www.current-biology.com
The work is reported by James K. Bowmaker of University College London, Karen L. Carleton of the University of New Hampshire, and their colleagues.
Cichlid fish of the East African Rift Lakes are renowned for their diversity: Owing to migrations of ancestor species out of Lake Tanganyika and into other lakes, such as Lake Malawi, it has been estimated that hundreds of new cichlid species have arisen in these lakes in the last 100,000 years. Thanks to the relatively recent colonization by these fish of different ecological niches, as well as the prominent role of nuptual coloring in the mating preferences of these species, the cichlids offer a unique opportunity to study how color vision can undergo change in rapidly evolving species. For example, because color plays a significant role in mate choice, differences in color vision could greatly influence and even drive cichlid speciation.
In the new work, the researchers performed physiological and molecular genetic analyses of color vision in cichlid fish from Lake Malawi and demonstrated that differences in color vision between closely related species arise from individual species' using different subsets of distinct visual pigments. The scientists showed that although an unexpectedly large group of these visual pigments are available to all the species, each expresses the pigments selectively, and in an individual way, resulting in differences in how the visual world is sensed.
The researchers identified a total of seven "cone" (color-sensing) visual pigments underlying color vision in these cichlids. They have measured the sensitivities of the cones to different wavelengths of light and isolated the seven genes that give rise to the pigment proteins. The seven cone types have maximum sensitivities ranging from the red end of the spectrum right through to the ultraviolet--light outside the range of human sensitivity. The researchers showed that in order to tune its color vision, each cichlid species primarily expresses three of the seven cone pigment genes encoded by their genomes.
It is not clear why such closely related cichlid species have evolved such different visual sensitivities, but the sensitivities most likely relate to such selective forces as foraging specializations and subtle differences in the underwater light environment. Evolutionary comparison of pigment genes suggests that other groups of fish may use a similar strategy for shaping their color vision.
###
The researchers included Juliet W.L. Parry, Aba Carboo, David M. Hunt, and James K. Bowmaker of University College in London, United Kingdom; Karen L. Carleton and Tyrone Spady of the University of New Hampshire, Durham, New Hampshire. This work was supported by the Leverhulme Trust and by the National Science Foundation.
Parry et al.: "Mix and match colour vision: tuning spectral sensitivity by differential opsin gene expression in Lake Malawi cichlids." Publishing in Current Biology, Vol. 15, pages 1734-1739, October 11, 2005. DOI 10.1016/j.cub.2005.08.010 www.current-biology.com
Adaptation To Parasites Drive African Fishes Along Different Evolutionary Paths
Captured from :
http://www.sciencedaily.com/releases/2007/08/070815101856.htm
ScienceDaily (Aug. 16, 2007) — An international team of scientists from Canada (Université Laval), the U.K. (University of Hull, Cardiff University) and Spain (Doòana Biological Station), have discovered that a pair of closely related species of East African cichlid fishes -- a group of fish whose diversity comprising hundreds of species has puzzled evolutionary biologists for decades -- evolved divergent immune gene adaptations which might explain why they do not interbreed, despite living side by side.
The two species ( Pseudotropheus emmiltos and Pseudotropheus fainzilberi ) are found in the north western part of Lake Malawi. Until now, the only known difference between them was the color of their dorsal fin. Many researchers believe that African cichlids recognize conspecifics from these kinds of colour differences, which are thought to result from sexual selection. However, recent mate choice experiments have shown that female P. emmiltos recognize males of their own species from P. fainzilberi males based on olfactory communication rather than color.
Some of the genes known to influence mating behavior through olfaction in other vertebrate species are genes of the major histocompatibility complex (MHC). These genes code for receptor that bound molecules produced by infectious agents and present them to specialized cells of the immune system which then launch an immune attack on the microbes.
They are the most diverse genes found in vertebrate genomes and individuals of some species, including humans, are able to "smell" other individuals' variability at these genes and adjust their mate choice in order to optimize the effectiveness of their offspring's immune system. Analysis of MHC genes between P. emmiltos and P. fainzilberi revealed that the two species were genetically more different at these sites involved in contacting and presenting molecules to immune cells than at other sites of the gene's DNA sequence that do not play functional roles.
These results show that natural selection has driven the evolution of these genes in different direction between the two species. Furthermore, the researchers showed that infecting parasites found on the two species were significantly different, as predicted based on the known immune function of MHC genes. "The mechanisms having produced the hundreds of species of East African cichlid fishes in a relatively short period of time are unclear", says Jonatan Blais, the senior author of the paper. "This is one of the first genetic adaptive differences between closely related East African cichlid species identified. As such, it improves our understanding of the recent evolution of this incredibly diverse group of fish by pointing to a trait that not only diverged for adaptive reasons but may also be involved in mating behavior."
"The precise role that this divergence played in the evolution of reproductive isolation has yet to be studied", comments Louis Bernatchez, co-author of the study." But it offers an exciting new perspective in the study of African cichlids speciation. "
Citation: Blais J, Rico C, van Oosterhout C, Cable J, Turner GF, et al (2007) MHC Adaptive Divergence between Closely Related and Sympatric African Cichlids. PLoS One 2(8): e734. doi:10.1371/journal.pone.0000734
http://www.sciencedaily.com/releases/2007/08/070815101856.htm
ScienceDaily (Aug. 16, 2007) — An international team of scientists from Canada (Université Laval), the U.K. (University of Hull, Cardiff University) and Spain (Doòana Biological Station), have discovered that a pair of closely related species of East African cichlid fishes -- a group of fish whose diversity comprising hundreds of species has puzzled evolutionary biologists for decades -- evolved divergent immune gene adaptations which might explain why they do not interbreed, despite living side by side.
The two species ( Pseudotropheus emmiltos and Pseudotropheus fainzilberi ) are found in the north western part of Lake Malawi. Until now, the only known difference between them was the color of their dorsal fin. Many researchers believe that African cichlids recognize conspecifics from these kinds of colour differences, which are thought to result from sexual selection. However, recent mate choice experiments have shown that female P. emmiltos recognize males of their own species from P. fainzilberi males based on olfactory communication rather than color.
Some of the genes known to influence mating behavior through olfaction in other vertebrate species are genes of the major histocompatibility complex (MHC). These genes code for receptor that bound molecules produced by infectious agents and present them to specialized cells of the immune system which then launch an immune attack on the microbes.
They are the most diverse genes found in vertebrate genomes and individuals of some species, including humans, are able to "smell" other individuals' variability at these genes and adjust their mate choice in order to optimize the effectiveness of their offspring's immune system. Analysis of MHC genes between P. emmiltos and P. fainzilberi revealed that the two species were genetically more different at these sites involved in contacting and presenting molecules to immune cells than at other sites of the gene's DNA sequence that do not play functional roles.
These results show that natural selection has driven the evolution of these genes in different direction between the two species. Furthermore, the researchers showed that infecting parasites found on the two species were significantly different, as predicted based on the known immune function of MHC genes. "The mechanisms having produced the hundreds of species of East African cichlid fishes in a relatively short period of time are unclear", says Jonatan Blais, the senior author of the paper. "This is one of the first genetic adaptive differences between closely related East African cichlid species identified. As such, it improves our understanding of the recent evolution of this incredibly diverse group of fish by pointing to a trait that not only diverged for adaptive reasons but may also be involved in mating behavior."
"The precise role that this divergence played in the evolution of reproductive isolation has yet to be studied", comments Louis Bernatchez, co-author of the study." But it offers an exciting new perspective in the study of African cichlids speciation. "
Citation: Blais J, Rico C, van Oosterhout C, Cable J, Turner GF, et al (2007) MHC Adaptive Divergence between Closely Related and Sympatric African Cichlids. PLoS One 2(8): e734. doi:10.1371/journal.pone.0000734
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