文章来源:未知 文章作者:meng 发布时间:2010-07-14 03:19 字体: [ ]  进入论坛

Scientists studying ancient species migration1 believe northern birds had the ability to colonise continents that southern species lacked. The research, published in Ecography(描述生态学) , reveals how the ancient 'land bridge' of Panama, which first connected North and South America, caused an uneven2(不均匀的) species migration, leading to a new understanding of species diversity today. The continents of North and South America were historically isolated3 until they were abruptly4(突然地) joined three million years ago through the tectonic(构造的) uplift of Central America and the formation of a land corridor in modern day Panama, creating a land bridge.

"This connection allowed an unprecedented5(空前的) degree of intercontinental exchange between species that had been isolated for millions of years," said lead author Brian Tilston Smith from the University of Nevada. "However the relatively7 poor fossil record has prevented us from understanding how the land bridge shaped New World bird communities."

Using molecular8 data and phylogenetic(系统发生的,动植物种类史的) evidence from 11 orders, 34 families, and over 100 genera(属,种) of bird species the team applied9 a 'molecular clock' to estimate the historical timing10 of the migration, giving a unique insight into how the ancient history of American bird migration led to present day species diversity across the equator(赤道) .

The results reveal that while ancient birds could fly most species did not cross the water between the two isolated continents, so were subject to the same constraints11 as their land based mammalian(哺乳类的) counterparts. The land bridge was therefore crucial in facilitating cross continental6 migration.

"This inter-continental migration was far from even. While within the tropics around the equator exchange was equal in both directions, between the temperate12 zones of North and South America it was not," said Smith. "Avian lineages(血统,家系) from the northern Nearctic regions have repeatedly invaded the tropics and radiated throughout South America. In contract species with South American tropical origins remain largely restricted to the confines of the tropical regions."

Existing studies show that in mammals 50% of modern South American species have Northern origins whereas only 10% of species from the North originated in the South. The team found that this pattern is also reflected in birds. When considering the perching birds oscine and suboscine(亚鸣禽亚目) the team found that despite having northern ancestral origins, 55% of New World oscine species now breed in South America, many of them in tropical habitats. In contrast, only 2.4% of suboscines have secondarily adapted to North American temperate zone habitats.

"Our study suggests the formation of the Panama land bridge was crucial for allowing cross continental bird migration," concluded Smith. "We believe that the ability of species to colonise and radiate(辐射,传播) across this area represents an important and underappreciated(未受到充分赏识的,未得到正确评价的) factor to the distribution of species around the equator."


1 migration mDpxj     
  • Swallows begin their migration south in autumn.燕子在秋季开始向南方迁移。
  • He described the vernal migration of birds in detail.他详细地描述了鸟的春季移居。
2 uneven akwwb     
  • The sidewalk is very uneven—be careful where you walk.这人行道凹凸不平—走路时请小心。
  • The country was noted for its uneven distribution of land resources.这个国家以土地资源分布不均匀出名。
3 isolated bqmzTd     
  • His bad behaviour was just an isolated incident. 他的不良行为只是个别事件。
  • Patients with the disease should be isolated. 这种病的患者应予以隔离。
4 abruptly iINyJ     
  • He gestured abruptly for Virginia to get in the car.他粗鲁地示意弗吉尼亚上车。
  • I was abruptly notified that a half-hour speech was expected of me.我突然被通知要讲半个小时的话。
5 unprecedented 7gSyJ     
  • The air crash caused an unprecedented number of deaths.这次空难的死亡人数是空前的。
  • A flood of this sort is really unprecedented.这样大的洪水真是十年九不遇。
6 continental Zazyk     
  • A continental climate is different from an insular one.大陆性气候不同于岛屿气候。
  • The most ancient parts of the continental crust are 4000 million years old.大陆地壳最古老的部分有40亿年历史。
7 relatively bkqzS3     
  • The rabbit is a relatively recent introduction in Australia.兔子是相对较新引入澳大利亚的物种。
  • The operation was relatively painless.手术相对来说不痛。
8 molecular mE9xh     
  • The research will provide direct insight into molecular mechanisms.这项研究将使人能够直接地了解分子的机理。
  • For the pressure to become zero, molecular bombardment must cease.当压强趋近于零时,分子的碰撞就停止了。
9 applied Tz2zXA     
  • She plans to take a course in applied linguistics.她打算学习应用语言学课程。
  • This cream is best applied to the face at night.这种乳霜最好晚上擦脸用。
10 timing rgUzGC     
  • The timing of the meeting is not convenient.会议的时间安排不合适。
  • The timing of our statement is very opportune.我们发表声明选择的时机很恰当。
11 constraints d178923285d63e9968956a0a4758267e     
强制( constraint的名词复数 ); 限制; 约束
  • Data and constraints can easily be changed to test theories. 信息库中的数据和限制条件可以轻易地改变以检验假设。 来自英汉非文学 - 科学史
  • What are the constraints that each of these imply for any design? 这每种产品的要求和约束对于设计意味着什么? 来自About Face 3交互设计精髓
12 temperate tIhzd     
  • Asia extends across the frigid,temperate and tropical zones.亚洲地跨寒、温、热三带。
  • Great Britain has a temperate climate.英国气候温和。
TAG标签: species birds ancient