如有一只老鼠和它的后代一直生活在适宜个头长大的环境中，那么到进化成大象那样的“大块头”需要多久？研究人员为这个有趣的问题找到了答案——2400万代。
2月1日，英国布里斯托尔大学发布公告说，该校研究人员参与的一个国际研究小组分析了地球历史上各种生物的进化情况，计算得出了这一结论。由于小型动物和大型动物每一代的年龄不一样，如老鼠只能活两三年，而大象可以活几十年，研究人员在描述这个结果时没有用具体的时间，而是用了“代”这个遗传学上的常用概念。
研究还显示，如果是兔子般的“身材”，那么进化到大象那么大，需要约1000万代。
不过，海洋动物朝大型方向进化的速度要快一些，研究人员在分析了鲸鱼的进化历史后认为，海洋动物在进化中个头增长的速率约是陆地动物的两倍。研究人员认为，这可能是因为水有浮力，更容易支撑巨大的身体和体重。
与变大相反的是变小，研究人员发现这较为容易。如果是大象一直生活在适宜变小的环境中，它在进化中变小的速率是前述变大速率的10倍以上。这种差异可能是因为从小变大需要比较复杂的进化，如骨骼需要变得能承受更大的体重，而从大变小则没有这么多麻烦，还会因为所需食物更少而更容易。
这项研究报告已发表在新一期美国《国家科学院院刊》（PNAS）上。（生物谷 Bioon.com）
doi:10.1073/pnas.1120774109
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The maximum rate of mammal evolution
Alistair R. Evans, David Jones, Alison G. Boyer, James H. Brown, Daniel P. Costa, S. K. Morgan Ernest, Erich M. G. Fitzgerald, Mikael Fortelius, John L. Gittleman, Marcus J. Hamilton, Larisa E. Harding, Kari Lintulaakso, S. Kathleen Lyons, Jordan G. Okie, Juha J. Saarinen, Richard M. Sibly, Felisa A. Smith, Patrick R. Stephens, Jessica M. Theodor, and Mark D. Uhen
How fast can a mammal evolve from the size of a mouse to the size of an elephant? Achieving such a large transformation calls for major biological reorganization. Thus, the speed at which this occurs has important implications for extensive faunal changes, including adaptive radiations and recovery from mass extinctions. To quantify the pace of large-scale evolution we developed a metric, clade maximum rate, which represents the maximum evolutionary rate of a trait within a clade. We applied this metric to body mass evolution in mammals over the last 70 million years, during which multiple large evolutionary transitions occurred in oceans and on continents and islands. Our computations suggest that it took a minimum of 1.6, 5.1, and 10 million generations for terrestrial mammal mass to increase 100-, and 1,000-, and 5,000-fold, respectively. Values for whales were down to half the length (i.e., 1.1, 3, and 5 million generations), perhaps due to the reduced mechanical constraints of living in an aquatic environment. When differences in generation time are considered, we find an exponential increase in maximum mammal body mass during the 35 million years following the Cretaceous–Paleogene (K–Pg) extinction event. Our results also indicate a basic asymmetry in macroevolution: very large decreases (such as extreme insular dwarfism) can happen at more than 10 times the rate of increases. Our findings allow more rigorous comparisons of microevolutionary and macroevolutionary patterns and processes.
