如果存在时间机器，那么把一种生物放进去，能不能使它逆转退回远祖的状态呢？
葡萄牙和美国的科研人员用果蝇作为对象研究了这个有趣的问题，他们在实验室中模拟重建了果蝇远祖生活的环境。实验用果蝇是1975年从野外捕捉的果蝇的后代，已经在各种环境中经历了500代的进化，许多特性已经随环境变化而发生了改变。
实验中，科研人员把这些果蝇放回类似其祖先曾经生活过的环境中，让它们再经历50代的进化。如果果蝇出现逆向进化，那么它们3号染色体上的DNA（脱氧核糖核酸）一个片断将会有所变化。
研究得出的结论是什么呢？果蝇的确会“返祖”，但到一定程度就停止了。
科研人员在新一期英国《自然·遗传学》杂志上报告说，一旦果蝇能自如适应新环境，逆向进化的时钟就会停止，但这时果蝇的DNA与其远祖并不一定一致。
科研人员说，平均来说，只有一半的基因序列逆转变回到和其祖先相同的状态。这项研究也表明，进化比人们想像的要复杂得多，甚至有时取决于偶然事件。
这项研究也从一方面表明，物种能够通过基因重组而非基因突变一代代地进化。（生物谷Bioon.com）
生物谷推荐原始出处：
Nature Genetics 11 January 2009 | doi:10.1038/ng.289
Experimental evolution reveals natural selection on standing genetic variation
Henrique Teotónio1, Ivo M Chelo1, Martina Bradi2, Michael R Rose3 & Anthony D Long3
Evolution depends on genetic variation generated by mutation or recombination from standing genetic variation. In sexual organisms, little is known about the molecular population genetics of adaptation and reverse evolution1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11. We carry out 50 generations of experimental reverse evolution in populations of Drosophila melanogaster, previously differentiated by forward evolution, and follow changes in the frequency of SNPs in both arms of the third chromosome. We characterize the effects of sampling finite population sizes and natural selection at the genotype level. We demonstrate that selection has occurred at several loci and further that there is no general loss or gain of allele diversity. We also observe that despite the complete convergence to ancestral levels of adaptation, allele frequencies only show partial return.
1 Instituto Gulbenkian de Ciência, Apartado 14, P-2781-901 Oeiras, Portugal.
2 Department of Biology, New York University, New York, New York 10003, USA.
3 Department of Ecology and Evolutionary Biology, University of California, Irvine, California 92697, USA.
