一项发表在2月20日PLoS Biology上的最新研究表明，一群4000万年来从未交配过的生物仍在在朝不同物种的方向进化。该研究挑战了关于交配是生物体多样化必需条件的假想，帮助科学家重新深入了解生物最初进化的原因。
这项由英国帝国理工学院（Imperial College London）研究人员进行的研究主要集中于蛭形轮虫（bdelloid rotifers），一种栖息在在池塘、河流、土壤和地衣中的微小水生动物。这些无性繁殖生物由母体卵细胞发育而来，是母亲的“克隆体”。化石记录和分子信息表明，蛭形轮虫在长达4000万年的时间里没有经历有性生殖过程，但是新的研究却发现，它们已经进化成了不同的物种。
利用DNA测序和电子显微镜等手段，科学家研究了生活在英国、意大利及世界各地不同水生环境中的蛭形轮虫的下颚，结果发现，遗传基因和下颚形态证明这些蛭形轮虫通过适应环境差异，已经进化成了不同的物种。
帝国理工学院生物系Tim Barraclough博士表示，“我们发现这种进化不仅仅是由于它们地域上的隔离，不同环境中选择压力（Selection pressure）的差异也是原因之一。最典型的例子就是生活在同一只水虱不同身体部位的两种蛭形轮虫，一种在腿上，一种在胸部。为了适应不同的生态小环境，它们在身体大小和下颚形态上发生了分化。我们的结论表明，经过数百万年，自然选择使无性繁殖的生物产生了与有性生殖生物类似的实体分化。”
此前，许多科学家认为有性繁殖是新物种形成所必需的，尽管像蛭形轮虫这样的无性繁殖生物也并非都完全相同，但仅仅是因为个体在“克隆”过程中发生的随机基因突变的累加。而新的研究证实了这些差异并非是任意的，而是一种所谓的“歧化选择”（Divergent selection）的结果，而这种“歧化选择”以前被认为是有性繁殖生物所特有的。
Barraclough表示，蛭形轮虫是一种神奇的生物，科学家一般认为无性繁殖生物会很快灭绝，而它们却活了几千万年。除此之外，它们适应环境和存活的能力也为科学家认识物种进化过程提出了新的研究课题。
Sex Optional For Evolutionary Adaptation
The pond-dwelling microscopic invertebrates known as bdelloid rotifers gave up on sexual reproduction more than 40 million years ago but have nevertheless managed to evolve into around 400 distinct species, say researchers in PLoS Biology. The new study challenges the notion that species diversification requires sexual reproduction, as asexual bdelloids display evolutionary patterns similar to those seen in sexually reproducing organisms.
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Using DNA sequencing and bdelloid jaw measurements taken using a scanning electron microscope, researchers Tim Barraclough and Diego Fontaneto examined a selection of the little critters from across Europe and other parts of the world. They found strong evidence that the rotifers had evolved into distinct species by adapting to differences in their environment.
"We found evidence that different populations of these creatures have diverged into distinct species, not just because they become isolated in different places, but because of the differing selection pressures in different environments. One remarkable example is of two species living in close proximity on the body of another animal, a water louse. One lives around its legs, the other on its chest, yet they have diverged in body size and jaw shape to occupy these distinct ecological niches. Our results show that, over millions of years, natural selection has caused divergence into distinct entities equivalent to the species found in sexual organisms," explained Barraclough.
Sexual reproduction was previously thought to be necessary for speciation, and significant differences in bdelloid rotifers were explained as arising solely through the chance build-up of random mutations that occur in the cloning process when a new rotifer is born. The new study, however, shows that these differences are not random, and are in fact the result of so-called "divergent selection."
Asexual reproduction was traditionally regarded by biologists as evolutionary suicide, so the new findings should prompt a reappraisal of our understanding of speciation. "[Their] very existence calls into question scientific understanding, because it is generally thought that asexual creatures die out quickly, but these have been around for millions of years. Their ability to survive and adapt to change certainly raises interesting questions about our understanding of evolutionary processes," noted Barraclough.
