一項(xiàng)發(fā)表在2月20日PLoS Biology上的最新研究表明，一群4000萬年來從未交配過的生物仍在在朝不同物種的方向進(jìn)化,。該研究挑戰(zhàn)了關(guān)于交配是生物體多樣化必需條件的假想,，幫助科學(xué)家重新深入了解生物最初進(jìn)化的原因,。
這項(xiàng)由英國帝國理工學(xué)院（Imperial College London）研究人員進(jìn)行的研究主要集中于蛭形輪蟲（bdelloid rotifers）,，一種棲息在在池塘、河流、土壤和地衣中的微小水生動(dòng)物,。這些無性繁殖生物由母體卵細(xì)胞發(fā)育而來,，是母親的“克隆體”?；涗浐头肿有畔⒈砻?，蛭形輪蟲在長達(dá)4000萬年的時(shí)間里沒有經(jīng)歷有性生殖過程，但是新的研究卻發(fā)現(xiàn),，它們已經(jīng)進(jìn)化成了不同的物種,。
利用DNA測序和電子顯微鏡等手段，科學(xué)家研究了生活在英國,、意大利及世界各地不同水生環(huán)境中的蛭形輪蟲的下顎，結(jié)果發(fā)現(xiàn),，遺傳基因和下顎形態(tài)證明這些蛭形輪蟲通過適應(yīng)環(huán)境差異,，已經(jīng)進(jìn)化成了不同的物種。
帝國理工學(xué)院生物系Tim Barraclough博士表示,，“我們發(fā)現(xiàn)這種進(jìn)化不僅僅是由于它們地域上的隔離,，不同環(huán)境中選擇壓力（Selection pressure）的差異也是原因之一。最典型的例子就是生活在同一只水虱不同身體部位的兩種蛭形輪蟲,，一種在腿上,，一種在胸部。為了適應(yīng)不同的生態(tài)小環(huán)境,，它們在身體大小和下顎形態(tài)上發(fā)生了分化,。我們的結(jié)論表明，經(jīng)過數(shù)百萬年,，自然選擇使無性繁殖的生物產(chǎn)生了與有性生殖生物類似的實(shí)體分化,。”
此前，許多科學(xué)家認(rèn)為有性繁殖是新物種形成所必需的,，盡管像蛭形輪蟲這樣的無性繁殖生物也并非都完全相同,，但僅僅是因?yàn)閭€(gè)體在“克隆”過程中發(fā)生的隨機(jī)基因突變的累加。而新的研究證實(shí)了這些差異并非是任意的,，而是一種所謂的“歧化選擇”（Divergent selection）的結(jié)果,，而這種“歧化選擇”以前被認(rèn)為是有性繁殖生物所特有的。
Barraclough表示,，蛭形輪蟲是一種神奇的生物,，科學(xué)家一般認(rèn)為無性繁殖生物會(huì)很快滅絕，而它們卻活了幾千萬年,。除此之外,，它們適應(yīng)環(huán)境和存活的能力也為科學(xué)家認(rèn)識(shí)物種進(jìn)化過程提出了新的研究課題。
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.
