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Bangalore: New research shows that natural agents such as bacteria have caused rapid shifting of the sex ratio in some animal species over time, providing one of the first examples of evolutionary change over a short span of about 100 years.

Tracking change: Museum specimen of butterflies in trays. British scientists have discovered that the sex ratio of a tropical butterfly called Great Eggfly has changed swiftly, driven by a bacterium called Wolbachia. Photo: Oxford University Museum of Natural History

“We were surprised at the speed with which change in sex ratio could occur," said lead author Emily Hornett of the University of Liverpool, in the UK. “Between 1886 and 1894 in Fiji, the male-killing bacterium rose from 50% to over 90% frequency, changing the sex ratio from 2:1 to 10:1."

“This is certainly one of the most dramatic examples of evolutionary change," said Suhel Quader, an evolutionary ecologist at the National Centre for Biological Sciences (NCBS) in Bangalore. “It also provides clues about how (natural) selection favours a suppressor when faced with such a massive fitness cost from the male killer."

Hornett says that besides bacteria, there are cases of meiotic drivers—genes that disrupt the normal rules of inheritance—that can sway the sex ratio in animals.

In contrast, human studies, said population geneticist Lalji Singh at the Centre for Cellular and Molecular Biology, show no evidence of the ratio—notably distorted in Indian and Chinese populations—being impacted by natural selection.

“The skewed male-female ratio is due to our own intervention," says Singh, who, as part of his research traces the origin of modern humans in different populations.

The British researchers inferred from the extracted DNA of museum butterflies what historical populations must have looked like.

Then they compared contemporary and museum samples, only to find a profound change in four out of five butterfly populations they examined.

Traditionally, for direct observation of evolution, scientists have required records over time from long-term population study. But resurrection ecology is a new approach with which evolution can be observed directly.

“For long, scientists have been constrained by the non-availability of historical data, so this technique, which allows us to examine how evolution has acted, is promising," said Kavita Isvaran, a professor at the Centre for Ecological Sciences at the Indian Institute of Science in Bangalore.

Quader says this method could be used to investigate a variety of questions in plants, insects and birds.

India has rich historical archives of these taxa in collections maintained by organizations like the zoological and botanical surveys and the Bombay Natural History Society, opening up vast opportunities, he says.

Incidentally, population geneticist Uma Ramakrishnan at NCBS is using museum tissue samples, ancient DNA so to say, for her study that is trying to construct a synthetic picture of evolution in the Indian subcontinent. Studying wild cats, she is stumbling upon data that show that the decline in India’s tiger population started 275 years ago.

At a broader level, the rapidity of evolution that the present study shows has a rather “sobering implication", particularly now when the rampant use of chemicals in agriculture and healthcare have put selection pressures on organisms.

“We are not sure what kind of evolution might we be prompting through the use of antibiotics, pesticides, genetically modified crops," said Quader.

Predictably enough, the organisms have responded to man-made chemicals by evolving, often rapidly, and developing resistance. Bacterial resistance to antibiotics or tuberculosis treatment, or resistance to DDT pesticide in mosquitoes, are examples of rapid evolution, says Quader

Comparing these organisms with their ancient counterparts using DNA extraction tool could reveal a lot, he added.

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