In 1870, a shipment of nursery stock from China arrived in San Jose, California, carrying an unsuspecting menace that would cause devastation for farmers across Northern America. The small fruit-eating insect, now known as the San Jose scale, soon spread rapidly across United States and Canada, destroying orchid trees as it went. Carl Zimmer, in his book, Evolution, recounts the fascinating story behind this small pest (pages 241-243).
At first, farmers could totally eradicate outbreaks of the San Jose scale by spraying their crops with a mixture of sulphur and lime. However, by the turn of the century, a number of farmers began to realise that this pesticide was beginning to loose its potency; a few scales would survive the spraying and then breed rapidly. Farmers in Washington state were convinced that manufacturers were supplying poor quality pesticide, so they built their own factory to produce a 'pure' form of sulphur-lime. But even their home-made concoction failed to curb the spread of scales. The question was: why did such an effective pesticide became totally useless in just a few years?
Enter the entomologist A.L Melander. In 1912, after studying the San Jose scale problem, he realised that the scales were developing a resistance to the pesticide. How? His short answer was that evolution was happening.
The long answer is as follows:
- In the scale population, there were a few individuals that possessed a mutation that made them resistant to the sulphur-lime pesticide.
- Under normal circumstances, the mutation would remain in only a few individuals. However, when farmers started applying the pesticide, most of the non-resistant scales – initially comprising a majority of the population – were killed off. But the resistant scales carrying the mutation survived.
- The surviving individuals would then breed with each other and other non-resistant scales to produce resistant offspring. Over time, with consistent application of the pesticide killing off non-resistant scales, the mutation would spread throughout the entire population, eventually making the pesticide ineffective against a growing number of resistant scales.
This is a classic example of Darwin's theory of natural selection, beautifully expressed in an example that was experienced by many people. And this scenario has occurred many times in humankind's struggle against germs and insects. One only has to think about the constant need to change medications in order to combat Malaria, to recent scares over drug resistant strains of Tuberculosis. These and many other cases show that biological evolution does happen.
But there is one very valid objection in response to this. One can argue that all I'm describing here is microevolution, the small changes that occur within species, and what I've argued above does not support macroevolution, the large changes that result in new species.
My response to this? Watch this space . . .
Next post: The unbelievability of change
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