Sunday, 2 August 2015

Evolution in the Blink of an Eye

A male Eggfly or Blue Moon Butterfly, Hypolimnas bolina
Photo credit: Sylvain Charlat/Science
Extraordinary Flux in Sex Ratio | Science.

This example of evolution from 2007, the evolution of the Blue Moon butterfly on the South Pacific Samoan islands of Upolu and Savaii, is one of the fastest on record other than the rare occasions of a stable hybrid giving rise to a new species.

It appears to have happened in just a few years, over some ten generations, in response to intense selection pressure from a killer bacterium.

In 2001, researcher found that in the Samoan Islands, in populations of this butterfly, only about 1% were males. This was caused by a bacteria of the, Wolbachia genus, which infects females and which causes male embryos to die before hatching. The resulting success rate for egg hatching was also low.

We usually think of natural selection as acting slowly, over hundreds of thousands of years, but the example in this study happened in the blink of the eye, in terms of evolutionary time, and is a remarkable thing to get to observe.

Gregory Hurst, senior author on the paper and researcher in evolutionary genetics, University College London.
Quoted in ScienceBlogs
In evolutionary terms, the strategy used by Wolbachia which infects females was to kill of the male competition for food resource with, of course, no planning or forethought that this could lead to the eventual extinction of the females too. If produced more Wolbachia with this capability, so more Wolbachia had this capability.

Wolbachia is a genus of bacteria that infects arthropods including many insects and normally adopts a strategy which involves favouring females. In one tiny species of wasp, Trichogramma, Wolbachia bacteria have replaced male gametes, so the wasp is unable to reproduce without them but produces only females. It turns up again in the tsetse fly where it inhabits the gonads although its effects are unknown. In other species Wolbachia causes cytoplasmic incompatibility between individuals infected with different strains.

But, in the Samoan Islands at least, the male Blue Moon butterflies are staging a fight back.

When the islands were again surveyed during 2005 it was found that on Upolu the male to female ratio was 1:1 but on Savaii out of 100 specimens caught, not a single male was found. The males on Upolu had acquired a gene which suppressed Wolbachia. But, and this is where the rate of evolution was most astonishing, by 2006, the ratio on Savaii was also approaching 1:1. In a single year the suppressor gene had reached Savaii and males were increasing rapidly in the population. These males were also carriers of the mutant, suppressor gene. Hatching success rates of the eggs had also improved in line with this recovery.

It's not know how the mutant gene was acquired. It could have come by chance migration from Southeast Asia where it is established, or it could have arisen by mutation in the local population. The important thing though is that natural selection is undisputedly the cause of the rapid increase of this allele in the population on these two islands.


The ratio of males to females in a species is often considered to be relatively constant, at least over ecological time. Hamilton noted that the spread of "selfish" sex ratio-distorting elements could be rapid and produce a switch to highly biased population sex ratios. Selection against a highly skewed sex ratio should promote the spread of mutations that suppress the sex ratio distortion. We show that in the butterfly Hypolimnas bolina the suppression of sex biases occurs extremely fast, with a switch from a 100:1 population sex ratio to 1:1 occurring in fewer than 10 generations.

The suppressor gene allows infected females to produce males. These males will mate with many, many females, and the suppressor gene will therefore be in more and more individuals over generations.

Sylvain Charlat, lead author on the study, post-doctoral researcher, University of California, Berkeley.
Now, for doubters of evolution, what else would you expect, where a chance mutation has conveyed protection on the males? With fewer males being killed and so more males hatching and being available to mate with the females, why would you not expect more males carrying this mutation to appear in the population? To doubt evolution you have to doubt the simple maths in this formula.

Is there a creationist prepared to explain why more males hatching would not lead to more males, and why, if the males hatching carry the mutant gene, more males would not carry the mutant gene?

Is there an Intelligent (sic) Design advocate who can explain why designing a bacterium to kill off the males of a species then having to redesign the males to avoid them being killed off is an example of intelligence? Is this perhaps an example of this 'Intelligent' designer thinking of a 'brilliant' design for making more Wolbachia bacteria, then panicking when it realised the stupidity of its design and having to rush through a work-around?

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