It was in 1967 that W.D. Hamilton published his seminal thoughts on "extraordinary sex ratios", suggesting ways of understanding why "Fisher's principle" for 1:1 being the equilibrium ratio has many exceptions in nature. Records of the butterfly Hypolimnas bolina living on the Samoan islands reveal extreme ratios and also extreme fluctuations of those ratios. Media reports have majored on the thought that this is evolution in action: "Evolution occurs in the blink of an eye" (LiveScience) and "Butterfly shows evolution at work" (BBC). Since the media have rarely heeded Phil Johnson's sound advice to distinguish the different meanings of the word "evolution", it is useful for us to consider the research with this in mind.
Sex ratios are distorted by the presence of a maternally inherited bacterium which has the effect of selectively killing male embryos. The authors report ratios of >99% female to nearly 1:1. These were different on different islands and at different times. The genetics of this shift of sex ratios is summarised in one paragraph with some supporting online data. There is not enough information here for anyone to either confirm or challenge their conclusions. However, more substantial evidence for "suppression genes" had previously been published in 2006, and for the sake of this discussion, we will accept that "the shift in sex ratio was caused by the spread of host suppressor genes."
What we should be aware of is that the source of these genes is unknown. Various options exist. A novel mutation is a possibility (the 2006 study considered the suppression was controlled by a single locus). Another avenue to explore is that the gene had existed previously in the population (or sub-populations) of butterflies.
So the story concerns a genetic trait restoring the sex ratio of a species of butterfly living in the Samoan Islands. This seems to be a straightforward case of natural selection of a gene whose presence is directly related to the survival of male butterflies. The lead author comments: "To my knowledge, this is the fastest evolutionary change that has ever been observed". It is true it is rapid, but we might be forgiven the thought that if it were not rapid, the species would last little longer than its lifecycle!
In terms of its significance for evolutionary theory, this research is on a par with the peppered moth: there is no insight into speciation and there is no change in complexity of the organism. It is an observation that is perfectly compatible with Darwinism, ID and creation-based biology. The lead author is quoted as saying: "We're witnessing an evolutionary arms race between the parasite and the host. This strengthens the view that parasites can be major drivers in evolution". This is much more controversial. There is widespread view in ID circles that these genetic skirmishes actually lead to genetic impoverishment. Parasites may drive change, but it is likely to be towards extinction. To justify the "major driver" claim, the genetics of the process need to be analysed in much greater detail.
Extraordinary Flux in Sex Ratio
Sylvain Charlat, Emily A. Hornett, James H. Fullard, Neil Davies, George K. Roderick, Nina Wedell, Gregory D. D. Hurst.
Science, 13 July 2007: Vol. 317, p. 214, DOI: 10.1126/science.1143369
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.
Bryner, J. Evolution Occurs in the Blink of an Eye, LiveScience, 12 July 2007
Butterfly shows evolution at work, BBC News, 12 July 2007
Hornett, E.A., Charlat, S., Duplouy, A.M.R., Davies, N., Roderick, G.K., Wedell, N. and Hurst, G.D., Evolution of Male-Killer Suppression in a Natural Population, PLoS Biology Vol. 4, No. 9, e283 doi:10.1371/journal.pbio.0040283
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