Last week at JournalPub we covered John Turner’s 1979 paper on contrasted modes of evolution:
This was one of the first papers to show that evolution can happen at different rates in different parts of the genome. It seemed strange to us, spoiled as we are by whole genome sequences, that this was considered a significant finding as late as 1979, but it’s difficult to reconstruct the context. We’d be interested to hear from anyone who was around at the time.
Mayr, for example, had argued for ‘genetic revolutions’ following the isolation of a founder population:
Isolating a few individuals (the “founders”) from a variable population which is situated in the midst of the stream of genes which flows ceaselessly through every widespread species will produce a sudden change of the genetic environment of most loci. This change, in fact, is the most drastic genetic change (except for polyploidy and hybridization) which may occur in a natural population, since it may affect all loci at once. Indeed, it may have the character of a veritable “genetic revolution”. Furthermore, this “genetic revolution”, released by the isolation of the founder population, may well have the character of a chain reaction. Changes in any locus will in turn affect the selective values at many other loci, until finally the system has reached a new state of equilibrium.
Heliconius species are usually more like the variable populations Mayr describes than the founder populations. However, single species can have many different wing patterns. By 1979, it was well known that these wing patterns were controlled by a handful of genetic loci, through a long series of genetic crosses showing the perfect segregation of loci with different wing pattern elements: red rays, yellow bands and so on (summarised in Sheppard 1985). Species such as Heliconius erato have races spread across South America with very different patterns, and yet these races can interbreed.
How much of the genome was responsible for these wing pattern differences? The method of choice for answering this question in the 1970s was allozyme electrophoresis, as DNA sequencing was still in its infancy. Proteins with variations in amino acid composition carry different electrical charges. Varying proteins could therefore be separated by running them on a gel. By 1979, a large library of protein variations had been reported in a wide variety of species (documented in detail in Chapter 3 of Lewontin’s The Genetic Basis of Evolutionary Change, available as full free PDFs).
Turner et al were the first authors to test allozyme variation in Heliconius. They took a set of 17 enzymes and tested them for variations in eight species of Heliconius, including multiple races of many species. They showed that none of the enzymes segregated with colour pattern.
For example, wing patterns in Heliconius erato were known to be controlled by between one and seven genetic loci, but the sampled populations of erato were found to be between 93 and 99% identical based on the selected allozymes. This was very similar to the genetic identity seen in Heliconius sara, a species with very few races and only minor wing pattern differences.
The enzymes chosen were the ones that were available, with no intentional bias towards genes that may or may not have been involved in wing patterning. From our point of view, it was unsurprising that such enzymes would not necessarily be involved in wing patterning. But perhaps this was surprising in 1979; perhaps it was expected that loci unrelated to colour patterning would be ‘carried along’ with sharp variation in leading colour pattern loci, following lines of thought like Mayr’s. After all, it is not so different from the ‘islands of speciation’ arguments we are having today.
It may be that we are now used to the idea that only small regions of a few chromosomes are responsible for the variation in colour patterns, with the rest of the genome being mostly very similar and freely flowing between Heliconius races. But we are definitely still struggling with the idea that different parts of the genome can evolve at different rates, under different contraints. And with that, I have to get back to thinking about ABBA-BABA windows…
