Many people wonder whether the butterflies that spend the summer in Minnesota or New York or Ontario stay together when they migrate to Mexico for the winter. There are a couple ways scientists have studied this question. One is the tagging project in which many of you participate. Tagging helps us trace where individual Monarchs go. By tracking many individuals over time, we will hopefully get a good picture of how whole groups of Monarchs move throughout the year.
Another way to answer this question involves looking at the population genetics of Monarchs. Population genetics, which combines theories from evolution and genetics, studies how genes are distributed in a population. By using the tools of population genetics, biologists can evaluate the distribution of genes in Monarch populations to get a better idea of how groups of Monarchs move around and mate. Some distributions would indicate that Monarchs stick together in groups and tend to mate within their own group, while other distributions would show that Monarch populations mix either in the summer, in the winter, or during both times.
Two experiments have investigated the population genetics of Monarch butterflies, and they found some interesting and surprising results. To help you better understand the ideas behind those studies, we encourage you to go review Theories in Evolution and Population Genetics before reading the summaries of these studies.
Genetic Structure of Summer and Migratory Monarchs
Eanes, W.F. and R.K. Koehn. 1978. An analysis of genetic structure in the Monarch butterfly, Danaus plexippus L. Evolution 32(4): 784-797.
Eanes and Koehn studied the genetics of different Monarch populations in the early 1970s. They collected 20 different sets of samples, both during the summer and during migration. Using electrophoresis to examine the same protein in different individuals, they found that Monarchs have allele frequencies that sort out into groups somewhat in the summer and become uniform again during migration. These results indicate that Monarchs divide into slightly isolated populations during the summer but mix together during migration (and, they assume, in the winter roosts although the roosts had not yet been discovered when they did this research). Migratory populations and roosts, therefore, include individuals from all over North America; all the Monarchs from a particular summer region do not necessarily overwinter in the same place, and their descendants may not return to the same region the next year. The mixing that happens during spring mating in the roosts overwhelms any genetic differentiation that occurs during summer in isolated populations.
Eanes and Koehn found another interesting pattern in allele frequencies. For three of the eleven proteins they studied, there were more heterozygotes than expected. In at least once case, males and females also differed in which allele they were likely to have (that is, males more often had one version of the protein while females more often had the other version). When alleles have different average frequencies in males and females, mating will more often produce heterozygotes. For Monarchs, there are still many unanswered questions about whether mating behavior results in different allele frequencies between sexes and what causes increased heterozygosity in the population.
DNA Variation in Monarch Butterflies
Brower, A.V.Z. and T.M. Boyce. 1991. Mitochondrial DNA variation in Monarch butterflies. Evolution 45(5): 1281-1286.
Brower and Boyce studied the mitochondrial DNA (mtDNA) of Monarch butterflies from the United States, Mexico, and the West Indies to see how similar or different their genetic material was. They were especially curious about whether the eastern and western populations of Monarchs in North America were genetically different; the eastern population overwinters in Mexico while the western one overwinters in California, and there is no evidence that these two populations ever interbreed. They looked at variation in mtDNA using restriction enzymes, a technique that identifies differences in DNA sequences. If one population, or individual, had a small change in its DNA, this technique can reveal that change. In some other insect species, studies have found that there are big differences in individuals' mtDNA between regional populations, and sometimes even within a region.
To their surprise, Brower and Boyce found almost no variation in any of the Monarch populations' mtDNA, including the ones from the West Indies. Using 13 restriction enzymes, they found only two individuals with a single difference in one site, and they attribute this difference to a single base substitution. This level of similarity in the DNA from geographically isolated populations is dramatically different from most other studied groups of animals. Vertebrates, for example, have differences at 10 times this level while other insects show differences in mtDNA even within a population.
The most plausible explanation Brower and Boyce have is that all of these Monarchs underwent a bottleneck in recent evolutionary time. Bottlenecks reduce the genetic diversity in a population (for another example, read about cheetahs) because only a small number of individuals and their DNA serve as the ancestors for the present populations. Since mtDNA is maternally inherited, it seems likely that sometime in the recent past there was a significant reduction in the number of females who reproduced. Since that bottleneck, enough time has not passed for major changes to have occurred.