Seasonal monarch populations (1991-1997), western monarch population, population genetics and theories in evolution and population genetics. |
SEASONAL POPULATIONS |
Spring 1997: Off to a Great Start! Observations of the spring migration are usually scattered and rather unexciting. Relatively few Monarchs are seen by most observers and the numbers of eggs and larvae reported from most locations are usually few. Not so this year! The numbers of adult Monarchs reported from TX, AR, GA, MD, OH and points in between have been astonishing and the numbers of eggs and larvae are unusually high. In many locations the leading edge of the northern migrants seem to be ahead of the milkweeds and there have been many observations of multiple ovipositions on single plants. This is certainly a great start to the season but is it too good? Some experts are worried that this is an ominous sign, that perhaps the Monarchs have left the roosts too early, that they will exhaust their food supply and so on. Perhaps there is reason for concern, but we are skeptical. Every population has its ups and downs and nature has a way of dealing with them. Winter 1996-1997 Spring 1996: Off to a Slow Start? Drought conditions from northern Mexico combined with an unusual jet stream and spring moisture pattern may have had the effect of causing Monarchs to move in an easterly and northeasterly direction rather than northward through the dry regions of Texas, Oklahoma and Kansas. At this time (20 May), there are more reports of Monarchs from the eastern states, and even Ontario, than there are for most of the midwest. This is the third year in a row that the Monarchs appear to have moved up through the eastern states faster than they have the central midwest. Is this an artifact of the number and distribution of observers or is this pattern a consequence of weather factors that have the effect of limiting movement in the midwest while favoring flight to the northeast? Does anybody have an enterprising student who would be interested in relating patterns of spring Monarch sightings to weather conditions? Winter 1995-1996 Assessment of the conditions at the roosts and the general state of the overwintering population were complicated and more than a little confused by the differing assessments of the effect of the snow storm which dumped 13-27 inches of snow throughout the Transvolcanic mountains from 29-31 Dec. Fortunately, the early estimates of 30% morality due to the storm were excessively high. Post storm assessments by several groups working independently placed the mortality rate at 5-7%. Fall 1995 The midsummer Monarch populations in the midwest seemed to be below normal and only at the end of August did it appear that populations were at the usual fall levels in the northern states. Nevertheless, the numbers of Monarchs seen moving south in Iowa and Nebraska were not remarkable and there was nothing to prepare taggers in Kansas for what happened in mid September. Monarchs arrived in large numbers on the 14th across central Kansas on a line extending from Olathe in the east to Sharon Springs in the west. Subsequent to their arrival, unseasonally cold weather delayed the migration for 4-7 days, depending on the location, and during this interval students at schools from Olathe to Hays tagged more than 6,000 Monarchs. The concentrations of Monarchs seen throughout central Kansas in this period appeared to be higher than in the previous three years of the Monarch Watch. Many observers reported seeing trees and hedgerows covered with thousands of Monarchs. In the previous season (1994), the migration stalled in Iowa for about a week and then stalled briefly again in southern Kansas before moving into Oklahoma. A unique aspect of the migration this year was the ability to receive email messages from observers to the north. These reports allowed us to predict the arrival of the "front" or mass of the migration with some accuracy and to warn taggers to the south of the imminent arrival of the Monarchs. Spring 1994 This year the northward migration is being monitored by "The World School" "Journey North" through Internet (see Internet address below). The same information can be accessed through Prodigy. The pattern of the migration this year indicates a more rapid spread into the southeastern rather than the midwestern states. This is not surprising since the weather patterns were not favorable for movement of Monarchs into states at the latitude of Kansas (37-40� N) until mid-April. In fact, at this writing (20 April), Monarchs arriving in eastern Kansas are ahead of their host plants as no new growth has appeared so far (However, 6 days later the milkweed plants were 2-6 inches in height). In general, it looks like the 1994 populations are off to a good start. Winter 1993-1994 Summer and Fall 1993 Spring 1993 Winter 1992-1993 In February, a team of scientists and Mexican authorities visited all the known Monarch roosting areas. Members of this group estimated that 85-93 million Monarchs overwintered at these roosts in '92-93. Whether these are high or low numbers is not clear, because in previous years not all the roosts were surveyed at the same time. Estimates of populations at the best known roosts, "El Rosario" and "Chincua", seemed to indicate that these overwintering populations were of "normal" size. Even though the Monarch population was low in the eastern United States, this reduced population did not have a clear impact on the number of overwintering Monarchs. Summer and Fall 1992 Several observers reported that extremely large numbers (perhaps millions) of Monarchs reached the United States/Mexican border area near Del Rio, Texas in the last few days of September. This occurred at the same time that large numbers of Monarchs reached Austin, and southeastern Texas. Reports from Austin indicated that Monarchs were abundant from the end of September until the first week of November. Overall, the pattern of reports from Texas seemed to indicate that the fastest-moving front of the southward migration was in west Texas, and that Monarchs funnel into northeastern and eastern coastal Texas from the North and East over much longer periods. Spring 1992 Fall 1991 Summer 1991 |
WESTERN POPULATION |
There are two geographically distinct Monarch populations in North America. The eastern population overwinters in Mexico and breeds east of the Rocky Mountains. The western population overwinters along the California coast and breeds in areas west of the Rockies. Contact between eastern and western Monarchs is minimal suggesting that there is little exchange, or what scientists call gene flow, between these populations. In recent years, several people have transplanted migrating Monarchs between east and west to determine, if for example, western Monarchs introduced in the east would be found in Mexico. Many scientists are concerned about this practice and cite numerous reasons, such as the potential introduction of diseases from one population into another which is why this practice should be stopped immediately. Under the leadership of Dr. Lincoln Brower, 14 scientists have coauthored an article which appeared in the September 1995 issue of BioScience and outlines numerous reasons for not mixing eastern and western Monarch populations. Teachers may find this article useful for class discussions. The question "why should transfer between eastern and western Monarch populations be discouraged?" could be presented to the class. The article could then be used as a guideline to lead students by questioning, through many of the same biological and practical considerations presented by the scientists. For individuals living west of the Rockies and interested in Monarch conservation, check out our page on Other Monarch Programs |
POPULATION GENETICS |
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 below 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. |
THEORIES IN EVOLUTION & POPULATION GENETICS |
Evolution, the science of how populations of living organisms change over time in response to their environment, is the central unifying theme in biology today. Evolution was first explored in its modern form in Charles Darwin's 1859 book, On the Origin of Species by Means of Natural Selection. In this book, Darwin laid out a strong argument for evolution, or "descent with modification" as he called it. He postulated that all species have a common ancestor from which they are descended. As populations of species moved into new habitats and new parts of the world, they faced different environmental conditions. Over time, these populations accumulated modifications, or adaptations, that allowed them and their offspring to survive better in their new environments. These modifications were the key to the evolution of new species, and Darwin proposed natural selection or "survival of the fittest" as the mechanism by which that change occurs. Under natural selection, some individuals in a population have adaptations that allow them to survive and reproduce more than other individuals. These adaptations become more common in the population because of this higher reproductive success. Over time, the characteristics of the population as a whole can change, sometimes even resulting in the formation of a new species. In the beginning of the last century, scientists began to understand genetics and how offspring inherited traits from their parents. Figuring out how offspring inherited traits from their parents had stumped Darwin, and twentieth-century biologists soon recognized the importance of genetic theory to natural selection and evolution. By the 1920s, a new field of science had emerged called population genetics. These scientists expanded the scope of genetics, which had focused previously on how individuals inherited and passed on genetic information, by studying how genetic change occurs in populations. As scientists combined ideas from population genetics with Darwin's ideas about evolution, a modern synthesis theory of evolution emerged in the 1940s. This synthesis, which includes most of Darwin's ideas but focuses on populations, combines population genetics with natural selection and creates a powerful tool for examining evolution in action in the natural world. One of the most important tools population genetics gave to the study of evolution was the principle of Hardy-Weinberg equilibrium. This principle states that there is nothing in gene replication, meiosis, fertilization, or reproduction that changes the frequency of gene alleles over time. As long as no other forces act upon the population, a gene in that today has 25% allele "A" and 75% allele "a" will still have 25% A and 75% in a million years. The Hardy-Weinberg equilibrium is based in five assumptions, which, if they held true in nature, would create a situation where no other forces were acting upon a population and no change in gene frequencies (evolution) would occur. These assumptions are:
Since these are the assumptions that must hold true for the Hardy-Weinberg equilibrium to be maintained, their opposites are the causes of evolution, or the change of allele frequencies in populations. For example, mutation can cause changes in allele frequency by creating new, altered genetic material while gene flow can change allele frequencies by introducing new alleles to the population through immigration or removing alleles through emigration. Of these five causes of evolution, only the last one involves natural selection, or directional change imposed by survival of the fittest in a harsh environment. The first four, all of which can play critical roles in evolution, involve chance events. By studying the allele frequency in a population, and doing some math, ecologists can determine whether allele frequencies occur as predicted by the Hardy-Weinberg equilibrium principle. Hardy-Wienberg allele frequencies are likely with large populations that have lots of gene flow and random mating, and are unlikely if populations are small, isolated, or have non-random mating patterns. If a study reveals allele frequencies that do not match Hardy-Weinberg predictions, the population is probably structured in some way (e.g., is made up of more than one isolated subpopulation or has non-random mating). This is what two teams of researchers have done with monarch butterflies. |
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