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Genetic Variation: the Raw Material of Evolution

Posted on Sunday, January 1, 2006 at 2:56 pm in Monarch Biology

For natural selection to occur there must be genetic variation in a population. To understand what this means, we need to understand the concept of phenotype. An organism’s phenotype is all of its structural and functional properties, like size, hair color and blood type. Phenotypes are the product of genotype, the genetic makeup of the organism, and its environment. 

The relative importance of these two factors is the basis of the nature/nurture debate about human intelligence. Think of eye color—in humans, genes code for blue or brown eyes, creating two different phenotypes. There is little or no environmental influence; if a person grows up in Alaska or sub Saharan Africa, their eye color won’t be affected. However size is a different story. A person with very tall parents is likely to be tall, but diet can also affect height. A person’s height is therefore the result of both genes and environment, or of both nature and nurture.

If a population is genetically identical, evolution can’t occur. Even if individuals have different phenotypes due to environmental variation, these differences could not be passed onto their offspring. However, if there is a genetic variation and one genotype makes individuals more apt to pass on their genes, then this genotype will increase in frequency. The genotype can be favored in a number of ways—by increasing frequency of mating, increasing the number of surviving offspring, or by more successfully avoiding predators. It can also come from many sources, including mutations, genetic drift and influx of genes from another population. If a population becomes different enough that t can no longer mate and create viable offspring with members of another population of its species, it has become a different species.

Phenotypic Variation in Monarchs

Close-up of monarch wing spots (Photo: M. Solensky)

Although at first glance all monarchs tend to look the same to humans (although you can probably tell a male from a female, a difference caused by genes in monarchs and most, but not all, other animals), they do in fact vary quite noticeably if you look closely.

One of the first things we notice is that monarchs vary in wing length and girth (some look quite fat). These differences, like human sizes, are a result of both genotype and environment. On close examination, there are also differences in wing coloration and pattern, and in stripe patterns in larvae. Recently, a Monarch Lab undergrad quantified the number of spots in different locations on monarchs’ wings, and found a surprising amount of variation. Spot number, color and size have been the subject of several excellent insect fair projects over the years. While we haven’t determined whether these differences are genetic or environmental, it’s a good bet that they are genetic; it’s hard to imagine how rearing conditions could affect the spots on a monarch’s wings. However, this could be tested.

Variation in spot size on monarch wings brings up another important evolutionary concept. If having a certain number of spots made a monarch more fit for some reason, and if spot number was caused by a monarch’s genotype, we would expect spot number to evolve. However, some traits don’t have a huge effect on an organism’s fitness, like blood type or eye color in humans. We expect that variation in these traits could last for a long time, whereas variation that makes a big difference would soon disappear as the more advantageous genotype would take over.

Investigating phenotypic variations is a great way to introduce students to evolutionary concepts. They can measure and record wing lengths or other traits such as spot characteristics, and analyze their data, looking at means, medians and mode, as well as the amount of variation. The resulting bell curve would provide discussion material for evolutionary concepts. 

White Monarchs: Unusual phenotypes

White monarch larva with smaller normal larvae

Nivous monarchs are grayish white in all areas of the wings that are typically orange. They are rare in Australia, New Zealand, Indonesia, and the Americas. In Hawaii, however, the frequency of white monarchs is 10%! It is believed that the gene responsible was present in the founder population of the mid-1800’s. The genetics of this variation appear to be a simple recessive gene. Interestingly, the white form of monarchs has increased in Hawaii over the past 20 years, perhaps as a result of selective predation on orange butterflies by bulbul birds, which find the white form more difficult to see.

Monarch Larvae also have a white form that was the subject of a paper published by Monarch Lab researchers and also observed by Monarch Larvae Monitoring Project volunteer, Sherry Skipper-Spurgeon. These larvae lack the yellow pigment found in most monarch larvae, and looked like caterpillar zebras. WE did study the genetics of this variation, and found that the trait is due to a single recessive gene. If this trait caused higher fitness among its bearers, it would be expected to increase in frequency. However, the fact that this is rare suggests that it is disadvantageous. 

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