Transitions: The Evolution of Life

Butterflies are used quite frequently in the study of evolution. For example, one type of study involves the evolution of mimicry - that is one form imitating another.

The above is a picture of a tiger swallowtail. They inhabit a large part of the United States. In one part of their range, however, they look a little different.

The butterfly on the bottom left is also a tiger swallowtail. As you can see it looks a little different from the tiger swallowtail on the bottom right. Why the difference? The range of the tiger swallowtail overlaps with the range of the pipevine swallowtail.

The pipevine swallowtail is somewhat poisonus - any predator that eats it soon winds up sick, so most predators have learned to avoid it. If you compare the pipevine swallowtail to the blue form of the tiger swallowtail you will notice that they look a lot alike. The tigerswallow tail has evolved a coloration that mimics the pipevine swallowtail in an effort to avoid predation. Interestingly enough, the blue form of the tiger swallowtail is female. Which brings us to what I really want to talk about.
Below is a picture of a male peacock. Note the large spectacular tail. How can evolution explain it. If natural selection were at work you would expect that this tail would interfere with, for example, the peacock escaping from predators.

How to explain it? There is another type of selection, called sexual selection, that researchers use to explain things like the peacock’s tail. Sexual selection involves competition for mates and can be broken down into two different.
First is male competition. Here males compete with each other for females. In the picture of the baboon below note the large canines.

Although they serve a defensive purpose (they help defend he animal against predators) they also serve a purpose in male competition for mates. They are displayed prominately towards other males in the baboon troop and their is some evidence to indicate that males with larger canines have more mates.

Another type of sexual selection is female choice. Females choose mates with some characteristic they find desirable in a mate. Over time the characteristic, such as the peacock’s tail, becomes exagerated in size or some other quality. The question is how can we tell if some trait or characteristic has evolved by means of sexual selection?

Enter the butterflies. Below is a picture of a butterfly called Bicyclus anynana. On the left is a wet season form (when the butterflies mate) and a dry seaon form.

Bicyclus has been used in studies of mimicry (such as those on the swallowtails mentioned above) but recently an clever experiment was performed to study sexual selection in Bicyclus anynana. The results were interesting. Notice along the edge of the wing there are round (eye) spots. Researches belived that females chose males based on the size and color of these spots. To test the theory researchers carefully altered, via painting, a wide variety of characteristics on male butterflies including: wing size, eyespot size, quantity of eyespots on the wing, eyespot and pupil color, and pupil reflectivity. They then introduced the altered males and kept track of which males were choosen and how often they were choosen by the females. A picture of one of the altered males is below.

The article goes on to point out:

“Once we found a trait that appeared to be important, we then would exaggerate it or reduce it to pin it down,” said Monteiro.

None of the variations induced on the ventral side appeared to have any affect on the females’ mating decisions, leading the researchers to conclude that the ventral side of the wing does not play a role in the decision making.

But when the researchers painted the white pupil on the dorsal side with black paint, thereby eliminating the pupil, these males were much less desirable to females by a ratio of two to one, demonstrating clearly that females preferred the presence of the white pupil.
However, a large white pupil, about twice the diameter of a natural pupil, also was not found desirable by females, indicating strong sensitivity to a set of rather narrowly defined features, such as eyespot pupils that measure approximately half of one millimeter.
The most conclusive finding resulted when the researchers painted the white pupils in male eyespots on the dorsal side with a plant extract, rutin, which maintained the pupils’ whiteness, but eliminated their ultraviolet reflectivity.

“When there was no UV reflectivity, which butterflies can see, females registered a strong distaste,” said Monteiro. “Selection against the absence of UV reflectivity was as strong as selection against the absence of a pupil altogether.”

The reasons for this phenomenon are complex, but Robertson noted that the UV reflectivity may be important in what is known as photic stimulation — a flashing light effect — during the series of events that lead up to mating.

“When the male approaches the female, he opens and closes his wings in rapid succession so she can observe his pupils,” she explained. “We believe the purpose of the fluttering of his wings is two-fold: to spread pheromones to her antennae and to stimulate her visually. The female appears to be very sensitive to this rapid flickering, which probably looks to her like a strobe-light effect.”

The important point here is that the researchers came up with a theory about evolution. Defined somes variables they felt were related to the theory and then systematically tested the theory by altering those variables. Another interesting point. Female Bicyclus anynana butterflies also have these eyspots so the results of this study have led researchers to start a study the effects of the eyspots on male competition and to see if they play a role in how male butterflies choose females. The point here iis that the results of the original study created new questions for future research.

Studying the evolution of insects can be difficult because they don’t fossilize well. But there are ways to study insect evolution. All life affects it’s environment in one form or another. In some cases the affect can be large, in others small. Occassionally, these affects remain behind long after the organism that caused them has died. Animal footprints, such as those of two dinosaurs below (from Glen Rose trackway), are good examples.

But organism leave other types of traces besides footprints. Below is a picture of a leaf. If you look closely you can see a black line zig-zagging around the leaf. This represents a track of an insect burrowing through the leaf and eating as it goes. The black bits are - well, I’ll leave that to your imagination.

The next picture shows what happens when an insect feeds on a leaf. The blackened edges, in this case, are caused by the plant trying to heal itself. The white parts (center middle) are plant cells that have become swollen and discolored due to the damage.

The above two picture are of fossil leaves found in Colorado and date to about 35 million years ago. Scientists interested in the study of insect evolution realized that different insects leave different kinds of damage and that the type of leaf damage could be used to identify the type of insect that caused the damage. You can go here for a quick overview on how this is done.
With this information you can learn a lot about how insects lived in the past. As one scientist puts it:

“Insect damage on leaves, the remains of insect meals, is uniquely valuable data,” … “While actual insect fossils can give us taxonomic information, leaf damage provides unique ecological data about which and how many kinds of insects were eating and interacting with ancient plant species in the deep past. Also, insect damage on fossil plants, which can be very abundant, can give us a great deal of information about insects at times and places with very few insect fossils.”

Recently, fossils in Patagonia were analyzed using the above ideas. Almost 3,600 plant fossils were collected and compared to fossils collected in North America. The fossils were examined for insect damage:

The researchers classified damage by feeding group and damage type. The four feeding groups are those insects that feed on the external leaf, chewing holes, edges and other leaf parts; those insects that mine tissues inside the leaf; those that produce bulbous galls and those that pierce and suck the leaves. Because different insects chew, mine, gall and pierce in different ways, the researchers recognized 52 discrete damage types from the four feeding groups. They applied these categories to both bulk samples from single quarries and to individual leaf species.

By comparing the Patagonian fossils with the North American fossils, researchers were able to learn about past environments and how past environments affected the number of different types of plants and insects in each area.

The current evidence from South America suggests that there were a large number of different insect lineages feeding on a large number of plant species.

Above is a 52 million year old fossil of a laurel leaf. The white circles with black centers represent the feedin activities of the fairy moth (pictured below)

“There was tremendous diversity and abundance of insects and plants in the Eocene,” … “Insects depend on plants to survive. If you have diverse plants, you get diverse animals. We know that plant and insect diversity are linked today and our study shows that plant and insect diversity were linked in the past as in today’s South America.”

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