Lab 1 - Evolution
 
 

Monarch butterfly, Photo by B. E. Fleury






Introduction

It's difficult to hold a meaningful lab on evolution, at least one that we can do in a single afternoon. Evolution usually extends over several human lifetimes, often over millions of years or more. One of the ways we can demonstrate the reality of evolution is to simply consider biodiversity, the large numbers of species, many of which may have similar forms, but are reproductively isolated from one another - like lions, tigers, leopards, cheetahs, house cats, lynx, mountain lions, bobcats, and all the other members of the Family Felidae. It is difficult to imagine any process other than evolution that could have produced such an amazing number of ways to be a cat. Organisms who live on different continents, but in similar environments, are often very similar to one another. Animals like the American bison and the African wildebeest, both large mammalian grazers who browse in open grasslands, hint at the broader patterns of evolution.

Further evidence of that pattern comes from a detailed study of biogeography, the geographic distribution of plants and animals. The plants and animals that we see in a particular place often traveled there from somewhere else, where conditions were somewhat different, and then evolved to adapt to their new environment.

An important line of evidence for evolution is the fossil record. The fossil record shows us the evolutionary history of life on earth. We find many extinct forms which are obviously related to more modern forms, evidence of descent with modification.

Another line of evidence for evolution comes from the study of embryology. In vertebrates, for instance, the early stages of development are extremely similar to one another, even though the adult stages are very dissimilar (like mammals and reptiles and birds). This implies a common ancestry for all mammalian species. Among the invertebrates, there are several similar examples. Certain annelid worms have a larval stage called a trochophore larva, which is essentially identical to the trochophore larval stage of the molluscs. This suggests that these two groups share a common ancestry. All the various types of crustaceans share a common larval stage, called a nauplius larva, which is one of the characteristics uniting that diverse group into a single class.

Comparative anatomy also provides proof of evolution. We find the same bones in many different types of animals, but these bones are often modified to do different things. The hopping legs of the frog contains the same bones as our own legs, but the frog's legs are highly modified to fulfill a different function (hopping). The wing of a bird and the forelimb of a bat contain exactly the same bones as the arm of a human, but the size, shape, and even internal structure of these bones are all adapted to play a different role in each animal.

We call structures like the wings of a bird and the forelimbs of a bat homologous structures. Homologous structures are structures that are derived from a common ancestor. Even if they are superficially different, they are developmentally related. Homology does not mean that these structures must share the same function. You can alter the same pieces to make different biological structures. The flippers of a whale are supremely designed to cut through the water, but they are homologous with our own human arms. You can trace out the same bones in each, in the same relative positions, and they develop in roughly the same fashion. This is strong evidence that we are closely related to whales.

But very often in nature we find structures that are superficially similar, even though the organisms are completely unrelated to one another. These structures may even serve the same function, like flying. We call these analogous structures. The wing of a bird and the wing of an insect are good examples of analogous structures. In every physical and biological way, these wings are radically different from one another. One is a flat plane of exoskeletal material, the other is a chordate forelimb shaped into an airfoil, with hollow internal bones and an outer covering of feathers. But they can both be used to fly. If you can fly, you have a huge advantage over animals that can't fly. You can escape from ground predators, grab your food out of mid air, and nest in relative safety in the treetops. So wings are a good idea, whether they evolve on an insect or a bird.

We often find unrelated animals converging on the same form or structure, because that form is very adaptive in their common environment. This special case of evolution is called convergent evolution. Another example of convergent evolution is the streamlined shape of sharks and dolphins. One is a fish, the other is a mammal, and they are related to one another in only the most distant sense. But if your life depends on swift movement through the water, then a streamlined shape is pretty much essential.

Convergent evolution produces analogous structures. Divergent evolution produces homologous structures. The same bones can be used in many ways, leading to several divergent evolutionary paths - frogs, bats, birds, men and so on. But this causes a real problem for evolutionary biologists. Just because two organisms have a similar structure, like a wing, does not necessarily mean that they are related to one another. We have to be very careful not to let these analogies confuse us when we puzzle out which animals are related to one another.

One final line of evidence for evolution, completely lacking in Darwin's day, is molecular evolution. Molecules themselves change over time, because the genes that code for them are changing or mutating. Mutations are an alteration in the genetic instructions that shape the molecules of living systems. Changes at the molecular level occur slowly. If these building blocks are altered too radically, they might lose their ability to work at all. So the pace of molecular evolution is often very slow. The greater the similarities between the biochemistry of two organisms, the more likely it is that they are related. For example, consider the reaction between antibodies and the invading antigen. If you take blood from one animal, and mix it with blood from another, you will get an antibody reaction, because some elements in the blood of the other species will be different enough for the blood of the first species to sense that it is "not-me", and attack it chemically. The more distantly these animals are related, the greater the reaction will be. So we can use the measure of the intensity of the reaction as a clue to the degree of their relatedness.

Another molecular test of common descent depends on the simple fact that proteins are made up of sequences of simpler molecules, the amino acids. Proteins are the molecules that compose the structural elements of living systems, and control the rate and direction of biochemical reactions in living tissue. By comparing the sequence of amino acids that compose various proteins in organisms, we can get a better idea of how closely they are related. The more similar the same protein is between two species, the more likely those species are closely related. Cytochrome c, for example, is an enzyme essential in cellular metabolism. The closer that two organisms are related, the fewer the differences between their version of this basic molecule.

There are several strong lines of circumstantial evidence that the branching pattern of relationships between organisms are an expression of a fundamental pattern. As Darwin discovered, that branching pattern is a simple consequence of their shared descent from a common ancestor. There is unity in diversity.

Terms
To Do and View

Watch the Video on the proofs of evolution. Be sure you understand why the four lines of evidence presented in the video offer good circumstantial evidence for evolution.

Examine the fossils on display. What do fossils tell us about life in the past? Why are fossils good evidence that evolution has taken place?

Examine the boxed forelimb bones of vertebrates. Try to find the homologous bones in each of these vertebrate forelimbs. Consider how these forelimbs are adapted to each animal's niche, its role in the ecosystem. These similarities in anatomy suggest that all of these vertebrates evolved from a common ancestor.

Examine the bat skeleton. Compare it to the bird skeleton. Each has evolved a very different way to use their forelimbs in flight. Birds fly using their entire forelimb, but the large bones are filled with air spaces, making them very lightweight, and many of these hollow bones are fused together for extra strength. The bat flies with its finger bones, which are more solid than bird bones, but just as lightweight because they are extremely slender.

Examine the skeletons on display. Start with the fish, and proceed to the frog, snake, other reptile (if available), cat, monkey and bird. Notice the different ways that the limbs are held in relation to the body, and compare them to the diagram below. In primitive amphibians, the legs are held out towards the side and flat on the ground, much like the fins of the fishes they evolved from. This primitive posture gives them a sprawling locomotion; the body winds back and forth like a snake. Reptiles have a semi-improved posture. Their legs are held away from the body at an angle, and they can push their bellies off the ground as they run. Dinosaurs, birds, and mammals have a fully improved posture. Their legs are held completely under their bodies. The efficient back and forth motion that results from this position allows the animal to run forward rapidly, a tremendous evolutionary step forward (as it were)

Things to Remember

Know the six proofs of evolution.

Consider This

Conscious awareness is the ultimate product of evolution. Born from stardust, we are truly the universe becoming aware of itself.

Links -- http://www.tulane.edu/~bfleury/diversity/diversitylinks.html#evolution

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