Chapter 1. Natural Selection

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In this lab you will work with an evolutionary biologist who specializes in tetrapod evolution. All life on Earth began in the oceans, but eventually it moved onto land. In this lab, you will investigate how that was accomplished. You will examine the changes required to move from water to land through the evolution of tetrapods, living organisms with four legs or limbs (humans, for example). We will then examine some of the evidence that supports this theory in the fossil record and in the arm or fore-limb structures of several different modern species. Lastly, we will examine another example of evolution that is occurring in the world around us today. Evolution affects all living life forms, so today’s lab will be quite diverse.

Are you ready to begin?

Background Info

The results of evolution are all around us. The animals and plants that we see today are the products of, in some cases, millions of years of evolutionary change. Occasionally, evolution can take a very unexpected turn. In 1941, the US Army created a 10,000 acre enclosure that included a population of white-tailed deer. Some of the deer were carriers of a rare gene that can cause their offspring to have completely white fur. Once the army noticed a couple of white deer, they helped matters along by protecting the white deer from hunters. This is a good example of artificial selection, where influences other than nature affect which individuals in a population survive. More than 70 years of inbreeding have now led to a deer population that is about ½ white, ½ brown.

[insert figure]

This figure will show a white deer, possibly with a brown one in the photo for contrast .

How do these types of changes occur? That is what you will be examining in today’s lab.

How does evolution happen?

Charles Darwin published Onthe Origin of Species in 1859. But what is a species, exactly? While there are many possible definitions, the biological species definition is a commonly used one. It states that a species is a group of organisms that can interbreed and produce fertile offspring. By this definition, the white deer are not a new species—they just have one trait that makes them look different from the brown ones. They would only become a new species if they stopped breeding with the brown deer for some reason: the two populations would then evolve separately, eventually becoming separate species. This is an important process for establishing new species, but not the focus of today’s lab. .

The theory of evolution states that species change over time. These changes come from changes in a population’s gene pool, the shared genetic information of all the animals in a population. Sometimes a trait in the gene pool is very homogenous, e.g. all deer have two eyes. However, sometimes there is variation in the gene pool. For example, in the deer population discussed above, half of the genes in the gene pool code for white hair, and half code for brown hair.

Variations in the gene pool initially come from mutations, random changes in the DNA sequence of a gene. Most genetic mutations happen at random. Some mutations will be very bad for the organism, so it may die without reproducing. For example, these white deer wouldn’t last long in the wild: they aren’t camouflaged like normal deer, so predators would target them first. If all of the white deer died before they could reproduce, their genes would drop out of the gene pool, and eventually there would be no more white deer. Other mutations will have little or no effect on the organism. For example, a mutation that makes a deer’s nose brown instead of black won’t really affect its survival rate or the number of children it has.

However, sometimes mutations will create a certain trait in the gene pool that confers an advantage on those organisms that have it, making them more likely to survive and reproduce.

An adaptive trait, or adaptation, helps an organism survive in its environment and produce offspring. Having brown fur is adaptive for deer because it hides them from predators. Other examples of adaptive traits include things like:

• A hummingbird having a long beak so it can reach flower nectar that other birds can’t
• A redwood tree growing taller so it can capture more sunlight than shorter trees
• A shark being able to replace lost teeth, otherwise it might starve

And so on.

It’s a tough world, and not all of the members of a species can survive and reproduce—there just isn’t enough food, water, and shelter to go around. The organisms that are most likely to reproduce are the ones that have some kind of advantage over the other members of their species: even a small advantage can have a big effect on survival. Living things are under a lot of pressure to survive and reproduce, and that pressure comes from their environment. That is what the term natural selection means. The environment—nature—selects the organisms that are best adapted to it. The others? Some will survive, but many of them will not make it.

When an organism reproduces, it passes down its genes to its offspring, including the genes for its useful adaptations. Some of the individuals who are not well-adapted to the environment die before they can reproduce, so their genes drop out of the gene pool. On the other hand, the individuals that have the adaptation produce lots of offspring, many of whom will have the beneficial trait. What this means is that in the next generation, the beneficial trait becomes more common in the gene pool and more individuals should possess the beneficial adaptations. Over time, the entire population changes, or evolves, so that most or even all of the organisms have the adaptation. Darwin called this “descent with modification.” Modifications—the adaptations caused by genetic mutations—will be passed down to an organism’s descendants. Then there may be another new adaptation, and then another new adaptation being passed down. Eventually there have been so many changes in the organism that it evolves into a new species.

This also brings up some very important points.

• Organisms do not evolve, populations do.
• Not every member of the next generation will possess the useful adaptation. But over several generations, more and more of them will.
• Because the population is not genetically identical, if the environment changes some individuals will possess adaptations that make them more suited to survive in the new environment.

How do we learn more about evolution?

One of the reasons that evolution is an established theory is that there is a great deal of supporting evidence for it. Let’s take a look at some of it.

1) The fossil record is an important link to the past. Fossils can teach us a great deal about life on ancient earth. We see things like:

• Extinct species such as trilobites. While the trilobites may remind us of insects, and they probably are the ancestors of insects, trilobites do not exist today. Many fossils have been found of organisms that were common at one point in time, but are no longer around. This means that life on Earth is always changing.

[insert figure] This figure will show one or more trilobites close up, enough to make them look like a bug. Familiar, but not identical to anything we know.

• Transitional forms. Fossils can vividly illustrate changes in a lineage over long periods of time. For example, the ancestor of the modern horse was a tiny animal with toes and teeth that were suited for chewing leaves. A series of fossils shows us the transition from this ancestor to the large, hooved, grass-eating horse of today.

[insert figure] This figure will show the major transitional species in the evolution of the horse, including changes in the teeth and feet.

• Geological Age: Fossils can be dated so their approximate age is known. That is one way to help put a transitional series of fossils in the correct order, and determine a timeline for various events. For example, carbon dating of rocks and fossils revealed that the Earth is much older than anyone first believed.
• Continental drift: The continents are in constant movement. At certain points in the Earth’s history, they all merged into one large supercontinent; eventually, they moved away from each other to form the 7 continents we see today. This explains why some of the fossils found in South America, Africa, and Antarctica are so similar. The continents were connected when these organisms were alive, and it was easy for the organisms to move from one location to another. (Click on this link to see a simple animation of the breakup of the supercontinent Pangea: https://en.wikipedia.org/wiki/File:Pangea_animation_03.gif) [Note: designers/developers: can we re-create a simple animation showing the breakup of Pangea rather than using the government one?]
• Evolutionary trends: By studying fossils, scientists have been able to identify some major themes in the history of life on earth. The trend that you will be examining today is the move from water to land and vice versa.

[Insert figure] This figure will show the location of fossils and Pangea.

2) Homologous structures are features that organisms inherited from a common ancestor. Why should cats, bats, and humans have such similar bones in their forelimbs, when they use those limbs so differently? Because all three are descended from the original four-legged animal. Over time, mutations in the forelimb bones occurred that allowed these animals to diversify into the species we see today. But the similarities we observe show us that these animals are truly related. Homologies can be identified in the physical characteristics of different organisms (such as bones), the proteins that they make, or in their genetic sequence.

[insert figure] This figure will show forelimb bones for several vertebrate species. Cat, bat and human are the ones mentioned in the text, but others may be included.

3) Artificial selection. Natural selection means that nature is selecting the best-adapted organisms. Artificial selection means that some other force is acting to determine which organisms get to pass down their genes. Usually that outside force is humans. For example, all modern dogs are descended from wolves. How did such different breeds as the toy poodle and the Irish wolfhound develop? Human breeders decide what traits they want to see in the next generation, and only dogs with those traits are selected for breeding. So toy poodle breeders are looking for small dogs to breed, while for an Irish wolfhound breeder, the bigger the better. A more disturbing case of artificial selection is seen in bacteria that become resistant to antibiotics. As bacterial populations are exposed to an antibiotic, some of them will be killed, while others will not. The survivors will pass the resistance genes to other bacteria, and soon that antibiotic has little effect on that population.

Modern classification systems are based on evolutionary relationships.

As a species, humans like to put things into categories. This helps us look for patterns and ultimately make sense out of our incredibly complex world. For example, a human who watched a lion eat a zebra would conclude that a lion is a dangerous predator. If that person later encountered a tiger, the similarities between the two cats would enable the person to deduce that a tiger is just as dangerous as a lion. That kind of reasoning has helped humans survive, and so classification is something that we do on a regular basis.

Evolution has led to a great diversity of organisms. How can we determine the relationships between them?

Cladistics is a type of classification system that focuses on evolutionary relationships. A cladogram is a diagram that focuses on a particular set of organisms and characters, or traits. A primitive character is one that appears in an ancestor older than the last common ancestor (in this case, “primitive” doesn’t necessarily mean simple or crude, just old). Therefore, a primitive character is one that evolved early on in this group. A derived character is one that arose from the last common ancestor. “Derived” means that evolution has modified this trait from a pre-existing one. The more derived characters two organisms share, the more closely related they are.

How do we interpret a cladogram? In the one below, the most primitive character is “does not photosynthesize.” This is true for all of the animals, so it will not tell us much about relationships between different groups of animals. The first new characteristic is “vertebrae.” Insects do not have this characteristic, but all of the other groups of animals do. This means that the insects branched off from the other animals early on, and have been evolving their own characteristics. The next new trait is “tetrapod,” having four legs. Fish do not have this trait, while the rest of the animals do. This means that the ancestor of the amphibians, birds, lemurs, and humans was a tetrapod. It also means that amphibians have a closer relationship to humans than they do to fish because they share more traits with humans than they do with fish. Eventually, we see that the last common ancestor of the lemurs and humans had hair, but only humans are bipedal. The cladogram shows us the relationships between these organisms; it also shows us approximately when these traits evolved.

[insert figure] In this case, the text is very specific to the figure used here. If a different figure is used, the text must be modified. Would like to change “develop from blastula” to “does not photosynthesize”. Also change label from “prosimians” to “lemurs”.

This cladogram was constructed by comparing physical characteristics. But just because two organisms look alike doesn’t necessarily mean they’re related. However, for a long time, that was all scientists had to work with.

Now that DNA analysis is possible, we can look at relationships between species at the most fundamental level. It seems reasonable that the more similar the DNA sequences are, the closer the relationship between two species. And since the DNA sequence determines the amino acid sequence of a protein, analysis of proteins can also show us relationships.

Therefore, cladograms can be used to compare DNA or protein sequences and reveal evolutionary relationships, as seen below. Cladograms can provide a lot of information in a compact, easy to interpret format. Because of this, reading and constructing cladograms will be part of your work for today!

[insert figure] This figure shows a molecular clock type of diagram. Could be either DNA or protein

Pre-lab Quiz

Before we can begin to design this experiment, you’ll have to understand the background material covering evolution and natural selection—how it works, what some of the possibilities are, what the supporting evidence is, etc. Make sure you've read the background material thoroughly, and then answer the following questions. You must get a score of 90% or better on these questions to proceed to the first experiment.

Use the following cladogram to answer the questions below.

Experiment Notebook

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Lab Report

Vocabulary List

Adaptation: A trait thathelps an organism survive in its particular environment.

Artificial selection: When humans choose only plants or animals with certain desired characteristics for breeding.

Cladistics: A method of classification that is based on evolutionary relationships.

Cladogram: A diagram showing the hypothesized relationships between different organisms.

Continental drift: The movement of the earth’s continental land masses.

Derived character: A trait that was inherited from the most recent common ancestor. This trait has been modified by evolution from a pre-existing version.

Descent with modification: Charles Darwin’s term for how new species form.

Evolution: Changes in a species that occur over time.

Heritable: A genetic trait that is capable of being inherited from a parent.

Homologous structure: A physical or molecular characteristic that two organisms inherited from a common ancestor.

Humerus: The upper arm bone that forms the shoulder joint.

Lobe-finned fish: A fish that has muscular fins like a coelacanth.

Lungfish: A fish with lungs that is capable of surviving for prolonged periods of time out of the water.

Metacarpal: A bone that connects the carpals to the phalanges.

Natural selection: How environmental pressures mean that the individuals best adapted to their environment are the most likely to reproduce and pass down their adaptations to their offspring.

Phalange: A bone that makes up one of the digits (fingers or toes).

Primitive character: A trait that evolved early in a lineage and is shared by most of the members of that lineage.

Radius: One of the two bones of the lower forelimb.

Ray-finned fish: A fish with fins made of webs of skin stretched over several small bones.

Tetrapod: An animal with four limbs; also any animal (living or extinct) descended from the first four-legged vertebrate organisms to walk the Earth.

Ulna: The bone of the lower forelimb that makes the elbow joint.