Field Note Collaborations for Week One (Natural Selection)

Welcome to the Week 1 lab. This week we will explore the concepts of evolution and natural

selection. So let’s start out with defining the term evolution. Evolution is simply change over

time. But since this class is about people, we are going to be looking specifically at the change

of species over time that ultimately led to the evolution of modern humans. When we talk about

human evolution, we are talking about the change in inherited traits in our ancestral species over

time, ultimately leading to Homo sapiens, or modern humans.

Scientists have determined that life began on earth about 3.5 billion years ago. From that time to

the present, species have formed, changed, and many species have gone extinct. As you will see

in subsequent weeks of this course, there were many species that were ancestral to modern

humans who eventually became extinct. So how did these species evolve, and why did all but

modern humans go extinct? One of the mechanisms to explain this is the theory of evolution

through natural selection. Of course, the next question is – what exactly is natural

selection? Let me give you a little bit of background information first.

Charles Darwin was a British naturalist who became famous his contribution to evolutionary

theory. Darwin introduced a scientific theory that explains how all species of life evolved from a

common ancestor in a process he called natural selection. While Darwin was in college, he

became friends with a biology professor, John Henslow, who invited him to go aboard the

H.M.S. Beagle – a ship outfitted to do a scientific expedition that lasted five years. The Beagle

sailed from England to the South American coast, and Australia from 1831-1836. Darwin was

the ship’s scientist. It was during this voyage that he came up with the idea that evolution is a

process that has been occurring for millions of years, and that life evolves by a process he called

natural selection.

While traveling to the Galapagos Islands, off the coast of Ecuador, Darwin noticed that there was

a lot of similarity between the plants and animals of South America and the Galapagos Islands,

especially the finches who were very similar but not identical. Originally, some finches from

South American managed to cross the ocean from South America, and then spread out across the

various Islands that make up the Galapagos. Darwin observed that the finches were not only

different from the mainland, but they also differed from island to island. The finches adapted to

each island based on various diets – seeds, insects, flowers, leaves, etc. If you look at this

picture of Darwin’s finches, you can see that the finches’ beaks look different. Some of the

finches’ beaks were adapted to eating seeds, like finch number one. Other finches, like finch

number 4 were adapted to eating nectar. Other finch beaks allowed them to eat fruit, cactus, and

a variety of other resources. One species of finch that originally arrived in the Galapagos

eventually evolved with different beak types so that they could all survive by eating different

resources instead of competing for the same resource.

After he returned to England, Darwin became interested in the writings of Thomas Malthus, an

economist, who wrote the “Essay on the Principle of Population” in 1798. In this book, Malthus

observed that plants and animals (including humans) produce far more offspring than can

survive, and that this establishes a competitive environment and that the variation among species

would produce some individuals with a slightly greater chance of survival. Darwin realized that

if all creatures born were to survive, the earth would fill up in a matter of time. Based on the fact

that this had not happened, Darwin realized that the key to evolution was whatever made the

difference between those who survive and reproduce and those who do not. He called this

natural selection.

Now, let’s talk about the key elements of Natural Selection, as outlined by Charles Darwin:

1. Populations have the potential to reproduce faster than the food supply.This means that there will be a competition for resources among a species

2. There is biological variation in all species. As you can see from this picture of parakeets, there is a lot of variation. We also see a lot

of variation in the human species as well.

3. Individuals must compete for limited resources. Not all individuals will have equal access to resources.

4. Those individuals with more favorable traits will have a better chance at survival. Favorable traits give those who possess them an advantage in obtaining resources over

those who do not.

5. The environment will determine if a trait is favorable or not. What is a favorable trait in one environment might not be in another.

6. Variation and favorable traits can be inherited by offspring. Favorable traits give those individuals a better chance at surviving to adulthood and

reproducing. Their offspring will inherit the favorable trait. Those without the favorable

trait will often not survive long enough to reproduce.

7. Over a long period of time, the accumulation of favorable traits will lead to the development of a new species.

When enough new traits have developed, the species becomes so different from the

original species that they can no longer mate and produce offspring. This is the process

of speciation.

8. Geographic isolation may also lead to the development of new species. If a species is geographically isolated, it will adapt to its environment, and over time may

become a new species.

One of the classic examples of natural selection is the peppered moth. The peppered moth is

found predominantly in England. The peppered moths are mostly white with black

speckles, which is the variety that you can see at the top of the picture. This coloring means it is

well camouflaged when resting on the trunks of light colored trees. But a genetic mutation

resulted in a variation of moth that has dark wings and bodies. As you can see from picture A,

the dark variation of peppered moth is not as well camouflaged as the lighter colored

moths. This means that the darker colored moths are more likely to be seen and eaten by its

main predator – birds.

However, during the 19 th

century, the increasing use of coal for heating and in factories caused

some pretty severe air pollution, which coated buildings and vegetation with a black dust. So

now it was the white peppered moth that became easy prey for birds, rather than the darker

moths, as you can see from Picture B. This meant that the black moths were more likely to

survive and produce offspring than the white moths. Over several generations, the black moths

became the predominant variety. And now, of course, because of cleaner energy sources, the

tree trunks became cleaner, and the black moths were once again easy prey and the white moths

are now more likely to survive and produce offspring. This is a classic example of natural

selection working in both directions!

Think about this in terms of natural selection. The moths with favorable traits (or the coloring

that made it able to be best camouflaged) had a better chance to survive and produce

offspring. And it was the environment that determined which trait was favorable – at one time it

was the light colored moths, and at another time it was the dark colored moth.

There are also a couple of other terms that you should become familiar with. I am sure you have

all heard the phrase “survival of the fittest.” In terms of natural selection, fitness refers to the

ability of an individual to survive and reproduce. In evolutionary terms, fitness does not refer to

physical strength. The fittest individuals will not only survive, but also pass their traits on to the

next generation.

Another term to become familiar with is selective pressures. Selective pressures are forces in

the environment that influence reproductive success. Think of the peppered moth example. The

selective pressure in this case was how easy it was for the birds to spot the moths.

For the other part of this lab, you will be completing a worksheet that has examples that illustrate

some of the things you have learned so far. Be sure to use what you have just learned when

completing your worksheet, and have a great week!