Anthropology 150

Concepts of Evolution

Our theme for this week is evolution in the Darwinian sense. It is important to note that Darwin

never used the term evolution. In the original concept of evolution had nothing to do with

biology, with rather with social change. Darwin used the expression “Descent with

modification.” He also used the term natural selection to describe the means by which descent

with modification took place. Nonetheless, evolution is the term we use today to denote the

process of descent with modification.

Let’s begin by saying what evolution is and what it isn’t. First, Contrary to recent

statements emanating from school boards in Kansas and Texas, evolution is not controversial or

unproven in the scientific community. On the contrary evolution the basis all modern biological

sciences. There is no concept, except perhaps gravity, which has more universal acceptance

among the scientific community.

When we discuss evolution as a theory, we mean theory in the scientific sense, not in the

vernacular sense of an unproven scheme. In scientific parlance a theory is an integrative or

conceptual framework that unifies or explains empirical or logical observations. There are many

different types of theory. There are testable theories, untestable theories, proven theories and

disproven theories. In science there is hardly anything that we can call a proven theory, because

science as a theory of knowledge is based upon constant skepticism. Newtonian physics worked

quite well for a long time and still works under most conditions, and for several hundred years it

was about as close to proven as anything can be. Einsteinian physics is more generalized and

works better on many scales than Newtonian.

My point is that in any science that deals with empirical phenomenon, all proofs and all

evidence is conditional and subject to re-examination. Mathematical proofs are of a different

sort and different from empirical observation.

Evolution happened and is still happening. That is the basis for everything to come in this

course. There are controversies about the precise mechanisms of selection. For example, there

may be arguments about how rapidly evolution takes place and whether it is gradual or rapid. In

science there are always controversies, but the basic principle of natural selection as put forward

by Charles Darwin is unquestioned in the biological sciences.

All of that is point number 1.

Point number 2 is that evolution is not progressive. It has no goal. It is not directional.

That is to say it is not teleological. Evolution didn’t start working on a population of primates 5

million years ago with the idea of producing us. We are here by accident. The process was

opportunistic. The traits that are adaptive at one point in time may not be adaptive at another.

We talk about survival of the fittest, but what is ‘fit’ at one time may not be fit at another.

There is a concept in evolution called “Anagenesis.” Anagenesis is the process by which

a population accumulates mutations over time. At a certain point, enough mutations have

occurred that for the sake of classification we have to give a new species name to the population.

Anagenesis occurs, and it is an important concept. But it is an error to focus only on anagenesis

and to think that evolution consist only of one species giving rise to a new and “better” species,

as is commonly depicted in images of hominid evolution.

3. Complexity isn’t necessarily better. The idea that “higher” life forms are better than “lower”

forms is a holdover from the concept of a “Great Chain of Being” in which all living things are

ranked by their similarity to God. In that line of thinking, humans are “higher” and most like

God because we were created by God in her image.

For a broad understanding of the mechanics of evolution, I recommend the many books

by Stephen Jay Gould. Gould, who died recently, was a paleontologist at Harvard who wrote for

many years in Natural History magazine.

The Modern Theory of Evolution has Three Major Components

1. Mutation – The development of new traits

2. Inheritance – the passing on of genetic material to the next generation

3. Natural selection – the culling of each generation, so that a portion of each generation

survives. Selection means death.

Mutation

The mechanics of mutation really fall beyond the boundaries of this class. All that

interests us is that mutations happen, in what is essentially a random process.What you need to

know about mutation for this class is that it happens and that it is random. That’s it.

Inheritance

As for the mechanisms of inheritance. You should have learned about Mendel and his

peas in high school. If you didn’t, you can take a biology class or the introduction to physical

anthropology in this department.

The basic aspect of Mendelian inheritence is that traits are passed from generation to

generation as discrete units. That is, they don’t blend. If you cross a big pea with small pea you

don’t get a medium sized pea, you get predicatable percentages of big peas and small peas.

Since Mendel of course we have better understanding of the mechanisms of transmission.

To rephrase Mendel’s observations, we would now say that individuals can carry two

expressions of a given genetic trait. We call these alleles. Of course in any population of

humans there can be more than two expressions of trait such as hair color or blood type, but any

individual can only carry two alleles because that is how chromosones work; they come in pairs

Alleles can be dominant or recessive. In some cases both alleles can be expressed.

Given two alleles A and B

There are four possible conditions for pairing the alleles : AA AB BA BB

The trait that is expressed depends on dominance. If A is dominant, A is expressed, if B

is dominant, B is expressed. If both are co-dominant both can be expressed, as in the AB blood

type.

What Mendel also showed was traits are passed from generation to generation, even if

they are not expressed. That is, recessive genes can be carried in a lineage without being

expressed every generation. A recessive trait won’t be expressed until both alleles are the

recessive form. The genetic package that an organism carries is called its genotype. The genetic

traits that are expressed are called the phenotype. So, what Mendel says is that there are traits in

the genotype that may not be expressed in the phenotype. Put another way, there are things in the

genotype that you may not see everything generation in the phenotype.

An important contribution building on Mendelian genetics is the Hardy-Weinberg

principle. H-W isn’t covered in your text, so let me explain it to you.

The Hardy-Weinberg principle is really a mathematical proof. It shows that in a

population with certain allele frequencies, the next generation of the population will have the

same allele frequencies if the following conditions are met:

1. The population must be sufficiently large for statistical trends to be apparent.

2. Breeding must take place at random.

3. There must be no introduction of new genetic material.

4. All individuals must have an equal chance of surviving and reproducing.

In real-world situations you almost never see a case of Hard-Weinberg equilibrium, but

the model does provide a baseline against which to evaluate changes in the genetic make-up of

populations.

Natural Selection

The Third Element of the modern theory of evolution is the idea of natural selection,

originally proposed by Charles Darwin. Modern evolutionary theory begins with Darwin, but

Darwin didn’t event the concept of evolution. Before him there were many theories of

biological change, but none of them held up to scrutiny. In this context we can mention the

theories of Jean Baptiste Lamarck , who wrote in the first part of the 19 century. Lamarck’sth

theory was teleological he believed evolution had a pre-ordained course. He also believed in the

inheritance of acquired characteristics. Put simply, Lamarck would say that if you go to Gold’s

gym and buff up, your children will be born buff. Sound silly now, but it made sense at the time.

A number of observations went into making up Darwin’s theory. Let me tell you some of

the more salient ones.

1. First, he noticed that there fossils representative of animals that no longer exist. That is, there

have been extinctions. In this observation, Darwin was influenced by Lyell’s (1830) Principles

of Geology, especially in noting that previously existing species no longer existed.

2. Every species produces more young than survive to maturity.

3. Individuals within a species are not identical, but show variation.

4. A number of species in adjoining areas show evidence of common ancestry with subsequent

change occurring locally. That is to say, local populations showed evidence of adaptation to their

local environment.

Remember that Darwin did not know of Mendel’s work on the expression of genetic

traits. He observed populations of species, he observed geology, and he observed the fossil

record. What he came up with was a way to explain why two populations with common ancestry

when isolated from one another and in different environmental circumstances could have

different frequencies of traits.

Darwin’s theory of natural selection is really concerned with population genetics and the

frequency of traits within a given population.

One way to think about is:

Mutation explains how variation is amplified. Natural selection explains how variation is

reduced. Variation is reduced because of differential reproductive success.

When we combine Darwin’s theory of natural selection is Mendel’s theory of genetics we

have something pretty close to the modern definition of evolution, which is that evolution is the

change in gene frequencies in a population.

Example 1: Darwin’s finches and the development of different beak shapes. When

Darwin visited the Galapagos Islands with the HMS Beagle, he noted that the population of

finches on each island had a unique shape of beak. The shape of the beak was directly related to

the type of food eaten by the finches on a given island. Darwin could see that all the finches were

descended from a seed-eating finch native to the mainland of South America. The original

population of finches in each island, therefore, looked like the South American finch, which feed

on seeds. The speciation of the finches in the Galapagos Islands occurred through the process

known as adaptive radiation. The ancestral population radiated into new environments,

subsequently separating into new species as each new population adapted to local environmental

conditions. How did that happen?

Let’s take the grub eating finches as an example. You can see in the slide presentation

(Lecture 3 Evolution and Natural Selection.pdf) that the grub eating finch has the longest beak.

This species of finch feeds on grub that occurs on rocky beaches. In order to feed on the grubs,

the finch must be able to reach between the rocks. Therefore optimal feeding depends upon a

long beak.

Let’s begin with Darwin’s observation that in each species there is variation. In the case

of the finches, that means that in any population of finches, there will be variability in the length

of the beak. Let’s hypothesize that the original population of finches had beaks whose length had

an average of 2.0 cm. Not all finches are going to have a beak exactly 2.0 cm long. Some will be

shorter, and some will be longer. Most likely, beak lengths will follow a normal distribution. But

let’s say that the minimum beak length that finch needs to reach between the rocks and get at

grubs is 2.2 cm. What that means is that finches in the population whose beak is less than 2.2 cm

can’t eat grubs. Those individuals will either find something else to eat, probably by flying to

another island, or they will die. Either way, they are now out of the population on Grub Island.

The next generation of finches will be descended entirely from individuals with beaks over 2.2

cm. Because of random mutations and genotypic traits not expressed phenotypically, the next

generation of finches will include some individuals with beaks less than 2.2 cm. Those

individuals will encounter the same conditions as the previous generation; they will either leave

the island to find food or they will die. Random mutations will also generate individuals with

beaks much longer than 2.2 cm. The latter group will be able to feed quite successfully and will

reproduce. Over time, the mean beak length of the population on Grub Island will increase: there

is a bottom limit (2.2 cm) on the beak length needed to survive, but there may not be an upper

limit on beak length (more on that later). The increase over time in mean beak length in the

population of finches is a example of directional selection. There is a trait that is moving in a

certain direction over time.

Example 2: Adaptation to a cold environment by Pleistocene humans.

Say you have a population of individuals from a temperate climate. They have only stone

tool technology. You move them to an arctic environment. What will happen?

Let’s say the population at generation A has a normal distribution for height, with a mean

of 5 ft. 5".

If you look at the physiology of cold weather adaptation you’ll learn a couple of things.

The first is that short, stocky people retain body heat better than tall thin people. They have less

surface area exposed relative to their body mass. What this means in an arctic environment is that

short stocky people have a better chance of survival because they are less prone to hypothermia.

So, we can expect that people in generation A at the left tail of the distribution will have a higher

survival rate than the people above the mean. So, we can expect that in generation B the mean

height will be significantly lower than in generation A. Variation has been reduced because one

part of the population was better fit to survive than another. Its genetic material survived, and the

rest died.

This is directional selection.

That is natural selection at work.

Direction selection vs. stabilizing selection.

Directional selection is best illustrated by traits that can be quantified, as in the discussion

of height above. Given a normal distribution for the variable, directional selection moves the

mean of the population in one direction or the other. That is, one tail of the distribution has a

selective advantage over the rest of the population.

In stabilizing selection, there is no advantage in being above or below the mean. The

central portion of the distribution, around the mean, is favored. That means any mutations

developing above or below the meaning are not reproduced at a higher rate than individuals at the

center of the distribution. Hence, the mean does not move over time.

Let’s go back to the finches. Imagine that over time the mean beak length of finches on

Grub Island has increased to 2.5 cm. We know that the bottom end of the distribution (the

shortest beak length is going to be 2.2 cm). Is there an upper limit? Perhaps. Let’s imagine that

a bead length of 2.8 cm lets a finch get at every grub available between the rocks. Also imagine

that a beak length of 3.0 cm is extremely fragile and has a tendency to break off while foraging

for grubs. In those circumstances, there is absolutely no advantage to having a beak longer than

2.8 cm and there is a known disadvantage to having a beak 3.0 cm or longer. In those

circumstances, you have both an upper and a lower limit to beak length, and the mean will

stabilize within that range. Of course, individuals with beaks shorter than 2.0 cm and longer than

3.0 cm will continue to be hatched. They will not survive, however, and their genetic material

will not be passed on. When selection favors individuals in the center of a frequency

distribution, stabilizing selection is at work. See the slide in the presentation for a graphic

representation of the process.

There is a lot more to genetics and evolution than we’ve mentioned here. As we go along

in the course we’ll explore a lot of other concepts such as branching, adaptive radiation, and the

like. But you have to grasp the concept of natural selection before moving on.