Download Chapter 3 - Cynthia Clarke

Chapter 3
The Forces of Evolution
Overview
• This chapter is concerned with the processes associated with microevolution
o
Microevolution contrasts with macroevolution.
o
While microevolution describes the changes within a species over a relatively short time (evolutionarily speaking),
macroevolution is on a larger scale.
o
Macroevolution is linked both to speciation and to longer periods of time evolutionarily.
• Starting with a brief description of populations and Hardy-Weinberg equilibrium, the chapter provides an intuitive,
relatively nonmathematical approach to microevolutionary theory.
• While some specific examples from human populations are given, most case studies are deferred to later chapters.
o
The introduction briefly lists a few examples.
o
These include contemporary research on Darwin’s finches, the link between dairy farming and lactose, and of course
the dog.
• While the author does not label it by name, he also refers to the New Synthesis
o
This is where the observations of the naturalists and the laboratory work of the early geneticists were merged.
o
The Synthesis reconciled how microevolution (population genetics) and macroevolution (paleontology) were linked
Population Genetics 1
• This chapter looks at the evolutionary forces that act to cause changes in the frequency of alleles over time.
o
Microevolution takes into account changes in the frequency of alleles from one generation to the next. In the study of
microevolution, the first step is to determine the population to be studied.
o
The evolutionary forces we will discuss are natural selection, mutation, gene flow, and genetic drift.
o
Because of the principle of uniformitarianism, the forces that act to create microevolution also apply over greater
periods of time and explain macroevolution.
o
This is the basis of the New Synthesis.
• What is a population?
o
The focus of microevolution is not on specific genotypes and phenotypes of individuals, but rather on the total pattern
of an entire biological population.
o
Often used is the term breeding population: A group of organisms that tend to choose mates from within the group.

Human populations are most often defined on the basis of geographic and political boundaries.

Often the definition used depends on the research question being addressed

A potential problem is determining the difference between total census and the breeding population.

Another problem is when one locale contains many sub-populations
Population Genetics 2
• Genotype frequencies and allele frequencies
o
Once a population has been defined, the next step in microevolutionary analysis is to determine the frequencies of
genotypes and alleles within the population
o
Remember a genotype is NOT the same term as allele.

The genotype is the combination of the allele from the father with that of the mother. Example using the ABO
blood group:

The alleles are: A, B, O

The possible genotypes are: AA, AO, BB, BO, OO, AB

The blood type is actually a phenotype label, which we will discuss later.

The two types of frequency:

The genotypic frequency is a measure of the relative proportions of different genotypes within a population.
Genotype frequencies are obtained by dividing the number of individuals within each genotype by the total
number of individuals in the sample.

The allele frequency is a measure of the relative proportions of different alleles within a population. Allele
frequencies are computed by counting the number of each allele and dividing that number by the total number
of alleles in the sample. The number of alleles is TWICE the number of genotypes in a population.
Population Genetics 3
• Genotype frequencies and allele frequencies (continued)
o
The term frequency is not the same as the percentage although we often mix them up!
Relethford Chapter 3 Page 1

o
The frequency is based on actual numbers (both in the numerator and denominator). For instance, 4 persons from a
population of 20.

The percentage is based on conversion to “per centum” (by the hundred).

To record this value one has to convert the actual observation. (4 persons in a population of 20 becomes
25/100 or 25%)

The reason for this effort is so that a standard is created to more easily compare across sets of observations.
Let’s do some calculations here. The frequency is determined in this way:

Example 1: Genotypic frequency in a co-dominant gene system

A population of 200 persons have their blood tested and we learn the following: 98 persons are MM, 84
persons are MN, and 18 persons are NN

Calculation:
• MM: 98/200 or 0.49 (49%)
• MN: 84/200 or 0.42 (42%)
• NN: 18/200 or 0.09 (9%)
Population Genetics 4
•
Genotype frequencies and allele frequencies (continued)
o
Example 2: Allelic frequency in a co-dominant gene system
o
Key concept: In a population of 200 persons, there are 400 alleles Calculation:

Number of M alleles:

MM had 98 persons and each has 2 M alleles = 98(2)

MN had 84 persons and each has 1 M allele = 84(1)

NN had 0 persons with M and each so = 0

So to chain this together and M is = 98(2) + 84(1) + 0 =280 M alleles

Number of N alleles:

NN had 18 persons and each has 2 N alleles = 18(2)

MN had 84 persons and each has 1 N allele = 84(1)

MM had 0 persons with N and so = 0

So to chain this together and N is = 18(2) + 84(1) + 0 = 120 N alleles
o
Special note: This method of determining the number of genotypes and alleles can only be used if the number of each
genotype can be determined.

This means when the alleles are co-dominant

This means it is not useful when there is a dominant-recessive situation (we will show this method by using HardyWeinberg equilibrium formula in the next few slides).
Population Genetics 5
• Hardy-Weinberg Equilibrium (see Box 3.1)
o
The Hardy-Weinberg equilibrium model is a mathematical statement relating allele frequencies to the expected
genotype frequencies in the next generation. Basically it predicts the values in the next generation based on the values
in the present population.
o
The Hardy-Weinberg model makes certain assumptions:

Assumes random mating with respect to the locus of the study

Assumes that no new alleles are introduced by mutation or natural selection.

Assumes that there are no changes caused by movement in or out of the population (migration, or what we will call
gene flow)

Assumes that there is no variation is caused by random sampling (population is large also)
o
Observed change means that one or more assumptions are incorrect.
o
Once the allelic frequencies of M and N are determined, the next question is: What are the expected genotype
frequencies in the next generation?

In Chapter 2, we used the Punnett square to answer this question for a specific male and female.

But how might we calculate the values for all possible pairings in the population? Answer: Use the formula
identified by Hardy and Weinberg.
Population Genetics 6
• Hardy-Weinberg Equilibrium (continued)
o
Example: The MN estimate in the next generation
Relethford Chapter 3 Page 2

o
As there are only two alleles in the MN blood group, then the total of the two frequencies can be written as: p + q =
100% OR p + q = 1

This is the allelic frequency formula for a 2-allele system.
Now, let’s use the formula for allelic frequency to create one for genotypes.

Think of this as the potentials for mating of each 2 persons

By this I mean: (p + q)(p + q) = 1

The first parenthesis represents the proportions possible for the first mate and the second parenthesis is that for
the second person.

Do the math to create the genotypic frequency formula: p2 +2pq + q2 = 1

p2 is the genotype frequency of the homozygote dominant genotype (or first of co-dominant genotypes)

2pq is the genotype frequency of the heterozygote genotype

q2 is the genotype frequency of the homozygote recessive genotype (or second of co-dominant genotypes)
Population Genetics 7
• Hardy-Weinberg Equilibrium (continued)
o
Using this formula for our MN values we find for the next generation:
• p2 = (0.7)2 or 0.49
• 2pq = 2(0.7)(0.3) or 0.42
• q2 = (0.3)2 or 0.09
o
Note that in our example the values remain the same? We demonstrated that no
evolution occurred.
• Review here: www.youtube.com/watch?v=BuiPA8FJ_1M&feature=fvwrel
• Demo here: www.explorelearning.com/index.cfm?method=cResource.dspView&ResourceID=517
• The table shows this in not true; note that differences between populations.
Population Genetics 8
• Up to this point we have been pretending that evolution does not occur (being in equilibrium) as this is a baseline of a
population.
• Changes in genotypic frequencies do change as a result of evolutionary forces and random mating. So, the population may
not be in Hardy-Weinberg equilibrium for two basic reasons:
o
The effects of evolutionary forces (mutation, natural selection, genetic drift, or gene flow

Evolutionary forces are the only mechanisms that can cause allele frequencies to change over time.

We have already discuss the first two (briefly) in other chapters.
o
Nonrandom mating involves patterns of mate choice that influence the distributions of genotype and phenotype
frequencies.

Allele frequencies change together with genotype frequencies, but genotype frequencies can change without
altering allele frequencies.

One form of nonrandom mating is inbreeding.

Assortative mating is another form of nonrandom mating that occurs when there is mating based on phenotypic
similarity or dissimilarity (i.e., blondes choosing blondes).

Inbreeding and assortative mating involve no change in actual allele frequency, but a change in genotype
frequency.

Nonrandom mating does not cause evolution, but it can affect the rate of evolutionary change.
The Evolutionary Forces 1
• Mutation
o
When it goes wrong:

Mutation is a change in the DNA sequence that produces an altered gene

Technically, this term refers to changes in DNA bases as well as changes in chromosome number and/or structure.

Can be a single change (point mutation), a cross-over between chromosomes or even addition of chromosomes
or deletion of them

Point mutation is the substitution of one DNA base for another
o
There are 4 evolutionary forces, but it is important to keep in mind that mutation is the ultimate source of all variation
in organisms.

Mutation introduces new alleles into a population, and changes the frequency of different alleles over time.

MUST occur in the sex cells to influence evolution.

Mutation typically effects variability, generally, by increasing it.
Relethford Chapter 3 Page 3

It is rare to see evolution by mutation alone, except in microorganisms.

Mutations are vital to evolution, but rates are low and, by themselves, they do not lead to major changes in
allele frequencies.

In large populations maybe 1 in 10,000 a mutation might be observed.

Mutations can also occur in reverse direction (mutating back to the original form). This is rare.
The Evolutionary Forces 2
• Mutation (continued)
o
The Evolutionary Forces 2

Example: The point mutation of an A allele to an a allele.

Generation 1: 200 A alleles (this means this allelic frequency is 100% or 1)

Then a mutation occurs:

This changes the allelic frequency, but if there is not there is no additional evolutionary change, the allelic
frequency will be the same over generations

Generation 2: 199 A alleles and 1 a allele
o
Now A allelic frequency is 199/200 (or 0.995)
o
The a allelic frequency is 1/200 (or 0.005

Without the other forces, only additional introductions of this same mutation can change (read
increase/decrease) the allelic frequency of a allele.

Faster rates of evolutionary change are very likely do to one or more of the evolutionary forces.
o
Many discrete genetic traits are polymorphisms (many forms)

A genetic polymorphism is a locus of 2 or more alleles with frequencies too high to be by mutation alone

The arbitrary frequency used is 0.01
The Evolutionary Forces 3
• Natural selection
• Natural selection does not create new genetic variation, but it can change the relative frequencies of different alleles.
Natural selection can be defined as differential survival and differential reproduction
o
The environment ultimately selects individuals with the best suited genotypes to survive to adulthood and to
reproduce.
o
Those who have more surviving offspring pass on more of their genes to the next generation.
• Natural selection filters genetic variation
o
Analysis of natural selection focuses on fitness, the probability of survival and reproduction of an organism.
o
It is measured from 1 (the most fit) to 0 (one who does not survive to reproduce successfully or who does not
reproduce even if they survive)

The record for the most live births by a single woman is 69 so she had a value of 1.

Of these, 67 lived past infancy.
• Depending on the fitness of each genotype, natural selection can have different effects.
• Natural selection is usually the most important mechanism of evolution.
o
We now know that its effect on individuals depends on their phenotypes which in turn are determined mostly by
their genotypes.
o
Different environments means different results
o
Thus, Darwin never said ‘survival of the fittest’, he said ‘struggle for survival’ because he was talking about
differing environments and different success rates.
The Evolutionary Forces 4
•
Examples of how natural selection works
o
Analysis of natural selection depends on the initial allele frequencies, whether one allele is dominant or not and the
exact fitness values for each genotype.
•
Simple genetic traits with only two alleles have a number of different models of natural selection to investigate. For
instance, use a 2-allele system (so 3 genotypes: AA, Aa, aa).
o
If there were no selection, each would have the same fitness.
•
If that were so, the allele frequencies would be the same each generation.
•
Selection against the homozygous recessive
•
What if the genotypes do not have different fitnesses? Suppose that the
genotypic fitnesses are as follows:
o
AA = 100%, Aa = 100% and aa = 50%
Relethford Chapter 3 Page 4
The aa individuals only have a 50% chance of survival.
Selection against an allele will lead to a reduction in the allele frequency over time.
o
The heterozygote will continue to pass on the allele to the next generation.
o
Is never totally eliminated as there is a slow, but present ‘back mutation’.
See Table 3.2 for a numerical example of this and Figure 3 (next slide) shows that over time the aa genotype.
Real example: Tay Sachs disease, a fatal lipid storage disorder.
o
Tay-Sachs is a deadly disease with no adaptive advantage.
o
Right now there is no treatment; death is usually in the first few years of life.
o
•
•
•
The Evolutionary Forces 5
•
Examples of how natural selection works (continued)
o
Selection for the dominant homozygote
•
Suppose AA = 100% fitness, Aa = 95% and aa = 90%
•
Over time the A allele will increase (see Figure 3.3 in the book)
o
Selection against the dominant homozygote
•
Selection against the dominant homozygote causes the frequency of the dominant allele to go down and the
frequency of the recessive allele to go up.
•
Achrondoplastic dwarfism is an example.

Achrondoplastic dwarfism (small body size and abnormal body proportions) is caused by a dominant allele
found in low frequencies in human populations.

The condition is often caused by a mutation occurring in the sex cells of one parent.

The allele is dominant, and individuals with one or two alleles will show the disease.

Low frequency of the disease is the result of natural selection acting to remove the harmful allele from the
population.
The Evolutionary Forces 6
•
Examples of how natural selection works (continued)
o
Selection for the heterozygote (therefore against the homozygotes)

What about a situation that creates a selection for the heterozygote?

Many loci show intermediate frequencies. This reminds us that fitness is most often NOT close to 1 or 0 in
populations.
o
Case of sickle-cell anemia (see Chapter 2)

The frequency of sickle-cell allele can reach 20% in some African populations and is also high in Indian and
Mediterranean populations as well.

This hereditary disability persisting at high values then in these populations. Why?

Plasmodium falciparum malaria is among the most deadly forms

Relative fitnesses in this example:

Healthy persons are better hosts, sickly from malaria

The homozygous dominant (AA) = 70% fitness in our example

The homozygous recessive (aa) die and have a fitness of 0%.

The heterozygotes (Aa) = 100% fitness.

Results in balancing selection (selection for the heterozygote and against both homozygotes) as the heterozygote is
most fit. See Figure 3.4 for a similar illustration.
The Evolutionary Forces 7
•
Selection against the heterozygote
o
Selection against the heterozygote results in a decrease in the less common allele.
o
An example is the Rhesus (Rh) blood group.

Inheritance of the Rh blood group involves three linked loci.

Those with Rh positive (DD or Dd genotypes) produce a certain chemical; those with Rh negative (dd) produce a
corresponding antibody.

Some Rh negative mothers carry Rh positive fetuses.
• The child is at risk and may be selected against. These offspring are heterozygous (D from father, d from
mother). The risks are as follows (among those untreated):
• 13.7-29.8% will develop the response (either in the womb or at birth)
Relethford Chapter 3 Page 5
•


During the next birth, 20-25% of the children will be exposed to the mother’s response (occasionally this
can happen in the original birth)
In geographic areas where biomedicine is available the treatment is RhoGAM which has greatly reduced the
problem since the 1960s.
Another reason for the decrease is smaller family sizes (less exposure).
The Evolutionary Forces 8

Selection and complex traits
o
Complex traits are continuous, and the effects of selection are examined
on the average value and on the extremes.
o
The effects of selection are examined on the average value and on the
extremes.
o
Stabilizing selection refers to selection against both extremes of a trait’s
range in values

Individuals with extreme values are less likely to survive; those
closer to average values are more likely to survive and reproduce.

The effect is the promotion of the a population with the same
average value over time (stability)

Human birth weight is a good example.

Both the environment and genetics play a role in birth weights

Underweight and overweight infants die more than those in the average
o
Directional selection is a form of selection in complex traits against one extreme and/or for the other extreme.
• A direct relationship exists between survival and reproduction and the value of the trait.
• Promotion of change in a population over time in one direction

The increase in human brain size in the last 4 million years is an example.

Another example is the lighter skin color of the prehistoric peoples who left Africa.
The Evolutionary Forces 9

Genetic drift
o
Overview of genetic drift:

Genetic drift is defined as the fluctuations in the percentages of two or more alleles in a population or as the
random change in allele frequency from one generation to the next.

This is based on luck and not on the actual characteristics of the parents (not based on fitness)

Genetic drift is random and it occurs in every generation.

Direction of allele frequency is random.

Given enough time, (and in the absence of other evolutionary forces) genetic drift leads to reduction in variation
within a population. for humans.
o
Changes are the result of the nature of probability, like a coin toss.

If you toss a coin the expectation is 50% heads, 50% tails for each toss.

But in the number of tosses is only 10, you are not surprised to see 7 heads and 3 tails.

If you tossed 1 million times, much closer to 50/50 (Law of really big numbers).
o
During the process of cell replication (meiosis), only one allele out of two is present at a given locus because only you
receive 1 of 2 chromosomes (and the alleles that are on it).

Think of meiosis as if it were a game of coin toss. The probability of receiving one allele is the same as a coin toss:
50%

If two parents are Aa, the expected distribution of genotypes over 4 children is 25% AA, 50% Aa, and 25% aa
[Think Punnett square].
The Evolutionary Forces 10

Genetic drift (continued)
o
Population size and genetic drift.
• The effect of genetic drift depends on the size of the breeding population.

In larger populations:
• The larger the population size, the less change will occur from one generation to the next.
• The larger the population size, the fewer deviations in allele gene frequencies caused by genetic drift.

In smaller populations:
• In small populations, genetic drift more often results in a quick loss of one allele or another.
Relethford Chapter 3 Page 6
•
o
In a small population, just by chance, a few individuals may leave behind more offspring than others in the
population.

Genetic drift has the greatest evolutionary effect in relatively small breeding populations.
Examples of genetic drift
• The founder effect is a type of genetic drift caused by the formation of a new population by a small number of
persons. The Dunker population and those in Tristan de Cunha
• A bottleneck is an event that decreases genetic variation. It can be a natural disaster that wipes out a population or
massive hunting. Toba eruption may be one
The Evolutionary Forces 11

Gene flow
o
Gene flow is the movement of alleles from one population to another.

Migration and gene flow are not exactly the same thing

Gene flow may or may not occur when there is migration

Gene flow can also occur after the migrants have gone home.

Genes may occasionally also flow between species.
o
When gene flow occurs, the two populations mix genetically and tend to become more similar.

Gene flow also introduces new variation into a population

One of the principal reasons humans are so similar is because of gene flow
o
The amount of gene flow between human populations depends on a variety of environmental and cultural factors.

Geographic distance is a major determinant of migration and gene flow.

Greater distance means less likelihood of exchange of mates.

Most marriages are between those local to each other.

Cultural factors such as ethnicity, social status and so forth are factors (think assortative mating).
o
Since 1492, gene flow has become a more important evolutionary
force among humans.
The Evolutionary Forces 12
• Interaction of the evolutionary forces
o
Sometimes the four evolutionary forces–mutation, natural selection,
genetic drift, and gene flow – act together and sometimes they act in
opposition.

Different evolutionary forces can produce the same, or opposite,
effects

Mostly we look at three of the forces: gene flow, genetic drift
and natural selection

When gene flow and genetic drive are both operating they
counter-balance each other

When mutation and genetic drift are present, genetic drift can either increase or decrease the mutation within a
population

Genetic drift can increase a harmful gene, even if it is being selected against
• Population genetics is all about the MATHEMATICS of these forces.
Relethford Chapter 3 Page 7