Exercise 3-2: Population Evolution
The Evolution of Populations
Evolution is the genetic change occurring in a population of organisms over many generations. So what
is a population? A population is all of the members of a single species in a given area at a given time. So
by the definition of evolution and what a population is, we now know that individual organisms do not
evolve but populations do.
We know that we get phenotypes from genotypes and that genotypes have alleles. All of the alleles at
all of the gene loci in all individuals of a population is called a gene pool. The amount of each allele in
the gene pool is known as the allele frequency.
Fig. 1: Gene Pool
Page | 87
Figure 2 shows an example of allele frequency.
Gray dots represent dominant alleles and white
dots represent recessive alleles. Knowing this
information, the following is true:
Phenotypes:
AA = 7 (homozygous black)
Aa = 3 (heterozygous black)
aa =5 (homozygous white)
Allele frequency:
A = 17/30 or 57%
a = 13/30 or 43%
Fig. 2: Allele Frequency
Notice that the frequencies of A and a add up to 1.
We describe this by using the formula p + q = 1,
where p is the frequency of the dominant allele
and q is the frequency of the recessive allele.
p = .57 and q = .43
Hardy-Weinberg Equilibrium
When allele frequencies do not change, or in other words evolution does not occur, populations are said
to be in Hardy-Weinberg Equilibrium. Hardy-Weinberg Equilibrium occurs only if the following
conditions within a population are met.
For Hardy-Weinberg to be true:
1.
2.
3.
4.
5.
No mutations within the populations gene pool.
No migration in or out of the population.
Large gene pool
Random mating
No selection
Now, p + q = 1 only shows us the allele frequency of a non-evolving population, to show the genotype
frequency of a non-evolving population we use another formula called the Hardy-Weinberg equation.
The equation is 𝑝2 + 2𝑝𝑞 + 𝑞 2 = 1. Remember that p is the dominant allele so 𝑝2 is the homozygous
dominant genotype and q is the recessive allele so 𝑞 2 is the homozygous recessive genotype. Therefore,
pq is the heterozygous genotype. Using the information from figure 2, our equation would now be
. 572 + 2(. 57)(.43) +. 432 = 1.
Page | 88
Microevolution and Hardy-Weinberg
Microevolution is the accumulation of
small changes in the allele frequencies in
the gene pool of a population. Basically,
the allele frequencies are changed in
small amounts from generation to
generation. We can use Hardy-Weinberg
to determine if microevolution has
occurred. We know that Hardy-Weinberg
cannot be used if any of the five
conditions are not true. Evolutionary
change can be caused by any of the
Hardy-Weinberg conditions are not met.
Fig. 3: Microevolution
A mutation is a change in the DNA of an organism. Mutations are not necessarily bad nor good, in fact
they are the basis for microevolution and therefore the evolution of populations. Keep in mind that
most mutations do not affect the gene pool because they are caught and fixed. The mutations that are
not caught and repaired can affect the gene pool only if they can be and will be transmitted from
generation to generation. Mutations do not have much influence on the evolution of populations
because there are so many genes and an allele change may not even have an effect at all on the
organism or even the population.
Gene flow is the movement of alleles out of a population into another population. There are a few ways
in which this can occur. Migration, or the movement of organisms from one area to another for finding
mates, food, and territory. There are some things that determine the amount of gene flow. The distance
between populations and the organisms ability to reproduce with other populations are two big factors.
The more gene flow that occurs between two populations, the more their gene pool becomes similar, so
therefore the more similar the populations become similar. The less gene flow occurs between
populations, the less their gene pool becomes similar, so therefore the less similar the populations
become similar. Eventually, the populations could become reproductively isolated, meaning the
population gene pools become so different that they lose their ability to interbreed.
Genetic drift is the change in the frequencies of alleles within a gene pool because of random or chance
events. Natural disasters play a major role in genetic drifts. The reason why Hardy-Weinberg needs a
large gene pool is because things like natural disasters do not choose which individuals will be affected
within a population. A black cow is just as likely to be affected by a hurricane as a brown cow. The larger
the population the less likely the population will be affected by genetic drift and therefore will be less
likely to result in evolution. For example, if 10 individuals are eliminated out of a population of 100, only
10% of the population has been eliminated, but if 10 individuals are eliminated out of a population of
1000, only 1% of the population has been eliminated. The smaller the population, the more genetic drift
will affect the allele frequencies of the population. There are two different special types of genetic drift:
the bottleneck effect and the founder effect. They are similar in there end results but differ in the way
that they occur.
Page | 89
The bottleneck effect is the loss of genetic diversity from natural disasters, disease, overhunting, etc.
The founder effect is the loss of genetic diversity from when individuals are separated from the original
population and form another population. The results from both types of genetic drift are gene pools
with a random assortment of alleles. The newly formed gene pools are usually very different from the
original gene pool from the original population.
Fig. 4: Bottleneck Effect
Fig. 5: Bottleneck Effect
Page | 90
Fig. 6: Founder Effect
Random mating is another requirement of Hardy-Weinberg, but just because population do not mate
randomly, does not mean that evolution is taking place. Nonrandom mating just gives you a new allele
frequency for certain traits in a population, not an evolving population. A specific type of nonrandom
mating is called assortative mating, which is when an individual chooses a mate that has a desired trait,
such as feather colors, winners of battles (strongest), and even body size.
Hardy-Weinberg says that phenotypes play no role in whether or not organisms will survive. We know
that nature does select certain traits to survive better, this is called natural selection. These chosen
traits gives the organism better reproductive fitness, which allows the organism to pass on the selected
trait to other generations. This in turn will raise the allele frequency of the selected trait.
Natural Selection
Scientists have determined that there are three distinct categories or types of natural selection. They
are stabilizing selection, directional selection, and disruptive selection. When measuring natural
selection we use a bell curve graph.
Stabilizing selection is when a middle phenotype is better suited to the environment than the extreme
phenotypes.
Directional selection is when one extreme phenotype is better suited to the environment than the
middle or other extreme.
Disruptive selection is when two or more extreme phenotypes are better suited to the environment
than the middle phenotype.
Page | 91
DISRUPTIVE SELECTION
Fig. 7: Types of Natural Selection
Sexual Selection
Changes in male and females that can lead to a better chance to acquire a mate is known as sexual
selection. In males, this selection may be a trait that better suits them to compete with other males for
a mate and then females may select the more fit male. Sexual dimorphism is where males and females
of the same species differ in size, coat color, and other characteristics.
Page | 92