Genetic Mechanisms of Population C hange
Transcript of Genetic Mechanisms of Population C hange
Unit 4: Mechanisms of Population Change
Lesson 4.2
Genetic Mechanisms of Population Change
Contents
Introduction 1
Learning Objectives 2
Warm Up 2
Learn about It! 3 Population Genetics 3 Genes, Genotype, and Alleles 4 Factors Affecting Genetic Structure of Population 6
Mutation 6 Genetic Drift 8 Founder Effect 9 Bottleneck Effect 9 Recombination 10
Key Points 14
Check Your Understanding 15
Challenge Yourself 16
Bibliography 16
Unit 4: Mechanisms of Population Change
Lesson 4.2
Genetic Mechanisms of Population Change
Introduction
Have you ever observed the organisms in your garden? You may have noticed that some
organisms become more prominent after certain events. For example, weeds may grow
more in months with lots of rainfall. Changes that occur in the environment may reflect the
changes that happen to entire populations. Most of the time, changes in the population of
organisms are driven by random natural events. A population may grow or shrink in
number or it can shift from having bigger to smaller individuals. These changes in the
structure and characteristics of the population are mostly governed by random events. For
example, a population of beetles may be altered due to random accidents where humans
may step on some of its members. Due to this, individuals having special genetic features
may disappear in the population.
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In this lesson, students are expected to have a better understanding of the genetic
mechanism behind population change and at the same time, understand that most of
these processes are happening in a random manner. At the end of the discussion, you
should know how the genetic structure of the population is affected by several factors.
Learning Objectives
In this lesson, you should be able to do the
following:
● Discuss the concept of genetic drift,
mutation, and recombination.
● Enumerate the effects of these processes
on the population.
DepEd Competency
Explain the mechanisms that
produce a change in populations
from generation to generation (e.g.,
artificial selection, natural selection,
genetic drift, mutation,
recombination)
(STEM_BIO11/12-IIIc-g-9)
Warm Up
Understanding Genetic Drift 15 minutes In this activity, the class will watch a video explaining genetic drift. This will help the students
to have a better understanding of the factors affecting the genetic structure of the
population.
Materials ● computer with an internet connection
● LCD projector
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Procedure 1. Watch the video regarding genetic drift using this link.
Genetic Drift Wyatt Loney, “Genetic Drift Example Video,” YouTube (April 24, 2017), https://youtu.be/IUCL-nfStFQ , last accessed on February 14, 2020.
2. After watching the video, group yourselves into two groups.
3. Each group will be assigned to one mechanism of genetic drift:
a. Founder effect
b. Bottleneck effect
4. The group must think of a story that would illustrate the assigned mechanism to
their group.
5. The group must present a skit of their story and explain how it is related to the
changes in the structure of the population.
6. Answer the guide questions for the activity.
Guide Questions 1. How was the founder effect reflected in the video?
2. How was the bottleneck effect reflected in the video?
3. Given the two mechanisms, how do you think genetic drift operates in changing the
population?
4. Do you think genetic drift can cause major changes in the characteristics of the entire
population?
Learn about It!
Population Genetics Population genetics is a field of science that deals with genetic variation in the populations
of organisms in the ecosystem. It deals with the examination and modeling of the spatial
and temporal variation in frequencies of genes and alleles in populations . This is
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possible due to the presence of minute differences in the DNA sequences of genes. Most of
the time, population geneticists utilize mathematical models to understand the occurrence
of specific alleles in the populations.
How do you think scientists study changes in the genetic structure of the entire population?
Genes, Genotype, and Alleles The DNA is made up of many molecules known as nucleotides. These nucleotides are the
building blocks of DNA and RNA. These nucleotides chain together and form protein-coding
segments of the DNA called genes. Genes, as shown in Fig. 4.2.1 , are segments of DNA that
regulate the expression of the traits of an organism through the identity and arrangement
of the nucleotides. A gene consists of a specific nucleotide sequence and has a definite
position in a given chromosome. This particular sequence codes for a specific protein for
phenotype expression.
Fig. 4.2.1. Genes, a segment of DNA Other important terms that must be defined in this lesson are genotype, phenotype, and
alleles. In most cases, the expression of certain traits in the organism is influenced and
regulated by a set of genes that are also called genotype . In basic biology classes, genotypes
are represented by combinations of letters which represent the genes present in the
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organism. Combined with the effects of environmental factors, the genotype determines the
phenotype. Phenotypes are the observable traits expressed in an individual. A gene
contains all the needed information that codes for a specific protein required in controlling
the expression of different phenotypes in an organism. In short, a phenotype is the physical
features of an organism. Lastly, alleles refer to the variant form of a given gene. Alleles are
formed due to the presence of different versions of mutation that took place in the same
position in the chromosome. Alleles are often represented by sequence variations for a
several-hundred base-pair or region of the DNA that codes for a protein. Alleles are
responsible for having variation in a specific trait regulated by a gene. This is illustrated in
Fig. 4.2.2 for the regulation of mustache in men.
Fig. 4.2.2. Difference between alleles, genotype, and phenotype
Given this knowledge, it is important to take note that population genetics deals with the
changes in allele and genotype frequency in the population over space and time scale.
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Factors Affecting Genetic Structure of Population This part of the lesson aims to enumerate the different factors that affect the population’s
genetic structure. Specifically, we aim to describe the effects of mutation, genetic drift, and
recombination in the genetic makeup of individuals in the population.
What is mutation? How can it affect the population?
Mutation Mutations as shown in Fig. 4.2.3 are events
where changes in the genetic code or in the
nucleotides of DNA occur. Mutations can be
represented by either a change in the
nucleotides present in the gene or the
nucleotides present in large segments of the
chromosome. It leads to genetic variation by
affecting a single nucleotide pair or a segment of
a chromosome.
Mutations happen at the DNA level and can alter
genes, which may result in the formation of allele
variants. This means that organisms are capable
of transferring these changes in the DNA of that
organism into their offspring. Mutations are essential for driving the evolution of organisms.
It is through mutations that genetic changes occur, whether they are advantageous, neutral,
or deleterious to the survival of the organism. Advantageous mutations increase the
fitness of organisms. Deleterious mutations are the opposite, decreasing the fitness of
organisms. Neutral mutations , on the other hand, do not impact fitness.
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In most cases, advantageous and deleterious mutations undergo natural selection and
have a low frequency or portion of the population. On the other hand, neutral mutations
or those with almost no effect on the fitness of the organisms are being fixed and
widespread in the population. As the number of mutations accumulates and is passed on,
new traits may arise and the occurrence of these traits may vary in number in the
population.
As discussed in the previous section, natural selection dictates the frequency of the traits
that arise from mutations. Some of these traits increase the fitness of the individuals and
remain in the population, while some of the traits from mutation are lost along the way. For
the case of the fixed traits caused by mutation, this may then lead to an increased number
of organisms with said traits, which changes the ratio of the organisms with those traits as
opposed to those that do not. On the opposing end, if the traits give the organisms certain
characteristics that are detrimental to their survival, then the chances of them passing these
traits onto their offspring are low since they may not survive long enough to reproduce in
the first place. This fate on the frequency of the different types of mutations is represented
in Fig. 4.2.4.
Fig. 4.2.4. Hypothetical distribution of different types of mutations in the population
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Genetic Drift Genetic drift is the change of allele frequencies as a product of random events in the
environment. Unlike natural selection where the cause for the change in allele frequencies
in the populations is based on the advantageous effect for adaptation, genetic drift acts by
pure chance. Genetic drift is capable of causing a decrease in the frequency of some alleles
or even to the complete loss of alleles in the population. The same thing can also happen to
beneficial alleles, where even helpful alleles can completely disappear from the population
in question. The effect of genetic drift can be shown through the founder and bottleneck
effects.
How does genetic drift cause changes in the population’s genetic structure?
Fig. 4.2.5 shows an example of a
genetic drift. A random event, which in
this example is the death of all the
green beetles by random chance, led
to the gene frequency changing from a
3:4 green to orange beetle ratio to a
purely orange beetle population. The
coloration of the beetles had nothing
to do with the event wiping out the
green beetles as it was purely by
chance, making it an example of
genetic drift.
Natural selection operates on adaptations. As was discussed in the previous lesson, these
adaptations allow the selection of organisms that have beneficial traits for survival. In
comparing natural selection and genetic drift, the former selects for organisms that have
beneficial traits while the latter is based on pure random chance.
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Founder Effect Founder effect refers to the loss of genetic variation in the new population that was
established by a very few individuals from a larger population. The loss of genetic variation
can be measured through the changes in the alleles and genotype frequency in the
population. As a consequence of these losses, the new population may exhibit distinct
differences, both genotypically and phenotypically, compared to the original founding
individuals. For some extreme situations, the founder effect can lead to speciation and the
emergence of new species. The effect of a limited number of founding individuals for the
newly derived population is illustrated in Fig. 4.2.6.
.
Fig. 4.2.6. . Effect of the founding population on the genetic structure of the future population
Bottleneck Effect Population bottleneck refers to an event where there is an abrupt reduction in the size of
a population caused by random environmental events. These events can be in the form of
famines, earthquakes, floods, typhoons, fires, diseases, or deadly human activities. These
random events cause a rapid decline in the population size that eventually reduces the
variation in the gene pool of a population. This means that the population becomes smaller
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having lower genetic diversity. The decline in genetic diversity may result in a reduction in
the robustness of the population and inhibit their ability to adapt to and survive in
ever-changing environments. The effect of a population bottleneck on the genetic structure
of the future population is illustrated in Fig. 4.2.7.
Fig. 4.2.7. Changes in the genetic structure of the population due to population bottleneck
Recombination Recombination refers to the process where pieces of DNA are segmented and recombined
to produce new combinations of alleles. Overall, recombination is important in creating
genetic diversity at the level of genes as reflected by the differences in the DNA sequences
of different organisms.
Recombination happens in different events in the life of an organism. One of the
recombination processes occurs during meiosis. During prophase I, paired chromosomes
called homologous chromosomes or simply homologues tightly join together through a
process called synapsis . During this event, arms of chromatids are observed to crisscross in
some parts. These intersecting regions are called chiasmata (singular: chiasma) . A
chiasma holds the homologues together until they are separated in anaphase I. Segments
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of the chromosomes in the chiasmata can be switched. This switching of genetic content
increases genetic variation between the parent and its offspring. This is illustrated in Fig.
4.2.8. The crossing over of genetic materials during this process is the one responsible for
the variation among siblings despite coming from the same set of parents.
Fig. 4.2.8. Crossing over during meiosis
Moreover, this process of recombination and segregation of alleles was perfectly described
by Gregor Mendel based on his observations in the pea plant. He proposed that alleles
must segregate somewhere between the production of sex cells and fertilization.
From this, the law of segregation was formed, which states that all the genes for all the
traits of an offspring are equally distributed or segregated in all the resulting gametes
after meiosis. Fig. 4.2.9 shows that the parent cell produces four daughter cells after
meiosis, where letters in each cell represent genes. In this illustration, only two genes (A and
B) are drawn, where all the genes of the parent cell segregate and distributed in each
gamete. Therefore, whatever gamete will be able to fertilize, it still carries all the genes of
the parents.
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Fig. 4.2.9. Gene segregation during meiosis
Taking the idea of the law of segregation, we can describe that during meiosis, two alleles of
a gene that codes for a certain trait segregate from one another to form gametes that
contain only one gene of the pair. Moreover, during fertilization, the offspring get one
genetic allele from each parent, the egg and the sperm cells. The cell with the combined
alleles from both parents now forms the offspring. With this recombination process, we can
conclude that the genetic structure of the individuals within the population varies from one
generation to another, thus causing higher genetic diversity as the population grows.
An illustration of Mendel’s law of segregation from parents to offspring
How does recombination help in increasing genetic diversity in the population?
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Did You Know? The mitochondrial genome in humans totally comes from the
maternal mitochondrial genome. The genetic materials in the
mitochondria do not undergo a series of recombination processes,
thus preserving the original DNA sequence from the ancestral to
the most recent population. Take note, however, that some
organisms follow a paternal inheritance for their mitochondrial
DNA.
An illustration of mitochondrial genome
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Key Points _____________________________________________________________________________________________
● Population genetics is a field of science that deals with genetic variation in the
populations of organisms in the ecosystem.
● Genes are segments of DNA that regulate the expression of the traits of an organism
through the identity and arrangement of the nucleotides.
● Genotypes are sets of genes that regulate the expression of certain traits in the
organism. Phenotypes are the observable traits expressed in an individual. A gene
contains all the needed information that codes for a specific protein required in
controlling the expression of different phenotypes in an organism.
● Alleles refer to the variant form of a given gene. Alleles are formed due to the
presence of different versions of mutation that took place in the same position in the
chromosome.
● Genetic drift is the change of allele frequencies as a product of random events in
the environment.
○ Founder effect refers to the loss of genetic variation in the new population
that was established by a very few individuals from a larger population.
○ Population bottleneck refers to an event where there is an abrupt reduction
in the size of a population caused by random environmental events.
● Recombination refers to the process where pieces of DNA are segmented and
recombined to produce new combinations of alleles.
_____________________________________________________________________________________________
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Check Your Understanding
A. Identify if the statements are true or false.
1. Meiosis I results in 2 daughter cells. On the other hand, meiosis II involves 2
daughter cells producing another 2 daughter cells which then results in a total of 4
cells.
2. Meiosis involves the recombination of genetic materials.
3. An allele controls similar traits but exhibits different phenotypes.
4. Genetic drift refers to changes in allele frequencies resulting from random chance.
This can possibly lead to changes in the allele frequencies and can even lead to the
disappearance of some of these alleles.
5. Genetic drift leads to random changes. This means that even traits that come from
beneficial alleles can disappear from a population.
6. Recombination happens in different events in the life of an organism. One of the
recombination processes occurs during meiosis.
7. For some extreme situations, the founder effect can lead to speciation and the
emergence of new species.
8. Mutations happen at the DNA level and can alter a certain gene leading to the
formation of allele variants.
9. Deleterious mutations have a low chance of being passed on since the organisms
may not survive long enough to reproduce in the first place.
10. Combined with the effects of environmental factors, genotype determines the
phenotype.
B. Classify if the given scenario is an example of the effect of the
mutation, genetic drift, or recombination.
1. new characteristics emerged after a population migrated into an area and
reproduced with the local population
2. low genetic diversity in an island population
3. variation among siblings from the same parents
4. variation among cousins from the same grandparents
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5. high genetic diversity in population with random mating
6. low genetic diversity in population with small population size
7. the appearance of rare genetic diseases
8. large populations with morphological abnormalities
9. low genetic diversity of populations of organisms in the zoo
10. low genetic diversity among the family that practice incest mating
Challenge Yourself
Answer the following questions.
1. Do you think humans can control the effect of genetic drift?
2. What could be the consequences of low genetic diversity in the population?
3. Do you think it is a good thing that the recombination process occurs in the human
population? Why?
4. Do you agree that some mutations are good for the organisms? Explain your
answer.
5. Do you think artificial selection can regulate the effects of the factors discussed in
this lesson?
Bibliography
Baum, David, Douglas Futuyma, Hopi Hoekstra, Richard Lenski, Allen Moore, Catherine L.
Peichel, Dolph Schluter, and Michael Whitlock. 2013. The Princeton Guide to
Evolution. Princeton University Press.
College of the Redwoods. Beaupre-Riggs. Lab 13: Evolution and Natural Selection.
https://redwoods.instructure.com/courses/2715/files/126998
Coyne, Jerry. 2009. Why Evolution Is True. Oxford University Press. Genetic Science
Learning Center. July 1, 2013.
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Johnson, G.B., and Raven, P.H. 2001. Biology: Principles & Explorations . Austin: Holt, Rinehart,
and Winston.
Klug, W.S., Spencer, C.A., and Cummings, M.R. 2016. Concepts of Genetics . Boston: Pearson.
Mader, S.S. 2014. Concepts of Biology . New York: McGraw-Hill Education.
Reece, J.B. and Campbell, N.A. 2011. Campbell Biology . Boston: Benjamin
Cummings/Pearson.
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