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


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.

4.2. Genetic Mechanisms of Population Change 1


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



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


https://youtu.be/IUCL-nfStFQ





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



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.



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.



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



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.



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



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



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.



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?



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



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.

_____________________________________________________________________________________________



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



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.


https://redwoods.instructure.com/courses/2715/files/126998


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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