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BIOLOGY 2
Mechanisms that Produce Change in Population
Albay_Gen.Biology2 _Q3_LP4
BACKGROUND INFORMATION FOR LEARNERS:
Populations evolve, but individual organisms do not. A population is an interbreeding group
of individuals of one species in a given geographic area at the same time. A population evolves because
the population contains the collection of genes called the gene pool. As changes in the gene pool
occur, a population evolves. Evolution is happening right here, right now!
Evolution doesn't mean that these populations are marching towards some final state of
perfection. Evolution means “descent with modification”. “Descent” means inheritance while
modification refers to changes in traits from generation to generation. Evolution therefore means that
a population is changing its genetic makeup over generations.
What causes populations to evolve? When a population is in Hardy-Weinberg
equilibrium (HWE) for a gene, it is not evolving, and allele frequencies will stay the same across
generations.
What is the difference between allele and genotype
frequency?
 Allele frequency is the fraction of all the gene copies in
a population that a particular allele makes up. For
instance, to get the frequency of allele A in the
population below, we count up all the A alleles. We
find 12 of them (out of the 40 total alleles in the
population). This gives us an allele frequency of 12/40 =
0.3.

Genotype frequency is the fraction of individuals in that
population that have a particular genotype. For
instance, to get the frequency of the genotype AA in the
population above, we count up all the AA individuals.
We find two beetles of this genotype out of 20 beetles
in the population. This gives us a genotype frequency
of 2/20 = 0.1
Albay_Biology2_Grade11_Q3_LP4
Hardy-Weinberg Equilibrium is a null model of the relationship between allele and genotype
frequencies, both within and between generations. In order for a population to be in Hardy-Weinberg
equilibrium, or a non-evolving state, it must meet five major assumptions:
1. No mutation. No new alleles are generated by mutation, nor are genes duplicated or deleted.
2. Random mating. Organisms mate randomly with each other, with no preference for particular
genotypes.
3. No gene flow. Neither individuals nor their gametes (e.g., windborne pollen) enter or exit the
population.
4. No finite population size (genetic drift). The population should be effectively infinite (large)
in size.
5. No natural selection. All alleles confer equal fitness (make organisms equally likely to survive
and reproduce).
Mechanisms of evolution
Violations of different Hardy-Weinberg assumptions correspond to mechanisms of evolution.
There are several key mechanisms that cause a population to exhibit a change in allele frequency from
one generation to the next. These are evolution by: mutation, non-random mating, gene flow, genetic
drift, natural selection, and genetic recombination. Each mechanism of evolution can be characterized
by how it affects fitness, adaptation, the average phenotype of a trait in population, and the genetic
diversity of the population.
1. Mutation. Evolution by mutation occurs whenever a mistake in the DNA occurs in the
genome- heritable complete set of genetic information an organism requires to function that
generates variation and can be passed on to the next generation. Mutation is the raw stuff of
evolution because it creates a new heritable phenotype, irrespective of fitness or adaptation.
It should be noted that mutation is the ultimate source of genetic variation in all population.
Image source: https://www.khanacademy.org/
Although mutation is the original source of all genetic variation mutation rate for most
organisms is pretty low. So, the impact of brand-new mutations on allele frequencies from
one generation to the next is usually not large. Nevertheless, mutation combined with one of
the other mechanisms of evolution can be a powerful mechanism of evolution.
Albay_Biology2_Grade11_Q3_LP4
2. Non-random Mating. Selecting a mate at random
is a pretty risky idea because half of your
offspring’s genes come from your mate. Nonrandom mating can occur when individuals prefer
mates with particular superior physical
characteristics or by preference of individuals to
mate with individuals similar to themselves. Nonrandom mating is a more common approach in
real populations: think about male birds being https://www.chegg.com/learn/biology/introductionto-biology/nonrandom-mating
selected as mates by females who choose males
for their vivid coloration or beautiful and complex birdsong. If individuals non-randomly mate
with other individuals in the population, choices can drive evolution within a population. In
animals, non-random mating can change allele frequencies as the choice of mates is often an
important part of behavior. Many plants self-pollinate, which is also a form of non-random
mating (inbreeding).
3. Gene flow. Another mechanism of evolution may occur during the movement of genes into
and out of a population due to migration of organisms or their gametes. For example, animals
may be driven off from a herd forcing them to migrate to a new population, thereby bringing
new genes to a gene pool. Gene flow tends to increase the similarity between remaining
populations of the same species because it makes gene pools more similar to one another.
During the Vietnam War in the 1960s and 1970s, many American servicemen had children
with Vietnamese women. Most of the servicemen returned to the United States after the war.
However, they left copies of their genes behind in their offspring. In this way, they changed
the allele frequencies in the Vietnamese gene pool. Was the gene pool of the American
population also affected? Why or why not?
Image source: https://www.khanacademy.org/
4. Non-infinite population size (genetic drift). Genetic drift involves random fluctuations in the
frequencies of alleles from generation to generation due to chance events. Genetic drift can
cause traits to be dominant or disappear from a population. The effects of genetic drift are
most pronounced in small populations.
Albay_Biology2_Grade11_Q3_LP4
Image source: https://www.khanacademy.org/
Examples of Genetic Drift
a. The Founder Effect. Small population that branches off from a larger one may or may
not be genetically representative of the larger population from which it was derived.
This reduced genetic diversity that results when a population is descended from a
small number of colonizing ancestors is known as the “founder effect.”
Amish child with Ellis Van Creveld syndrome, is a
recessive condition whose symptoms include
dwarfism, extra fingers and toes (polydactyly),
abnormalities of nails and teeth, and a hole
between the upper two chambers of the heart.
Image source: https://learn-biology.com/apbiology/module-20-population-genetics/geneticdrift-the-founder-effect-and-populationbottlenecks/
b. Population Bottleneck occurs when a population
undergoes an event in which a significant
percentage of a population or species is killed or
otherwise prevented from reproducing.
Ex. cheetahs
Albay_Biology2_Grade11_Q3_LP4
Image source: https://dragonflyissuesinevolution13.fandom.com/wiki/Bottleneck_Effect
Because genetic drift acts more quickly to reduce genetic variation in small
populations, undergoing a bottleneck can reduce a populations’ genetic variation by a lot,
even if the bottleneck doesn’t last for very many generations. The event may eliminate alleles
entirely or also cause other alleles to be over-represented in a gene pool.
5. Natural selection. Finally, the most famous mechanism of evolution! Natural selection occurs
when one allele (or combination of alleles of different genes) makes an organism more or less
fit, that is, able to survive and reproduce in a given environment. If an allele reduces fitness,
its frequency will tend to drop from one generation to the next.
Image source: https://www.khanacademy.org/
Charles Darwin was a British naturalist who proposed the theory of biological evolution
by natural selection. Darwin defined evolution as "descent with modification," the idea that
species change over time, give rise to new species, and share a common ancestor.
The mechanism that Darwin proposed for evolution is natural selection. Because
resources are limited in nature, organisms with heritable traits that favor survival and
reproduction will tend to leave more offspring than their peers, causing the traits to increase in
frequency over generations.
Albay_Biology2_Grade11_Q3_LP4
Natural selection causes populations to become adapted, or increasingly well-suited, to
their environments over time. Natural selection depends on the environment and requires
existing heritable variation in a group.
Darwin's concept of natural selection was based on several key observations:

Traits are often heritable. In living
organisms, many characteristics are
inherited, or passed from parent to
offspring. (Darwin knew this was the
case, even though he did not know that
traits were inherited via genes.)

More offspring are produced than
survive. Organisms are capable of
producing more offspring than
their environments can support.
Thus, there is competition for
limited
resources
in
each
generation.

Offspring vary in their heritable traits. The
offspring in any generation will be slightly different
from one another in their traits (color, size, shape,
many of these features will be heritable.
can
etc.),
and
Based on these simple observations, Darwin concluded the following:
 In a population, some individuals will have inherited traits that help them survive and
reproduce (given the conditions of the environment, such as the predators and food
sources present). The individuals with the helpful traits will leave more offspring in the
next generation than their peers since the traits make them more effective at surviving
and reproducing.
 Because the helpful traits are heritable, and because organisms with these traits leave
more offspring, the traits will tend to become more common (present in a larger fraction
of the population) in the next generation.
 Over generations, the population will become adapted to its environment (as individuals
with traits helpful in that environment have consistently greater reproductive success
than their peers).
Albay_Biology2_Grade11_Q3_LP4
How natural selection can work
To make natural selection more concrete, let's consider a simplified, hypothetical
example. In this example, a group of mice with heritable variation in fur color (black vs. tan)
has just moved into a new area where the rocks are black. This environment features hawks,
which like to eat mice and can see the tan ones more easily than the black ones against the
black rock.
Because the hawks can see and catch the tan mice more easily, a relatively large
fraction of the tan mice are eaten, while a much smaller fraction of the black mice are eaten.
If we look at the ratio of black mice to tan mice in the surviving ("not-eaten") group, it will be
higher than in the starting population.
Schematic based on similar schematic in Reece et al. ^44start superscript, 4, end superscript. Hawk outline traced from "Black
and white line art drawing of Swainson hawk bird in flight," by Kerris Paul (public domain).
Fur color is a heritable trait (one that can be passed from parent to child). So, the
increased fraction of black mice in the surviving group means an increased fraction of black
baby mice in the next generation. After several generations of selection, the population might
be made up almost entirely of black mice. This change in the heritable features of the
population is an example of evolution.
Key points about natural selection
 Natural selection depends on the environment
Natural selection doesn't favor traits that are somehow inherently superior. Instead, it
favors traits that are beneficial (that is, help an organism survive and reproduce more
effectively than its peers) in a specific environment. Traits that are helpful in one environment
might actually be harmful in another.

Natural selection acts on existing heritable variation
Albay_Biology2_Grade11_Q3_LP4

Natural selection needs some starting material, and that starting material is heritable
variation. For natural selection to act on a feature, there must already be variation
(differences among individuals) for that feature. Also, the differences have to be heritable,
determined by the organisms' genes.
Heritable variation comes from random mutations
The original source of the new gene variants that produce new heritable traits, such as fur
colors, is random mutation (changes in DNA sequence). Random mutations that are passed
on to offspring typically occur in the germline, or sperm and egg cell lineage, of organisms.
Sexual reproduction "mixes and matches" gene variants to make more variation.
6. Artificial Selection. Artificial selection is the identification by humans of desirable traits in plants
and animals, and the steps taken to enhance and perpetuate those traits in future generations.
Artificial selection works the same way as natural selection, except that with artificial selection it
is human interference, that makes decisions on which individuals reproduce.
A prime example of artificial selection is dog breeding. Although it was traced that all dogs are
descendants of the wolf, the use of artificial selection has allowed humans to dramatically alter
the dogs, phenotype. Dogs have been bred for various desired characteristics, leading to the
creation of a wide range of dogs, from the tiny Chihuahua to the massive Great Dane. Desirable
or fashionable traits of dogs nowadays are product of artificial selection.
There are three different types of breeds that exist:



Purebred comes from a lineage of the same breed and that has never mated with another
breed. For example, a purebred German shepherd is all German shepherd and nothing else.
A cross-breed is an offspring of two different types of purebreds. A purebred german
shepherd mated with a purebred husky resulting to half german shepherd, half husky.
Mixed-breeds are a combination of multiple breeds, where their parents were not
purebreds. One advantage to choosing cross-breeds and mixed-breeds over purebred dogs
is that harmful genetic mutations that tend to frequently occur in certain lineages can be
covered up, or “bred out”, by the genetic background of the other dog breed(s) in the
individual.
In agriculture, artificial selection has long been used to produce animals and crops with
desirable traits. Artificial selection has increased carrots’ beta carotene content by 50% since
1970 (Thomson et al., Genetic Literacy Project, 2016). Artificial selection appeals to humans since
it is faster than natural selection and allows humans to mold organisms to their needs.
7. Genetic Recombination. Genetic recombination refers to the rearrangement of DNA sequences
by the breakage and rejoining of chromosomes or chromosome segments which is a programmed
feature of meiosis in most sexual organisms, where it ensures the proper segregation of
chromosomes. Recombination also serves as a mechanism to repair some types of potentially
lethal damage to chromosomes. This allows nonfunctional (less functional) alleles to be cleared
from a population.)
Albay_Biology2_Grade11_Q3_LP4
Meiotic recombination is an example of
a reaction that involves DNA sequences
that are paired and homologous over
very extended lengths. This type of
process, which is illustrated in Figure 5,
is
termed
as
homologous
recombination. Each participating
chromosome receives information
comparable to what it donates to the
other partner which is termed as
reciprocal. The event shown in Figure 7
is also designated as a crossover, since
all the information on both sides of the
effective break has been exchanged.
Recombination
during
meiosis
generates new combinations of alleles
in the offspring.
Simplified diagram of a meiotic recombination event.
Image source: https://ars.els-cdn.com/content/image/3-s2.0B9780123749840006276-f00627-01-9780123749840.jpg
Gene conversion is a form of homologous
recombination that is nonreciprocal. This is
recognized by the recovery of unequal numbers of
the parental markers at a particular locus, and a
simple example is shown in Figure 6 . Conversion
events can be accompanied by a crossover, or not
(as shown in Figure 8). In the latter case,
conversion looks like a very localized double
crossover, but it is nonreciprocal and is likely the
result of a single event.
Homologous recombination can occur
between homologous chromosomes or sister
Illustration of a gene conversion event
https://ars.els-cdn.com/content/image/3-s2.0chromatids in mitotic cells. Recombination that
B9780123749840006276-f00627-02-9780123749840.jpg
involves very limited or no homology between the
interacting DNA sequences is termed illegitimate or non-homologous recombination.
MOST ESSENTIAL LEARNING COMPETENCY WITH CODE:
MELC: Explain the mechanisms that produce change in the population from generation to
generation (eg. artificial selection, natural selection, genetic drift, mutation and
recombination)
Specifically, the learners are expected to:
1. Describe the mechanisms of evolution: selection, mutation, genetic drift, and gene flow
and recombination.
Albay_Biology2_Grade11_Q3_LP4
2. Explain how each mechanism can influence the allele frequencies of a population
3. recognize the consequences of each mechanism of evolution in terms of fitness,
adaptation, average phenotype, and genetic
ACTIVITIES/ EXERCISES:
ACTIVITY 1:
A. Say it with a picture
INSTRUCTION: Determine what type of mechanism of evolution each picture represents.
1.
4.
2.
3.
5.
B. Multiple Choice. Choose the letter of your answer then write it on your paper.
1. Members of the same species which are capable of interbreeding is best described as a(n):
a. Community
c. population
b. Ecosystem
d. biosphere
Albay_Biology2_Grade11_Q3_LP4
2. The total aggregate of alleles in a population is referred to as:
a. the gene pool
c. the allelic frequency
b. the genotypic frequency
d. the genetic structure
3. Which of the following is not a requirement for the maintenance of Hardy–Weinberg equilibrium?
a. large population size
b. non-random migration
b. absence of mutation
d. no natural selection
4. Which of the following is a likely result of genetic drift?
a. Gene fixation
b. Introduction of new alleles
b. Natural selection
d. Gene flow
5. How do mutations most directly contribute to natural selection?
a. They change the base sequence of DNA within a gene.
b. They create new alleles that may be more suitable for a given environment.
c. They increase the variation within a population.
ACTIVITY 2: Critical thinking
Instruction: Read the following paragraph and answer the questions that follow.
(Adapted from the University of California, Los Angeles Life Sciences 1 Demonstration Manual 2010)
1. A characteristic which is influenced by genes and passed from parents to offspring is called
heritable. Over many generations heritable adaptive characteristics become more common in a
population. This process is called evolution by natural selection. Evolution by natural selection
takes place over many, many generations.
Evolution by natural selection leads to adaptation within a population. The term evolution by
natural selection does not refer to individuals changing, only to changes in the frequency of
adaptive characteristics in the population as a whole. For example, for the mice that lived in the
beach area with tan sand, none of the mice had a change in the color of their fur; however, due to
natural selection, tan fur was more common for the pups than for the mother mice.
In summary, a heritable characteristic that helps an animal or plant to have more offspring
which survive to reproduce will tend to become more common in a population as a result of
evolution by natural selection.
Question:
 Explain why a characteristic which helps an animal to live longer will generally tend to
become more common in the population as a result of evolution by natural selection.
2. Describe what is happening in
figures 1-3. Is the population of
mice different in figure 3 than in
figure 1? Explain why.
Questions:
 Describe what is happening
in figure 3
 Is the population of mice
different in figure 3 than in
figure 1? Explain why.
Albay_Biology2_Grade11_Q3_LP4
3. Living things that are well adapted to their environment survive and reproduce. Those that are
not well adapted don’t survive and reproduce. An adaptation is any characteristic that increases
fitness, which is defined as the ability to survive and reproduce.
Question:
What characteristic of the mice is an adaptation that increased their fitness?
Scoring Rubrics for Activity
Exceptional
(10 points)
Characteristics
Very
Satisfactory
(8 points)
Satisfactory
(6 points)
Needs
Improvement
(4 points)
Poor
(-3 points)
1.Answers the questions in cohesive
and coherent manner by citing some
examples related to the concept
being discussed. There is none
grammatical, spelling and
punctuation errors.
2. Workmanship. Handwriting is
clear
and the output is presented neatly
and organized.
TOTAL
ACTIVITY 3: LET’S LEVEL UP!
Instruction: Make either (infographic, concept map, collage (digital or non-digital) or an
artwork (eg. poster art) (digital or non-digital) illustrating mutation of the Novel Corona Virus
resulting its variants that impacted the whole world today.
Notes:


CATEGORY
Graphics –
Originality
Graphics –
Relevance
Attractiveness
Use ½ Cartolina for collage making and poster art.
Use a 1 whole cartolina for infographic or concept map
Scoring sheet and Rubrics for Activity 3. Show the World
POINTS
Exceptional (10 pts)
Very satisfactory (8 pts)
CHARACTERISTICS
Satisfactory (6 pts)
Needs Improvement
4 points
Several of the graphics used on the poster
reflect an exceptional degree of creativity in
their creation and/or display
All graphics are related to the topic and
make it easier to understand
The masterpiece is exceptionally attractive in
terms of design, layout, and neatness.
Albay_Biology2_Grade11_Q3_LP4
Writing Mechanics
(For Concept Map/
Infographics HPS 20
pts))
Originality of
Message
(for Collage and
Poster Art Work only)
Persuasiveness of the
message
(for Collage and Poster
Art Work only)
TOTAL POINTS
Capitalization, spelling, grammar, and
punctuation are correct throughout the
poster
Poster depicts an important and original
message in a novel, creative manner
Poster conveys a clear and important
message in a powerful and persuasive
manner
Highest Possible Score
Collage Making
Poster Art Work
Digital and Non- Digital Poster
50 points
50 points
50 points
REFLECTION: Share your insights and queries.
Things I have learned today
__________________________________________________________________________________
I wish to ask about
__________________________________________________________________________________
REFERENCES FOR LEARNERS:
Books:
Hoefnagels, M.(2019).General Biology(Book I and II)(2nd ed.).McGraw-Hill Education
Internet Sources:
Gregogy, R. (2009). Understanding natural Selection: essential concepts and common
misconception Https://Evolution-Outreach.Biomedcentral.Com/Articles/10.1007/S12052009-0128-1.
Ridley, M. (n.d.). Evolution - A-Z - Random sampling. Www.Blackwellpublishing.Com.
Retrieved January 31, 2022, from https://www.blackwellpublishing.com/ridley/az/Random_sampling.asp
Evolution(module 1). (n.d.). Https://Bioprinciples.Biosci.Gatech.Edu. Retrieved January 28,
2022, from https://bioprinciples.biosci.gatech.edu/module-1-evolution/neutralmechanisms-of-evolution/
Albay_Biology2_Grade11_Q3_LP4
ANSWER KEY:
ACTIVITY 1.
A. Say it with a picture
1. Natural selection
2. Genetic drift (founder effect)
3. Gene flow
4. Mutation
5. Non-random mating
1.
2.
3.
4.
5.
c
a
b
a
b
ACTIVITY 1.
B. Multiple Choice
Activity 2
Sample Answers
1.Since favorable characteristics help the individual to live longer, there is a greater chance that it will
reproduce than those with lesser favorable traits.
2. Yes. There are more brown or tan mice in figure 3 than in figure 1. Because those mice that are not
well adapted to the environment will soon die and will not able to reproduce.
3. Camouflage is what allowed the brown/tan mice to blend in with their environment and increased
their fitness. Camouflage is a mechanism of some animals to avoid detection by the predators that
helps an animal to survive in its environment.
DEVELOPMENT TEAM
Author/ Writer:
Content Editor:
Language Editor:
Education Program Supervisor:
Albay_Biology2_Grade11_Q3_LP4
MARGIE R. LLENO, TIII, ANHS, Bascaran High School Ext.
ROCHELLE L. DE VERA, MT II San Jose NHS
EUGENE MINAS
JADE O. ALBERTO, EdD