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Science 9th grade
LEARNING UNIT
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Language
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the LO
Curricular axis
Standard competencies
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Vocabulary box
LEARNING OBJECT
How has the theory of evolution changed in the 20th
century?
 SKILL 1: Understand Darwin’s theory of natural
selection.
 SKILL 2: Describe the basics of Mendel’s theory
of heredity
 SKILL 3: Evaluate the mutation phenomenon
within the framework of speciation process.
 SKILL 4: Define speciation models.
 SKILL
5:
Define
the
continental
drift
phenomenon as a speciation mechanism.
 SKILL 6: Summarize the theories that make
part of the modern evolution theory and debate
about the topic.
English
Colombia
Living Surroundings.
Explain the variability of populations and biological
diversity as a consequence of reproduction strategies,
genetic adaptation and natural selection.
•
Know the different mechanisms of cellular
division.
•
Identify different structures of genetic
information storage (chromosomes)
•
Recognize the reciprocity of genetic factors with
environmental factors.
Superlatives and comparatives.
 Offspring: A descendant.
 Species: A class of individuals having some
common characteristics or qualities.
 Allopatric: Originating in or occupying different
geographical areas.
 Sympatric: Originating in or occupying the
same geographical areas.
 Speciation: The formation of new species as a
result of geographic, physiological, anatomical
or behavioral factors that prevent previously
interbreeding with each other.

Population: All the
individuals of one species in a
given area.
NAME: _________________________________________________
GRADE: ________________________________________________
INTRODUCTION
Why has the theory of evolution changed?
During the 19th and 20th centuries, several scientists dedicated most of
their lives to formulate hypotheses and develop experiments which would
allow them to verify some of the processes that triggered changes in the
history of living creatures.
Some of the most relevant contributions to the understanding of evolution
were made by Darwin and Mendel in the 19th century. However, it was
only until the 20th century, thanks to the re-discovery of the principles
suggested by Mendel, that one of the biggest gaps in Darwin’s evolution
theory could be explained.
Meanwhile, the addition of discoveries in other disciplines that were not
considered before, as well as research breakthroughs in genetics, allowed
to explain the mechanisms that originated biodiversity on earth.
Figure 1. Main contributions to the reinforcement of the theory of
evolution.
OBJECTIVES
1. To summarize the foundations of the modern evolution theory.
2. To describe the processes that lead to the creation of new species.
3. To recognize the contributions of paleontology, geography and
population genetics to the concept of the synthetic theory of evolution.
ACTIVITY 1
NATURAL SELECTION
Natural selection is the process in which those individuals who are better
suited to their environment have greater possibilities of survival and
reproduction.
This idea is based on four events:
1. A bigger amount of individuals are born than those who can survive.
2. Individuals of one same species differ one from the other and some
variations are more favorable than others.
3. Resources are limited and species must compete for them.
4. Favorable variations are transmitted from one generation to the next
(selection).
Figure 2: Events in natural selection.
Variations are caused as a consequence of errors in DNA transcription:
Even if these errors are random, the environment rewards those who
improve their chances of survival. This means that nature selects those
living organisms that are better suited to survive.
Example
Nowadays we know the polar bear and the grizzly bear as two separate
species. However, approximately 150,000 years ago the polar bear did
not exist.
How did this happen?
Inside the ovaries of a female grizzly bear, a mutation occurred modifying
the gene that controlled the fur color, thus affecting the production of
dark pigment. As a result,, one of the cubs was born with white fur.
The change in fur color gave this white bear an advantage when being
hunted compared to the grizzly bear because the dark one could be seen
easier in the snow.
Thanks to this advantage, the white bear survived and transmitted these
genes to its offspring. This process occurred multiple times, causing the
white fur gene to spread between the populations of polar bears.
Both populations of bears drifted apart and, after thousands of years,
evolved other characteristics that now separate them.
Did you know that…?
If artic ice continues to disappear because of global warming, polar bears
could become extinct and, very likely, would be replaced by grizzly bears
better suited to environments with no snow?
Figure 3: Polar bears’ reduction of habitat.
Exercise
1. Classify the next statements as FALSE (F) or TRUE (T).
The theory of evolution by natural selection states that:
a) More individuals than the number that can survive are born (
b) One individual cannot change by itself. (
)
)
c) Favorable variations are transmitted from generation to generation. (
)
d) Nature selects the individuals who are better suited. (
)
e) The environment decides which variations occur in organisms. (
)
2. Fill in the blanks with the correct words to complete the sentences.
Organisms with favorable ______________ have more possibilities to
______________ and ___________ these characteristics to their
descendants.
MENDEL’S THEORY OF HEREDITARY TRANSMISSION
In the lapse of 10 years, Gregor Mendel, performed crosses between
different types of pea plants. Based on the results, he established the
principles of hereditary transmission.
While performing crosses between pea plants with yellow seeds and green
seeds, Mendel discovered that in the first generation all the individuals
showed only one of the two characteristics (yellow seeds). Mendel named
these characters dominant and represented them with uppercase letters
(A).
When the plants of the first generation were self-pollinated, the
individuals of the second generations showed the characters that had
disappeared in the first generation, being these, green seeds. These
characters were named recessive and were represented with lowercase
letters (a). (Curtis & Barnes, 1993).
Figure 4. Dominance and recessivity. Principle of segregation.
The appearance or disappearance of alternative characteristics, as well as
the steadiness of the percentages in which they appear, are generated
because the hereditable characteristics are determined by discreet factors
(genes).
These factors are present in pairs and are separated during the formation
of sexual cells.
This principle is known as Mendel’s first law or Principle of
segregation. (Curtis & Barnes, 1993).
Mendel crossed pea plants that differed in two characteristics: round,
yellow seeds and wrinkled, green seeds.
The round, yellow seeds are dominant and are represented with the
letters AABB, whereas the wrinkled, green seeds are recessive and are
represented with the letters aabb.
As a result of the cross between these two types of plants, the first
generation consisted of 100% of round yellow seeds.
Figure 5. Crosses between plants that differ in two characteristics.
When the plants of the first generation were self-pollinated, round, yellow
seeds were obtained (dominant characters) in greater percentage than
wrinkled, green seeds (recessive characters). However, new seeds with
combination of characters were obtained, like wrinkled, yellow seeds and
round, green seeds.
Figure 6. Offspring of plants that differ in two characteristics.
Based on these results, Mendel concluded what is known as Mendel’s
second law: Principle of independent assortment, which establishes that
the characteristics of a gene are split in an independent fashion as of other
genes. (Curtis & Barnes, 1993).
Exercise.
1. Choose the correct Answer Key:
Which of the following statements is NOT a principle in Mendel’s theory of
hereditary transmission?
a) The characteristics of a gene (alleles) can be dominant or recessive.
b) Mutations are produced by errors in DNA transcription.
c) Hereditary characteristics are determined by discreet factors
(genes).
d) The characteristics given to a gene (alleles) are distributed
independently to the characteristics of another gene.
ACTIVITY TWO
How are new species formed?
In order to understand the processes that allow the creation of new
species (speciation), the biological concept of species must be
understood, which is defined as a group of natural populations whose
members can breed among each other and generate viable and fertile
offspring. (Campbell & Reece, 2005).
Figure 7: Reproduction among biological species.
Allopatric or geographic speciation occurs when a segment of the
population makes way to a new species as a result of geographic isolation
of the parent population.
This speciation can happen when: the water level of a lake descends
creating new, smaller lakes; a river changes its course and divides a
population of animals that cannot cross it; or when the movement of
tectonic plaques generates the rise of mountain chains isolating two
population groups of the same species. (Campbell & Reece, 2005)
Continental drift theory.
A progressive event that still happens in earth, and has generated
different processes of geographic speciation, is known as continental drift.
Approximately 225 million years ago, all the continents we know today
were part of one super-continent called Pangea. However, this broke apart
creating two smaller continents: Laurasia and Gondwana. Furthermore,
Laurasia separated creating Eurasia, North America and Greenland in the
north; whereas Gondwana formed Africa-South America, AustraliaAntarctica and India in the south. (Curtis & Barnes, 1993).
A successive series of separations and displacements situated the
continents as we know them nowadays.
Figure 8: Continent disposition due to continental drift.
Did you know that…?
The Himalayans, the tallest mountain range on earth, was formed when
India moved north and collided with Asia?
The creation of this mountain chain isolated geographically some
populations that, with, time, were separated from other species.
Figure 9: Creation of the Himalayans.
Exercise 1
Classify the following statements as false (F) or true (T):
a) Geographical barriers intervene in the creation of new species. ()
b) Species are defined as a group of natural populations that breed among
each other and produce infertile individuals. ()
c) All the continents we know today were united in one super-continent.
()
d) The creation of a mountain range is not considered as a speciation
mechanism. ()
Remember that…
Mutations appear spontaneously as a result of errors in DNA replication
processes.
Plants, just as humans, are diploid organisms. This means that each
chromosome (structure that storages DNA) can be found forming pairs.
Creation of new species when there are no geographic barriers
separating
them.
Sympatric speciation results as the creation of genetic, ecological or
behavioral barriers that isolate reproductively a segment of the
population.
Mutation as a mechanism of formation of new species.
Some vegetable species are originated from accidents during the cellular
division process, which increase the number of chromosomes in the
subject; this is a mutation known as polyploidy. If these organisms are
crossed with normal organisms of the same species, they will produce
infertile offspring. But if they self-pollinate, they could originate fertile
offspring, thus creating a new species. (Campbell & Reece, 2005).
Creation of new species due to habitat differentiation.
The natural habitat of the fly larvae (Rhagoletis pomonella) was native
thorns. However, 200 years ago some populations of flies colonized apple
trees.
Female flies deposit their eggs in the fruits they grew in and male flies
reproduce in the fruits they grew in. This generates a reproduction
behavioral barrier, based on different preference of habitat between the
two populations. (Audesirk, et al, 2003).
Sexual selection as a mechanism of new species creation.
Two populations of cichlid fish are different only by color; one species has
a blue hind while the other has a reddish hind.
Female fish of this sub-species choose the males to breed with based on
their color.
In this way, the reproductive barrier that separates these populations is
the election based on color. (Campbell & Reece, 2005).
Exercise
1. Fill in the blanks with the following words.
_________________ speciation produces _____________ barriers in the
absence __________________ barriers.
Suggested words: sympatric, allopatric, behavioral, reproductive,
ecologic, geographic.
2. Choose the correct answer. Remember that you can choose more than
one.
A mutation known as polyploid produces an extra pair of chromosomes in
the subject. Of the following statements, choose the results of this
mutation.
a) It is a lethal mutation; it therefore causes death to the organisms that
have it.
b) If a polyploid plant is crossed with a normal subject of the same
species, its offspring will be infertile.
c) Polyploidy is a harmless mutation; it does not create advantages or
consequences on the organisms that have it.
d) If a polyploid plant is self-pollinated, it could produce viable and fertile
offspring.
ACTIVITY THREE
SYNTHETIC THEORY OF EVOLUTION OR NEO-DARWINISM
This makes reference to the integration of Darwin’s theory of evolution
with Mendel’s principles of genetics. Additionally, it links the contributions
of other disciplines to explain evolutionary processes of species. (Curtis &
Barnes, 2007).
Darwin’s theory of evolution suggests natural selection as the mechanism
of adaptive change.
Mendel’s genetics explains how characters are transmitted from one
generation to the next.
Geneticist Dobzhansky suggested that variations in organisms originate
from mutations and genetic recombination.
Simpson related paleontological results of fossil registry with the
principles of population genetics.
Mayr linked bio-geography through the definition of allopatric speciation
as a mechanism of reproductive isolation.
Did you know that...?
The main weakness of Darwin’s theory of evolution was the lack of
explanation regarding the persisting variations in populations?
The principles on heredity proposed by Mendel filled the gaps in Darwin’s
theory by explaining how variation remains from generation to generation
(segregation) and how the individuals can present characteristics the
parents did not have; this concept is known as independent distribution.
(Curtis & Barnes, 1993).
Synthetic theory of evolution is based on three principles or
postulates.
1. Natural selection, which acts as a source of variation through favorable
mutation.
2. Evolutionary processes do not occur in individuals but in populations.
3. Evolution occurs gradually in the genetic composition of species,
meaning that these processes are lengthy.
Types of mutations: Some mutations are harmless, other lethal, but
could randomly give an organism some advantage over the competition.
Exercise
On the following link you will find a document about the theory of
evolution and the origin of the human being:
http://amesweb.tripod.com/ccmc02.pdf
Read points 2.4, 2.5, 2.6 and 2.7, starting on page 17 and based on the
information presented, do the following activities.
a) Create a diagram to summarize the theories and principles on which
the synthetic theory of evolution was founded.
b) Name at least two of the doubts proposed against the synthetic theory
of evolution.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_______________
SUMMARY
The theory of natural selection establishes that individuals that possess
more favorable characteristics have greater possibilities of surviving,
reproducing and transmitting these characteristics to the next generation.
Mendelian genetics state two principles: segregation, which sustains that
hereditary characteristics are determined by genes presented in pairs and
are separated during the creation of sexual cells: and the principle of
independent distribution, which declared that the characteristics of a gene
(alleles), are separated independently from the alleles of another gene.
Two mechanisms of new species creation have been proposed: allopatric
speciation, in which a geographic barrier isolates reproductively a
segment of a population from its parent generation; and sympatric
speciation, in which a part of the population isolates from its parent
generation by means of ecologic, genetic or behavioral barriers.
Synthetic theory of evolution compiles Darwin’s theory of natural selection
and the principles of Mendel’s genetics. Additionally, other disciplines are
linked, such as population genetics, systematics, paleontology etc.
It establishes that evolution is a gradual process during which favorable
mutations are sources of variation and a population is the unit where the
evolution occurs.
HOMEWORK
1. Create a timeline based on the history of the theory of evolution and
locate its main exponents, as well as their contributions to the topic.
Your timeline must include the creationist theory as an historical
reference, as well as the contributions done by Lamarck, Darwin, Wallace,
Mendel, De Vries and Dobzhansky.
2. On the following link you will find a chapter about the origin of species,
in which the different speciation processes are described:
http://www.curtisbiologia.com/node/261
Based on the suggested reading, answer the following questions:
a) Which are the two main categories in which you can classify speciation
processes?
_________________________________________________________
_________________________________________________________
__________________
b) Name the three models of speciation by divergence.
_________________________________________________________
_________________________________________________________
_________________________________________________________
___________________________
c) What is the parapatric speciation model?
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
____________________________________
d) Mention the situations that could generate peripatric speciation.
_________________________________________________________
_________________________________________________________
_________________________________________________________
___________________________
3. Research what were the main contributions done by Ernest Mayr,
George Simpson and Theodosius Dobzhansky to the synthetic theory of
speciation.
a) Draw a comparative chart that shows the disciplines through each
author made their main contribution.
SYNTHETIC THEORY OF EVOLUTION
Ernest Mayr
George Simpson
Theodosius
Dobzhansky
b) Explain the relevance of paleontology, geography and population
genetics towards the synthetic theory of evolution.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
______
EVALUATION
1. Classify the following statements as FALSE (F) or TRUE (T).
a) For a natural selection process to occur, random variations must
appear. ( )
b) Evolution happens in individuals. (
)
c) A mutation may isolate reproductively a population from its parent
generation ( )
d) The creation of a mountain range could create a reproductive barrier
between two populations of the same species. ( )
e) Hereditary characteristics are determined by discreet factors (genes).
( )
2. Choose the correct answer.
Synthetic theory of evolution states that:
a) Mutations are originated by symbiosis and gene re-combination.
b) Mutations appear or disappear because of natural selection.
c) Mutations are the result of an evolutionary process opposite to
natural selection.
d) Environment selects those individuals who present the most
adaptive characteristics.
3. Choose the correct word and place it where it belongs:
a) In ____________ speciation, one population isolates reproductively
from its parent generation as a result of geographic barriers.
b) In ____________ speciation, the reproductive isolation of a population
from its parent generation is caused by ecologic or behavioral barriers.
c) Continental drift is an example of _____________ speciation.
d) Sexual selection is a ___________ mechanism that generates
reproductive barriers in sympatric speciation.
Words: Allopatric, sympatric, geographic, behavioral.
4. Choose the correct answer.
Of the following disciplines, choose those that contributed in the definition
of synthetic theory of evolution. Remember that you can choose more
than one answer.
a) Systematics and genetics
b) Paleontology and zoology
c) Geography and paleontology
d) Genetics and taxonomy
e) Taxonomy and phylogeny
5.
Fill in the blanks to complete the phrases:
a) One of the gaps in the evolution theory proposed by ___________ was
the absence of mechanisms that could explain how ____________ remain
in
populations.
The
principles
of
the
theory
of
____________
transmission proposed by ____________ filled these gaps.
BIBLIOGRAPHY.
Audesirk, T., Audesirk, G., & Byers, B. (2003). Biología. La vida en la
tierra. México: Pearson Educación.
Campbell, N., & Reece, J. (2005). Biología. Madrid: Editorial Médica
Panamericana S.A.
Curtis, H. & Barnes, N. (1993). Biología. Buenos Aires: Editorial Médica
Panamericana S.A.
GLOSSARY
Allele: The different forms of a gene.
Self-pollination: transference of pollen from an anther to a stigma of a
same flower, or from flower to flower of the same plant, leading to selffertilization.
Chromosomes: Structures that storage genes.
Cellular division: Reproduction process of cells in which two or more
offspring cells are created.
Diploid: Cells that contain a double number of chromosomes.
Genetic frequency: Frequency of each allele in the population.
Gene: DNA sequence which makes up the functional unit for the
transmission of hereditary characters.
Polyploid: Cells that contain more than two pairs of chromosomes.
Cub: Baby bear
DNA replication: Mechanism that allows DNA do duplicate; in other
words, to create an identical copy.