Week 5 - Patterns of Descents

MIGUEL ANGELAN B. CORPUZ, JR. | miguelangelan.corpuz@deped.gov.
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Learning Objectives
MELC: Show patterns of descent with modification from common

ancestors to produce the organismal diversity observed today
(STEM_BIO11/12-IIIc-g-10)
Objective:
1. Define speciation according to the biological species concept
2. Identify the various type of reproductive isolating mechanisms
that can lead to speciation
3. 3. Discuss the importance of reproductive isolating
mechanism in diversification of organisms
What are Species?
Species are groups of

interbreeding natural populations
that are reproductively isolated
from other such groups. (Ernst
Mayer’s definition)
Is a closely related organism that
are very similar and capable of
producing fertile offspring
What are Speciation?
It is a process of evolution wherein a

genetically homogenous population is
divided into two or more populations,
each of which undergoes a genetic
differentiation and eventually leads to
reproductive isolation.
The creation of new populations
(species) with greater adaptive
efficiency than their ancestors
determines the trajectory of evolution.
Reproductive Isolating Mechanism
 The mechanisms of reproductive
isolation are a collection of evolutionary
mechanisms, behaviors and
physiological processes critical for
speciation.
 They prevent members of different
species from producing offspring, or
ensure that any offspring are sterile.
 These barriers maintain the integrity of a
species by reducing gene flow between
related species.
1. Pre-zygotic Isolation Mechanisms
prevent fertilization and

zygote formation.
happens before fertilization
occurs between gametes
a. Geographic/Ecological/ Habitat Isolation
Occurs when two species that

do not interbreed because the
species live in different areas.
The two species live in different
habitats and will not encounter
one another: each is isolated
from the other species
b. Temporal or Seasonal Isolation
different groups may not be

reproductively mature.
For example:
 Two populations of plants may
produce flowers in different
seasons, making mating
between the populations
impossible
c. Behavioral Isolation
Patterns of courtship is
different.
For example:
Eastern & Western
meadowlark songs differ.
d. Mechanical Isolation
differences in reproductive
organs prevent successful
interbreeding.
Mechanical isolation occurs
when mating is physically
impossible.
e. Gametic Isolation
incompatibilities between egg and sperm

prevent fertilization.
Often this occurs because the female
immune system recognizes sperm as
foreign and attacks it.
Example:
Sea cucumbers releases eggs into open
water. The sperm recognizes their own
species through chemical markers
2. Post-zygotic Isolation Mechanisms
 allows fertilization but nonviable or

weak or sterile hybrids are formed.
 In these cases, the zygote formed
is called a hybrid.
 However, even after a hybrid
zygote forms, reproduction may still
not be successful.
a. Hybrid Inviability
fertilized egg fails to develop past

the early embryonic stages.
For example:
When tigers and leopards are
crossed, the zygote begins to
develop but the pregnancy ends in
miscarriage or stillborn.
b. Hybrid Sterility
 their hybrids are sterile because

gonads develop abnormally or
there is abnormal segregation of
chromosomes during meiosis.
For example:
A horse and a donkey may
produce a hybrid offspring, a mule.
Mules are sterile
c. Hybrid Breakdown
 F1 hybrids are normal, vigorous and viable, but F2

contains many weak or sterile individual
Importance of Reproductive
Isolating Mechanism
Importance of Reproductive Isolating
Mechanism
1. Speciation: By preventing gene flow

between populations, these mechanisms
allow for genetic divergence and the
accumulation of differences that ultimately
lead to speciation.
• Without reproductive isolation, interbreeding
between populations could result in genetic
homogenization and the loss of distinct
species identities.
Importance of Reproductive Isolating Mechanism
2. Preservation of Genetic Diversity:

Reproductive isolating mechanisms help
preserve genetic diversity within species
by limiting gene flow between different
populations.
• This diversity is important for the long-term
survival of species, as it provides the raw
material for adaptation to changing
environmental conditions and the
emergence of new traits that enhance
fitness.
3. Ecological Adaptation:
• By maintaining reproductive isolation
between populations, these
mechanisms allow for ecological
adaptation and specialization.
• Populations may evolve unique traits
or adaptations in response to local
environmental conditions, such as
habitat preferences, resource
availability, or predator pressures.
4. Prevention of Hybridization:
• Reproductive isolating mechanisms
prevent hybridization between distinct
species or populations, which can often
result in the production of unfit or sterile
offspring.
• Hybridization can disrupt local adaptation
and lead to the breakdown of species
boundaries over time. By maintaining
reproductive isolation, these mechanisms
help prevent the negative consequences
associated with hybridization.
5. Promotion of Reproductive Success:

• Reproductive isolating mechanisms
promote reproductive success by ensuring
that individuals mate with conspecifics
(members of the same species).
• By reducing the likelihood of mating with
individuals from different species or
populations, these mechanisms increase the
chances of successful fertilization and the
production of viable offspring.
• This ultimately contributes to the survival
and persistence of species over time.
Performance Task #2: Infographic of a Domesticated Crop
(Submission Date: March 13, 2024)
Make an infographic showing the evolution of a domesticated crop and explain

one of the mechanisms that produces change in populations from generation to
generation that is involved in the evolution of the domesticated crop. (e.g., artificial
selection, natural selection, genetic drift, mutation, recombination).
TIME’S
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MIGUEL ANGELAN B. CORPUZ, JR. | miguelangelan.corpuz@deped.gov.

MELC: Show patterns of descent with modification from common

Species are groups of

It is a process of evolution wherein a

prevent fertilization and

Occurs when two species that

different groups may not be

incompatibilities between egg and sperm

 allows fertilization but nonviable or

fertilized egg fails to develop past

 their hybrids are sterile because

 F1 hybrids are normal, vigorous and viable, but F2

1. Speciation: By preventing gene flow

2. Preservation of Genetic Diversity:

5. Promotion of Reproductive Success:

Make an infographic showing the evolution of a domesticated crop and explain

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