Mechanisms of Evolution and Their
Effects on Populations
Part 2
SBI3U Evolution - 8
Factors That Cause Evolutionary
Change
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Mutations
Gene Flow (migration)
Non-random Mating
Genetic Drift
Natural Selection
Natural Selection
• Populations have a wide range of phenotypes
and genotypes
• Some individuals in a population produce
more offspring than others
• Selective forces such as competition and
predation affect populations
As a result, some individuals are more likely to
________ and ______________ than others
Natural Selection
• If an organism has even one allele that gives a
slight, yet consistent, selective advantage, the
frequency of that allele in the population will
increase from one generation to the next
• Less favourable alleles may be lost
• Thus, natural selection causes changes in the
________________________ of a population,
which can lead to evolutionary change
Natural Selection
• Several types of natural selection affect the
frequency of a heritable trait in a population:
- stabilizing selection
- directional selection
- disruptive selection
Stabilizing Selection
-Favours an ____________ phenotype
- Acts against __________ variants of
the phenotype
The most common phenotype (the
intermediate phenotype) is made
more common in the population by
removing the extreme forms.
This type of selection:
- Reduces ______________
- Improves adaptation of the
population to aspects of the
environment that remain constant
Directional Selection
- Favours the phenotypes at one
________________ over the other
- Common during times of
environmental__________ or when a
population ___________ to a new
habitat that has different
environmental conditions and niches
to exploit
Examples:
- The changes in coloration of
peppered moths
- Antibiotic resistance in infectioncausing bacteria
Disruptive Selection
- Takes place when the __________ of
a range of phenotypes are favoured
over intermediate phenotypes
- Thus, intermediate phenotypes can
be eliminated from the population
Example: The extreme size differences of
mature male coho salmon. The smaller
phenotype averages 500g, and the larger
phenotype averages 4500g. This difference in
size reflects the means by which each
phenotype gains access to females. The
smaller specialize in a “sneaking” technique
to fertilize eggs. The larger specialize in
fighting for access to eggs.
Practice Problem:
Identify the Type of Selection
The graph below shows a representation of the changes that
have occurred in a population of bentgrass over a period of
time.
Use your understanding of the three types of natural selection
(stabilizing, directional, disruptive) to identify the type of
selection that is acting on this bentgrass population.
Sexual Selection
The different phenotypes of the male coho
salmon are also a specific example of natural
selection referred to as sexual selection.
In general, sexual selection involves:
- Competition between males through combat
or visual displays
- Females choosing their mates
Sexual Selection
Males and females of many animal species often have
very different __________ characteristics, such as
colourful plumage in male birds and antlers in male
deer.
This difference between males and females is called
________ _____________
Speciation: How Species Form
SBI3U Evolution - 8
Grevy’s Zebra
Plains Zebra
These two zebras look very similar but they are actually two
different species.
How is a species defined? A biological species is a population or
group of populations in nature whose individual members can
interbreed to produce viable, fertile offspring – offspring that can
also interbreed.
We learned that various factors cause changes
within populations.
These factors are:
- Mutations
- Gene flow (migration)
- Non-random Mating
- Genetic Drift
- Natural Selection
These factors are what cause microevolution to
occur.
Speciation
• Formation of new species from existing
species
• Occurs when some members of a sexually
reproducing population change so much that
they are no longer able to produce________,
_________ offspring with members of the
original population
• The formation of new species is sometimes
called _________________
Speciation
Two populations may become reproductively
isolated over time (meaning they become two
species) if there is little or no ______
________ between populations.
These reproductive isolating mechanisms may
be ____________ or ______________.
Pre-zygotic Isolating Mechanisms
• Either inhibit mating between species or
prevent fertilization of eggs if individuals from
different species attempt to mate
• There are 5 types of pre-zygotic isolating
mechanisms:
- Behavioural
- Habitat
- Temporal
- Mechanical
- Gametic
• Behavioural: Any special _______ or
_____________ that are species specific
prevent interbreeding with closely related
species
Some examples of signals/behaviours: Songs of
birds, courtship rituals of elk and the chemical
signals (phermones) of insects
• Habitat: Two species may live in the same
general region, but in different habitats, so
they may encounter each other rarely, if at all
• Temporal: Many species are kept separate by
temporal (_________ ) barriers
Example: two species may inhabit the same
habitat but mate at different times of the day
• Mechanical: Species that are closely related may
attempt to mate but fail to achieve fertilization
because they are _____________ incompatible
• Gametic: If gametes (egg and sperm) from
different species do meet, gametic isolation
ensures they will rarely fuse to form a ________
Practice Question
Based on the description given, what type of
pre-zygotic isolating mechanisms is operating?
a) The sperm of one species may not be able to
survive in the environment of the female
reproductive tract of another species.
b) The eastern and western meadowlark’s are
identical in shape, coloration and habitat but
have different mating call songs
Post-zygotic Isolating Mechanisms
• In rare cases in nature, the sperm of one
species successfully fertilizes an egg of
another species and a zygote is produced
• Post-zygotic isolating mechanisms prevent
these hybrid zygotes from developing into
viable, fertile individuals.
• There are 3 types:
- Hybrid Inviability
- Hybrid Sterility
- Hybrid Breakdown
• Hybrid Inviability: Genetic incompatibility of
the interbred species may stop ____________
of the hybrid zygote (normal mitosis cannot
occur)
• Hybrid Sterility: Sometimes two species can
mate and produce hybrid offspring. The hybrid
is often __________ because meiosis fails to
produce normal gametes (different
chromosomal numbers)
• Hybrid Breakdown: In some cases, the first
generation hybrids are _______ and _______ .
When these hybrids mate with each other or
with an individual from one of the parent
species, offspring of the second generation
hybrid are either ________ or __________.
Practice Question
Based on the description given, what type of postzygotic isolating mechanism is operating?
a) Hybrid embryos between sheep and goats die in
early development before birth
b) Different species of cotton plants can produce
fertile hybrids, but the offspring of the hybrids
die as seeds or in early development
c) A mule is the offspring of a donkey and a horse.
Mules are sterile.