Unit 5:
Evolution through Natural
Selection and other
Mechanisms
Natural Selection
• Natural selection is the
process by which those
heritable traits that make it
more likely for an organism to
survive and successfully
reproduce become more
common in a population over
successive generations.
• It is a key mechanism of
evolution.
Genetic variation
-Genetic variation is essential
for natural selection.
-The natural genetic variation
within a population of
organisms means that some
individuals will survive and
reproduce more successfully
than others in their current
environment.
-Natural selection acts on the
phenotype, or the observable
characteristics of an organism,
but the genetic (heritable) basis
of any phenotype which gives a
reproductive advantage will
become more common in a
population.
-Over time, this process can
result in adaptations that
specialize organisms for
particular environment.
Sample Question # 1
The idea that evolution takes place at
one point in time, followed by a long
period without change is ______.
From: Test bank 07-08
A
B
C
D
gradualism
punctuated equilibrium
artificial selection
succession
Sample Question 2
Charles Darwin proposed his theory of
evolution based on observations of
nature. Which observation that
contributed to his theory is illustrated by
this population of beetles?
A. Species produce more offspring than
can survive.
B. Populations remain stable over time.
C. Individuals within a population may
vary widely.
D. Environmental resources are limited.
Genetic Drift
-Genetic drift, along with natural
selection, mutation, and migration, is
one of the basic mechanisms of
evolution.
-In each generation, some individuals
may, just by chance, leave behind a
few more descendents (and genes, of
course!) than other individuals. The
genes of the next generation will be
the genes of the “lucky” individuals,
not necessarily the healthier or
“better” individuals.
Genetic drift affects the genetic makeup of the
population but, unlike natural selection,
through an entirely random process. So
although genetic drift is a mechanism of
evolution, it doesn’t work to produce
adaptations.
Bottleneck and Founder Effects
-Genetic drift can cause big
losses of genetic variation
for small populations.
-Population bottlenecks
occur when a population’s
size is reduced for at least
one generation.
-A founder effect occurs
when a new colony is
started by a few members
of the original population.
Sample Question 3
A small population of chimpanzees lives in a
habitat that undergoes no changes for a long
period. How will genetic drift probably affect
this population?
A It will accelerate the appearance of new traits.
B It will promote the survival of chimpanzees with
beneficial traits.
C It will increase the number of alleles for specific
traits.
D It will decrease genetic variation.
Sample Question 4
Genetic drift is a ______________
process.
A. directed
B. selection-driven
C. random
D. co-evolutionary
Speciation
-A species is often defined
as a group of individuals
that actually or potentially
interbreed in nature. In this
sense, a species is the
biggest gene pool possible
under natural conditions.
-Speciation is a lineagesplitting event that
produces two or more
separate species.
Causes of Speciation
1. Geographic Isolation
2. Reduction of Gene Flow
Kinds of Speciation
Allopatric
speciation is just
a fancy name for
speciation by
geographic
isolation
Sympatric speciation
refers to the formation
of two or more
descendant species
from a single ancestral
species all occupying
the same geographic
location.
Sample Question 5
River
This diagram illustrates what kind of speciation?
A
Sympatric
B
Allopatric
C
Geographic
D
Catastrophic
Cladogram
A cladogram is a
diagram used to
show ancestral
relations between
organisms, to
represent the
evolutionary tree
of life.
These are two
identical cladograms.
1
2
3
Analyzing a cladogram…
1.Which species are most closely related?
2.Which is the common ancestor of flies and bees?
3.Which insects share the most recent ancestor?
Here’s a more complicated one…
What
conclusions
can you draw
from this
‘tree of life’?
Sample Question 6
According to the cladogram above, which two species are most
closely-related?
A
B
C
D
clubmosses and spikemosses
spikemosses and ferns
ferns and flowering plants
flowering plants and clubmosss
Fossil Record
Fossils are the preserved remains or traces of
animals, plants, and other organisms from the
remote past. The totality of fossils, both
discovered and undiscovered, and their
placement in fossiliferous (fossil-containing)
rock formations and sedimentary layers
(strata) is known as the fossil record.
• Fossil evidence indicates
that over time organisms of
increasing complexity
appeared on Earth.
Bacteria and blue-green
bacteria are the first fossils
that were preserved from
the Precambrian era.
• The fossil record contains
many examples that could
be interpreted to mean
that species evolved from
more ancient organisms.
Sample Question 7
In a section of the Grand Canyon, scientists
have found the fossil remains of several
different groups of organisms. The diagram
on the left represents the number and age
of the fossils the scientists found. The
width of each shaded area in the diagram
below indicates the relative number of
fossils found.
Which of the following statements is supported by the fossil record?
A
B
C
D
Group C is the only group that no longer exists for it’s now extinct.
Group B was the most numerous organism 10 million years ago.
Group D has been in existence the longest.
Group A is the most recent organism to come into existence.
Sample Question 8
Which of the following is usually most helpful
in determining the age of the fossils found in
rock strata?
A the size of the fossils
B the color of the fossils
C the water content of the rock layer where the
fossils were located
D the depth of the rock layer where the fossils
were found
Sample Question 9
Which of the following best explains how the fossil
record provides evidence that evolution has
occurred?
A It indicates the exact cause of structural and
behavioral adaptations of organisms.
B It shows that the form and structure of groups of
organisms have changed over time.
C It shows how the embryos of many different
vertebrate species are very similar.
D It indicates that forms of life existed on Earth at
least 3.5 billion years ago.
Sample Question 10
Orchids were studied to determine if the
amount of humidity affected the flowering of
these plants. Which of these was the
independent variable in this study?
A The percentage of humidity
B The length of time required for flowering
C The amount watered
D The number of flowers on each plant
Remember…
INDEPENDENT
VARIABLE
-is the
manipulated
variable in the
experiment.
DEPENDENT
VARIABLE
-is the variable
that is
measured at
the end of the
experiment.