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Announcements
Objectives
Web
Resources
Conditions
Causing
Evolutionary Change
Review of
Natural Selection
Artificial
Selection
Sexual
Selection
Mate Selection
Mutation
Genetic
Drift
Microevolution
Recap
Lecture
Syllabus
IB 100/101
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Announcements
Text Readings in Lewis et al.
Chapter 15, The Evolution of Evolutionary Thought
Chapter 16, The Forces of Evolutionary Change: Microevolution
Much of the material cited in this lecture outline came from
your
textbook (Lewis et al). It is highly beneficial to read these chapters
carefully before your final exam.
The "Reviewing Concepts" boxes are valuable summaries of the
main
ideas in these sections of the text.
You have open access (no log-in or password needed) to
instructional
materials on the Text web site. Select the text chapter you want and
use the links to the e-learning modules or other available materials.
There is also a collection of study materials called the "Essential
Study Partner" that you may find useful.
Web Crossing
You may also ask questions and see answers to your classmates'
questions in Web Crossing in the "Talk to Sarah and Ed" discussion.
Objectives:
The content of today's lecture will help you complete these
assignments:
After studying this material you should be able to:
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list and describe the effects of the conditions that cause
evolutionary change in populations, including:.
| Natural Selection |
Nonrandom Mating |
| Mutation |
Genetic Drift |
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Distinguish between Natural Selection and Artificial
Selection.
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Describe the role of nonrandom mating and sexual selection
in the
process of microevolution.
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Explain how the random events of genetic drift, including
bottleneck
events, founder effect, and gene flow via migration could change allele
frequencies of a population.
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Describe how genetic drift that does not invovle drastic
reduction
in population size or migration into or out of a population could
result
in a change in allele frequencies of a population.
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Understand the relationships among these terms:
| biological evolution |
artificial selection |
natural selection |
| microevolution |
allele frequencies |
genetic drift |
| bottleneck |
founder effect |
gene flow |
non-random mating
|
Sexual Selection |
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Web Resources:
These links would provide good sources for Extra Credit Projects (due in Web Crosing by 8:00
AM Monday Dec. 3)
or the
Fourth Web Crossing Assignment (due 8:00 AM, Wednesday Dec. 5).
Conditions that Cause Evolutionary Change in Natural
Populations
Natural Selection is an important, but not the only force
that
results in biological evolution.
Microevolution occurs when the frequency of an allele in a
population
changes. This may happen through:
1. Natural selection (and artificial selection)
2. Sexual Selection, Mate Choice, Nonrandom Mating
3. Mutation
4. Genetic drift
1. A Review of Natural Selection, Change in Allele Frequency,
and
Microevolution
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Lewis et al. (page 966) define natural selection
as, "The
differential survival and reproduction of organisms whose genetic
traits
better adapt them to a particular environment"
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Figure 16.6f, pg. 297Diagram representing changing
allele frequency in a population
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More specifically, we mean a change in the number
(statistically
speaking, the "frequency") of individuals in a population that
carry one or two copies of a specific allele or variant of a specific
gene on their chromosomes.
-
As we learned earlier in the course, an individual's
specific
versions of their traits are determined by the specific combination of
the alleles they have for each gene locus.
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Change in allele frequency in a popuation is called microevolution.
Microevolution can take place over a
relatively
short time periods (even one or only a few generations of a
population).
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Recall, a population is ".....a group of members
of
a species
that interbreed" (Lewis et al., Life, pg. 288). The Glossary on
pg. 968 gives a slightly different wording of the same concept, "A
group
of interbreeding organisms living in the same area." Because an
individual cannot change his or her genes, we focus on populations as
the functional units of evolutionary change.
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If a population contains variation, and
if the variation is at least partly heritable, and
if some variants survive to reproduce at higher rates than others,
then the population will evolve. (EvoDots Tutorial, Jon C.
Herron
2002)
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Simply, some phenotypes are better adapted to a particular
environment than others. Natural selection favors some phenotypes and
the alleles that produce them and removes others from the population.
Therefore, allele frequencies will change in response to environmental
change.
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Two fundamental forces are operating: genetic variation
and environmental change. Both are constantly occurring at
random in
every natural population. Those with more adaptive traits survive in
the
new environment.
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The environment of a population includes more than
the
obvious factors of climatic conditions. We also need to consider the
populations of parasites, predators, and competitors that
affect
the population we are considering.
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Parasites, predators, and competitors also evolve.
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Natural selection reflects adaptation to a prevailing
environmental
condition. The direction of natural selection can change. A phenotype
that is adaptive in one set of circumstances may be a liability in
another.
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Over time, the population would change so that it could no
longer
breed with the original group. Eventually, a new species would
arise.
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Macroevolution, the subject of our final set of
lectures,
represents accumulated changes in allele frequency in two populations
that preclude their interbreeding, leading to the formation of new
species (or their extinction). Often, macroevolutionary changes
tend
to span very long time periods (many generations), but in some
instances, new species can evolve in only a few generations.
Artificial Selection: Proof of the Power
of Selection
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Also called selective breeding, domestication, or
selection by
humans.
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The process of intentional or unintentional modification
of a
species through human actions which encourage the breeding of certain
inherited traits over others. The breeding potential of individuals who
possessed desirable characteristics is intentionally encouraged,
whereas
the breeding of individuals with less desirable characteristics is
discouraged.
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The many breeds of domestic dogs and cats existing today
are a
consequence of artificial selection. Darwin raised pigeons and
artificially selected several new breeds.
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Darwin
used his observations of artificial selection in animals and
plants to help him think about his observations of what was going on in
nature. From Darwin's Diary, part of the
PBS Evolution Program.
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Comparison
of artificial and natural selection from Wikipedia, the Free Encyclopedia. -
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Artificial selection underscores the power of selection
in
generating evolutionary change.
2. Sexual Selection, Mate Choice, Nonrandom Mating
(A variation of Natural Selection)
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Completely random matings (where each individual has as
equal chance
of mating with every other member of the population) are nearly
impossible to achieve.
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Sexual selection: the "natural selection of traits
that
increase an individual's reproductive success." Text Glossary, pg 971.
These traits contribute to attraction, courtship, or mating. Most
species exhibit some sort of preferences in mate choice; the alleles
for
these desired traits will become more common in future generations.
Lecture Activity
In the previous
evolution lecture we looked at the effect of predator fish on the
evolution of the color of male guppies in a stream.
The mate choice of female guppies also affects the
evolution of
guppies.
Observe two "runs" of the "Sex and the Single Guppy" simulation from PBS
Evolution
Series.
Both "runs" will start with an even mix of "Brightest",
"Bright",
"Drab", and "Drabbest" male guppies.
Both "runs" will start with the same female guppies.
In the FIRST "run" we will set predator species and
numbers as "30
rivulus, 30 acara, 30 cichlid" (Lots of predators).
In the SECOND "run" we will set predator species and
numbers as "30
rivulus" (Only a few predators).
Work with one or two classmates to predict how the
proportions of
"Brightest", "Bright", "Drab", and "Drabbest" male guppies will change
in each of the two simulation runs lasting at least 10
generations.
|
Brightest
+ or -? |
Bright
+ or -? |
Drab
+ or -? |
Drabbest
+ or -? |
Lots of
Predators |
|
|
|
|
Few
Predators |
|
|
|
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Why do you think natural selection produced such
different results
in the two different environments?
Indicate your predicted changes in male guppy colors in
the table
provided and write your explanations below.
Print AND sign your name.
Pass your paper to the aisle when requested.
3. Mutation
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Review our lecture on mutations. Remember, most mutations are neither
beneficial nor useful, with no effect on phenotype. Some are harmful,
resulting in defects in protein production that can lead to genetic
disease.
4. Genetic Drift (RANDOM survival or
reproduction)
Changes in allele frequency in a population that result from
RANDOM survival or reproduction of individuals with
certain
characteristics.
Survival or reproduction of those individuals in the face of
some
environmental change is just a matter of LUCK or CHANCE, not
because of their phenotype or genotype.
For example: if a Florida Panther is killed by a truck on
a highway,
that is bad luck. The panther did not get hit because of some allele it
carried.
Genetic Drift contrasts with natural selection.
In
selection the environmental events that affect a population may be
random, but the survival or reproduction of the individuals depends on
their phenotypes and genotypes.
An example of selection: If the panther population is
infected with
FIV (feline AIDS), individuals with alleles that give them resistance
to
the disease will survive. The introduction of the virus is a random
event, but survival is based on genes.
Types of Genetic Drift:
Gene Flow Resulting from Migration
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Individuals migrate between populations and take their
alleles with
them.
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Immigrating individuals introduce new alleles by
mating
with members
of the population they are joining.
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Emigrating individuals remove alleles from the
population they leave
behind.
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Gene Flow is the movement of alleles from
population to
population.
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Any advantage given to individuals with new alleles
will change the
population due to subsequent natural selection.
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Because geographic barriers greatly influence
migration
patterns,
allele frequencies may differ between adjacent but separated geographic
regions.
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Migration (text figure 16.6C, pg 297)
Founder Effect:
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A type of genetic drift resulting in the establishment
of a new,
geographically isolated population from a single or very few
individuals. It is very unlikely that the gene pool of a founding
population is representative of the total genetic diversity of the
original population.
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Founder effect is different from gene flow because the
migrating
individuals are establishing a new population where none existed
before.
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White
Deer of the former Seneca Army
Depot
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Ellis-van Creveld Syndrome. Small groups of people
founding new settlements may have different allele frequencies than the
original population, and may also have higher incidents of certain
traits (such as genetic disorders) because they marry within the group.
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Ellis-van Creveld is an autosomal recessive disease
and occurs in
7% of the people in the Amish community of Lancaster County,
Pennsylvania. The occurrence of the disease is high because these Amish
marry among themselves. See page 292, text, for more information.
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The Founder Effect (text figure 16.2A, pg 292)
Population Bottleneck
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A type of genetic drift occurring when many members of
a population
die, and a few surviving individuals mate, eventually restoring the
population numbers. The new population has lost much of the genetic
diversity that was present in the larger ancestral population as a
result of this inbreeding.
"Plain Ol'e Genetic Drift"
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In the absence of migration into or out of the
population (founder effect or gene flow) or drastic changes
in population
size resulting from some catastrophic die-off (bottle neck), the
allele frequencies of a population can change because of genetic
drift.
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Chance events affect which individuals survive and/or
reproduce in a
population independent of the genetic make up of those individuals
affected. Small, random changes in allele frequency can "build up" over
generations and result in a significant change in allele
frequency.
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Such chance changes in allele frequency tend to have
small efects on
the overall genetic makeup of large populations.
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In contrast, small populations of endangered species
are much more
susceptible to drastic changes in allele frequency resulting from
chance
events. Small populations tend to be inbred and have little gentic
variation anyway. What little genetic variation that exists can easily
be lost from the gene pool as a result of accidental loss of
individuals
carrying the variant alleles.
-
Because such changes in allele frequency are not
related to
phenotype or genotype, they are classified as genetic drift.
Microevolution Recap
Microevolution (a change in allele frequency in a
population) can
occur as a result of several factors that fall into two broad
categories.
1. Factors in which an individual's chance of survival
and, more
importantly, participation in reproduction, depend on the
individual's phenotype and genotype.
2. Genetic Drift Factors in which an individual's
chance of
survival and, more importantly, participation in reproduction, are just
that, a matter of CHANCE. An individual's phenotype and
genotype has nothing to do with it.
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