Download Chapter 12: Processes of Evolution Part 1

CHAPTER 12:
PROCESSES OF
EVOLUTION
Part 1
Making Waves in the Gene Pool
• A group of individuals of the same species in a specified
area is a population.
• Individuals in a population have similar traits because
they have similar genes (Genotype gives rise to
phenotype).
• All of the genes of a population make up the gene pool
(or pool of genetic resources) of a population.
Making Waves in the Gene Pool
• Even though all of the individuals in a population have similar
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genes, their genes are not identical.
Therefore, every trait in a population varies a little bit in the
individuals of the population.
Different alleles (or forms) of genes are the reason behind this
variation.
A trait with only two alleles has only two forms (or morphs) and is
called dimorphic.
A trait with more than two alleles and therefore, forms or morphs, is
called polymorphic.
When traits are determined by many genes (and different forms of
each of these many genes), it will vary continuously among
individuals in the population (like height or weight in humans) due
to interaction among the genes.
Variations also exist due to the influence of environmental factors.
Making Waves in the Gene Pool
• Add the fact that there are many processes that introduce
variations in traits among individuals (see the next slide)
and you see just how unlikely it is that another individual
has or will ever have the exact genetic makeup and/or
combination of variations of traits as you!!
Sources of Variation of Traits
• Genetic Event
Effect
• Mutation
Source of new alleles
• Crossing over (Meiosis I)
Introduces new combinations of alleles into
chromosomes
• Independent Assortment (Meiosis I)
Mixes maternal and paternal chromosomes
• Fertilization
Combines alleles from two parents
• Changes in chromosome number/structure
Transposition, duplication, loss of chromosomes
Mutations
• Mutation is the original source of new alleles and is
the raw material of evolution.
• Mutations are rare events, are spontaneous, and are
unpredictable, so we cannot predict when a mutation will
occur, where in the DNA it will occur, or which individual it
will occur in.
• But we can predict the average mutation rate of a
species, which is the probability that a mutation will occur
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• The average mutation rate in humans is 175 mutations
per person per generation.
Mutations
• A mutation that drastically alters the phenotype of an
individual is called a lethal mutation because it results in
death. (ex. Collagen mutation)
• A mutation that does not affect survival or reproduction is
called a neutral mutation. (ex. Earlobes)
• Natural selection does act upon neutral mutations since
they do not affect survival or reproduction.
• This is the reason their accumulation can be used to
determine the relatedness of organisms when looking at
their DNA sequences.
Mutations
• Sometimes, a change in the environment can favor a
mutation that had previously been neutral or even
harmful.
• When this happens, natural selection increases the
frequency of this mutated form of the gene in the gene
pool of a population over time.
Allele Frequencies
• Allele frequency is the abundance of an allele in a
population.
• Change in allele frequency is the same thing as change in
a line of descent- EVOLUTION.
• Microevolution refers to evolution within a population or
species.
Allele Frequencies
• We can determine how much a population has evolved by
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considering a theoretical genetic equilibrium, called
Hardy-Weinberg equilibrium.
Remember, this is a theoretical equilibrium, it is never
actually present in a population.
This equilibrium is what would would be true if NO
EVOLUTION ever happened in a population.
Since all populations evolve, no real world population
would ever be in Hardy-Weinberg equilibrium.
This is just a theoretical control population that we can
compare real world populations to in order to determine
just how much a population is evolving.
Allele Frequencies
• In order for a population to be in Hardy-Weinberg
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equilibrium, five conditions must be met in order for there to
be no evolution occurring:
1) very large population
2) no emigration or immigration
3) no mutations
4) random mating
5) no natural selection
Since all five condition are never met by a real world
population, real world populations are never in HardyWeinberg equilibrium; that’s why this is a theoretical
equilibrium.
Hardy-Weinberg Equilibrium
Condition 5: No Natural Selection
• Natural selection does occur in natural populations.
• With natural selection, there is differential survival and
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reproduction of individuals in a population so that allele
frequencies change over many generations.
We observe different patterns of selection depending
upon selection pressures in the environment and the
organisms involved.
There are three types of natural selection:
1) directional selection
2) stabilizing selection
3) disruptive selection
Condition 5: No Natural Selection
• Directional selection: allele frequencies shift in a
consistent direction so that forms at one end of a range of
phenotypic variation become more common.
• In other words, out of a range of phenotypes, from one
extreme to another, one of the extremes becomes the
most common.
Examples: peppered moths, rock pocket mice, warfarinresistant rats, antibiotic resistant bacteria
Condition 5: No Natural Selection
• Stabilizing selection: intermediate form becomes more
frequent than the two extremes out of a range of possible
phenotypes.
• Examples: body weight of weaver birds
Condition 5: No Natural Selection
• Disruptive selection: both extreme forms of a trait
become more frequent, while the intermediate form is
selected against
• Example: black-bellied seedcracker (finches)
Condition 4:
Random Mating
• Natural selection occurs not only as a result of interactions
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between species and their environment, but also as a result of
competition.
For example, some traits, though energetically costly and
attractive to predators, persist in a population due to sexual
selection.
In sexual selection, the most adaptive forms of a trait are the
ones that help individuals defeat same-sex rivals for mates or are
the ones most attractive to the opposite sex.
In sexual selection, the genetic winners are those that outreproduce others in the population because hey are better at
attracting a mate.
By choosing among mates, a male or female acts as a selective
agent on its own species and, therefore, mating is not random.
Condition 4:
Random Mating
• For example, one sex of a species may “shop” for a mate
among individuals of the opposite sex for those that display
specific cues such as more colorful, larger, and more flashy
body parts.
• Even though these body parts may actually hinder the
animal in escaping predators and may even attract
predators, it also implies health and vigor to the opposite
sex, which implies that its offspring will also be healthy and
more vigorous.
• Therefore, the individuals with these traits, even though they
may act as a “handicap” in some aspects, actually results in
these individuals producing more offspring, passing the
genes for these traits on to the next generation and
increasing the frequency of the alleles for these traits over
generations.
Condition 4:
Random Mating
• Example: stalk-eyed flies, eyespots on male peacocks
Homework 1
• On the next two slides you will find three scenarios and
three graphs.
• For each scenario, indicate whether stabilizing,
directional, or disruptive selection is at work and explain
why.
• Indicate whether each graph (1, 2, and 3) represents
stabilizing, directional, or disruptive selection and explain
why.
Homework 2
• In a population of deer, solid brown deer and spotted
white/brown deer (intermediate phenotype) are camouflaged,
while solid white deer are almost always seen and eaten by
predators.
• In the human population, very short (dwarfism) and very tall
(gigantism) people suffer from genetic disorders and have
shortened life spans, while average height people live longer
life spans.
• In a population of chickens, chickens that lay eggs with a
medium thick shell are more desirable and are bred by humans
because these eggs don’t break as easily. (Chickens that lay
very thin shelled eggs aren’t bred because the eggs break very
easily and those that lay very thick shelled aren’t bred because
they are too hard to break).
Homework 3
Disruptive, Directional, or Stabilizing Selection?
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Homework 4
• Locate and explain another example of sexual selection
that results in nonrandom mating.