Homework Questions submitted to biovcc@yahoo.com
- Explain how the
principle of gradualism and C. Lyell's theory
of uniformitarianism influenced
- Describe how
Alfred Russel Wallace influenced
- State 3
inferences
- Using some
contemporary examples, explain how natural selection results in
evolutionary change
- Describe how
molecular biology can be used to study evolutionary relationships among
organisms
Notes
Overview: The Smallest Unit of Evolution
One common misconception about evolution is that individual organisms evolve,
in the Darwinian sense, during their lifetimes
Natural selection acts on individuals, but populations evolve
EVOLUTION
A genetic change in a population of organisms that occurs over time, often
adapting to an environment or way of life.
Evolutionary changes must be genetically inherited, not acquired.
Evolution of Evolutionary thinking (Pre-Darwinian)
Jean-Baptiste Lamarck
(1744-1829) – French naturalist, proposed a theory that organisms were driven
by some inner force toward greater complexity. But thought that org. could pass
on traits to their offspring that they acquired during their lives.
(“Lamarckism”, proposed in 1809)
Lamarckism
Lamarckism holds that traits acquired (or diminished) during the lifetime of an
organism can be passed to its offspring.
Lamarck based his theory on two observations thought
to be true in his day:
“Use it or lose it” - Individuals lose characteristics they do not require and
develop those which are useful.
Inheritance of acquired traits - Individuals inherit
the acquired traits of their ancestors.
Lamarckism
Examples include: the stretching by giraffes to reach leaves leads to offspring
with longer necks;
Strengthening of muscles in a blacksmith's arm leads to sons with like muscular
development.
Charles Darwin
Charles Darwin (1809-1882)
Born in
Takes a 5-year trip around the world as a naturalist on the
HMS Beagle.
Observes plant and animal species in Galapagos Islands,
Observed:
On the Origin of Species…
Came home, worked for 16 years analyzing his data
(The entire printing (2500 copies) was sold that same day!)
Controversy…
The Origin of Species… caused great arguments between scientists and
philosophers – both noting the theories failures and strengths.
Could not believe that organisms today appeared as they have always appeared
Natural Selection
Has four premises:
1) Variation – Members of a population have individual differences that are
inheritable
2) Overproduction – Natural populations reproduce geometrically
3) Competition – Individuals compete for limited resources
4) Survival to reproduce – Only those individuals that are better suited to the
environment survive and reproduce
Natural Selection
1. Variation: Member within a species exhibit individual
differences – these differences must be inheritable
Natural selection won’t work in a population of clones! Remember that a key to
variation is sexual reproduction.
Natural Selection
2. Overproduction: Natural populations increase geometrically, producing
much more offspring than will survive…
Natural Selection
3. Competition: Individuals compete for the same, limited natural
resources.
Natural Selection
4. Survival to reproduce: Only those individuals that are better suited to the
environment will survive and reproduce (Survival of the fittest).
Fit individuals pass on to a portion of their offspring the advantageous
characteristics.
Natural Selection
Works on the individual phenotype à which in turn changes the population gene pool.
Time – long periods of time must be available in order to change to a
completely different species; changes are slow..
Natural Selection
Offspring that inherit the advantageous traits (“favorable genes”) are selected
for
Their chances of survival are greater
May live to reproductive age
May pass on those desirable attributes to future generations
Natural Selection
Those that do not inherit these traits (“unfavorable genes”), are not likely to
survive/reproduce.
Gradually, the species evolves (changes) as more individuals carry these
traits.
Over time, enough changes à New species
Artificial Selection
Selective breeding as practiced by humans on domesticated plants and animals….
For example: Dogs
Plant artificial selection
Teosinte vs. modern corn
Rates of evolution
Two interpretations about the pace/speed of evolution – based on the fossil
record:
1. Gradualism (a traditional view) states that
Evolution occurs as a slow and steady accumulation of changes in organism
(Darwinian evolution)… Not much evidence.
Rates of evolution
2. Punctuated Equilibrium – evolution proceeds with periods of
inactivity, followed by periods of very rapid evolution (Gould & Eldridge
model).
Punctuated Equilibrium
Fossil record supports this view:
Long periods of stasis (no change in species)
Followed by rapid change
However, fossil record is evidence only of Morphology (structure), while
evolution encompasses: morphology, ecology, biochemistry, and behavioral
changes…
That is, there may be stasis in morphology, while there is
still active evolutionary changes going on…
Part II
Evidence for evolution: extant and extinct organisms
Adaptations
Coevolution
2. Mimicry and protective coloration
Mimicry: a harmless species may resemble a dangerous species.
Ex. Some moths resemble wasps
Monarch butterfly is toxic, Viceroy is not,
But Viceroy mimics the Monarch
Protective coloration
Coloration that allows an organism to blend with environment
Moths in bark in polluted England
3. Developmental Biology
Early embryos of different mammal species look very much alike – they share
common features (gills, tail, etc.).
3. Developmental Biology
4. Biogeography
Unequal distribution of organisms on earth
Kangaroos in
Saguaro Cacti in southwestern
Each species originated only once, in one place – point of origin…
Species spread out until they encounter a barrier (physical, environmental,
ecological)
5. Biochemistry & Molecular Biology
Our genes provide an ‘evolutionary record’
If we evolved from a common ancestor:
We should have same genetic molecule (DNA)
We should use the DNA in the same way (dogma)
Portions of our DNA should be the same
Closely related organisms share large portions of DNA sequence…
FOSSIL EVIDENCE FOR EVOLUTION
Fossils – any trace left by a previous organism
Rocks, ice, amber, bogs, tar, etc.
Most are preserved in sedimentary rocks
Oldest rocks (fossils) have simplest life forms
Most recent rocks – have more complex life forms
ADAPTATIONS
Adaptation: A process by which genetic changes occur…
ADAPTATIONS are traits that promote the survival and reproductive success of an
organism in a particular environment.
Specific anatomical, physiological or biochemical
structures/mechanisms that arise during evolution, as a response to specific
environmental pressures.
Adaptations
Adaptations may originate as mutations in one individual organism.
Adaptations are universal: life occurs in everywhere on earth…
Organisms adapt to a specific niche (place in the environment)
Without adaptations, species can become extinct.
Examples of plant adaptations
Plants try to avoid predation by herbivores. For example: desert plants with
thorns; fruits distasteful when not ripe.
Coloration: Different flower colors attract different pollinators.
Morphological adaptations. For example: Strawberries
grow underground stems (stolons) that break so that a
new plant grows asexually. Also, some leaves of desert plants are hairy, to
reduce water loss, leaves in tropics are smooth
Plant adaptations
Leaves: Adapted to many functions in different plants
Coevolution
Coevolution: the long term evolutionary adjustment of
one group of organisms to another.
Coevolution is a reciprocal process in which
characteristics of one organism evolve in response to specific characteristics
of another
Examples of co-evolution
There’s ANTS in PLANTS!
Acacia and ants – coevolution.
Flowers & insects – coevolution
for pollination.
Bluejays & monarch/viceroy butterflies
More than just Human Cognition
A Brief History of Life
Basics of Darwinian Evolution
“Fitter?”
The Blind Watchmaker
The Origin of Needs and Wants
Simple beginnings
Cognition?
Plant Cognition
Speeding up …
Social behavior and Trust
Insects
A strange species
Continuity of Cognition
A population is a group of organisms of the same species living in the same
place at the same time
Millions of different populations all evolving according to their own self
interest in a particular environment.
But each population is a part of the environment of its neighbors, so any
evolutionary change has a ripple effect.
A Population
Group of organisms of the same species living in the same place at the same
time
Individuals may come and go, but the population can remain the same
The Nakuru Flamingos each year, for Example
Population Growth
Since each organism of a population is governed by the selfish gene,
populations tend to grow
If unlimited resources are present, growth will be exponential
It will proceed very quickly for rapidly reproducing organisms and more slowly
for slowly reproducing ones
The curve, however, will always be a “J” curve or an exponential growth curve
Population Growth 2
Resources are never unlimited, though.
As population rises, resources decline.
If the growth is too rapid, resources are rapidly depleted and a population
crash can occur
This pattern occurs often with many populations (including humans)
For example...
Population Growth 3
More often what happens is that the resources slowly
decrease, the growth rate slowly decreases, and they meet.
This point that they oscillate around is the carrying capacity of the
environment for that particular organism
So when would you “harvest” these individuals? (1,2,3,4,or 5)
Population Mortality
Organisms differ on strategies of reproduction and differ on types of predation
Those organisms that put much care into their few young tend to have good
survivorship of young
Those organisms that spread their young all over tend to have poor survivorship
of their young
A graphic representation of the rates of survival at different ages is called a
survivorship curve
Population Density and Dispersion
Population density is simply the number of individuals measured per unit of
area or volume
Additionally, the population can clump in different ways
Random
Clumped
Regular
Growth Rate Limiting Factors (effecting birth or mortality rates)
Density-Dependent
Predation
Increased competition for scarce resources
Sickness
Others?...
Density-Independent
Weather
Ice Age
Global Warming
Flood
El Nino
Etc.
Human Growth Patterns
Microevolution
A change in a populations gene pool over a succession of generations
Population is the smallest unit of evolution
A population is a localized group of individuals of the same species
Species is a group of populations whose members are capable of interbreeding.
The gene pool is the term for all the genes present in a population at any
given time.
Diploids à two alleles at each locus
If all individuals are homozygous for the same allele à “fixed”
More often see alleles in some relative proportion or frequency
The Evolution of Populations
1900’s à many geneticists believed
Darwin’s focus on inheritance of quantitative traits that vary on a continuum
could not be explained by the inheritance of discrete Mendelian
traits
1930’s à population genetics –
emphasis on quantitative inheritance and genetic variation in populations à Mendelism + Darwinism
1940’s à modern synthesis –
comprehensive theory of evolution – emphasized importance of populations as
units of evolution, the essential role of natural selection and the gradualness
of evolution
The Darwinian Evolution
The Voyage of the Beagle
1844 à essay on the origin of
species and natural selection
1858 à Darwin + Wallace
presented identical theories of natural selection
1859 à “On The Origin of
Species” published
Darwin’s Focus on Adaptation
The Origin of Species
Developed two main points
The occurrence of evolution
Natural selection as its mechanism
Descent with Modification
All organisms were related through descent from some unknown ancestor and had
developed increasing modifications as they adapted to various environments
Natural Selection and Adaptation
Ernst Mayr (biological species concept) summarized
Darwin’s theory
Observation 1: Species have the potential for their population size to increase
exponentially.
Observation 2: Most population sizes are stable.
Observation 3: Environmental resources are limited.
Inference 1: Since only a fraction of offspring survive, there is a struggle
for limited resources.
Natural Selection and Adaptation
Observation 4: Individuals vary within a population.
Observation 5: Much of the variation is inherited.
Inference 2: Individuals whose inherited characteristics fit them best to the
environment are likely to leave more offspring.
Inference 3: Unequal reproduction leads to the gradual accumulation of
favorable characteristics in a population over generations.
Support for Natural Selection
Artificial Selection à breeding of domesticated plants and animals for specific traits
Natural Selection à measured only as a change in the relative proportions of variations in
a population over time
Affects only those traits that are heritable (acquired characteristics cannot
evolve)
Local and temporal, depending on specific environmental factors present at a
given time and place
Natural Selection in Action
Favor some heritable traits over others, changes populations over successive
generations
The Peppered Moth
Snails
Light-colored and striped à well-lighted areas
Dark-colored, lacking stripes à shady places
Natural Selection in Action
Examples provide evidence for evolution à Directed Selection
Evolution of insect resistant strains
Evolution of antibiotic resistant bacteria
Evolution of drug-resistant HIV
Other Evidence of Evolution
Biogeography + Fossil Record
Comparative Anatomy
Comparative Embryology
Molecular Biology
Darwinism
What is theoretical about the Darwinian view of life?
The evolution of modern species from ancestral forms is supported by facts –
fossils, biogeography, comparative anatomy and embryology and molecular
biology.
The second of
The Evolution of Populations
Although it is individuals that are selected for or
against by natural selection à it is populations that actually evolve.
Hardy-Weinberg Equilibrium
In the absence of selection pressure, the gene pool of a population will remain
constant from one generation to the next
Equation: p2 + 2pq + q2 = 1
Also (p + q = 1)
Population Genetics & Public Health
If know the frequency of individuals born with a certain inherited disease
(homozygous, recessive) à estimate the frequency of a harmful allele in a population
The q2 = genotype frequency of affected individual (1 in 10,000 births)
PKU à q2 = 0.0001 then q = 0.01
Carriers à 2pq = 2 (0.99) (0.01) =
0.02 à 2% of the population are
carriers
Hardy-Weinberg Equilibrium
Five conditions are required to maintain:
The population is very large.
The population is isolated à no migration in or out.
Mutations do not alter the gene pool.
Mating is random.
All individuals are equal in reproductive success à no natural selection.
These 5 conditions are rarely (if ever) met in nature.
Causes of Microevolution
Five causes correspond to a deviation of the 5 causes that maintain
Hardy-Weinberg Equilibrium.
Genetic drift
Gene flow
Mutation
Nonrandom mating
Natural selection
Genetic Drift
Change in the gene pool of a small population due to chance
Chance event can have a disproportionately large effect
Genetic drift is most likely to play a role in populations with 100 or fewer
individuals
Genetic Drift
Bottleneck à genetic drift resulting
from an event that drastically reduces population size à natural or man-made disasters à small surviving population unlikely to have the same
genetic make-up as original population à reduces genetic variation
Founder Effect à colonization of a new location by a small number of individuals à reduces genetic variation
Genetic Drift
Statistically, the smaller a sample
The greater the chance of deviation from a predicted result
Genetic drift
Describes how allele frequencies can fluctuate unpredictably from one generation
to the next
Tends to reduce genetic variation
Gene Flow
Gain or loss of alleles from a population by the movement of individuals or
gametes
Occurs when fertile individuals move into or out of a population, or transfer
of gametes, (pollen), from one population to another
Gene flow tends to reduce genetic differences between populations
Gene Flow
Gene flow
Causes a population to gain or lose alleles
Results from the movement of fertile individuals or gametes
Tends to reduce differences between populations over time
: Natural selection is the primary mechanism of adaptive evolution
Natural selection
Accumulates and maintains favorable genotypes in a population
Mutation
Random change in an organism’s DNA that creates a new allele
Rare event – alone usually does not have much impact on a population
Natural selection or genetic drift à may increase the frequency of the allele
Mutation
Mutations
Are changes in the nucleotide sequence of DNA
Cause new genes and alleles to arise
Point Mutations
A point mutation
Is a change in one base in a gene
Can have a significant impact on phenotype
Is usually harmless, but may have an adaptive impact
Mutations That Alter Gene Number or Sequence
Chromosomal mutations that affect many loci
Are almost certain to be harmful
May be neutral and even beneficial
Gene duplication
Duplicates chromosome segments
Mutation Rates
Mutation rates
Tend to be low in animals and plants
Average about one mutation in every 100,000 genes per generation
Are more rapid in microorganisms
Nonrandom Mating
Selection of mates other than by chance
Nonrandom mating is more the rule in populations à “like” tends to mate with “like”
Stationary organisms or geographically restricted organisms à tend to mate with their neighbors rather than more
distant members of the population à tends to promote interbreeding
Natural Selection
Differential success in reproduction
Factor most likely to result in adaptive changes in a gene pool
The Bottleneck Effect
In the bottleneck effect
A sudden change in the environment may drastically reduce the size of a
population
The gene pool may no longer be reflective of the original population’s gene
pool
Understanding the bottleneck effect
Can increase understanding of how human activity affects other species
Hardy Weinberg Principle:
The work of mathematician G.H. Hardy and German doctor W. Weinberg, it explains
some basic properties of populations and how to describe them mathematically.
The Law:
"The frequencies of alleles that make up a gene will remain the same in a stable population. When p and q
stand for the frequency of each allele in a gene then: p+q=1
The Hardy-Weinberg Theorem
The Hardy-Weinberg theorem
Describes a population that is not evolving
States that the frequencies of alleles and genotypes in a population’s gene
pool remain constant from generation to generation provided that only Mendelian segregation and recombination of alleles are at
work
Mendelian inheritance
Preserves genetic variation in a population
Preservation of Allele Frequencies
In a given population where gametes contribute to the next generation randomly,
allele frequencies will not change
Hardy-Weinberg Equilibrium
Hardy-Weinberg equilibrium
Describes a population in which random mating occurs
Describes a population where allele frequencies do not change
A population in Hardy-Weinberg equilibrium
If p and q represent the relative frequencies of the only two possible alleles
in a population at a particular locus, then
p2 + 2pq + q2 = 1
And p2 and q2 represent the frequencies of the homozygous genotypes and 2pq
represents the frequency of the heterozygous genotype
Conditions for Hardy-Weinberg Equilibrium
The Hardy-Weinberg theorem
Describes a hypothetical population
In real populations
Allele and genotype frequencies do change over time
The five conditions for non-evolving populations are rarely met in nature
Extremely large population size
No gene flow
No mutations
Random mating
No natural selection
Population Genetics and Human Health
We can use the Hardy-Weinberg equation
To estimate the percentage of the human population carrying the allele for an
inherited disease
: Mutation and sexual recombination produce the variation that makes evolution
possible
Two processes, mutation and sexual recombination
Produce the variation in gene pools that contributes to differences among
individuals
p + q= 1 (100% for homozygous matings)
p 2 + 2pq +
q 2 = 1
( heterozygous matings)
In a sexually reproducing population, the frequencies will stay the same in
generation after generation provided these conditions are met:
1. mutations can not occur
2. matings are at random
3. natural selection can not occur
4. no genes may enter or leave the population
5. the population must be large.
Hardy- Weinberg Worksheet
The Hardy- Weinberg model is much easier to teach if
the students calculate gene frequencies along with the instructor. This means
that you (me) must pause frequently to allow plenty of time for students (you)
to actively process the information and practice the calculations.
In the absence of other factors, the segregation and recombination of alleles
during meiosis and fertilization will not alter the overall genetic makeup of a
population.
·
The frequencies of alleles in the gene pool will remain
constant unless acted upon by other agents; this is known as the Hardy-
Weinberg theorem.
The Hardy-Weinberg model describes the genetic structure of nonevolving
populations. This theorem can be tested with theoretical population models.
To test the Hardy-Weinberg theorem, imagine an isolated population of
wildflowers with the following characteristics:
· It is a diploid species with both pink and white
flowers.
The population size is 500 plants: 480_ plants have pink flowers, 20 plants
have white flowers.
· Pink flower color is coded for by the dominant allele
"A," white flower color is coded for by the recessive allele
"a."
Of the 480 pink-flowered plants, 320 are homozygous (AA) and 160 are
heterozygous (Aa). Since
white color is recessive, all white flowered plants are homozygous aa.
· There are 1000 genes for flower color in this population, since each of the
500 individuals has two genes (this is a diploid species).
A total of 320 genes are present in the 160 heterozygotes
(Aa): half are dominant (160
A) and half are recessive (160 a).
·800 of the 1000 total genes are dominant.
The frequency of the A allele is 80% or 0.8 (800/1000).
· 200 of the 1000 total genes
are recessive.
The frequency of the a allele is 20% or 0.2
(200/1000).
Assuming that mating in the population is completely random (all male-female
mating combinations have equal chances), the
frequencies of A and a will remain the same in the next generation.
· Each gamete will carry one gene for flower
color, either A or a.
Since mating is random, there is an 80% chance that any particular gamete will
carry the A allele and a 20% chance that any particular gamete will carry the a allele.
The frequencies of the three possible genotypes of the next generation can be
calculated using the rule of multiplication.
The probability of two A alleles joining is 0.8 x 0.8
= 0.64; thus, 64% of the next generation will be AA.
The probability of two a alleles joining is 0.2 x 0.2
= 0.04; thus, 4% of the next generation will be aa.
· Heterozygotes
can be produced in two ways, depending upon whether the sperm or ovum contains
the dominant allele (Aa or aA). The probability of a heterozygote being produced is
thus (0.8 x 0.2) + (0.2 x 0.8) = 0.16 + 0.16 = 0.32.
The frequencies of possible genotypes in the next generation are 64% AA, 32% Aa and 4% aa.
The frequency of the A allele in the new generation is 0.64 + (0.32/2) = 0.8,
and the frequency of the a allele is 0.04 + (0.32/2) =
0.2. Note that the alleles are present in the gene pool of the new population
at the same frequencies they were in the original gene pool.
· Continued sexual reproduction with
segregation, recombination and random mating would not alter the frequencies of
these two alleles: the gene pool of this population would be in a state of
equilibrium referred to as Hardy-Weinberg equilibrium.
If our original population had not been in equilibrium, only one generation
would have been necessary for equilibrium to become established.
From this theoretical wildflower population, a general formula, called the HardyWeinberg equation, can be derived to calculate allele
and genotype frequencies.
The Hardy-Weinberg equation can be used to consider loci with three or more
alleles.
· By way of example, consider the
simplest case with only two alleles with one dominant to the other.
· In our wildflower population,
let p represent allele A and q represent allele a, thus p = 0.8 and q = 0.2.
The sum of frequencies from all alleles must equal 100% of the genes for that
locus in the population: p + q = 1.
• Where only two alleles exist, only the frequency of one must be known since
the other can be derived:
1 -p=q or 1 -q=p
When gametes fuse to form a zygote, the probability of producing the AA
genotype is p2; the probability of producing aa is
q2; and the probability of producing an Aa
heterozygote is 2pq (remember heterozygotes may be
formed in two ways Aa or aA).
· The sum of these
frequencies must equal 100%, thus:
p2 + 2pq + q2 = 1
Frequency Frequency Frequency
of AA of Aa of aa
The Hardy-Weinberg equation permits the calculation of allelic frequencies in a
gene pool, if the genotype frequencies are known. Conversely, the genotype can
be calculated from known allelic frequencies.
For example, the Hardy-Weinberg equation can be used to calculate the frequency
of inherited diseases in humans (e.g., phenylketonuria):
· 1 of every 10,000 babies in the United States is born with phenylketonuria (PKU), a metabolic disorder that, if left
untreated, can result in mental retardation.
· The allele for PKU is recessive, so babies with this disorder are
homozygous recessive = q2.
Thus q2 = 0.0001,
with q = 0.01
(the square root of 0.0001).
· The frequency of p can be determined since p = 1 - q:
p = I - 0.01 = 0.99
· The frequency of carriers (heterozygotes)
in the population is 2pq.
2pq = 2(0.99)(0.01) = 0.0198
Thus, about 2% of the
http://science.nhmccd.edu/biol/hwe/q1d.html
Chapter 23
The Evolution of Populations
Genetic variations in populations
Contribute to evolution
: Population genetics provides a foundation for studying evolution
Microevolution
Is change in the genetic makeup of a population from generation to generation
The Modern Synthesis
Population genetics
Is the study of how populations change genetically over time
Reconciled Darwin’s and Mendel’s ideas
The modern synthesis
Integrates Mendelian genetics with the Darwinian
theory of evolution by natural selection
Focuses on populations as units of evolution
Gene Pools and Allele Frequencies
A population
Is a localized group of individuals that are capable of interbreeding and
producing fertile offspring
The gene pool
Is the total aggregate of genes in a population at any one time
Consists of all gene loci in all individuals of the population
Sexual Recombination
In sexually reproducing populations, sexual recombination
Is far more important than mutation in producing the genetic differences that
make adaptation possible
: Natural selection, genetic drift, and gene flow can alter a population’s
genetic composition
Three major factors alter allele frequencies and bring about most evolutionary
change
Natural selection
Genetic drift
Gene flow
Natural Selection
Differential success in reproduction
Results in certain alleles being passed to the next generation in greater
proportions
The Founder Effect
The founder effect
Occurs when a few individuals become isolated from a larger population
Can affect allele frequencies in a population
Genetic Variation
Genetic variation
Occurs in individuals in populations of all species
Is not always heritable
Variation Within a Population
Both discrete and quantitative characters
Contribute to variation within a population
Discrete characters
Can be classified on an either-or basis
Quantitative characters
Vary along a continuum within a population
Polymorphism
Phenotypic polymorphism
Describes a population in which two or more distinct morphs for a character are
each represented in high enough frequencies to be readily noticeable
Genetic polymorphisms
Are the heritable components of characters that occur along a continuum in a
population
Measuring Genetic Variation
Population geneticists
Measure the number of polymorphisms in a population by determining the amount
of heterozygosity at the gene level and the molecular
level
Average heterozygosity
Measures the average percent of loci that are heterozygous in a population
Variation Between Populations
Most species exhibit geographic variation
Differences between gene pools of separate populations or population subgroups
Some examples of geographic variation occur as a cline, which is a graded
change in a trait along a geographic axis
A Closer Look at Natural Selection
From the range of variations available in a population
Natural selection increases the frequencies of certain genotypes, fitting
organisms to their environment over generations
Evolutionary Fitness
The phrases “struggle for existence” and “survival of the fittest”
Are commonly used to describe natural selection
Can be misleading
Reproductive success
Is generally more subtle and depends on many factors
Fitness
Is the contribution an individual makes to the gene pool of the next
generation, relative to the contributions of other individuals
Relative fitness
Is the contribution of a genotype to the next generation as compared to the
contributions of alternative genotypes for the same locus
Directional, Disruptive, and Stabilizing Selection
Selection
Favors certain genotypes by acting on the phenotypes of certain organisms
Three modes of selection are
Directional
Disruptive
Stabilizing
Directional selection
Favors individuals at one end of the phenotypic range
Disruptive selection
Favors individuals at both extremes of the phenotypic range
Stabilizing selection
Favors intermediate variants and acts against extreme phenotypes
The three modes of selection
The Preservation of Genetic Variation
Various mechanisms help to preserve genetic variation in a population
Diploidy
Diploidy
Maintains genetic variation in the form of hidden recessive alleles
Balancing Selection
Balancing selection
Occurs when natural selection maintains stable frequencies of two or more
phenotypic forms in a population
Leads to a state called balanced polymorphism
Heterozygote Advantage
Some individuals who are heterozygous at a particular locus
Have greater fitness than homozygotes
Natural selection
Will tend to maintain two or more alleles at that locus
The sickle-cell allele
Causes mutations in hemoglobin but also confers malaria resistance
Exemplifies the heterozygote advantage
Frequency-Dependent Selection
In frequency-dependent selection
The fitness of any morph declines if it becomes too common in the population
An example of frequency-dependent selection
Neutral Variation
Neutral variation
Is genetic variation that appears to confer no selective advantage
Sexual Selection
Sexual selection
Is natural selection for mating success
Can result in sexual dimorphism, marked differences between the sexes in
secondary sexual characteristics
Intrasexual selection
Is a direct competition among individuals of one sex for mates of the opposite
sex
Intersexual selection
Occurs when individuals of one sex (usually females) are choosy in selecting
their mates from individuals of the other sex
May depend on the showiness of the male’s appearance
The Evolutionary Enigma of Sexual Reproduction
Sexual reproduction
Produces fewer reproductive offspring than asexual reproduction, a so-called
reproductive handicap
If sexual reproduction is a handicap, why has it persisted?
It produces genetic variation that may aid in disease resistance
Why Natural Selection Cannot Fashion Perfect Organisms
Evolution is limited by historical constraints
Adaptations are often compromises
Chance and natural selection interact
Selection can only edit existing variations
