Homework Questions 1 submitted to valenciabiologyhw@gmail.com
The upper forelimbs of humans and bats have fairly similar skeletal structures, whereas the corresponding bones in whales have very different shapes and proportions. However, genetic data suggest that all three kinds of organisms diverged from a common ancestor at about the same time. Which of the following is the most likely explanation for these data?
7. Within six months of effectively using methicillin to treat S. aureus infections in a community, all new infections were caused by MRSA. How can this result best be explaine
9. The volcano is currently dormant, but in a hypothetical future scenario, satellite cones at the base of Mt. Kilimanjaro spew sulfurous gases and lava, destroying all life located between the base and 6,000 feet above sea level. As a result of this catastrophe, how should the frequency of the sickle-cell allele change in the remnant human population that survives above 6,000 feet, and which phenomenon accounts for this change in allele frequency?
Questions addressed by Evolutionary Biology
Why are there so many different kinds of organisms?
Why are there so many species of flies and so few species of elephants?
How have they become good at what they do? Find food, mates, avoid predators?
What is related to what? And where do new species come from?
How old are they?
Overview: Darwin Introduces a Revolutionary Theory
A new era of biology began on November 24, 1859
o The day Charles Darwin published On the Origin of Species by Means of Natural Selection
The Origin of Species
o Focused biologists attention on the great diversity of organisms
The Origin of Species
Darwin developed two main ideas
o Evolution explains lifes unity and diversity
o Natural selection is a cause of adaptive evolution
Descent with Modification
The phrase descent with modification
o Summarized Darwins perception of the unity of life
o States that all organisms are related through descent from an ancestor that lived in the remote past
In the Darwinian view, the history of life is like a tree
o With multiple branchings from a common trunk to the tips of the youngest twigs that represent the diversity of living organisms
Natural Selection and Adaptation
Evolutionary biologist Ernst Mayr
o Has dissected the logic of Darwins theory into three inferences based on five observations
Observation #1: For any species, population sizes would increase exponentially
o If all individuals that are born reproduced successfully
Observation #2: Nonetheless, populations tend to be stable in size
o Except for seasonal fluctuations
Observation #3: Resources are limited
Inference #1: Production of more individuals than the environment can support
o Leads to a struggle for existence among individuals of a population, with only a fraction of their offspring surviving
Observation #4: Members of a population vary extensively in their characteristics
o No two individuals are exactly alike
Observation #5: Much of this variation is heritable
Inference #2: Survival depends in part on inherited traits
o Individuals whose inherited traits give them a high probability of surviving and reproducing are likely to leave more offspring than other individuals
Inference #3: This unequal ability of individuals to survive and reproduce
o Will lead to a gradual change in a population, with favorable characteristics accumulating over generations
Artificial Selection
In the process of artificial selection
o Humans have modified other species over many generations by selecting and breeding individuals that possess desired traits
Summary of Natural Selection
Natural selection is differential success in reproduction
o That results from the interaction between individuals that vary in heritable traits and their environment
Natural selection can produce an increase over time
o In the adaptation
o of organisms to their environment
If an environment changes over time
o Natural selection may result in adaptation to these new conditions
Darwins theory explains a wide range of observations
Darwins theory of evolution
o Continues to be tested by how effectively it can account for additional observations and experimental outcomes
Natural Selection in Action
Two examples
o Provide evidence for natural selection
Differential Predation in Guppy Populations
Researchers have observed natural selection
o Leading to adaptive evolution in guppy populations
The Evolution of Drug-Resistant HIV
In humans, the use of drugs
o Selects for pathogens that through chance mutations are resistant to the drugs effects
Natural selection is a cause of adaptive evolution
Researchers have developed numerous drugs to combat HIV
o But using these medications selects for viruses resistant to the drugs
The ability of bacteria and viruses to evolve rapidly
o Poses a challenge to our society
Homology, Biogeography, and the Fossil Record
Evolutionary theory
o Provides a cohesive explanation for many kinds of observations
Homology
Homology
o Is similarity resulting from common ancestry
Anatomical Homologies
Homologous structures between organisms
o Are anatomical resemblances that represent variations on a structural theme that was present in a common ancestor
Comparative embryology
o Reveals additional anatomical homologies not visible in adult organisms
Vestigial organs
o Are some of the most intriguing homologous structures
o Are remnants of structures that served important functions in the organisms ancestors
Molecular Homologies
Biologists also observe homologies among organisms at the molecular level
o Such as genes that are shared among organisms inherited from a common ancestor
Homologies and the Tree of Life
The Darwinian concept of an evolutionary tree of life
o Can explain the homologies that researchers have observed
Anatomical resemblances among species
o Are generally reflected in their molecules, their genes, and their gene products
Biogeography
Darwins observations of the geographic distribution of species, biogeography
o Formed an important part of his theory of evolution
Some similar mammals that have adapted to similar environments
o Have evolved independently from different ancestors
The Fossil Record
The succession of forms observed in the fossil record
o Is consistent with other inferences about the major branches of descent in the tree of life
The Darwinian view of life
o Predicts that evolutionary transitions should leave signs in the fossil record
Paleontologists
o Have discovered fossils of many such transitional forms
What Is Theoretical about the Darwinian View of Life?
In science, a theory
o Accounts for many observations and data and attempts to explain and integrate a great variety of phenomena
Darwins theory of evolution by natural selection
o Integrates diverse areas of biological study and stimulates many new research questions
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 England, studied Theology
Takes a 5-year trip around the world as a naturalist on the HMS Beagle.
Observes plant and animal species in Galapagos Islands, Australia, NZ, etc.
Observed: Island animals are similar to mainland animal species (descended), but they show differences due to the conditions on their island.
On the Origin of Species
Came home, worked for 16 years analyzing his data
Darwin publishes the most influential text of all times: On the Origin of Species by Means of Natural Selection in November 24, 1859.
(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.
Darwins revolutionary thoughts
Darwin thought of organisms not as
constant, unchanging or specially created beings.
Could not believe that organisms today appeared as they have always appeared
Darwin changed biological thought forever with the concept of Natural Selection!
Natural Selection
Has four premises:
Variation Members of a population have individual differences that are inheritable
Overproduction Natural populations reproduce geometrically
Competition Individuals compete for limited resources
Survival to reproduce Only those individuals that are better suited to the environment survive and reproduce
Natural Selection
Variation: Member within a species exhibit individual differences these differences must be inheritable
Natural selection wont work in a population of clones! Remember that a key to variation is sexual reproduction.
Natural Selection
Overproduction: Natural populations increase geometrically, producing much more offspring than will survive
Natural Selection
Competition: Individuals compete for the same, limited natural resources.
Darwin called it: Struggle for existence
Natural Selection
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 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 New species
Artificial Selection
Selective breeding as practiced by humans on domesticated plants and animals .
For example: Dogs
Artificial Selection
Plant artificial selection
Teosinte vs. modern corn
There are five recognized species of teosinte: Zea diploperennis, Zea perennis, Zea luxurians, Zea nicaraguensis and Zea mays. The last species is further divided into four subspecies: ssp. huehuetenangensis, ssp. mexicana, ssp. parviglumis and ssp. mays. The first three subspecies are teosintes; the last is maize, or corn, the only domesticated taxon in the genus Zea.
Rates of evolution
Two interpretations about the pace/speed of evolution based on the fossil record:
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
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
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
Developmental Biology
Early embryos of different mammal species look very much alike they share common features (gills, tail, etc.).
3. Developmental Biology
Biogeography
Unequal distribution of organisms on earth
Kangaroos in Australia;
Saguaro Cacti in southwestern U.S. deserts
Each species originated only once, in one place point of origin
Species spread out until they encounter a barrier (physical, environmental, ecological)
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
Theres 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?
Simple beginnings
Multicellular
Commonwealth of interests
Cognition?
Plant Cognition
Speeding up
Social behavior and Trust
Insects
A strange species
Continuity of Cognition
End part 1
Sponge Activity-What did you learn today?
In evolutionary terms, an organism's fitness is measured by its ______
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 27,532 people per square mile new york city, The population density was 1,108.1 inhabitants for apopka
Additionally, the population can clump in different ways
o Random
o Clumped
o Regular
Growth Rate Limiting Factors
(effecting birth or mortality rates)
Density-Dependent
o Predation
o Increased competition for scarce resources
o Sickness
o Others?...
Density-Independent
o 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.
o Diploids two alleles at each locus
o If all individuals are homozygous for the same allele fixed
o More often see alleles in some relative proportion or frequency
The Evolution of Populations
1900s many geneticists believed Darwins focus on inheritance of quantitative traits that vary on a continuum could not be explained by the inheritance of discrete Mendelian traits
1930s population genetics emphasis on quantitative inheritance and genetic variation in populations Mendelism + Darwinism
1940s 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 essay on the origin of species and natural selection
1858 Darwin + Wallace presented identical theories of natural selection
1859 On The Origin of Species published
Darwins Focus on Adaptation
The Origin of Species
Developed two main points
o The occurrence of evolution
o Natural selection as its mechanism
Descent with Modification
o 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 Darwins 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.
o 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.
o Inference 2: Individuals whose inherited characteristics fit them best to the environment are likely to leave more offspring.
o Inference 3: Unequal reproduction leads to the gradual accumulation of favorable characteristics in a population over generations.
Support for Natural Selection
Artificial Selection breeding of domesticated plants and animals for specific traits
Natural Selection measured only as a change in the relative proportions of variations in a population over time
o Affects only those traits that are heritable (acquired characteristics cannot evolve)
o 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
o The Peppered Moth
o Snails
Light-colored and striped well-lighted areas
Dark-colored, lacking stripes shady places
Natural Selection in Action
Examples provide evidence for evolution Directed Selection
o Evolution of insect resistant strains
o Evolution of antibiotic resistant bacteria
o Evolution of drug-resistant HIV
Other Evidence of Evolution
o Biogeography + Fossil Record
o Comparative Anatomy
o Comparative Embryology
o Molecular Biology
Darwinism
What is theoretical about the Darwinian view of life?
o The evolution of modern species from ancestral forms is supported by facts fossils, biogeography, comparative anatomy and embryology and molecular biology.
o The second of Darwins claims, that natural selection is the main mechanism of evolution, is a theory that explains the historical facts.
The Evolution of Populations
Although it is individuals that are selected for or against by natural selection it is populations that actually evolve.
Hardy-Weinberg Equilibrium
o In the absence of selection pressure, the gene pool of a population will remain constant from one generation to the next
o 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) estimate the frequency of a harmful allele in a population
The q2 = genotype frequency of affected individual (1 in 10,000 births)
PKU q2 = 0.0001 then q = 0.01
Carriers 2pq = 2 (0.99) (0.01) = 0.02 2% of the population are carriers
Hardy-Weinberg Equilibrium
Five conditions are required to maintain:
o The population is very large.
o The population is isolated no migration in or out.
o Mutations do not alter the gene pool.
o Mating is random.
o All individuals are equal in reproductive success 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.
o Genetic drift
o Gene flow
o Mutation
o Nonrandom mating
o Natural selection
Genetic Drift
Change in the gene pool of a small population due to chance
o Chance event can have a disproportionately large effect
o Genetic drift is most likely to play a role in populations with 100 or fewer individuals
Genetic Drift
Bottleneck genetic drift resulting from an event that drastically reduces population size natural or man-made disasters small surviving population unlikely to have the same genetic make-up as original population reduces genetic variation
Founder Effect colonization of a new location by a small number of individuals reduces genetic variation
Genetic Drift
Statistically, the smaller a sample
o The greater the chance of deviation from a predicted result
Genetic drift
o Describes how allele frequencies can fluctuate unpredictably from one generation to the next
o 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
o Causes a population to gain or lose alleles
o Results from the movement of fertile individuals or gametes
o Tends to reduce differences between populations over time
: Natural selection is the primary mechanism of adaptive evolution
Natural selection
o Accumulates and maintains favorable genotypes in a population
Mutation
Random change in an organisms DNA that creates a new allele
Rare event alone usually does not have much impact on a population
Natural selection or genetic drift may increase the frequency of the allele
Mutation
Mutations
o Are changes in the nucleotide sequence of DNA
o Cause new genes and alleles to arise
Point Mutations
A point mutation
o Is a change in one base in a gene
o Can have a significant impact on phenotype
o Is usually harmless, but may have an adaptive impact
Mutations That Alter Gene Number or Sequence
Chromosomal mutations that affect many loci
o Are almost certain to be harmful
o May be neutral and even beneficial
Gene duplication
o Duplicates chromosome segments
Mutation Rates
Mutation rates
o Tend to be low in animals and plants
o Average about one mutation in every 100,000 genes per generation
o Are more rapid in microorganisms
Nonrandom Mating
Selection of mates other than by chance
Nonrandom mating is more the rule in populations like tends to mate with like
Stationary organisms or geographically restricted organisms tend to mate with their neighbors rather than more distant members of the population 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
o A sudden change in the environment may drastically reduce the size of a population
o The gene pool may no longer be reflective of the original populations gene pool
Understanding the bottleneck effect
o 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
o Describes a population that is not evolving
o States that the frequencies of alleles and genotypes in a populations gene pool remain constant from generation to generation provided that only Mendelian segregation and recombination of alleles are at work
Mendelian inheritance
o 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
o Describes a population in which random mating occurs
o 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
o p2 + 2pq + q2 = 1
o 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
o Describes a hypothetical population
In real populations
o Allele and genotype frequencies do change over time
The five conditions for non-evolving populations are rarely met in nature
o Extremely large population size
o No gene flow
o No mutations
o Random mating
o No natural selection
Population Genetics and Human Health
We can use the Hardy-Weinberg equation
o 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
o 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
o ( heterozygous matings)
In a sexually reproducing population, the frequencies will stay the same in generation after generation provided these conditions are met:
o mutations can not occur
o matings are at random
o natural selection can not occur
o no genes may enter or leave the population
o 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 U.S. population are carriers for PKU.
http://science.nhmccd.edu/biol/hwe/q1d.html
Chapter 23
The Evolution of Populations
Genetic variations in populations
o Contribute to evolution
: Population genetics provides a foundation for studying evolution
Microevolution
o Is change in the genetic makeup of a population from generation to generation
The Modern Synthesis
Population genetics
o Is the study of how populations change genetically over time
o Reconciled Darwins and Mendels ideas
The modern synthesis
o Integrates Mendelian genetics with the Darwinian theory of evolution by natural selection
o Focuses on populations as units of evolution
Gene Pools and Allele Frequencies
A population
o Is a localized group of individuals that are capable of interbreeding and producing fertile offspring
The gene pool
o Is the total aggregate of genes in a population at any one time
o Consists of all gene loci in all individuals of the population
Sexual Recombination
In sexually reproducing populations, sexual recombination
o 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 populations genetic composition
Three major factors alter allele frequencies and bring about most evolutionary change
o Natural selection
o Genetic drift
o Gene flow
Natural Selection
Differential success in reproduction
o Results in certain alleles being passed to the next generation in greater proportions
The Founder Effect
The founder effect
o Occurs when a few individuals become isolated from a larger population
o Can affect allele frequencies in a population
Genetic Variation
Genetic variation
o Occurs in individuals in populations of all species
o Is not always heritable
Variation Within a Population
Both discrete and quantitative characters
o Contribute to variation within a population
Discrete characters
o Can be classified on an either-or basis
Quantitative characters
o Vary along a continuum within a population
Polymorphism
Phenotypic polymorphism
o 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
o Are the heritable components of characters that occur along a continuum in a population
Measuring Genetic Variation
Population geneticists
o Measure the number of polymorphisms in a population by determining the amount of heterozygosity at the gene level and the molecular level
Average heterozygosity
o Measures the average percent of loci that are heterozygous in a population
Variation Between Populations
Most species exhibit geographic variation
o 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
o 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
o Are commonly used to describe natural selection
o Can be misleading
Reproductive success
o Is generally more subtle and depends on many factors
Fitness
o Is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals
Relative fitness
o 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
o Favors certain genotypes by acting on the phenotypes of certain organisms
Three modes of selection are
o Directional
o Disruptive
o Stabilizing
Directional selection
o Favors individuals at one end of the phenotypic range
Disruptive selection
o Favors individuals at both extremes of the phenotypic range
Stabilizing selection
o 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
o Maintains genetic variation in the form of hidden recessive alleles
Balancing Selection
Balancing selection
o Occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population
o Leads to a state called balanced polymorphism
Heterozygote Advantage
Some individuals who are heterozygous at a particular locus
o Have greater fitness than homozygotes
Natural selection
o Will tend to maintain two or more alleles at that locus
The sickle-cell allele
o Causes mutations in hemoglobin but also confers malaria resistance
o Exemplifies the heterozygote advantage
Frequency-Dependent Selection
In frequency-dependent selection
o The fitness of any morph declines if it becomes too common in the population
An example of frequency-dependent selection
Neutral Variation
Neutral variation
o Is genetic variation that appears to confer no selective advantage
Sexual Selection
Sexual selection
o Is natural selection for mating success
o Can result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics
Intrasexual selection
o Is a direct competition among individuals of one sex for mates of the opposite sex
Intersexual selection
o Occurs when individuals of one sex (usually females) are choosy in selecting their mates from individuals of the other sex
o May depend on the showiness of the males appearance
The Evolutionary Enigma of Sexual Reproduction
Sexual reproduction
o Produces fewer reproductive offspring than asexual reproduction, a so-called reproductive handicap
If sexual reproduction is a handicap, why has it persisted?
o 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
Sponge Activity-What did you learn today?
In evolutionary terms, an organism's fitness is measured by its ______
2 In the Hardy-Weinberg theorem, 1 represents
__________
3 The smallest unit that can evolve is a __
In evolutionary terms, an organism's fitness is measured by its _____.
The ultimate source of all genetic variation is __