Chapter 21 Genes within populations
Jan 04, 2016
Chapter 21Genes within populations
Question?
How did the diversity of life originate?
Through the process of Evolution.
Evolution
The processes that have transformed life on earth from its beginnings to today's diversity.
Evolution is the most pervasive principle in biology.
Theory vs theory
Hypothesis supported repeatedly by data.
Makes testable predictions
Layperson’s definition.
Confused with hypothesis in Science.
Examples of Theory
Cell Theory Big Bang Theory Atomic Theory Theory of Gravity Theory of Evolution
Teaching Philosophy on
Evolution
Evolution
Has itself "evolved" or changed over time.
Illustrates “Science as a Process”.
Students should be able to give the main points of several views.
Pre-Darwinian Views
1. Greeks
2. Fixed Species
3. Catastophism
4. Hutton and Lyell
5. Lamarck
Greek Philosophers
1. Plato - Organisms are already perfectly adapted to their environments.
2. Aristotle - Organisms arranged on a “scale of life” from simple to complex.
Result
No evolution. Life is already perfect and
doesn’t need to change. All the rungs on life's "ladder" are already occupied.
Fixed Species Concept
The creator had designed each and every species for a particular purpose.
Result
No evolution. Created the viewpoint that all
species could be identified and named (Taxonomy). A major factor in the Linnaeus classification system.
Catastrophism
Georges Cuvier (1769-1832).
Attempted to relate fossils to current life.
Theory Fossils were the remains of
species lost due to catastrophe.
No new species originated; species could only be lost over time.
Result - No evolution.
James Hutton 1795 - Gradualism Profound change is the
cumulative product of slow, but continuous processes.
Result
Changes on the earth were gradual, not catastrophic.
Charles Lyell
1797 - 1875. Incorporated
Hutton’s gradualism into a theory called Uniformitarianism.
Uniformitarianism
Geological processes have operated at the same rate over the Earth’s history.
Result
The Earth must be VERY old. (much older than 6000 years of the fixed species concept).
Idea that slow and subtle processes can cause substantial change.
Jean Baptiste Lamarck
Published theory in 1809.
Theory - Life changed from simple to complex over time.
Lamark
Fossils were the remains of past life forms.
Evolution did occur.
Mechanisms
1. Use and Disuse - Body parts used to survive
become larger and stronger. Body parts not used to survive
deteriorate.
Mechanisms
2. Acquired Characteristics Modifications acquired by
use/disuse were passed on to offspring.
Problem
No knowledge of genetics. Acquired traits are not
transmitted offspring.
Lamarck’s Credits
Did suggest correctly the role of fossils in evolution.
Did suggest that adaptation to the environment is a primary product of evolution.
Charles Darwin
Father of the modern theory of evolution.
Theory - Descent with Modification.
Darwin's Background
Trained as a Naturalist (after trying religion and medicine).
Voyage of the Beagle
Result
Darwin's training and travel opportunities allowed him to formulate and support his ideas on Natural Selection.
Galapagos Finches
Alfred Wallace - 1858
Paper on Natural Selection identical to Darwin's ideas.
Result - July 1, 1858
Dual presentation of the Wallace-Darwin ideas to the Linnaean Society of London.
Darwin - 1859
Publication of "The Origin of Species”
Comment
Darwin best remembered for the theory because of his overwhelming evidence and because he published.
Darwinian View
History of life is like a tree with branches over time from a common source.
Current diversity of life is caused by the forks from common ancestors.
Example
“The Origin of Species”
Documented the occurrence of evolution.
Suggested that the mechanism for evolution was Natural Selection.
The Facts:
Fact 1 - All species reproduce themselves exponentially.
Fact 2 - Most populations are normally stable in size.
Fact 3 - Natural Resources are limited (finite).
Inference 1
The large number of offspring must compete for the finite resources.
Result - Most offspring die.
Thomas Malthus Essay on human population
growth in 1798. Disease, famine, homelessness,
and war are inescapable because human populations grow faster than food supplies.
Darwin read Malthus.
More FactsFact 4 - No two individuals in a
population are exactly alike.
Fact 5 - Variation is inheritable.
Inference 2
Those individuals whose inherited characteristics fit them best to their environment survive and reproduce.
Inference 3
Offspring inherit the favorable characteristics. Populations shift over time as the favorable characteristics accumulate.
Nature
Determines which characteristics are favorable.
Determines who survives. Result - “Natural Selection”
Artificial Selection
When man determines the characteristics that survive and reproduce.
Result - the various breeds of animals and plants we’ve developed.
Ex - Mustard Plant
Original Cultivars
Evolution Success Measured By
Survival Reproduction Whoever lives long enough
and has kids is the “winner” in evolution.
Requirements
In order for Natural Selection to work, you must have: Long periods of time. Variations within a population.
Subtleties of Natural Selection
1. Populations are the units of Evolution.
2. Only inherited characteristics can evolve.
Comment
Acquired characteristics may allow a species to evolve "outside" of Natural Selection.
Ex: culture, learning
Additional Signs
1. Biogeography
2. Fossils
3. Taxonomy
4. Comparative Anatomy
5. Comparative Embryology
6. Molecular Biology
Biogeography
The geographical distribution of species.
Problem: Species mixtures on islands Marsupials in Australia
Evolution Viewpoint
Biogeographical patterns reflect descent from the ancestors that colonized that area.
Fossils
Relics or impressions of organisms from the past.
Problem: Show changes over time from
simple to complex. Many fossils don't have
descendants.
Evolution Viewpoint
Life has changed over time.
Many species failed to survive and became extinct.
Comments1. Fossilization is a rare event.2. Only hard parts fossilize well.3. Problem in finding fossils.4. Interpretation.5. Missing Links.
Taxonomy Science of Classification.
Main Categories Domain Kingdom Phylum or Division Class Order Family Genus Species
Problem - Why can life be grouped this way?
Evolution Viewpoint -Hierarchy reflects the branching genealogy of the tree of life.
Question?
Is the unit of evolution the individual or the population?
Answer – while evolution effects individuals, it can only be tracked through time by looking at populations.
So what do we study?
We need to study populations, not individuals.
We need a method to track the changes in populations over time.
This is the area of Biology called
population genetics.
Population Genetics
The study of genetic variation in populations.
Represents the reconciliation of Mendelism and Darwinism.
Modern Synthesis
Uses population genetics as the means to track and study evolution.
Looks at the genetic basis of variation and natural selection.
Population A localized group of individuals
of the same species.
Species
A group of similar organisms. A group of populations that
could interbreed.
Gene Pool
The total aggregate of genes in a population.
If evolution is occurring, then changes must occur in the gene pool of the population over time.
Microevolution
Changes in the relative frequencies of alleles in the gene pool.
Hardy-Weinberg Theorem
Developed in 1908. Mathematical model of gene
pool changes over time.
Basic Equation
p + q = 1 p = % dominant allele q = % recessive allele
Expanded Equation
p + q = 1 (p + q)2 = (1)2
p2 + 2pq + q2 = 1
Genotypes
p2 = Homozygous Dominants2pq = Heterozygousq2 = Homozygous Recessives
Example Calculation
Let’s look at a population where: A = red flowers a = white flowers
Starting Population
N = 500 Red = 480 (320 AA+ 160 Aa) White = 20 Total Genes = 2 x 500
= 1000
Dominant Allele
A = (320 x 2) + (160 x 1)
= 800
= 800/1000
A = 80%
Recessive Allele
a = (160 x 1) + (20 x 2)
= 200/1000
= .20
a = 20%
A and a in HW equation
Cross: Aa X Aa Result = AA + 2Aa + aa Remember: A = p, a = q
Substitute the values for A and a
p2 + 2pq + q2 = 1
(.8)2 + 2(.8)(.2) + (.2)2 = 1
.64 + .32 + .04 = 1
Dominant Allele
A = p2 + pq
= .64 + .16
= .80
= 80%
Recessive Allele
a = pq + q2
= .16 + .04
= .20
= 20%
Result
Gene pool is in a state of equilibrium and has not changed because of sexual reproduction.
No Evolution has occurred.
Importance of Hardy-Weinberg
Yardstick to measure rates of evolution.
Predicts that gene frequencies should NOT change over time as long as the HW assumptions hold (no evolution should occur).
Way to calculate gene frequencies through time.
Example
What is the frequency of the PKU allele?
PKU is expressed only if the individual is homozygous recessive (aa).
PKU Frequency
PKU is found at the rate of 1/10,000 births.
PKU = aa = q2
q2 = .0001
q = .01
Dominant Allele
p + q = 1
p = 1- q
p = 1- .01
p = .99
Expanded Equation
p2 + 2pq + q2 = 1
(.99)2 + 2(.99x.01) + (.01)2 = 1
.9801 + .0198 + .0001 = 1
Final Results
Normals (AA) = 98.01% Carriers (Aa) = 1.98% PKU (aa) = .01%
AP Problems Using Hardy-Weinberg
Solve for q2 (% of total). Solve for q (equation). Solve for p (1- q). H-W is always on the national
AP Bio exam (but no calculators are allowed).
Hardy-Weinberg Assumptions
1. Large Population
2. Isolation
3. No Net Mutations
4. Random Mating
5. No Natural Selection
If H-W assumptions hold true:
The gene frequencies will not change over time.
Evolution will not occur. But, how likely will natural
populations hold to the H-W assumptions?
Microevolution
Caused by violations of the 5 H-W assumptions.
Causes of Microevolution
1. Genetic Drift
2. Gene Flow
3. Mutations
4. Nonrandom Mating
5. Natural Selection
Genetic Drift
Changes in the gene pool of a small population by chance.
Types: 1. Bottleneck Effect 2. Founder's Effect
By Chance
Bottleneck Effect
Loss of most of the population by disasters.
Surviving population may have a different gene pool than the original population.
Result
Some alleles lost. Other alleles are over-
represented. Genetic variation usually lost.
Importance
Reduction of population size may reduce gene pool for evolution to work with.
Ex: Cheetahs
Founder's Effect
Genetic drift in a new colony that separates from a parent population.
Ex: Old-Order Amish
Result
Genetic variation reduced. Some alleles increase in
frequency while others are lost (as compared to the parent population).
Importance
Very common in islands and other groups that don't interbreed.
Gene Flow
Movement of genes in/out of a population.
Ex: Immigration Emigration
Result
Changes in gene frequencies within a population.
Immigration often brings new alleles into populations increasing genetic diversity.
Mutations
Inherited changes in a gene.
Result
May change gene frequencies (small population).
Source of new alleles for selection.
Often lost by genetic drift.
Nonrandom Mating
Failure to choose mates at random from the population.
Causes
Inbreeding within the same “neighborhood”.
Assortative mating (like with like).
Result
Increases the number of homozygous loci.
Does not in itself alter the overall gene frequencies in the population.
Natural Selection
Differential success in survival and reproduction.
Result - Shifts in gene frequencies.
Comment As the Environment changes,
so does Natural Selection and Gene Frequencies.
Result
If the environment is "patchy", the population may have many different local populations.
Genetic Basis of Variation
1. Discrete Characters – Mendelian traits with clear phenotypes.
2. Quantitative Characters – Multigene traits with overlapping phenotypes.
Polymorphism
The existence of several contrasting forms of the species in a population.
Usually inherited as Discrete Characteristics.
Examples
Garter Snakes Gaillardia
Human Example
ABO Blood Groups Morphs = A, B, AB, O
Other examples
Quantitative Characters
Allow continuous variation in the population.
Result – Geographical Variation Clines: a change along a
geographical axis
Yarrow and Altitude
Sources of Genetic Variation
Mutations. Recombination though sexual
reproduction. Crossing-over Random fertilization
Preserving Genetic Variation
1. Diploidy - preserves recessives as heterozygotes.
2. Balanced Polymorphisms - preservation of diversity by natural selection.
Example
Heterozygote Advantage - When the heterozygote or hybrid survives better than the homozygotes. Also called Hybrid vigor.
Result Can't bred "true“ and the
diversity of the population is maintained.
Ex – Sickle Cell Anemia
Comment
Population geneticists believe that ALL genes that persist in a population must have had a selective advantage at one time.
Ex – Sickle Cell and Malaria, Tay-Sachs and Tuberculosis
Fitness - Darwinian
The relative contribution an individual makes to the gene pool of the next generation.
Relative Fitness
Contribution of one genotype to the next generation compared to other genotypes.
Rate of Selection
Differs between dominant and recessive alleles.
Selection pressure by the environment.
Modes of Natural Selection
1. Stabilizing
2. Directional
3. Diversifying
4. Sexual
Stabilizing
Selection toward the average and against the extremes.
Ex: birth weight in humans
Directional Selection
Selection toward one extreme. Ex: running speeds in race
animals. Ex. Galapagos Finch beak size
and food source.
Diversifying
Selection toward both extremes and against the norm.
Ex: bill size in birds
Comment
Diversifying Selection - can split a species into several new species if it continues for a long enough period of time and the populations don’t interbreed.
Sexual Mate selection
May not be adaptive to the environment, but increases reproduction success of the individual.
This is a VERY important selection type for species.
Result Sexual dimorphism. Secondary sexual features
for attracting mates.
Comments
Females may drive sexual selection and dimorphism since they often "choose" the mate.
Question
Does evolution result in perfect organisms?
Answer - No
1. Historical Constraints2. Compromises3. Non-adaptive Evolution
(chance)4. Available variations – most
come from using a current gene in a new way.
Summary
Know the difference between a species and a population.
Know that the unit of evolution is the population and not the individual.
Summary
Know the H-W equations and how to use them in calculations.
Know the H-W assumptions and what happens if each is violated.
Summary
Identify various means to introduce genetic variation into populations.
Know the various types of natural selection.
Summary
Darwin's ideas now a "Theory”. Predictions of a Theory are
tested by experiments and observations.
Be familiar with the pre-Darwin views of evolution.