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.