Chapter 17

Population Genetics

Genes & Variation

What were the two big gaps in Darwin’s theory?

1. He had no idea how heritable traits were passed on to the next generation.

2. He had no idea how the variation within a population appeared.

The connection between Mendel & Darwin wasn’t made until the 1930s.

Charles Darwin

Genes & Variation

Key Ideas:

1. Natural selection acts on phenotypes, not genotypes.

- Natural selection acts on the whole organism, not a single a gene.

2. A single gene pair could affect the phenotype to the point that the organism is no longer fit.

Genes & Variation

What is a population?

A group of individuals of the same species that live in the same area & interbreed.

What is a gene pool?

All of the genes (alleles) in a population.

Population of wild pigs

Gene Pool

Genes & VariationWhat is relative frequency (allele

frequency)?

1.The number of times an allele occurs in the gene pool

2.Often expressed as a percentage or a decimal.

3.Example: 100 alleles in the pool. 75 dominant alleles has a frequency of .75 (75/100).

Allele frequency has nothing to do with whether the allele is dominant or recessive.

Genes & Variation

What is evolution in genetic terms?

Any change in the relative frequencies of alleles within a population over time.

Genes & VariationWhat are the sources of genetic/heritable variation?

1. Mutation – any change in DNA base sequence. Mutations within the germ line (eggs & sperm) are the ones that can be passed on to the next generation.

2. Genetic Recombination – This is the gene shuffling that occurs during meiosis.

- independent assortment- crossing-over- some alleles from Dad & some from Mom

3. Lateral Gene Transfer – Transformation in bacteria is a form of this. Many bacteria pick up antibiotic resistance from lateral gene transfer.

Genes & Variation

What is the relationship between genotype & phenotype?

The genotype determines the phenotype.

Genes & Variation

What is a single-gene trait?1. A trait determined by a single gene that

has two alleles.2. There can only be two or three different

phenotypes.2 = complete dominance3 = incomplete

or codominance

Genes & Variation

What is a polygenic trait?

1. A trait controlled by two or more genes.

2. Produces many possible phenotypes.

3. The range of phenotypes typically creates a bell-shaped curve (normal distribution).

4. Human height & skin color are two examples of a polygenic trait.

Evolution as Genetic ChangeDoes natural selection act directly on genes?

No

Why?

Natural selection works

directly on the entire

organism.

What is an adaptation?

Genetically controlled

trait that increases fitness.

Evolution as Genetic Change

Natural Selection on single-gene traits can lead to changes in allele frequencies, and thus evolution.

Evolution as Genetic Change

Natural Selection on Polygenic Traits:

1. Phenotype range creates a bell-shaped curve.

2. Fitness can vary from one end of the curve to the other end.

Evolution as Genetic Change

There are three ways in which natural selection can affect phenotype distribution:

1. Directional Selection

2. Stabilizing Selection

3. Disruptive Selection

Evolution as Genetic Change

What is directional selection?

1. One end of the distribution curve has higher fitness.

2. Selection against one of the extremes.

3. The range of phenotypes will shift.

Evolution as Genetic Change

What is stabilizing selection?

1. The individuals in the center of the curve has higher fitness.

2. Selection against the extreme phenotypes at both ends of the curve.

Evolution as Genetic Change

What is disruptive (diversifying) selection?

1. Both of the extreme phenotypes have higher fitness.

2. The average phenotype is selected against.

Evolution as Genetic Change

What is genetic drift?

The random change in allele frequency within a small population.

Evolution as Genetic Change

What is the founder effect?

A change in allele frequencies as a result of migration.

Evolution as Genetic Change

What is the Hardy-Weinberg Principle?

1. It is a model in which no evolution occurs.

2. No evolution = genetic equilibrium

- no change in allele frequencies

3. This never really occurs in nature, but it helps science understand how evolution occurs.

Evolution as Genetic ChangeWhat are the five conditions for genetic equilibrium?

1. Large Population (no genetic drift)

2. Random Mating (no sexual selection)

3. No Immigration or Emigration

(no gene flow)

4. No Mutations (no new alleles)

5. No Natural Selection (all traits aid fitness)

Evolution as Genetic Change

How is the Hardy-Weinberg Principle expressed mathematically?p2 + 2pq + q2 = 1

p = frequency of one alleleq = frequency of the other allele

p2 = frequency of homozygous dominant2pq = frequency of heterozygousq2 = frequency of homozygous recessive

p + q = 1

This formula can be used to calculate changes in allele frequencies.

Demo Question

Speciation

What is a species?

A group of organisms that breed with each other and produce fertile offspring.

What is speciation?

1. The process that creates new species.

2. The key part of the process is reproductive isolation.

Speciation

What are the three types of reproductive isolation?

1. Behavioral Isolation

2. Geographical Isolation

3. Temporal Isolation

Speciation

What is behavioral isolation?

Mating rituals and/or other strategies keep populations from interbreeding.

Speciation

What is geographic isolation?

A geographic barrier keeps populations from interbreeding.

Speciation

What is temporal isolation?

The populations don’t mate at the same time.

Speciation

Speciation in Darwin’s Finches:1. Founders arrive on the Galapagos2. Separation of populations3. Changes in the gene pools of each

population4. Reproductive Isolation5. Ecological Competition6. Continued Evolution

Section 19-3: Early Earth’s History

Earth’s Early History

Formation of the Earth

• Geologic evidence shows Earth = 4.6 Billion years old

• Not “born” in a single event; cosmic collisions attracted & accumulated elements; arranged by density

• Early atmosphere = hydrogen cyanide, CO2, CO, N2, hydrogen sulfide, H2O vapor

Evolution of Life Concept MapEvolution of Life

Early Earth was hot; atmosphere contained poisonous gases. (4.6 BYA)

Earth cooled and oceans condensed. (3.8 BYA)

Simple organic molecules may have formed in the oceans..

Small sequences of RNA may have formed and replicated.

First prokaryotes may have formed when RNA or DNA was enclosed in microspheres.

Later prokaryotes were photosynthetic and produced oxygen.

An oxygenated atmosphere capped by the ozone layer protected Earth.

First eukaryotes may have been communities of prokaryotes.

Multicellular eukaryotes evolved.

Sexual reproduction increased genetic variability, hastening evolution.

Earth’s Early History

The First Organic Molecules• Stanley Miller & Harold Urey set up an

experiment to simulate early Earth conditions to see how organic molecules (building blocks of life) formed.

• Filled sterile flask with a mixture of gases found in early atmosphere; sparked with electricity to simulate lightning

• Results: amino acids (building blocks of protein formed); Suggests how mixtures necessary for life could have arisen from compounds present on primitive Earth.

Mixture of gases simulating atmospheres of early Earth

Spark simulating lightning storms

Condensation chamber

Cold water cools chamber, causing droplets to form

Water vapor

Liquid containing amino acids and other organic compounds

Miller-Urey Experiment

Earth’s Early History

The Puzzle of Life’s Origin: How might cells have arisen?

• Under certain conditions, large organic molecules can form tiny bubbles called proteinoid microspheres which have some cellular characteristics:

1. Selectively permeable membranes

2. Simple means to store/release energy

The Puzzle of Life’s Origins

Evolution of RNA & DNA (See Fig 17-10, pg 425)

• Scientists don’t know how these molecules evolved, but under certain conditions, RNA can help DNA replicate

• Experiments show that small sequences of RNA could have formed & replicated on their own in the early Earth conditions, so scientists think RNA evolved before DNA

Evolution of Prokaryotes & Free Oxygen

1. Microfossils, or microscopic fossils of unicellular prokaryotes that resemble modern bacteria have been found in rocks > 3.5 billion years old!

• They were anaerobic since Earth’s 1st atmosphere contained little oxygen

• These photosynthetic bacteria, called cyanobacteria, evolved in shallow seas; they released O2 which accumulated in the atmosphere & removed iron from the oceans

• O2 drove some life forms to extinction, while new ones evolved

Photosyntheis Equation

Recall: Prokaryotes

• Single-celled• Lack membrane-

bound organelles

• Lack nucleus but have DNA

• Also called “bacteria”

Recall: Eukaryotes

•Larger•Complex internal membranes•DNA enclosed within a nucleus •Most have mitochondria•Some have chloroplasts

Evolution Eukaryotic Cells

About 2 billion years ago, prokaryotes began evolving internal cell membranes (ancestor of eukaryotes)

Origin of Mitochondrion & Chloroplasts (Endosymbiotic Theory)

Endosymbiotic Theory – American biologist Lynn Margulis proposed this theory that states:

1.Mitochondria are descendants of symbiotic aerobic bacteria

2.Chloroplasts are descendents of symbiotic, photosynthetic bacteria

Origin of Mitochondrion & Chloroplasts (Endosymbiotic Theory)

3. Bacteria entered larger cells as parasites/undigested prey; they began to live inside the host where they performed either cellular respiration (mitochondria) or photoysnthesis (chloroplasts)

4. Explains why mitochondria & chloroplasts have their own DNA

Aerobic bacteria

Ancient Prokaryotes

Ancient Anaerobic Prokaryote

Primitive Aerobic Eukaryote

Primitive Photosynthetic Eukaryote

Chloroplast

Photosynthetic bacteria

Nuclear envelope evolving Mitochondrion

Plants and plantlike protists

Animals, fungi, and non-plantlike protists

Endosymbiotic Theory

Observations Supporting the Endosymbiotic Theory

1. Size & Structure – mitochondria are about the same size as most bacteria & its membrane is like that of aerobic bacteria

2. Genetic material- mitochondria & chloroplasts have circular DNA similar to bacterial DNA & genes different from nuclear DNA

3. Ribosomes in mitochondria & chloroplasts have similar size & structure of bacterial DNA

4. Reproduction- Like bacteria, mitochondria & chloroplasts reproduce by binary fission; Takes place independently of cell cycle of the host cell