Chapter 16Population Genetics

OBJECTIVES: • Relate the study of genetics to that of

population genetics and discuss factors that can affect gene-pool equilibrium

• Explain the Hardy-Weinberg model• Discuss evolution through natural selection• Explain genetic drift and contrast its effects

on large and small populations.• Discuss the role of quantitative traits in

microevolution.

Population Genetics

Recall: variation among individuals allows populations to adapt to new environmental conditions or to be selectively bred for desirable traits.

2 Types of Evolution

• Microevolution: change within a species. Occurs over dozens or hundreds of generations*

• Macroevolution: Much longer time period. Results in a new species

A More Precise Definition

Microevolution is a change in the genetic composition of

populations.

Studied by population geneticists.

Gene poolAll alleles in a population

of organisms.

Allele frequency

Percentage of a particular allele in one population.

Ex: In a population of pea plants that are all homozygous for purple flowers, allele freq.

for purple flowers is 100%

A change in an allele frequency is an indication of evolutionary

change.

Allele Frequencies within Beetle Population

Polymorphic Populations

• Have 2 or more alleles for a particular trait.

• Ex: humans are polymorphic for blood type.

• Ex: apple trees are polymorphic for fruit color.

Hidden Genetic Variations

• Mutation in non-coding regions of DNA

• Silent mutations code for the same amino acid

• Unseen polymorphisms

The Hardy-Weinberg ModelAn idealized mathematical model of

gene pools.

•Mathematician Godfrey H. Hardy

•Physicist Wilhelm Weinberg

Use allele freq. to calculate genotype freq.

Allele frequency p and q,

p + q = 1

In the next generation:

p2 + 2pq + q2 = 1

Homozygous p: p2

Heterozygous: 2pq

Homozygous q: q2

Hardy-Weinberg Equilibrium

Allele and genotype frequency will stay constant in the absence of

disturbing influences.

p2 + 2pq + q2 = 1

Hardy-Weinberg Model

Makes some assumptions about the population. No “disturbing influences.”

• random mating • no mutation (the alleles don't change) • no migration or emigration • infinitely large population size• no selective pressure for or against

any traits.

Hardy-Weinberg Equilibrium

Predictions of the model1. Predicts allelic and genotypic

frequencies 2. Genetic variation remains in the

population (unless selective pressures)

Good news for Darwin! (Assumed blending inheritance.)

Genetic equilibrium: a constant state of allele

frequencyThe following conditions must be met in order that genetic equilibrium not be

disrupted.1)No natural selection

2)Random mating

3)No migration

4)No mutation

5)Large population sizeNote: does not occur in nature.

So, why use Hardy-Weinberg model?

Quick Quiz

1. Would a change in allele frequencies be more likely to produce microevolution or macroevolution?

2. What is the difference between gene pools and allele frequencies?

3. Why does the concept of gene pools apply to populations but not to species?

A Normal Distribution results

from Stabilizing Selection: Natural

selection that favors average individuals in a

population.

Normal Distribution

IQ

Directional Selection in Peacocks

Females only mate with males with the largest tails. Over time, tails have gotten continually larger due to this selective pressure.

Peppered Moths: color variations

1850: allele frequency was 95% light, 5%

dark

1900: allele frequency 95% dark, 5% light

Disruptive Selection: Natural

selection that favors either extreme trait.

Disruptive Selection in Snails

Limpets with light-colored shells blend in with light rocks and sand. Dark shells blend in with dark rocks.

Limpets with medium-colored shells are easily seen on both rocks and eaten by birds.

Disruptive Selection in Spiders

• When spiders are small, they are not as easily seen by predators.

• When spiders are large, they are often too big to be eaten.

• Spiders in the middle are the most vulnerable to predators.

Other Factors Affecting Gene Pools

• Gene Flow: Migration to a new population, organism may bring new alleles with it.

• Mutation: If beneficial, will be favored by natural selection and gradually increase in frequency

• Genetic Drift: Spontaneous changes in allele frequencies. Small populations only.

Other Factors Affecting Gene Pools

• Inbreeding: Gradual increase of homozygotes. Ex: California Condor

• Population bottleneck: Population size reduced for a few generations. Inbreeding results. Ex: Buffalo in the 1800’s.

• Inbreeding increases frequency of harmful recessive alleles. Leads to Inbreeding Depression: reduced fertility and survival.

Did you know…

The average human is estimated to have 7 alleles that would be lethal if they were homozygous? In inbred populations, inheriting 2 of these alleles is more likely.

(BSCS Biology: A Molecular Approach)

Population Genetics

CONCEPT REVIEW:

• Evolution results from a disruption in genetic equilibrium.

• The normal distribution of variations in a population can be changed by natural selection, gene flow, mutations, and genetic drift

Chapter 18Diversity and Variation

Outcomes• Explain homology and give examples

of homologous structure• Describe how the general

characteristics of the 5 kingdoms differ.

The 5 (or 6) Kingdoms

• Archaebacteria• Eubacteria• Protista• Fungi• Plantae• Animalia

Bacteria/monera

Bacteria

• Prokaryotes• First organisms to evolve

Protista

• Earliest eukaryotes. • Usually single celled. • No organ systems• Nucleus developed• Mitochondria, flagellates, and

plastids became incorporated.• Ex: amoeba, paramecium, algae

Fungi

• Usually multicellular (except yeast)• Eukaryotic• Heterotrophs• Evolved from fungus-like protists

(slime molds)• Ex: mushroom, mold, yeast

Plantae

• Multicellular, with complex body systems (roots, stem, leaves)

• Autotrophs• Eukaryotes• Evolved from photosynthetic

bacteria• Ex: Flowering plants, Conifer,

Mosses, Ferns

Animalia

• Multicellular with complex systems• Heterotrophic• Eukaryotic• Ex: Fish, Amphibian, Reptile, Bird,

Mammal

There are still lobe-finned fish today called mudskippers. 34 species have

been identified. Unlike those we evolved from, most of today’s species have only 2 appendages (front lobe-

fins)

Fish to Amphibians

Reptiles to Birds

Reptiles to Birds

Evolution

of

Mammals

Eventually, some mammals returned to the

water.

• Today’s whales had an ancestor similar to a wolf.

Chapter 19Changes in Species

Outcomes:• Cite evidence from fossils, ecology,

and homologies that support the theory of evolution

• Discuss the genetic and molecular evidence for evolution

• Discuss isolation mechanisms that can cause speciation

• Describe the patterns in evolution such as punctuated equilibrium

Fossils

as evidence of evolution

Fossils are the preserved remains or imprints of ancient organisms.

Fossils: the only evidence we have that tells us directly about life in the

past.

This extinct dragonfly had a wingspan of 3 feet!

Life first appeared on earth more than 3 billion years ago.

Fossils of algae and diatoms

Millions of now extinct creatures lived on earth before humans came along.

Some fossils are the actual preserved remains of the organism

Soft-Tissue Fossils

Ice in the Arctic has preserved some fossils for 1,000s of years.

In 1999 a wooly mammoth was discovered intact.

Can it be revived by crossing with an elephant? Different #s of chromosomes (58 and 56), but 95% similar DNA.

Scientists have found 250,000 species of extinct orgnisms

Estimate that only 1 in 10,000 have been found.

Evolution of the Horse

In these pictures, there appears to be a straight line progression from the first horse ancetor to the modern horse species.

Such a progression implies an evolutionary

goal, since there is a trend toward larger body

size and fewer toes.

Evolution of the Horse

However, evolution rarely follows a straight line to a

goal.

Remember,

There are no goals in evolution !

1) homologous2) vestigial 3) analogous

Evidence for Evolution:

Body Structures

Homologous Structures: Traits that are similar in different species because they share a common ancestor.

Note how the bones have adapted to different niches

This is evidence of a common ancestor.

Vestigal Structures: No longer used.

The Human “Tailbone”

This is evidence that humans evolved from an ancestor that

had a tail.

appendix

Vestigal Organ: human appendix

A whale has a pelvic bone too,

and tiny leg bones.

Analogous Structures: structures that are similar in function but are not inherited from a common ancestor.

NOTE: Analogous structures indicate that organisms are not related.

Embryology is also used as Evidence of Evolution: Similar development of the embryo is

evidence of a common ancestor

All three embryos have “gill pouches” in the folds of the neck. All three have

tails.

Perhaps the clearest biochemical

evidence of the common origin of living things is the genetic code. The

same nitrogen bases of adenine, thymine,

guanine, and cytosine exist in

every form of life.

In addition, the genetic code itself – the codons for the amino

acids – is almost universal.

The genetic code is the same in every known organism. Every organism uses the same base

codes for amino acids

Degree of RelatednessCan be determined by

• Amino acid sequence• Homologous proteins• Nucleotide sequence• Homologous genes.

More Genetic Evidence: Pseudogenes

Gene duplication: produces multiple copies of DNA sequences.

Pseudogenes: gene copies that don’t function, so aren’t subject to natural selection.

Mutations occur unchecked. According to natural selection, these non-

coding sections should accumulate mutations faster than functional genes – and they do!

Amino Acid Sequence• Can be used to determine

relatedness

How fast do evolutionary changes take

place?

Based upon Darwin’s theory it has long been

believed that evolutionary changes were slow and gradual: Gradualism.

1972: scientists Stephen Jay Gould and Niles Eldridge advanced a different explanation

about the rate of evolution called

Punctuated Equilibrium

•Punctuated Equilibrium:

populations remain genetically stable for long periods of time, interrupted by brief

periods of rapid change.

•sudden changes in the environment

•increased mutation rate.

stasis

Speciation is the evolution of one or more species from a

single common ancestor species.

Patterns of Evolution

How do species remain separate?

(1) Potential mates do not meet. Grizzly and Polar bears.

(2) Potential mates meet but do not breed. Nocturnal and diurnal birds. Leopard frog populations that breed during different months.

(3) Potential mates meet and breed, but do not produce viable offspring.

Divergent Evolution:

Occurs when isolated populations of a species evolve independently.

Divergent evolution is responsible for polar bears. A northern population of grizzly bears became isolated from

others of the species and adapted to the Arctic regions.

Grizzly Bear Polar Bear

Coevolution:

Interactions with other organisms effect

evolution.

Coevolution is responsible for mimicry

one of the most fascinating topics in biological evolution.

CoevolutionThe pronuba moth and the

yucca flower

Depend on one another for

reproduction.

CoevolutionThe Orchid Fly

Coevolution: Cactus and Galapagos Tortoise

Saddleback shell

Cleaner Wrasse

Sabre Toothed Blennie

Adaptive Radiation: Many diversely related species from one common ancestor

Polyploidy in PlantsIf plants inherit an extra chromosome

from parents, they are said to be polyploidy.

Often, these plants can only mate with other polyploids, or use asexual reproduction.

Convergent Evolution:

Unrelated species display similar features. No common ancestor.

How does this happen?

Convergent Evolution

Disruptive markings make it hard for predators to single out

a victim.

Similar niches usually contain similar evolutionary pressures (selective

pressures).

If ancient niches were similar to modern niches then organisms today could resemble organisms now long

extinct.

Similar niches found on different continents can produce organisms

that are fairly similar.

Modern dolphins and prehistoric ichthyeosaurs (marine reptiles) look very

similar due to the types of niche they inhabited.

. .

..

.

Analogous structures can be caused by niches. Similar niches create similar body forms.

.

..

Note how similar niches created long necks in both sauropods and giraffes.

.

Similar foods (similar niches) create similar teeth in herbivores.

Similar foods (similar niches) create similar teeth in carnivores.

Quick Quiz

1. What are isolating mechanisms, how do they operate?

2. What is polyploidy? What is its connection to speciation?

3. Explain the statement: “Populations evolve, not individuals within a population.”

Origin of Species

Concept Review:

• New species can develop when populations become separated and isolated.

• Similar traits can develop in unrelated species occupying comparable niches.

• Interactions with other organisms affect evolution.• Many diverse species can evolve from one ancestral species.

Chapter 17The Origin of Life

Objectives:• Describe the origin of the universe and

probable conditions on early Earth• Evaluate hypotheses about the origin

of life and identify the probable characteristics of early life-forms

• Distinguish between chemical and biological evolution

• Describe the fossil record for prokaryotes and eukaryotes.

The Origin of Life

•Can’t be observed

•Inferences

•Probably needed energy, C, H, O, N, and lots of time.

The Big BangThe Expanding Universe

• Edwin Hubble, 1920. The Hubble Telescope was named for him.

• Wavelengths of light can be measured, spread out as objects move farther away

• The rate of expansion is known, used to calculate the time when universe was tiny.

The Big Bang

• 15 billion years ago• Universe was condensed into a tiny

“singularity”• An infinitely hot, dense mass.• When it exploded, The Big Bang,

hurled energy and mass into space.• What was there before the Big

Bang?

Early Earth

• 4.6 Billion years ago• Meteorites and the oldest rocks

from the Moon confirm this

History of the EarthEra Million Years Ago First evidence of

Cenozoic 7-565

Human-like apesPrimates

Mesozoic 140220235

Flowering PlantsMammalsDinosaurs

Paleozoic 300360400430520

ReptilesAmphibiansLand AnimalsLand PlantsVertebrates

Precambrian 210025003500

EukaryotesFree O2 in atmosphere by prok.Prokaryotes

The Early Atmosphere

• Gasses from volcanoes: N2, CO2, H2O, H2, CO, probably methane (CH4)

• No O2

• No ozone layer – intense radiation, extreme temperature changes.

The First Living Things

• Anaerobic Organisms• 1 billion years later, some

photosynthetic organisms began releasing free oxygen.

How did those living things come to be?

3 possible explanations:1. Life originated on some other planet,

then traveled to Earth through space.2. Life originated by unknown means on

Earth3. Life evolved from nonliving

substances through interaction with their environment.

2 of these cannot be tested, only one can be stated as a hypothesis. Which one?

Chemical Evolution Life evolved from nonliving

substances • Small, inorganic molecules were

heated via cosmic radiation, volcanoes, radioactivity and lightning.

• Gasses in the atmosphere react, forming organic compounds

• Compounds accumulate in oceans, forming a hot soup

• Life evolved by chemical reactions and transformations in the organic soup

Chemical EvolutionThe oceans became “soup” of

organic compounds

The Heterotroph Hypothesis

The first living things were probably heterotrophs that fed on organic compounds in the ocean.

With no competition, autotrophs would not have an advantage over heterotrophs

The Heterotroph Hypothesis(or Oparin-Haldane

hypothesis)3 Requirements1. There had to be a supply of

organic molecules, produced by nonbiological processes.

2. Some processes had to assemble those small molecules into polymers such as nucleic acids and proteins.

3. Other processes had to organize the polymers into a system that could replicate itself

Evidence for the Heterotroph Hypothesis

Stanley Miller’s experiment in 1950. Early Earth conditions were simulated in an airtight apparatus.

• Water vapor• Lightning• CH4, NH3, H2O, H2After circulating for a week, new

compounds were found in the water, including some amino acids.

More recent experiments have produced the 5 bases of DNA and RNA too.

Other Sources of Organic Molecules

• Meteorites from space – amino acids have been discovered

• Volcanic vents – release gases at high temperatures

Remember 1st requirement: There had to be a supply of organic molecules, produced by nonbiological processes.

The rest of the hypothesis:

#2. Some processes had to assemble those small molecules into polymers such as nucleic acids and proteins.

Clay – repeating crystalline structure that could attract then connect monomers

#3. Other processes had to organize the polymers into a system that could replicate itself

RNA – Can form spontaneously. Can reproduce itself. Probably came before DNA

Biological Evolution• When did chemical evolution become

biological evolution?• When organic molecules became living things• Self reproduction, mutation that can be

inherited, and natural selection = life• Cells? Today all living things are made of cells.• It is unknown when/how cell membranes

developed.• The first living things may have had

membranes, or not. They may have been DNA, RNA, proteins…who knows?

Prokaryotic Fossils

3.5 Billion years old. Single-celled prokaryotes.

Suggest life was already diverse and thriving.

Probably methanogens:Use CO2 to oxidize hydrogen

Fossils of Eukaryotes• 2.1 Billion years old• Lynn Margulis of UMass, Amherst

developed the endosymbiont hypothesis: Chloroplasts and mitochondria were once free-living prokaryotes. Photosynthesis and respiration of the small cells have benefited the host cells.

• Mitochondria probably evolved from aerobic, heterotrophic purple bacteria.

• Plastids probably evolved from autotrophic cyanobacteria.

Endosymbiont Hypothesis

The evidence: Both have their own DNA and ribosomes, which are similar to other bacteria. Also both have a double membrane; their outer membranes may have evolved from vacuoles when host cells took them in.

Evolution of Eukaryotes

Quick Quiz

1. Why is it believed that methanogens might have been the first organisms?

2. How might mitochondria and plastids have originated?

3. What evidence supports the idea that mitochondria and plastids originated from free-living prokaryotes?

Chapter 20Human Evolution

Outcomes:• Describe how modern humans

differ from other primates• Evaluate the techniques used to

study evolutionary relationships in humans

• Compare early hominids with Homo erectus and Homo sapiens

• Give reasons for the difference in the gene pools of modern human populations.

What are Primates?

• Opposable Thumbs• Fingers and toes have nails, not

claws• Flexible shoulder and hip joints• Binocular, 3-D vision for accurate

depth perception• Color vision

Humans vs. Other Primates

• Bipedal: Hands are free• Have a hippocampus: brain region

for memory and learning. Absent in most primates (not chimps and gorillas)

• More fine motor control in hands• Language, well developed vocal

chords

Molecular SimilaritiesHuman vs. Chimpanzee

Protein Number of amino acids

Amino-acid difference

Hemoglobin 579 1

Myoglobin 153 1

Cytochrome C 104 0

Serum Albumin 580 7

Molecular Similarities Among Primates

Species compared

Difference in DNA sequence (%)

Estimated time since divergence

Chimpanzee vs. Bonobo

0.7 3 million years

Human vs. Chimpanzee

1.6 7 million years

Human vs. Gorilla

2.3 10 million years

Gorilla vs. Chimpanzee

2.3 10 million years

Early Hominids

• Lived in Africa• Genera in Hominid family: Homo

and Australopithecus (larger teeth, smaller brains).

The Hominids

Hominids – The Human-like Primates

Comparing SkeletonsSkeletal fossils – clues to how

organism moves, eats, behaves.Footprint fossils – How organism

moved, how heavy it was.Who was Lucy?

Comparing SkeletonsSkeletal fossils – clues to how

organism moves, eats, behaves.Footprint fossils – How organism

moved, how heavy it was.Who was Lucy?

Australopithecus afarensis found in Ethiopia, 1974

The First Humans

Hominids

“Hence, both in space and time, we seem to be brought somewhat near to

that great fact - that mystery of mysteries - the first appearance of new beings on this

earth.”Charles Darwin