CH 17

POPULATIONS GENETICS AND

SPECIATION

IMPACTS, ISSUESRISE OF THE SUPER RATS

WARFARIN RATS

Killed By Warfarin

SurvivedWhy?

ResistanceImmunityThe MOST FIT

ANTIBIOTIC RESISTANCE

Left to FOUND new

population

Killed by Antibiotic

What the rat example illustrates is a concept that turns up again and again in nature and provides proof of evolution.

Many populations contain the traits necessary to survive when the environment turns on them.

Which traits will be good or more common normally can’t be predicted but through variation populations can survive and evolve.

HOW VARIATION ARISES FROM SIMILARITIES…

Evolution is predicated on the concept that genetic variation leads to the environment choosing which version of a trait is best.

What mechanism(s) allow for variation?There has to be a way for nature to allow for the creation of new alleles.

Evolution starts with mutations in individuals; mutation is the mechanism for the creation of new alleles.

The penetrance of this mutation is completely dependent on SEXUAL reproduction.

I. INDIVIDUALS DON’T EVOLVE, POPULATIONS DO

Population Interbreeding individuals of the same species in the

same area.All individuals of a species have to share certain

traitsBut individuals of a population vary in the details of

these shared traits.These variations of course are because of alternative

versions of the alleles that cause these shared traits.

VARIATION IN POPULATIONS

Mutations

Mutations

Where does VARIATION come from?

Mutations

Mutations

Mutations

Mutations

II. HOW TO DETECT EVOLUTION: VARIATION CAN BE CALCULATED

Variation is distributed within a population.This fact is a way evolution can be studied.A distribution is an overview of the relative

frequency and range of a set of values.Often, some values in a range are more

common than others. A normal distribution, or bell curve, is

one that tends to cluster around an average value in the center of the range.

This sets the range of what’s normal and standard for a population.

NORMAL DISTRIBUTIONMean = averageMedian = middle valueMode = the number most repeated

THE GENE POOL

The variation is founded in alleles Alleles

• Different forms of the same gene

• Determines genotype and phenotype

Gene pool

• All alleles found in one population Within the gene pool, variation is due

to various alleles. Mutations are responsible for the

various alleles.The Black Jaguar6% of the South American population

The dominant phenotype

VARIATION & INHERITANCE

You have to also consider recombination in changes in genetics…

Variation is constantly changing because of natural selection but also genetic shuffling, through new combinations of alleles.

Proving evolution is partially achieved through investigating the change in allelic frequency over several generations.

The change is measured against the population in genetic equilibrium.

Allele frequencies• Relative abundance of alleles of a given gene in a

population• Represented as percentages, much like the %

composition of compounds. Natural populations are never in genetic

equilibrium• Genetic equilibrium = the allele frequency stays the

same.• A theoretical state which occurs when a population is

not evolving

VARIATION & ALLELES ARE TRACKED BY STUDYING ALLELE

FREQUENCIES

Researchers know whether or not a population is evolving by tracking deviations from a baseline of genetic equilibrium.

Five conditions required for a stable gene pool:1. Mutations do not occur2. Population is infinitely large3. No gene flow4. Random mating5. All individuals survive and reproduce equally

This doesn’t happen but the rate these change will affect the rate of evolution.

III. GENETIC EQUILIBRIUM

GENOTYPE FREQUENCIES VS. ALLELE FREQUENCIES

The thing you need to know is that genotypic and allelic frequencies always add up to 1.

Genotype Frequency(frequency of EE) + (frequency of Ee) + (frequency of ee) = 1Allele Frequency: (frequency of E) + (frequency of e) = 1

Genetic equilibrium is affected by several processes that alter allelic frequency.

This is how evolution and speciation happens.

Tracking evolution is studied by and defined by changes in allele frequencies over time.

MICROEVOLUTION

Determining Genetic Equilibrium… the BASELINE

The Hardy-Weinberg formula can be used to determine if a population is in genetic equilibrium

p2(AA) + 2pq (Aa) +q2(aa) = 1.0

The frequency of the dominant allele (A) plus the recessive allele (a) equals 1.0

p + q = 1.0

THE HARDY-WEINBERG FORMULA

Homozygous dominant

Homozygous recessive

Heterozyg

ote

The Hardy-Weinberg principle describes a population that is not evolving.

If a population does not meet the criteria of the Hardy-Weinberg principle, it can be concluded that the population is evolving.

You will learn more about the Hardy-Weinberg formula soon…

THE HARDY-WEINBERG PRINCIPLE

A POPULATION IN EQUILIBRIUM

Fig. 18-3a, p. 280

Finding out whether a population is evolving. The frequencies of wing-color alleles among all of the individuals in this hypothetical population of morpho butterflies are not changing; thus, the population is not evolving.

Fig. 18-3b, p. 280

490 AA butterflies dark-blue wings

490 AA butterflies dark-blue wings

490 AA butterflies dark-blue wings

420 Aa butterflies medium-blue wings

420 Aa butterflies medium-blue wings

420 Aa butterflies medium-blue wings

90 aa butterflies white wings

90 aa butterflies white wings

90 aa butterflies white wings

Starting Population Next Generation Next Generation

Just as the Hardy-Weinberg principle allows us to see a population that’s not evolving, so we can measure changes, these forces act against equilibrium:

1. Gene Flow2. Genetic Drift3. Mutation4. Non-random Mating5. Natural Selection

IV. MICROEVOLUTIONFORCES OF GENETIC CHANGE

Gene flowPhysical movement of alleles caused by individuals moving into and away from populations Immigration, etc.

Tends to counter the evolutionary effects of mutation, natural selection, and genetic drift on a population

Example: Movement of acorns by blue jays

1. GENE FLOW

GENE FLOW BETWEEN OAK POPULATIONS

Genetic driftThe random drifting of allele frequenciesVery much dependent on population sizeLike a Pendulum…

Leads to…Fixation has occurred when all

individuals in a population are homozygous for one allele

2. GENETIC DRIFT—THE CHANCE CHANGES

GENETIC DRIFT AND POPULATION SIZE

GENETIC DRIFT AND POPULATION SIZE

BottleneckA drastic reduction in population size brought about by severe pressure

After a bottleneck, genetic drift is pronounced when a few individuals rebuild a population, fumbling to figure out beneficial traits.

Example: Northern elephant sealsExample: The Great Dinosaur Extinction

GENETIC DRIFT & BOTTLENECKS

AB

B

A

A

C

B

A

A

A

A

A

A

B

B

C

B

B

OriginalPopulation

Catastrophic Event

New PopulationDynamics

A

AA

B

B

A

A

C

B

A

A

A

A

A

A

B

C

Founder effectGenetic drift is pronounced when a few individuals start a new population

(Often in conjunction with bottlenecks)

InbreedingBreeding or mating between close relatives who share a large number of alleles

Example: Old Order Amish in Lancaster County, Pennsylvania (Ellis-van Creveld syndrome)

GENETIC DRIFT & THE FOUNDER EFFECT

Mutations are the source of new alleles that give rise to differences in details of shared traits

Lethal mutations usually result in deathDecrease fitness but these rarely survive the gene pool.

Neutral mutations have no effect on survival or reproduction

Often considered “silent” mutations. In humans, the average person undergoes 50-100 mutations in their lifetime.

Less than 3% have any consequence.Remember, ~95% of our genome is “junk” DNA

Beneficial mutations convey an advantage.Rare but does occur… has to.

3. MUTATION REVISITED

4. NON-RANDOM

MATING = SEXUAL

SELECTION

With sexual selection, some version of a trait gives an individual an advantage over others in attracting mates

Distinct male and female phenotypes (sexual dimorphism) is one outcome of sexual selection

SEXUAL SELECTION

MATING DANCE OF BIRD OF PARADISE

http://youtu.be/nS1tEnfkk6M

SEXUAL SELECTION

sexual dimorphism

Females are attracted to males with larger, dark manes

Correlation with higher testosterone levels better nutrition & health more muscle & aggression better sperm count / fertility more successful young

But imposes a cost to male It’s HOT! Is it worth it??

THE LION’S MANE…

The Golden RatioSymmetry

“SEXY” = SUBCONSCIOUS FITNESS MARKERS

This contributes to non-random mating:Humans have preferences, just like all other sexual organisms.

Balanced polymorphismA state in which natural selection maintains two or more alleles at relatively high frequencies

Occurs when environmental conditions favor heterozygotes

Example: SexesExample: Sickle cell anemia and malaria

HbA/HbS heterozygotes survive malaria more often than people who make only normal hemoglobin

BALANCED POLYMORPHISM

Natural selectionThe variable survival and reproduction among individuals of a population that vary in details of their shared traits.

This is a driving force of evolution.Occurs in recognizable patterns depending on the organisms and their environment.

5. NATURAL SELECTION REVISITED

Natural Selection Acts in 3 Major Ways:All Populations Have Variation

Sometimes hard to detect, not so in humansIndividuals Tend To Produce Too Many

Offspring Individuals in a population will always have a hard time surviving

All Populations Depend on the Reproduction of IndividualsFitness = the reproductive success of grandchildren.

NATURAL SELECTION

Fig. 18-4, p. 281

population before selection

directional selection

stabilizing selection

disruptive selection

V. Three Patterns of Natural SelectionNatural selection

Takes 3 forms:DirectionalStabilizingDisruptive

We’ll take a quick peak at all three types.

Directional selectionChanging environmental conditions, the selective PRESSURE put on by the environment, can shift allele frequencies in one consistent direction

Forms of traits at one end of a range of trait variation become more common in the population.

The other becomes less or drops out completely.

DIRECTIONAL SELECTION

Fig. 18-5a, p. 282

Directional selection. These bell-shaped curves signify a range of continuous variation in a butterfly wing-color trait. Red arrows indicate which forms are being selected against; green, forms that are being favored.

Fig. 18-5b, p. 282

Fig. 18-5c, p. 282

Once the norm…

Becomes this with directional selection

Casualties of Natural SelectionNow extinct…

Light color is adaptive in areas of low pollution; Dark color is adaptive in areas of high pollution

PREDATION AND PEPPERED MOTHS

Fig. 18-6a (1), p. 283

Natural selection of two forms of the same trait, body surface coloration, in two settings. (a) Light moths (Biston betularia) on a nonsooty tree trunk are hidden from predators. Dark ones stand out. (b) The dark color is more adaptive in places where soot darkens tree trunks.

Fig. 18-6a (2), p. 283

Fig. 18-6b (1), p. 283

Fig. 18-6b (2), p. 283

In rock-pocket mice, two alleles of a single gene control coat color.

Night-flying owls are the selective pressure that directionally shifts the allele frequency.

PREDATION AND ROCK-POCKET MICE

DIRECTIONAL SELECTION

IN ROCK-POCKET MICE

Fig. 18-7a, p. 283

Fig. 18-7b, p. 283

Visible evidence of directional selection in populations of rock pocket mice. (a) Rock pocket mice that have dark fur are more common in these areas of dark basalt rock. (b, c) The two color types of rock pocket mice, each posed on the dark and light rocks of the area.

A typical two-week course of antibiotics can exert selection pressure on over a

thousand generations of bacteria.If ANY remain, they do so because they may have a mutated gene that causes a resistance to these antibiotics.

Antibiotic resistant strains are now found everywhere, in hospitals and schools.

The rate of these superbugs are exacerbated by people not completing their prescriptions.

ANTIBIOTIC RESISTANT BACTERIA

Stabilizing selection Natural selection that favors an intermediate phenotype and eliminates extreme forms

Disruptive selection Natural selection that favors extreme forms of a trait and eliminates the intermediate forms

SELECTION AGAINST OR IN FAVOR OF EXTREME

PHENOTYPES

Time 2

Time 3Fig. 18-8, p. 284

Stepped Art

Stabilizing Selection

Range of values for the trait

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Time 1

STABILIZING SELECTION: BODY WEIGHT OF SOCIABLE

WEAVERS

Fig. 18-9a, p. 284

What is the optimal weight of a sociable weaver bird?

Fig. 18-9b, p. 284

Stabilizing selection in sociable weavers. Graph shows the number of birds (out of 977) that survived a breeding season. Figure It Out: What is the optimal weight of a sociable weaver bird?Answer: About 29 grams

DISRUPTIVE SELECTION

Fig. 18-10a, p. 285

Disruptive selection eliminates midrange forms of a trait, and maintains extreme forms.

Fig. 18-10b, p. 285

Fig. 18-10c, p. 285

DISRUPTIVE SELECTION: BILL SIZE IN AFRICAN FINCHES

In African seedcracker populations, birds with bills that are about 12 or 15 millimeters wide are favored. The difference is a result of competition for scarce food during dry seasons.But the middle phenotypes are not good at eating the food.

Fig. 18-11a, p. 285

lower bill 12 mm wide = the “splitter”

Fig. 18-11b, p. 285

lower bill 15 mm wide = the “crusher”

Natural selection theory helps explain diverse aspects of nature, including differences between males and females, and the relationship between sickle-cell anaemia and malaria.

MAINTAINING VARIATION

SICKLE CELL ANEMIA AND MALARIA

Fig. 18-13a, p. 287

Distribution of malaria cases reported in Africa, Asia, and the Middle East in the 1920s,

Fig. 18-13b, p. 287

Distribution (by percentage) of people that carry the sickle-cell allele.

Fig. 18-13b, p. 287

Notice the close correlation between the maps.

Individuals with SS (normal RBCs) get malaria and can die.Individuals with ss (sickle cell anemia) usually die early on.The hybrid (Ss) can survive with malaria and suffer little side affects associated with sickle cell anemia.

Fig. 18-13c, p. 287

Malaria and sickle-cell anemia. (c) Physician searching for mosquito larvae in Southeast Asia.

The data would indicate that sickle cell anemia and malaria respond to each other.Both alleles have success so natural selection favors a mix of both over one or the other.

Natural Selection supports the best traitsBut Natural Selection can be limited because:

Indirect Force: Natural selection doesn’t act on genes. Natural selection acts on unsuccessful physical traits… Not the genes that make them. Since traits (phenotypes) are determined by genes

(genotypes), sometimes these unsuccessful phenotypes can pop up if they are recessive.

Role of Mutation; Usually recessive alleles Recessive alleles can be hidden from targeting by natural

selection.

NATURAL SELECTION REVISITED

What is microevolution?How populations change due to changes in allele

frequencies.

Which type deals with allele frequencies that result from a population of rats that stow away on a barge that lands on an island?

Founder aff ect… genetic drift.

Some years black moths are favored, in other years white moths are favored. This form of natural selection is what?

Directional stabilization.

CONCEPT CHECK

VI. SPECIATION

Once Genetic Drift Ends…

SO WE’VE DISCUSSED THE MECHANISMS THAT CAUSE ALLELE

CHANGES

Now…Once gene flow ends, reproductive isolation occurs.

Once reproductive isolation occurs, speciation soon follows

SpeciationEvolutionary process by which new species form

Reproductive isolating mechanisms are always part of the process

Reproductive isolationThe end of gene exchange between populations

Beginning of speciation

REPRODUCTIVE ISOLATION

FOUR BUTTERFLIES, TWO SPECIES

Reproductive isolating mechanisms prevent interbreeding among groups. Heritable aspects of body form, function, or behavior

that arise as populations diverge… they start to change because of the different selection pressures of the different ecosystem and the traits available the founding population.

What are the isolating mechanisms?1. Mechanical2. Behavioral3. Hybridization 4. Allopatric5. Sympatric/ Polyploidy (genetic)6. Parapatric

REPRODUCTIVE ISOLATING MECHANISMS

1. MECHANICAL ISOLATION

2. BEHAVIORAL ISOLATION

Reduced hybrid viability (ligers, tigons) Extra or missing genes

Reduced hybrid fertility (mules) Robust but sterile offspring

Hybrid breakdown Lower fitness with successive generations

3. HYBRIDIZATION

Geographic Isolation= “Allopatric speciation”

A physical barrier arises and ends gene flow between populations

Genetic divergence results in speciationExample: llamas, vicunas, and camels

4. ALLOPATRIC SPECIATION

ALLOPATRIC SPECIATION

Winds or ocean currents carry a few individuals of mainland species to remote, isolated islands chains (archipelagos) such as Hawaii

Habitats and selection pressures that differ within and between the islands foster divergences that result in allopatric speciation

THE INVITING ARCHIPELAGOS

ALLOPATRIC SPECIATION ON AN ISOLATED ARCHIPELAGO

Fig. 18-21a, p. 293

B Later, a few individuals of a new species colonize nearby island 2. Speciation follows genetic divergence in the new habitat.

C Genetically different descendants of the ancestral species may colonize islands 3 and 4 or even invade island 1. Genetic divergence and speciation may follow.

A A few individuals of a mainland species reach isolated island 1. In the new habitat, populations of their descendants diverge, and speciation occurs.

Populations sometimes speciate even without a physical barrier that blocks gene flow. Sympatric speciationParapatric speciation

OTHER SPECIATION MODELS

In sympatric speciation, new species form within a home range of an existing species, in the absence of a physical barrier

Polyploidy An example when there is a change in chromosome number that can cause instant speciation .

On Lord Howe Island, species of palms are reproductively isolated because of polyploidy

5. SYMPATRIC SPECIATION

SYMPATRIC SPECIATION IN PALMS

In parapatric speciation, populations in contact along a common border evolve into distinct species

Hybrids in the contact zone are less fit than individuals on either side

6. PARAPATRIC SPECIATION

PARAPATRIC SPECIATION

Fig. 18-24c, p. 295

T. barretti

hybrid zone

T. anophthalmus

DIFFERENT SPECIATION MODELS

We have looked at microevolution, small allele frequency changes.

Microevolutionary events that occur independently lead to genetic divergences, which are reinforced as reproductive isolation mechanisms evolve

Now let’s look at macro, large-scale changes.Macroevolution refers to the appearance of new

species over timeNew species evolve mainly because of DIVERGENCE.These show how relationships influence the creation

of new species.Speciation, the formation of new species due to

evolution, links the two.

KEY CONCEPTS HOW SPECIES ARISE

Macroevolution Large-scale patterns of evolutionary change Includes patterns of change such as one species

giving rise to multiple species, the origin of major groups, and major extinction events

Coevolution Stasis Exaptation

Adaptive radiationKey innovationExtinctions

VII. MACROEVOLUTION

MACROEVOLUTION…THE PATTERNS OF EVOLUTION

Patterns of Macroevolution: that aff ect species. Co-evolution: Organisms are part of one other’s

environment, so they can aff ect one another’s evolution. Species that live in close contact often have clear adaptations to one another’s existence.

Adaptive Radiation: Over time, species may split into two or more lines of descendants, or lineages. As this splitting repeats, one species can give rise to many new species. The process tends to speed up when a new species enters an environment that contains few other species (meaning less competition). Helps explain fi nches.

Extinction: If all members of a lineage die off or simply fail to reproduce, the lineage is said to be extinct. The fossil record shows that many lineages have arisen and radiated, but only a few of their descendants survived and evolved into the species present today. Scientist estimate that 99% of all species that have lived, went extinct.

Two species in close ecological contact act as agents of selection on each other (coevolution)Predator and preyHost and parasitePollinator and flower

Over time, the two species may come to depend on each other.

COEVOLUTION

Fig. 18-25, p. 296

proboscis

nectar tube

10 cm

CoevolutionHawk moth & Orchid

Coevolved species. The orchid Angraecum sesquipedale, discovered in Madagascar in 1852, stores its nectar at the base of a floral tube 30-centimeters (12 inches) long. Charles Darwin predicted that someone would eventually discover an insect in Madagascar with a proboscis long enough to reach the nectar and pollinate the flower. Decades later, the hawkmoth Xanthopan morgani praedicta was discovered in Madagascar. Its proboscis is 30–35 cm long.

BEE & THE ORCHID

A niche is the entire collective habitat where a population lives.

When populations inhabit different niches, divergence often follows…because there are different selection pressures.

ECOLOGICAL NICHES

Adaptive radiation A burst of speciation that

occurs when a lineage encounters a new set of niches because of a Key Innovation: A structural or functional adaptation that allows individuals to exploit their habitat in a new way

Again, caused by variation and natural selection

Fig. 18-26a, p. 297

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Evolutionary tree diagram showing the adaptive radiation of mammals following the K–T extinction event. Branch widths indicate the range of biodiversity in each group at different times. We show only a sample of modern mammals. The entire mammalian lineage includes more than 4,000 modern species. The photograph shows a fossil of Eomaia scansoria (Greek for ancient mother climber), complete with the imprint of its fur. About 125 million years ago, this mouse-sized insect-eater crawled on low branches. It is thought to be an offshoot of the lineage that led to mammals.

How do Species Evolve?There are two main schools of though…1. Gradualism2. Punctuated Equilibrium

PATTERNS OF MACROEVOLUTION

MACROEVOLUTION

Gradualism : In Darwin’s day, the idea of slow, gradual change was new to geology as well as biology. Darwin had argued that large scale changes, such as the formation of new species, must require many small changes to build up gradually over a long period of time. This model is called gradualism.

MACROEVOLUTION

Gradualism conflicts with another more modern theory that suggests evolution can happen in bursts over a very fast period of time.

Punctuated Equilibrium: Some biologists argue that species do not always evolve gradually. Species may remain stable for long periods until environmental changes drastically and suddenly create new pressures. Then, many new species may “suddenly” appear. This model is called punctuated equilibrium.

Gradualism Punctuated Equilibrium

Small evolutionary changes accumulate over long periods of time. These result from mutations present in alleles present in population.

Catastrophic events, like volcanoes or meteor strikes, suddenly change the environment, creating new selection pressures. The fittest survive.

Which way is correct?Probably both but there is a lot of evidence that PE is the

more correct.

Eliminated Evolution…StasisExaptation

Extinction

VII. HOW DOES EVOLUTION STOP?

Stasis A lineage exists for millions of years with little or no

change (e.g. coelacanth)

Exaptation (preadaptation) Some complex traits in modern species held different

adaptive value in ancestral lineages (e.g. feathers in birds and dinosaurs)

STASIS AND EXAPTATION

Extinction The irrevocable loss of a species from Earth

Mass extinctions Extinctions of many lineages, followed by adaptive radiations Five catastrophic events in which the majority of species on

Earth disappearedK–T extinction, abbreviation of Cretaceous–Tertiary

extinction, also called K–Pg extinction or Cretaceous–Paleogene extinction a global extinction event responsible for eliminating approximately 80 percent of all species of animals at or very close to the boundary between the Cretaceous and Paleogene periods, about 65.5 million years ago.

EXTINCTION

http://www.britannica.com/EBchecked/topic/1314796/K-T-extinction

SO…

Exhausted Yet?We’ve covered a great deal today- The Parameters of Genetic Equilibrium- The Mechanisms of Microevolution- The Methods of Genetic Change- The Major Patterns of MacroevolutionDo you have to remember EVERYTHING?- Pretty much.Should you be aware of these terms for later investigations?- Yep. Review the list on the board and be prepared to use these next week.

Questions?

Reflect on what you’ve learned…

Go to the website & download the ppt to make up anything you missed.

Block periods: Natural Selection LabFriday: Bacterial lab: Last Check

CLOSURE