Chapter 8: Evolution and Natural Selection

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CHAPTER 8: EVOLUTION AND NATURAL SELECTION

Darwin’s dangerous idea: evolution by natural selection

Lectures by Mark Manteuffel, St. Louis Community College ; Clicker Questions by Kristen Curran, University of Wisconsin-Whitewater

EVOLUTION IN ACTION

We can see evolution occur right before us.

COULD YOU BREED FRUIT FLIES WHO COULD LIVE LONGER THAN 20 HOURS ON AVERAGE?

When these eggs hatch, do you think the flies in this new generation will live longer than 20 hours without food?

A POPULATION OF FRUIT FLIES WAS STARVED UNTIL 80% OF THE FLIES WERE DEAD. THE REMAINING FLIES WERE FED AND OFFSPRING WERE PRODUCED.

AFTER 60 GENERATIONS THE AVERAGE STARVATION RESISTANCE OF FRUIT FLIES WAS 160 HOURS

1. They are genetically identical to the original population.

2. They are genetically different from the original population.

WHAT HAPPENED? o Evolution

o Natural selection

EXPERIMENTS IN EVOLUTION

o Dogs?o Rabbits?

EVOLUTIONo How does evolution occur?

o What types of changes can evolution cause in a population?

o Five primary lines of evidence

o Evolution by natural selection

It is indeed remarkable that this theory [evolution] has been progressively accepted by researchers, following a series of discoveries in various fields of knowledge.

The convergence, neither sought nor fabricated, of the results of work that was conducted independently is in itself a significant argument in favor of this theory.

—Pope John Paul II, 1996

NATURAL CHANGESo The characteristics of individuals in a

population can change over time.

o We can observe such change in nature and can even cause such change to occur.

DARWIN’S JOURNEY TO AN IDEA

A job on a ’round-the-world survey ship allowed Darwin to indulge and advance his love of nature.

A JOB ON A ‘ROUND-THE-WORLD SURVEY SHIP ALLOWED DARWIN TO INDULGE AND ADVANCE HIS LOVE OF NATURE.

o Age 16, University of Edinburgh, medical studies

o Studied theology at Cambridge University

o His real love: study of nature

Before Darwin, most people believed that all species had been created separately and were unchanging.

JEAN-BAPTISTE LAMARCK

o Biologist, early 1800s

o Living species might change over time.o If we need to jump higher we will learn to jump

higher.

CHARLES LYELL & JAMES HUTTONo Geologist

o 1830 book Principles of Geologyo Geological forces had shaped the earth and were

continuing to do so.

o Gradual but constant changeo Darwin had this book with him on the Beagle

EMERGING BELIEFSo People used to think that the earth was 6,000 years

old and that species were unchanging.

o In the 18th and 19th centuries, scientists began to change their beliefs.

o These changes helped shape Darwin’s thinking.

DARWIN’S 5-YEAR JOURNEY

http://www.nhm.ac.uk/nature-online/science-of-natural-history/expeditions-collecting/beagle-voyage/

DIFFERENT INFLUENCES ON DARWIN’S IDEAS

Observing geographic similarities and differences among fossils and living plants and animals, Darwin developed a theory of evolution.

TWO IMPORTANT AND UNEXPECTED PATTERNS:1. THINGS THAT LOOK ALIKE ARE NOT THE SAME SPECIES.

1. THINGS THAT LOOK ALIKE ARE NOT THE SAME SPECIES. 13 Different Finch Species

2. THE FOSSILS OF EXTINCT SPECIES AND THE LIVING SPECIES IN THAT SAME AREA RESEMBLE EACH OTHER.

o Glyptodonts and armadillos

2. THE FOSSILS OF EXTINCT SPECIES AND THE LIVING SPECIES IN THAT SAME AREA RESEMBLE EACH OTHER. If no giants exist today, then living things

must change.

DARWIN’S FOSSILS

34The slides were made by slicing and polishing the fossils into translucent sheets and then placing them between two glass plates so they could be studied under a microscope.

DARWIN’S FOSSILS

These images were made available by the Royal Holloway, University of London on Jan. 17, 2012 . and were collected by Charles Darwin during the course of his famous "Voyage of the Beagle." (University of London, Kevin D'Souza Ho,AP Photo/Royal Holloway)

THOMAS MALTHUSo Economist

o Essay on the Principle of Populationo - populations can grow much faster than the

food supply. We will eat ourselves out of existence in the foreseeable future.

THE BOOK THAT WOULD “ROCK THE WORLD”o 1842 first draft

o 14 years in a drawer

In 1858, Darwin published his thoughts on natural selection after decades of percolating and procrastinating

HISTORICAL PROGRESSo After putting off publishing his thoughts on

natural selection for more than 15 years, Darwin did so only after Alfred Russel Wallace independently came up with the same idea.

o They published a joint presentation on their ideas in 1858 and Darwin published a much more detailed treatment in The Origin of Species in 1859, sparking wide debate and discussion of natural selection. 41

THE OBSERVATIONS DARWIN MADE DURING HIS TRIP ON THE HMS BEAGLE?

1. The earth is shaped by gradual forces. (Lyell)2. The earth is older than 6,000 years. (Buffon)3. Populations could grow beyond the ability of

the environment to support them. (Malthus)Malthus thesis was that the population of England would soon reach a point that was impossible to feed with the island’s resources.

FOUR MECHANISMS CAN GIVE RISE TO EVOLUTION.

Allele frequencies in a population change.

ALLELE FREQUENCIESIncrease the white phenotype through breeding.

o As the generations go by

o - Higher proportion of white tigers

o Evolution = change in allele frequencies of the population

INDIVIDUALS DO NOT EVOLVE.o Populations evolve – get better with the resources

they have

o Allele frequencies – dark colored moths on dark tree trunks

4 MECHANISMS OF EVOLUTIONARY CHANGE

1. Mutation

2. Genetic drift

3. Migration

4. Natural selection

Evolution is genetic change in a population.

FOUR MECHANISMS OF EVOLUTIONARY CHANGE

1. Mutation—a direct change in the DNA of an individual—the ultimate source of all genetic variation.

MUTATIONo An alteration of the base-pair sequence of an

individual’s DNA

o When this alteration occurs in a gene, the change in the DNA sequence may change the allele.

MUTATION IS THE ULTIMATE SOURCE OF GENETIC VARIATION IN A POPULATION.

WHAT CAUSES MUTATIONS? o The process of cells dividing can go awry.

o Environmental phenomenao mutagens

o Mutations are randomo Beneficial?o Detrimental?

TANNING BEDS BOMBARD THE BODY WITH ULTRAVIOLET RADIATION.Can they cause mutations?

MOBILE PHONES RELEASE RADIATION.

Can they cause brain tumors?

MUTATIONo An alteration of the base-pair sequence in an

individual’s DNA.

o It is Evolution if it changes an allele the individual carries.

NEARLY ALL MUTATIONS REDUCE AN ORGANISM’S FITNESS.

2. Genetic drift is a random change in allele frequencies in a population.

GENETIC DRIFT IS NOT NATURAL SELECTION:

The change in allele frequencies does not influence reproductive success.

THE IMPACT OF GENETIC DRIFT IS MUCH GREATER IN SMALL POPULATIONS THAN IN LARGE POPULATIONS.

FIXATIONo Genetic drift can lead to fixation for one allele for

a gene in a population.

o If this happens, there is no more variability in the population for this gene.

o Genetic drift reduces the genetic variation in a population.

GENETIC DRIFT IN POPULATIONSTHE FOUNDER EFFECT

POPULATION BOTTLENECKS

FOUNDER EFFECT

AMISH PEOPLE ARE MORE LIKELY TO HAVE EXTRA FINGERS AND TOES

POPULATION BOTTLENECKS

RETURN OF THE MICHIGANSANDHILL CRANES

GENETIC DRIFTo A random change in allele frequencies within a

population,

o A significant agent of evolutionary change primarily in small populations.

3. Migration into or out of a population may change allele frequencies.

Migration, also called gene flow, is the movement of some individuals of a species from one population to another.

MIGRATIONo Migration, or gene flow, leads to a change in allele

frequencies in a population as individuals move into or out of the population.

4. Natural Selection –random changes in a population produce an individual better suited to its environment

NATURAL SELECTION1. There must be variation for the particular

trait within a population.

2. That variation must be inheritable.

3. Individuals with one version of the trait must produce more offspring than those with a different version of the trait.

CONDITION 1: VARIATION FOR A TRAIT

CONDITION 2: HERITABILITYWe call the transmission of traits from parents to their children through genetic information inheritance or heritability.

CONDITION 3: DIFFERENTIAL REPRODUCTIVE SUCCESS

1. There are more organisms born than can survive.

2. Organisms are continually struggling for existence.

3. Some organisms are more likely to win this struggle and survive and reproduce.

DIFFERENTIAL REPRODUCTIVE SUCCESS- The runt of the litter, less chance of survival.

MOST AGRICULTURAL PESTS EVOLVE RESISTANCE TO PESTICIDES.

How does this happen?

RABBITS HAVE EVOLVED DUE TO INTERACTIONS WITH PREDATORS

1. Speed2. Coat color

THROUGH NATURAL SELECTION, POPULATIONS OF ORGANISMS CAN BECOME ADAPTED TO THEIR ENVIRONMENT.

Traits causing individuals to have more offspring than others become more prevalent.

“SURVIVAL OF THE FITTEST”o Reproductive success

o Fitness

FRUIT FLY EXAMPLEo One fly carries the genes for a version of a trait

that allows it to survive a long time without food.

o The other fly has the genes for a different version of the trait that allows it to survive only a short while without food.

o Which fly has the greater fitness?

THERE ARE THREE IMPORTANT ELEMENTS TO AN ORGANISM’S FITNESS:

1. An individual’s fitness is measured relative to other genotypes or phenotypes in the population.

2. Fitness depends on the specific environment in which the organism lives.

3. Fitness depends on an organism’s reproductive success compared to other organisms in the population.

"SURVIVAL OF THE FITTEST" IS A MISNOMER.

Organisms in a population can become better matched to their environment through natural selection.

ADAPTATION TO ENVIRONMENT

Natural selection does not lead to perfect organisms.

Evolution in general, and natural selection specifically, do not guide organisms toward “better-ness” or perfection.

If the environment changes, the alleles causing the traits favored by natural selection may change, too.

FACTORS THAT PREVENT POPULATIONS FROM PROGRESSING INEVITABLY TOWARD PERFECTION

1. Environments

2. Variation

3. Multiple different alleles for a trait,

Artificial selection is just a special case of natural selection.

ARTIFICIAL SELECTIONo Animal breeders and farmers

o Determined by humans and not nature,

Natural selection can change the traits seen in a population in several ways.

DIRECTIONAL SELECTIONIndividuals with one extreme from the range of variation in the population have higher fitness.

TURKEYS ON POULTRY FARMS HAVE SUCH LARGE BREAST MUSCLES THAT THEY CAN’T GET CLOSE ENOUGH TO EACH OTHER TO MATE.

STABILIZING SELECTIONIndividuals with intermediate phenotypes are most fit.

IS MEDICAL TECHNOLOGY UNDOING THE WORK OF NATURAL SELECTION IN OPTIMIZING THE NUMBER OF BABIES WITH NORMAL BIRTH WEIGHTS?

DISRUPTIVE SELECTIONIndividuals with extreme phenotypes experience the highest fitness, and those with intermediate phenotypes have the lowest.

NATURAL SELECTION CAN CHANGE POPULATIONS IN SEVERAL WAYS:

1. Directional selection,

2. Stabilizing selection,

3. Disruptive selection,

MOST MAMMALS DO NOT DRINK MILK AFTER THEY ARE WEANED BECAUSE THEY LOSE THE ABILITY TO DIGEST LACTOSE. THIS IS TRUE FOR ABOUT 60% OF PEOPLE. BUT MOST PEOPLE OF NORTHERN EUROPEAN DESCENT CAN DRINK MILK (AREAS WHERE DAIRY FARMING IS HISTORICALLY PREVALENT). WHAT TYPE OF EVOLUTION WOULD THIS EXEMPLIFY? 1. Directed selection2. Stabilizing selection3. Disruptive selection

Natural selection can cause the evolution of complex traits and behaviors.

HOW CAN A WING EVOLVE IF 1% OF A WING DOESN’T HELP AN ORGANISM FLY OR GLIDE AT ALL?

OFTEN, STRUCTURES APPEAR BECAUSE THEY SERVE SOME OTHER PURPOSE.

FUNCTIONAL SHIFTS

ADAPTING TO THE ENVIRONMENTo Natural selection can change allele frequencies for

genes involving roundabout unrelated paths.

o A trait that has been selected for one function is modified at a later time to serve a completely different function.

THE EVIDENCE FOR THE OCCURRENCE OF EVOLUTION IS OVERWHELMING

1. The fossil record documents the process of natural selection.

RADIOMETRIC DATING

2. Geographic patterns of species’ distributions reflect their evolutionary histories.

BIOGEOGRAPHY

3. Comparative anatomy and embryology reveal common evolutionary origins.

CHICK EMBRYOS AND HUMAN EMBRYOS BOTH HAVE GILLS BECAUSE…

HOMOLOGOUS STRUCTURES

ONTOGENY RECAPITULATES PHYLOGENY

Why are we all born with one?- Gills

- appendixes

VESTIGIAL STRUCTURES

CONVERGENT EVOLUTION

Analogous structures all developed from different original structures.

WE CANNOT HIDE WHERE WE CAME FROMo Similarities in how organisms look and develop

shows their common evolutionary origins.

4. Molecular biology reveals that common genetic sequences link all life forms.

The genetic code provides our fourth line of evidence that evolution occurs.

DNA SIMILARITIES AND DIFFERENCESo Related vs. unrelated individuals

o The more distantly you and another individual are related, the more your DNA differs.

DNA SIMILARITY BETWEEN TWO SPECIESo Compare their DNA sequences for individual

genes.

o In Rhesus monkeys, 138 amino acids are the same as those found in human hemoglobin.

RECENCY OF COMMON ANCESTRYo Estimates of evolutionary

relatedness made from: oComparative Anatomy oEmbryologyoThe Fossil Record

o “Molecular Clocks”

5. Laboratory and field experiments enable us to watch evolution in progress.

A fifth line of evidence for the occurrence of evolution comes from multi-generation

experiments and observations.

CHANGES IN THE FREQUENCY OF THE VARIOUS ALLELES

FIVE PRIMARY LINES OF EVIDENCE:1. The fossil record

2. Biogeography

3. Comparative anatomy and embryology

4. Molecular biology

5. Laboratory and field experiments

Chapter 8: Evolution and Natural Selection

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