DARWIN IN HISTORICAL CONTEXT

Unit 5: Evolutionary BiologyChapter 12: The Theory of Evolution

Evolution• One theory that is still debated today is if and

how organisms change over time

• Early in history people realized that organisms display a huge variation in complexity

• People thought (and some still think) that every organism was created for a purpose, was permanent and unchanging

New information…

• New information was made available suggesting that over time species change in response to their environment

• Building on ideas and observations made by himself and other people, Darwin proposed his theory of evolution in 1859

Evolution• Evolution: The changes that have

transformed life from single-celled organisms of the past to complex organisms seen today

• Darwin proposed that populations of organisms change over time in response to environmental pressures– These changes occur within a population due to

differences of reproductive success– i.e. “Survival of the fittest”

Survival of the Fittest

• Organisms that most fit to survive in their environment are more likely to have offspring and pass on their genes

Why the controversy?• Darwin’s Theory of Evolution is widely

accepted by the scientific community

• However, acceptance of his theory took a while and many people still do not agree with his ideas

• Darwinian evolution takes place over very long periods of time, and is difficult to test in a lab

Why the controversy?

• The theory contradicts the religious beliefs of many people

• Perhaps one of the most difficult ideas for some people to accept is that humans descended from ape-like ancestors

Pre-Darwin

• The conventional belief was that the Earth was only a few thousand years old

• It was thought that each type of organism was created for a specific purpose, was unchanging and permanent

Aristotle• In 380 BC, the Greek philosopher Aristotle

began classifying organisms by complexity

• Each type of organism was a rung on the ladder of life called “The Scale of Nature”– On this ladder of life there are no vacancies and

no mobility along the ladder

Species classification

• For over 200 years, scientists classified organisms by arbitrary criteria– For example, all domestic animals were grouped

together

• In 1735 the Swedish botanist Linnaeus developed a new way to name organisms based on their physical characteristics

Linnaeus’ classification

• Species: A specific kind of organism• Each species was given a two-part name made

up of a generic name and a specific name– Similar organisms could have the same generic

name, but each had a unique specific name– Example: lions, tigers and panthers have the

names Panthera leo, Panthera tigris and Panthera pardus, respectively.

Hutton and Lyell

• Geologists that agreed that the Earth is very old and constantly changing

• In 1795 Hutton suggested that geological changes occur over time due to mechanisms such as volcanic eruptions and erosion– Felt that the Earth was changing slowly, but

continually

• In 1830, Lyell agreed with Hutton• Both believed that life stayed the same

Cuvier• A big step in understanding evolution was the

discovery and study of fossils• Fossil: An organism or trace of an organism

preserved in the Earth’s crust• In 1804, the French biologist George Cuvier

proposed that the history of life was recorded in the layers of the Earth’s sedimentary rock– He noticed that each layer was characterized by

unique species– Proposed that the boundary between two layers

indicated a catastrophic event that caused the extinction of some of the species

Charles Darwin

• Born in England in 1809• Attended Cambridge University (studying to

become a clergyman)• Became a student of botanist Rev. John

Henslow• Henslow suggested that Darwin go on the

World-wide science expedition aboard the steamship the HMS Beagle

Darwin

• From 1831 to 1836, Darwin travelled on the Beagle and studied the plants and animals that he found

Darwin• Darwin’s most important findings were made

along the coast of South America and during his time on the Galapagos Islands

• The Galapagos Islands are located about 500 miles from the West coast of South America

• Darwin noticed that while many of the organisms were unique to the islands, they resembled the organisms of South America

• Many organisms were specific to a single island, but were still similar to mainland organisms

Darwin’s finches• Darwin collected information about the 13

species of finches on the islands• Found that a finches beak was adapted for

its food supply on its home island– Some islands had large, hard-shelled seeds

that fell to the ground – finches had large powerful beaks

– Other islands had small seeds that had to be picked out of cacti – finches had small thin beaks

– On other islands, finches ate insects or insect larvae – beaks adapted to prey capture

Darwin’s finches

Evolution of Darwin’s ideas

• Darwin catalogued the different beak shapes and how they related to the food sources

• While on the Beagle, Darwin read Lyell’s paper with the idea that the world is very old and constantly changing

• Darwin speculated that constant change, driven by adaptation to different environments, could cause be occurring in organisms

Darwin’s ideas

• Darwin thought that the process of adaption was related to the formation of new species

• When two populations of the same species were isolated from one another, they would adapt to their new environments and become increasingly dissimilar

• Eventually, the populations would diverge into different species

Darwin

• In 1844, Darwin wrote an essay on his theory of evolution

• He called the theory “Descent with modification”

• He believed that all organisms were descended from a single unknown prototype

• Over time, organisms have acquired modifications that make each species unique

Natural Selection

• Darwin proposed that evolution occurs through a process called “natural selection”– There is variation within a population– Individuals with advantageous traits produce

more offspring– The unequal ability of organisms to survive and

reproduce leads to gradual changes in a population

Constraints

• There are constraints to the process of natural selection

– Only works on variations present in a population– Only affects traits that are passed on to offspring– Causes changes in a population, not an individual

Evidence to support evolution

• Evolution leaves observable signs as clues to the past

– Fossils support the theory of evolution• Help to establish the order of when organisms

appeared, even if now extinct• Scientists can see how similar organisms have changed

over time

Evidence to support evolution

• Anatomical similarities between species also supports evolution– Example: Humans, whales, bats and all other

mammals have similar forelimbs– Structures are similar, even though they perform

very different functions– Some organisms possess vestigial structures

• An ancestral structure that has lost its use• For example, some snakes have the remnants of a

pelvis and legs, suggesting that they evolved from lizards

Evidence to support evolution

• Molecular evidence supports evolution– Today scientists can compare the DNA and protein

sequences of organisms– Closely related organisms often have similar

amino acid sequences between certain proteins– Certain fundamental processes, like cell division,

have been conserved throughout evolution from yeast, to plants to mammals

Evidence to support evolution• While evolution is usually difficult to observe,

there are some examples that are easy to see

– Example: English peppered moth population before and after the Industrial Revolution

– These moths spend much of their time on Birch tree bark (normally have light colored bark)

– Before the Industrial Revolution, 99% of the moths were light colored and were difficult for predators to see and catch

Evolution of the English Pepper Moth• The Industrial Revolution introduced many soot-

producing factories• The soot coated the birch trees, making them black• Light colored moths became easy to see• After the Industrial Revolution, 99% of the moth

population was dark colored

Artificial Selection• Artificial selection is used in the selective

breeding of domesticated plants and animals• Examples:

– Farmers breed cows to increase milk production and generate leaner beef

– Crops are selected for higher yields and for taste– Flowering plants are selected for their large, showy

flowers– Often, characteristics of organisms can be changed in

just a few generations• This suggests that the same process could occur

via natural selection

MECHANISMS OF EVOLUTION

Unit 5: Evolutionary BiologyChapter 12: The Theory of Evolution

Evolution

• When viewed macroscopically, organisms on Earth are incredibly diverse

• Example: Compare bread mold and a spider– Their differences are obvious– However, they have similarities, too– Both are eukaryotic organisms– Their cells are composed of proteins, lipids,

carbohydrates and nucleic acids– At this level, the spider and the mold are very

much alike!

Common origin

• The similarities between bread mold and spiders reflects their common origin

• Both the mold and spider arose from a common ancestor

• Their divergence reflects the divergent evolutionary pressures that acted on their ancestors

Definitions

• Macroevolution: Major change over very long periods

• Microevolution: A change in the gene pool of a population over a few generations.– Smaller change

Population evolution

• Evolution: The change in the genetic makeup of a population over time

• In order for a population to evolve:– The population must have genetic variation– Genetic variation: Genetic dissimilarity among

members of a population– Mutations are a major source of genetic variation– Also, sexual reproduction contributes to genetic

variation (independent assortment and crossing over)

Population evolution

Genetic Diversity

Mutation

Sexual ReproductionIndependent assortment

Sexual ReproductionRecombination

(aka crossing over)

Evolution

Individuals do not evolve

• Populations, not individuals, evolve

• Selection occurs at the level of the individual

• The reproductive success of individuals alters the frequencies of certain traits within a population, and the population evolves

Allele frequency• How is the genetic makeup if a population

determined?• Consider the allele frequency of a single gene• Diploid organisms have two copies of each

gene (may or may not be identical)• Allele: alternative form of the same gene• In a population, the relative number of copies

of each allele may be different• Allele frequency indicates the amount of

genetic diversity in a population

Eye color

• In humans is determined by 3 genes and multiple alleles

• To simplify, let’s consider just 1 gene– bey 2 gene– B allele is dominant (brown eyes)– b allele is recessive (blue eyes)

Allele frequency

• How many B alleles?9

• How many b alleles?11

• How many total?20

BB

BBBbBb

BbBbBb

bbbb

bb

Allele frequency• 9 B alleles• 11 b alleles• 20 total alleles (= B + b)• Let p = the frequency of the dominant allele B

= 9/20 = 0.45• Let q = the frequency of the recessive allele b = 11/20 = 0.55

Allele frequency• The sum of allele frequencies always equals 1• p + q = 1• 0.45 + 0.55 = 1• This is possible because we know the

genotypes• However, we don’t always know genotypes• Can be difficult to distinguish between

homozygous dominant individuals and heterozygous individuals

Hardy-Weinberg Equation• Possible to estimate alleles frequencies using

the Hardy-Weinberg equation• p2+2pq+q2 = 1.0• This equation can only be used when there

are no selective pressures acting on a population causing it to change

• i.e. this equation describes a population at equilibrium

• Population that is not changing is at equilibrium

Equilibrium

• Conditions for equilibrium (not changing):– Population must be very large– Mating must be random– No mutations occurring nor evolutionary

pressures acting on the population

Back to eye color…

• p2 = Frequency of BB genotype (homozygous individuals)

• q2 = Frequency of bb genotype (homozygous individuals)

• 2pq = Frequency of Bb genotype (heterozygous individuals)

Is this population at equilibrium?

• p2+2pq+q2 = 1.0 (at equilibrium)• p = 0.45• q = 0.55• (0.45)2 + 2(0.45)(.055) + (0.55)2 = ?• ? = 1; therefore, the population is at equilibrium

BB

BBBbBb

BbBbBb

bbbb

bb

Evolutionary change

• Populations are rarely at equilibrium• Four pressures that can cause populations to

change– Genetic drift– Gene flow– Non-random mating– Natural Selection

Genetic drift

• Genetic drift: Changes in gene frequencies of a small population due to chance

– In small populations, chance events can permanently change the populations gene pool or allele frequencies

Bottle Neck Effect• The bottle neck effect can cause genetic drift• Due to an event that leads to a significant

reduction in the population• Only a few individuals survive to pass on their

genes – alters allele frequencies

Founder Effect

• The Founder Effect: Due to the migration of a few individuals away from a population

• The new population is established in a new location

• The allele frequency of the new population may be very different from that of the old, depending on the allele frequency of the founding individuals

Gene Flow

• Gene flow: The change in a population’s allele frequency resulting from migration– Can be due to individuals entering or leaving a

population– The frequencies of alleles change when individuals

enter or leave a population

Nonrandom mating

• Nonrandom mating: The selection of mates on the basis of a trait or group of traits, not by chance

Nonrandom mating

• Assortative mating: Form of nonrandom mating; partners resemble each other– Although it does not change allele frequencies, it

does reduce the number of heterozygous individuals

– Can lead to inbreeding: Mating between genetically related individuals; increases the risk that an individual will be born with a homozygous damaging recessive trait

Nonrandom mating

• Sexual selection: another type of nonrandom mating; selection based on the evolution of traits among individuals of the same sex– Results from either

• Selection by the opposite sex, or• Competition among individuals of the same sex

Natural selection

• Natural selection– Adapts organisms to their environments– Facilitates evolution by increasing or decreasing

the odds that the organism successfully reproduces

• Darwinian fitness: The contribution an individual makes to the gene pool of the next generation relative to others– Difficult to measure– Therefore, scientists look at “relative fitness”

Relative fitness• Relative fitness: The contribution of a

genotype to the next generation relative to other genotypes

• The reproductive success of an individual ultimately depends on its genotype

• Differences in genotype arise from mutation– Some mutations are harmful, some have no effect

and some are beneficial– When a beneficial mutation occurs, the individual

has a reproductive advantage

Natural selection

• When an individual has a reproductive advantage, the population is not at equilibrium (Hardy-Weinberg equation does not apply)

• The population evolves• Favorable mutations accumulate and persist

in a population

Natural selection• Natural selection changes allele frequency in

three ways– Stabilizing selection: Minimizes extreme

phenotypes (example – average birth weights – higher mortality rates in very small or very large babies)

– Directional selection: Shifts phenotype to one extreme (often caused by environmental shifts – example – moth color)

– Diversifying selection: favors both extremes (example – beak sizes of finches in different habitats)

How is diversity maintained?

• What prevents natural selection from eliminating unfavorable traits?

• Are several mechanisms at work:– The diploid character of most eukaryotes hides

recessive alleles in heterozygous individuals– Polymorphism: The co-existence of different

alleles in the same population• Balanced polymorphism: The coexistence of different

alleles without any change in their frequency

Balanced polymorphism

• How does a balanced polymorphism occur?• Heterozygote advantage: The reproductive

advantage of heterozygous individuals over homozygous individuals– Example: the allele for sickle cell disease is

maintained at relatively high levels in some African countries

– Heterozygous individuals have the selective advantage of surviving Malaria

Balanced polymorphism

• In plants, inbreeding often results in reduced yields and increased sensitivity to disease

• Crossing of two inbred varieties produces hybrid offspring that are more vigorous than either parent

• Hybrid vigor: The superiority of a hybrid offspring– Unfavorable recessive alleles are “hidden”– Promotes heterozygote advantage

Balanced Polymorphism

• The environment plays a role• Different habitats

• Drive divergent evolution• Act to preserve different phenotypes

Balanced polymorphism

• Frequency-dependent selection: A type of selection in which the frequency of a certain phenotype determines the reproductive success of the organism– Reproductive success declines when a phenotype

becomes too common– Example: HIV evolves rapidly to avoid immune

system detection and destruction and drug intervention

– When a variant becomes too common it is more easily recognized by the host’s immune system, or targeted by drugs

POPULATION GENETICS AND EVOLUTION

Unit 5: Evolutionary BiologyChapter 12: The Theory of Evolution

Consider Blood Rh Factor

• When a person receives foreign blood, their immune system may recognize the blood as being foreign

• The blood may be recognized because of proteins and sugars present on the outer surface of the blood cells

• The molecules are classified into blood groups• Example: ABO group or the Rh factor group

Rh factor

• The Rh blood group is a very complex blood group polymorphism

• The molecules in this group are collectively referred to as the Rh factor

• About 85% of the population posses the factor and are Rh+

• About 15% lack these molecules and are Rh-– Serious complications can arise when Rh- mothers

carry Rh+ babies as the mother’s immune system targets the babies blood as being foreign

For our example…

• We will simplify the Rh factor down to just two alleles– One dominant (A) and one recessive (a)– AA– Aa– aa Rh-

Rh+

Remember…

• If population size (N) = 100, the total number of alleles = 200

• The number of A alleles in a population of 100 people = 120

• How many a alleles?• 80• Allele frequency = the number of allele copies

total alleles

Remember…

• The frequency of A = 120/200 = 0.60• The frequency of a = 80/200 = 0.40• In this example, we are given the allele

frequencies• It can be difficult to determine allele

frequencies and genotype frequencies based on phenotype alone

• We can use the Hardy Weinberg formula to calculate genotype and allele frequencies

Hardy-Weinberg Conditions• Random mating• Large breeding population• No differential migration (when individuals with

specific traits leave the population)• No mutation of the alleles• No natural selection• When these conditions are met, populations do

not evolve and allele frequencies do not change– The Hardy-Weinberg formula can be used to predict

frequencies of future generation

SPECIATION

Unit 5: Evolutionary BiologyChapter 13: The Origin of Species

The Grand Canyon

• The sides of the grand canyon are about 10 miles apart

• Each side has very different plants and animals

• Example: Abert Squirrels on the south side and Kaibob squirrels on the north side

Squirrels

Abert Squirrel Kaibob Squirrel

5 million years ago

• Before the Grand Canyon existed, the common ancestor of these squirrels was found on both sides of the Colorado River

• Some squirrels could get across the river• The gene pool from the north side was mixing

with the gene pool from the south side• Gene pool: All the genes present in a

population• Gene flow: Transfer of alleles between two

populations

Barrier was formed…

• Over time, the river cut into the ground and made it impossible for the squirrels to cross over this new boundary

• Prevented mixing the gene pools• As the canyon became wider, the conditions

on each side became different, and the populations on each side adapted to the changing environments

The species evolved…

• Species: A specific kind of organism• Speciation: The origin of a new kind of

species through evolution

Geographic isolation

• In order for speciation to occur, gene flow between two populations must be blocked

• Geographic isolation occurs when two populations are physically separated

• Allopatric speciation: A type of speciation caused by geographic isolation

• Types of geographic barriers can include:– Formation of a mountain range– Movement of a glacier– Division of a large lake into smaller lakes

Reproductive isolation

• Reproductive isolation: genetic changes cause two populations to become unable to mate with each other, even though they are not separated geographically

• Sympatric speciation: Speciation due to reproductive isolation

• Blockage before fertilization (formation of a zygote) (pre-zygotic barrier) or after fertilization (post-zygotic barrier)

Pre-zygotic barrier

• Pre-zygotic barrier: A kind of reproductive barrier that prevents organisms from mating with each other and forming viable zygotes– In order for reproduction to occur, individuals

must be• In same location within a habitat• Must mate at the same time of the day or season of the

year• Must have same mating rituals• Must have compatible anatomic parts

Post-zygotic barrier

• Post-zygotic barrier: A kind of reproductive barrier that prevents a hybrid from developing into a viable, fertile adult

• Hybrid: The product of breeding by organisms of different species– Hybrids tend to not completely develop– Those that develop tend to not be healthy– Hybrids tend to be not fertile

Speciation

• Allopatric and sympatric isolation stop gene flow

• In the case of the Grand Canyon squirrels, two populations were isolated from one another

• Their environments differed and each population adapted to their environment

Speciation• Allopatric speciation is most likely to occur when

a small population is separated from the parent population

• Splinter population: A small group that is isolated from the parent population

• Environment of the splinter population is often at the extreme range of the parental population

• Genetic drift due to the Founder Effect will act on the smaller population until the population size increases

• The resulting population will have different allele frequencies

Allopatric speciation

• Allopatric speciation is common on island chains

• Adaptive radiation: The evolution of many different species from a common ancestor

• Example: Darwin’s finches (13 different species)

Sympatric speciation

• A new species can arise within a population, even if there is no geographical isolation

• Sympatric speciation: due to reproductive isolation

• Many plant species arose this way• Polyploidy: An accident in cell division

resulting in an organism with more than two sets of chromosomes

Polyploidy

• 2n = 14 -> 2n = 28 (polyploid)• Polyploids cannot breed with individuals from

the parent population• Wheat, oats, potatoes and tomatoes are all

polyploids!

Sympatric speciation in animals

• In animals, reproductive isolation usually does not involve the formation of polyploids

• Reproductive isolation can occur in animals by other means

• For example, a mutation in the ancestor Drosophila heteroneura caused some males to have a wider head, preferred by certain females in mate selection

What criteria need to be met to be considered a new species?

• Criteria considered differs, depending on the organism being studied (and the scientist making the call!):

– Ability to reproduce– Anatomic differences– Mate selection– Physical behavioral components

What happens when geographic barriers are removed?

• If the ranges of the Abert and Kaibab squirrels overlapped, three possible outcomes could be predicted– The squirrels breed with one another and the

gene pools mix freely – speciation has not occurred

Kaibab Abert

Overlap of ranges…

• The second possibility is that the squirrels do not breed with one another and speciation has occurred (reproductive barrier may have developed

Kaibab Abert

Overlap of ranges…• The third possibility is that the squirrels will

breed with one another, but gene flow will only occur in the region where the populations overlap

• Hybrid zone: A region where two related populations come into contact with one another after geographic isolation and interbreed where their ranges overlap

Kaibab Abert

Hybrid zone

• If species form a hybrid zone, gene flow will only occur in the region where the two populations overlap

• Hybrids make it difficult to define the term “species”!

Rate of speciation

• Two theories1. Speciation occurs gradually and the big

changes observed in a new species are the result of many small changes over a long period

2. Speciation occurs in rapid bursts followed by long periods of little change (aka “Punctuated equilibrium”)

Support for punctuated equilibrium

• In a small splinter population, genetic drift and natural selection can cause significant changes in a relatively short time (1000’s of years)

• Only rarely is it possible to find gradual transitions between fossil forms– BUT, it is possible that the fossil record is

incomplete and does not record all information (only skeletal…)