What is Evolution?

A change in the genetic make-up of a population over time.

How Does Evolution Occur?

There were two hypotheses

Evolution by acquired traitsby Jean Baptiste de Lamarck

Do we acquire traits?

ClimbingEating

Show affection

Act nutty

Defend yourself

These traits are not genetic so evolution does not occur

How Does Evolution Occur?

• NATURAL SELECTION !!! (major mechanism of evolution)– Darwin’s hypothesis (now a theory):

• Survival of the fittest…..how is fitness measured?– By reproductive success

• Inheritable variations occur in individuals in a population. Due to competition for limited resources, individuals with more favorable variations or phenotypes are more likely to survive & produce more offspring, thus passing traits to future generations.

Darwin’s View of History

• Like a Tree with multiple branches.– Common trunk– Tips of twigs =

diversity of organisms living in the present.

– Forks = most recent common ancestor

Natural Selection Summary

• A process in which individuals that have certain heritable traits survive & reproduce at a higher rate than others because of those traits.

• Over time, natural selection can increase the match between organisms & their environment.

• If an environment changes, or if individuals move to a new environment, natural selection may result in adaptation to these new conditions, sometimes giving rise to new species.

Natural Selection acts on phenotypic variations in populations

• Environments change and act as selective mechanism on populations.

• Phenotypic variations are not directed by the environment but occur through random changes in the DNA and through new gene combinations.

Some phenotypic variations significantly increase or decrease fitness of the

organism and the population.

Examples of Natural Selection

• Antibiotic resistance in bacteria– Develops in several steps:

• Person is sick from a bacterial infection• Takes antibiotics• Gets better• One bacteria is not killed by the antibiotics, due to

genetic modification & reproduces• Person gets sick again & goes to the doctor again• Same antibiotic is prescribed but there’s no effect• Doctor prescribes a different antibiotic which hopefully

works.• If bacterium continues to change it could become

resistant to more antibiotics

Examples of Natural Selection

• Pesticide resistance in rats– Apply pesticide…most rats killed– Due to natural variations, a few rats are not

affected by the poison.– They reproduce passing on the trait to some if

not all of their offspring– Seeing the rats, you apply the pesticide again

with worse results.– A new pesticide must be used.

Artificial Selection

Darwin used these examples to help him derive another piece of his theory.

Humans impact the gene variation.

What Evidence is there that Evolution by Natural Selection occurs?

• Direct observations– Peppered moths

– Drug resistant bacteria

• Analysis of similarities among different organisms.– Homologous structures

– Vestigial structures

• Fossil Record– Documents the pattern of evolution

– Shed light on origin of new groups of organisms

• Biogeography– Geographical distribution of species

• Continental drift (slow movement of the continents over time)– Help make predictions of where fossils of groups of organisms can be found

Direct observations

Analysis of similarities among different organisms

Homologous structures: represent variations on a structural theme that was present in their common ancestor.

Vestigial structures: remnants of features that served important functions in the organism’s ancestors.

Convergent Evolution: independent evolution of similar features in different lineages.

Species that share features because of convergent evolution are said to be analogous. Analogous features share similar function but not common ancestry.

Fossil Record

Allow us to take a look at how new groups came about & when characteristics showed up.

EX: Cetaceans are closely related to even-toed ungulates.

Fossils can be dated, this helps us provide evidence of evolution by dating rocks where fossils are found, it helps indicate the relationships within phylogenetic trees, as well as chemical properties &/or geographical data.

BiogeographyWe can use our understanding of evolution and continental drift to predict where fossils of different groups of organisms might be found.

EX: Horses

Fossil dating indicates horses originated in North America 5 million years ago.

Since North & South America were not yet connected it was predicted that the oldest horse fossils would be located in North America.

So far the prediction has been upheld

The environment does not direct the changes in DNA,

but acts upon phenotypes that occur through random

changes in DNA

These changes can involve alterations in DNA sequences, changes in gene

combinations, and/or the formation of new gene combinations.

Environments are always changing

• No “perfect” genome (entirety of an organism's hereditary information). • Diverse gene pool = long term survival in a changing

environment.• Mutations contribute to diversity

– Some are silent– Some result in phenotypic difference

• Interaction of the environment & phenotype determines fitness.

What does “survival of the fittest” mean?

The species that are most adapted in their environment by reproducing

more successfully will survive.

Change in the nucleotide sequence (are rare; change from gen. to gen. is very small.

In multicellular organisms, only mutations occurring in gametes will be passed on to offspring

Most occur in somatic cells.

Some are “silent” changes

Mutation

Point mutation

Genes duplicating• Due to errors in meiosis

• During DNA replication

• Activities of transposable elements (wandering DNA segments)

• Large chromosome segment duplications are usually harmful

• Smaller pieces of DNA may not be.– Gene duplications that don’t have severe effects can

persist & accumulate = potential new genes with new functions.

Example of Beneficial Increases in Gene Numbers

A remote ancestor had a single gene for detecting odors & have duplicated over time. Today we have about 1,000 olfactory receptor genes.

Evolving Populations

What is a population?

A group of individuals of the same species that live in the same area & interbreed, producing fertile offspring.

What is a gene pool?

• The total number of genes of every individual in an interbreeding population.

We can characterize a population’s genetic make-up by describing its gene pool.

The size of the gene pool affects the rate of mutation

Why is it vital to have diversity in the gene pool?

• Environmental conditions change– What kills one would kill them all.

Clearing Potential Misunderstandings

• Natural selection acts on ___________– Ex: pepper moths

• _____________not individuals evolve.

• Organisms that are _______________ fit will pass on their _________.

PHENOTYPE

POPULATIONS

REPRODUCTIVELYGENES

G.H. Hardy & W. Weinberg (1908)

• Both men independently suggested a scheme whereby evolution could be viewed as changes in the frequency of alleles in a population of organisms.

Calculating changes in allele frequency

• Hardy-Weinberg– For populations to be at equilibrium (remain

constant) the following conditions must be met.

• Population must be large• Absence of migration• No net mutations• Random mating• Absence of selection

– These conditions are seldom if ever met.

Of what value is this?

Provides a yardstick by which changes in allele frequency, &

therefore evolution, can be measured.

The Hardy-Weinberg Equation

Tests whether a population is evolving.

The Hardy-Weinberg Principle• The equation determines what the genetic make-up of a

population would be if it were not evolving at that locus.

• Those results are compared to data from an actual population

• If there are no differences, it can be concluded that the “real” population is not evolving

• It is also used in medical applications:– Est. the % of a population that carries the allele for an inherited

disease.

p + q = 1

•Knowing the frequency of the phenotype, you can calculate the frequency of the genotypes and alleles in the population.

•Because there are only two alleles, the sum of p and q must always equal 1.

= frequency of the dominant allele

=frequency of the recessive allele

This equation finds the allele frequency

The Hardy-Weinberg equilibrium

can be written as an equation

p2+ 2pq + q2 = 1

% of Individuals homozygous for allele

BB written as a decimal% of Individuals

heterozygous for alleles Bb written as a decimal

% of Individuals homozygous for allele bb written as a decimal

By convention

The more common allele (B) is designated p

The less common allele (b) is designated q

This equation lets us calculate genotypic/phenotypic frequencies in a very simple way.

Let’s say we have a population of 546 frogs. 142 of these frogs have dark spots on them. The plain green frogs are completely dominant to the spotted frogs. Determine the genotypic frequencies within this population.

142/546 = .26 which represents q2 or gg

In order to get the homozygous dominant & heterozygous we need to use the p + q = 1 equation.

q2 = .26 take the square root of each side to get q which is .51

p + q = 1 p= 1- q p= 1- .51 p = .49

2pq = 2(.51 x .49) = .50

p2 = .24

So, 26% of the frogs are recessive; 50% are heterozygous; 24% are homozygous dominant.

To determine the frequency of gametes carrying the dominant allele. Meaning the percent of the population carrying at least one

dominant allele:

You would take half of the 2pq frequency and add it to the p2 frequency.

.25 + .24 = .49

Which means, 49% of the frog population carry at least one dominant allele.

.25 + .26 = .51Which means 51% of the population

carries at least one g allele.

To determine the recessive frequency take half of the 2pq & add to q2

CALCULATING THE FREQUENCY OF CYSTIC FIBROSIS

Cystic fibrosis is caused by the recessive allele b.

If q2, the frequency of recessive homozygotes, is 0.00048, then q is , or 0.022.√ 0.00048

p + q = 1 , p = 1-q

SO

p = 1 – 0.022, OR 0.978

Calculate the frequency for the dominant allele B:

Calculate the frequency of the recessive allele

Determine the frequency of heterozygotes

2pq = 2 x 0.978 x 0.022 = 0.043

Meaning: 43 of every 1,000 Caucasian North Americans are predicted to carry the cystic fibrosis allele unexpressed.

Try to curl your tongue upwards.

How would we determine q2?Divide the total number of non-curler students

by the total number of students.

How do you determine q?Calculate the square root

of q2.Since p + q= 1; determine p.Now plug in numbers for 2pq.

What percentage of the class are carriers for the tongue curling trait? (Tongue curling is a dominant trait.)

Time for a simulation!!!

Let’s Get It On!!!!

Number of alleles present after several generations

Number of offspring with AA _____X 2= _________ A alleles

Number of offspring with Aa _____X 1= __________A alleles

Total= __________A alleles

p = TOTAL number of A alleles

TOTAL number of alleles in the population

(number of students X 2)

Altering Allele Frequencies in Populations Directly & cause most

Evolutionary Change

• Natural Selection

• Genetic drift

• Gene Flow

Bottleneck effect A sudden decrease

in population size to natural forces

More likely to occur in smaller population

Genetic Drift

Founder effect Small group of

individuals establishes a population in a new location

Random fluctuation of alleles due to chance events

Case Study: Genetic Drift

The land was being converted to farmland and other uses.

Led to significant loss of genetic variation & an increase in the frequency of harmful alleles.

A reduction of genetic variation within a given population can increase the differences between populations of the same species.

Gene Flow

The transfer of alleles into or out of a population due to the movement of fertile

individuals or their gametes.

If there is gene flow between two populations there is a tendency for the amount of genetic variation between the populations to decrease.

Natural Selection is the only mechanism that consistently causes adaptive evolution

In humans, infants with intermediate weight at birth have the highest survival rate

In chicken, eggs of intermediate weight have the highest hatching success

Fig. 13.13

Increase in the frequency of the

intermediate phenotype

Stabilizing Selection

In the African seed-cracker finch, large- and small-beaked birds predominate

Intermediate-beaked birds are at a disadvantage Unable to open large

seeds Too clumsy to open

small seeds

More adept at handling small seeds

Can open tough shells of large seeds

Disruptive Selection

Drosophila flies that flew toward light were eliminated from the population

The remaining flies were mated and the experiment repeated for 20 generations

Common when environment changes or members of a population migrate out.

Phototropic flies are far less frequent in the population

Directional Selection

Sexual Selection

Sexual selection is often powerful enough to produce features that are harmful to the individual’s survival.

Sexual selection acts on an organism's ability to obtain (often by any means necessary!) or successfully copulate with a mate.

Australian Peacock Spiders

Sexual dimorphism- a difference in the 2 sexes

Operations of Sexual Selection

• Intrasexual selection:

• Intersexual selection:

Individuals of one sex compete for mates of the opposite sex. Mostly males compete but there are some species where females compete (ring-tailed lemurs and broad-nosed pipefish)

“mate choice” – individuals of one sex (usually females) are choosy in selecting their mates of opposite sex. Many cases it falls to the male’s showiness or behavior that wins the female.

Why natural selection cannot fashion perfect organisms

• Limited by species ancestors– Birds ancestors are reptiles & only had 4 legs. Wings were opted

for flight so that left only 2 legs left.

• Adaptations are often compromises– Seals spend a lot of time on rocks so legs would be a better trait

than flippers but then they would not swim as well.

• Chance & natural selection interact– Snakes are carried to different islands on seaweed rafts but the

snakes that are carried are not necessarily the snakes best suited to the environment.

• Selection can edit out only existing variations– New alleles do not arise on demand

The biological species concept

Emphasizes reproductive isolation

Belonging to the same biological species

• Means you are reproductively compatible, potentially.

But, it all hinges on…

• Reproductive barriers:– Impede members of two species from

producing viable, hybrids.– One barrier may not impede but a

combination of several can isolate a species gene pool.

Prezygotic & Postzygotic Barriers

• PREZYGOTIC:– Impede mating or hinder the

fertilization.

• POSTZYGOTIC:– If the sperm is able to

overcome the prezygotic barriers then the zygote may be prevented from developing.

Two main ways by which new species form

Speciation can take place with or without geographic separation

Geographic isolation -reproductive isolation may occur; preventing interbreeding

Reproductive barrier must be present. -allopolyploidy -reproductive isolation -habitat, food source, or other source not used by parent pop. -temporal or behavioral isolation

Example of sympatric speciation

Adaptive Radiation

Tempo of Speciation

Phylogeny

The study of the evolutionary past of a species.

Systematics

• An approach to looking at the diversity & relationships between organisms living & extinct.– Morphology– Biochemical resemblances– Molecular comparisons (ex: DNA)

It’s also good to look at fossils- they reveal ancestral characteristics that may have been lost over time.

If similar with any they are likely to be closely related

Sorting Analogy from Homology

• How can you tell if the structure is similar because of a common ancestor or due to convergent evolution?

Search for corroborating similarities between the speciesFossil evidenceLook at the complexity of the characters & note points of similarities.

Molecular comparisons of Nucleic Acids

Species 1 & 2 DNA are identical Begin to diverge

Mutations shift the matching sequences

Homologous regions (in yellow) no longer align

Using a computer program homologous regions now align because appropriate gaps were created.

Connecting Classification with Evolutionary History

Binomial nomenclature

• Two part naming system – genus and specific

epithet

• Canis lupus

or

• Canis lupus

Hierarchy of Classification

Fig. 14.3

Using & making a dichotomous key

Phylogenetic trees & cladograms are graphical

representations (models) of evolutionary history that can

be tested

Each branch point represents the divergence of two species from a common ancestor

Phylogenetic Tree

Cladogram – depicts patterns of shared characteristics

Does not imply evolutionary history.

If characteristics are homologous the cladogram can serve as a basis for a part of the phylogenetic tree.

Outgroup?Ingroup?

Making cladogramsOrganism Multicellular Vertebra

l columnHair Placenta Totals

Sponge Yes No No No 1

Sailfish Yes Yes No No 2

Wombat Yes Yes Yes No 3

Elephant Yes Yes Yes Yes 4

sponge sailfish wombat elephant

multicellular

Vertebral column

hair

placenta

Constructing a cladogram

• Using the following animals, list as many characteristics which each organism possesses .

• Create & fill in a chart with the animals & the characteristics you have come up with.

• Build your cladogram

Rhesus monkey Snapping TurtleKangaroo HumanBull frogTuna

TunaBull Frog

Snapping Turtle

Kangaroo

Rhesus Monkey Human

Paired

legs

Amniotic sac

Mammary glands

Placenta

Short canine teeth

Structural evidence supports the relatedness to all eukaryotes

• Cytoskeleton

• Membrane-bound organelles

• Linear chromosomes

• Endomembrane systems, including nuclear envelope.

Sometimes the most obvious evidence isn’t

the best

Which of these is most likely in your opinion?

Molecular & genetic evidence from existing and extinct organisms indicates all organisms on Earth

share a common ancestral origin of life

Molecular building blocks are common to all life forms

Common genetic code are shared by all modern organisms.

Metabolic pathways are conserved across all currently recognized domains (bacteria, archea, & eukarya)

The Universal Tree of Life

1. Last common ancestor of all living things.

2. Possible fusion of bacterium with archea making eukaryotes

3. Symbiosis of mitochondrial ancestor with ancestor of eukaryotes

4. Symbiosis of chloroplast ancestor with ancestor of green plants

The History of Life on Earth

Earth formed approximately 4.6 billion years ago. The earliest

fossil is dated 3.5 billion years old.

Primitive Earth

• Provided inorganic precursors from which organic molecules could have been synthesized due to the presence of available free energy & the absence of a significant quantity of oxygen.– These molecules served as monomers of building

blocks for the formation of more complex molecules, including amino acids & nucleotides.

– Joining of the monomers produced polymers with the ability to replicate, store & transfer information.

– The RNA World hypothesis proposes that RNA could have been the earliest genetic material.

Miller/Urey Experiment

This experiment showed it was possible to form complex organic molecules from inorganic molecules in the absence of life.

How are rocks & fossils dated?

Radiometric dating

Fig. 13.4 Whale “missing links”

Fossils have been found linking all the major groups

The forms linking mammals to reptiles are particularly well known

Radiometric Dating• Certain atoms are known to decay (break down) at

a specific rate. Scientists can look at these atoms to determine how old an organic object is. – Radioactive isotope 14C- gradually decays over time

back to 14N (known as Carbon Dating)• It takes ~5600 years for half of the 14C present in a sample to

be converted to 14N.• This length of time is called the half-life.

• Half life (t1/2): the time needed for half of the atoms of the isotope to decay

• For fossils older than 50,000 yrs scientists use other isotopes such as, potassium isotope– t1/2 of 40K = 1.3 billion years to turn to argon (40Ar)

Mass Extinctions• Are rapid during times of ecological stress.

• Fossil records chronicles mass extinctions

1st single-celled organism• Prokaryotes

1st Eukaryotes• About 2.1 billion years old.

• The endosymbiosis theory