Evolution

To understand evolution you have to first have an understanding of the age of the Earth.

The Earth is estimated to be approx. 4.6 billion years old.

The first fossil record is believed to be about 3- 3.5 billion years old

This is a very long time, and Earth has changed tremendously since its origin.

History of the Earth

What were some events that led to the development of Darwin’s theory of evolution?◦ Lamark- inheritance of acquired traits through use and disuse,

1st theory of evolution◦ Malthus- write an essay “Principles of Population” the idea that

people compete for a limited number of resources, and population growth rates depend on this flux in resources

◦ Lyell- wrote “Principles of Geology” established that the Earth has undergone tremendous changes, making it much older than once believed

◦ Wallace- through his own research came to the same conclusion as Darwin about Natural selection

Who was Charles Darwin?◦ Lived from 1809-1882◦ Developed the theory of evolution by means of natural selection

Day 1

Look for references to ◦ 1. Charles Darwin’s grandfather Erasmus Darwin,

and Darwin’s brother as an influence◦ 2. Darwin’s religious beliefs and changes in his

belief system◦ 3. Geological time, Lyell’s work on changes in

geological features◦ 4. Common ancestor◦ 5. Malthus’ writings about population

Day 1: Darwin’s Dangerous Idea DVD

Define Evolution:Cumulative change in groups of organisms through time

or - descent with modification- differential reproductive success

Principles behind evolution:1. Individuals in a population show variation

among others in the same species2. Variations are inherited3. Animals have more young than can survive on

the available resources4. Variations that increase reproductive success

will be more common in the next generation

Day 2:

The theory of evolution is supported with the following evidence (see handouts for more explanation)

1. Fossil record- using relative dating and carbon-14 dating to determine age of extinct and extant groups of organisms.

2. Biogeography- comparing differences in groups of organisms in line with the migration of continents and other changes in geography

3. Comparative Anatomy- looking at common anatomical structures (homologous or vestigial features)

4. Comparative Embryology- looking at common tissue development

5. Molecular Biology- comparing the DNA and protein sequencing of extant organisms and determining the accumulation of mutations since they shared a common ancestor (phylogeny- an evolutionary tree)

Day 2: Evidence of Evolution

Darwin’s theory was based on Natural Selection Natural Selection: differential reproductive

success; the result of selective pressures from the environment, that favor one phenotype over another

Artificial Selection: a man made selection of a phenotype, like selective breeding of agricultural plants and animals, horses, or dogs. This is also the case with antibiotics or antiviral medicines that change the body’s internal environment.

Day 2: continued

Modes of Action Natural selection has three modes of action:

1. Stabilizing selection2. Directional selection3. Diversifying selection

Number ofIndividuals

Size of individualsSmall Large

1. Stabilizing Selection Acts upon extremes and favors the

intermediate.

Number ofIndividuals

Size of individualsSmall Large

2. Directional Selection Favors variants of one extreme.

Number ofIndividuals

Size of individualsSmall Large

3. Diversifying Selection Favors variants of opposite extremes.

Number ofIndividuals

Size of individualsSmall Large

Day 3: Speciation The evolution of new species due to a

barrier, either geographical or reproductive.

What is a species: organisms that can and will mate to form a fertile offspring.

Barriers

Geographical Barrier: Any geographical formation that physically separates a population. Ex river, mountain range, valley

Reproductive Barrier: Any mechanism that impedes two species from producing fertile and/or viable hybrid offspring.

Two type of reproductive barriers:1. Pre-zygotic barriers- before fertilization2. Post-zygotic barriers- after fertilization

1. Pre-zygotic Barriers (5types)a. Temporal isolation:

Breeding occurs at different times for different species. *Often controlled by hormones that are sensitive to temperature and light availability.

b. Habitat isolation:Species breed in different habitats.

*Habitats are dictated by the adaptations organisms have for shelter, food, and protection.

c. Behavioral isolation:Little or no sexual attraction between species.

*Behaviors can be different without the organisms physical features being different

1. Pre-zygotic Barriers

d. Mechanical isolation:Structural differences prevent

gamete exchange. This can be true also with the pollinator that a plant employs.

e. Gametic isolation:Gametes die before uniting with

gametes of other species, or gametes fail to unite.

2. Post-zygotic Barriers (3 types)

a. Hybrid inviability:the Hybrid zygote fails to develop or

fails to reach sexual maturity.

b. Hybrid sterility:the Hybrid fails to produce functional

gametes. Ex. Mule a hybrid of horse and donkey, all mules are male and sterile

c. Hybrid breakdown:Offspring of hybrids are weak or

infertile.

Two models of speciation:1. Allopatric Speciation Induced when the ancestral population

becomes separated by a geographical barrier.

If separated for a long period of time they will become reproductively isolated

Example:Grand Canyon and ground

squirrels

2. Sympatric Speciation Result of a radical change in the genome

that produces a reproductively isolated sub-population within the parent population (rare).

Example: Plant evolution - polyploidA species doubles it’s chromosome

# to become tetraploid.

reproductive sub-population

Parent population

Day 4: Patterns of Evolution

Two theories:1. Gradualist Model (Neo-Darwinian):

Slow gradual changes accumulate in species overtime.

2. Punctuated Equilibrium:

Evolution occurs in spurts of relatively rapid change followed by a long period of no change.

Gradualism Punctuated Equilibrium

Organisms separated either reproductively or geographically are under different selective pressures and evolve in different directions

This is often called Adaptive radiation◦ Ex. Darwin’s Galapagos finches who descended

from a small group of mainland finches of South America

Divergent evolution

Adaptive Radiation Emergence of numerous species from

a common ancestor introduced to new and diverse environments, where land formations, food and predators may be different.

Example:Darwin’s Finches of the

Galapagos

Convergent Evolution

Species from different evolutionary branches may come to resemble one another if they live in very similar environments.

These are analogous characteristics

Example:Shark , Dolphin

Under similar environmental pressures these two very different organisms have developed similar body styles and predatory habits

Coevolution Evolutionary change, in which one

species act as a selective force on a second species, inducing adaptations that in turn act as selective force on the first species.

Example: symbiotic relationships are everywhere. Think “Circle of Life”1. Acacia ants and acacia trees2. Humming birds and plants with flowers with long tubes

Microevolution is change in the allele frequencies of a population over generations. This is on a small scale.

Allele frequencies refer to the actual number of a particular allele within a population’s gene pool ( all the alleles at all loci in all the members of a population)

Day 5: Microevolution

1. Genetic Drift- loss of variation (allele frequencies) due to a sudden environmental act that reduces the population

2. Gene Flow – change in variation (allele frequencies) due to immigration or emigration, movement of individuals into or out of the population

3. Mutation- introduction of a new allele that becomes established in the gene pool

4. Natural Selection- differential reproductive success, due to environmental pressure on a favorable phenotype

5. Non-Random mating -mate choice is no longer based on equal chance or opportunity. Mate choice has become selective and based on some characteristic

Day 5: Causes of Microevolution

Evolution occurs at the population level, population is defined as a group of the same species that live in the same area and interbreed, producing fertile offspring.

Hardy Weinberg equilibrium theory states that a population’s allele frequencies will remain unchanged generation after generation, no evolution, if the following 5 conditions are held constant:

Mutations do not change gene pool Mating is random and each organism has equal opportunity No natural selection, no phenotype is more favorable Population is large and contains variation No gene flow (emigration, immigration in/out of population)

Day 5: HW equilibrium

p2 + 2pq + q2 = 1

p = dominant allele q = recessive allele

p2= Homozygous dominant genotype (AA)

2pq= Heterozygous genotype (Aa)q2= Homozygous recessive

genotype (aa)

Day 5: The equationused to determine if a population’s allele frequency is changing, in other words is the population “evolving”?

Each letter represents the frequency of a particular allele in the population p+q=1

We can look at a population and identify specific traits or phenotypes.

We can actually count the number of individuals with those specific traits.

Ex. If there are 100 pigs 25 of them are black and 75 of them are pink, or 25% is black and 75% is pink.

What if you knew the black allele was Dominant and the pink allele was Recessive. Could you determine which ones had which genotype?

Day 5: using the equation

If B= Black skin and b= pink skin in a pig If 25% of the population were black and 75% were

pink, how many of them are ◦ Homzygous recessive bb◦ Homzygous dominant BB◦ Heterzygous Bb

• Remember that p is the dominant allele and q is the recessive allele.

• What does bb, BB, and Bb look like?• BB- black Bb- black bb- pink• p2- black 2pq- black q2= pink • Can we calculate q? yes if we know q then we can find p

Day 5: HW equilibrium problem

Day 5: practice problems

q= 0.6a. a.0.6, b. 0.4, b. c. AA=.16 and Aa= .48

1. A randomly mating population has an established frequency of 36% for organisms homozygous recessive for a given trait. What is the frequency of the recessive allele in the gene pool ?

2. You have sampled a population in which you know that the percentage of homozygous recessive genotypes (aa) is 36%. Calculate the following

a. The frequency of the a allele

b. The frequency of the A allele

c. Frequency of AA and Aa