The Theory of Evolution

TheTheory of Evolution

Darwin’s Ideas

1. Natural Selection– A process in which some individuals have

genetically-based traits that improve survival or reproduction

– Thus, they have more offspring surviving to reproductive age than other individuals.

Darwin’s Ideas

2. Common Ancestry– All life forms share a

common ancestor– We are all in the same “tree of life”

The Definition of Evolution

Evolution is….• Descent with Modification

– Small-scale evolution: Changes in gene frequency in a population from one generation to the next. (microevolution)

– Large-scale evolution: The descent of different species from a common ancestor over many generations. (macroevolution)

Which of these represents descent with modification?

Big Beetles go through a drought and have limited food

Next generation is physically smaller due to lack of food

90% of the beetles have the gene for green color

30% of the beetles have the gene for green color

In this example, what exactly is being modified?

or

Phenotypic traits of a population are modified over time.

This is measurable as changes in “allele frequencies”

A change in allele frequencies in a population is evidence of

microevolution

Let’s further study how allele frequencies can change in an

activity called “Breeding Bunnies”

Basic requirements of Micro-evolution

1. There is variation in traits.(diversity)

2. There is differential reproduction.Due to selective forces in the environment, not all individuals get to reproduce to their full potential.

3.There is heredity.

How do populations introduce variation or diversity?

1. Mutation• A change in a DNA sequence

– usually because of errors in replication or repair.

• Mutation is the ultimate source of genetic variation.

2. Migration (Gene Flow) • Individuals from one group move into

another group.– Brings in new genes or changes frequency

of genes in the population.

How do populations introduce variation or diversity?

How do populations introduce variation or diversity?

3. Genetic DriftIn each generation, some individuals may, just by chance, leave behind a few more descendents (and genes, of course!) than other individuals.

But does genetic drift always increase diversity?

Let’s consider the

following game of

chance to find out

Genetic Drift: A game of chance• Imagine a game in which you have a bag holding 100

marbles– 50 of which are brown and 50 green.

• You are allowed to draw 10 marbles out of the bag. • Now imagine that the bag is restocked with 100

marbles– with the same proportion of brown and green marbles as you

have just drawn out. • The game might play out like this:

How does the diversity of the 4th generation compare to that of the

starting population?The variation has __________ in response to selective pressures

decreased

What effect does genetic drift have on smaller populations?

1. Genetic drift acts faster and has more drastic results in smaller populations. This effect is particularly important in rare and endangered species.

Examples of Genetic Drift

1. Population Bottleneck– An event in which a population’s

size is greatly reduced

Initial Diverse Population

Event that causes the population to be reduced in size

Final population

Northern Elephant Seal

Cheetah

Examples of Genetic Drift

2. Founder Effect– Changes in gene

frequencies that usually accompany starting a new population from a small number of individuals.

Driftworm Activity

• Turn your intNB to the driftworm activity, and get out: – Five colored pencils

• Color in Generation 0 of your driftworms

Driftworm Activity

• Each of these worms reproduces asexually

• The population size is constant• The generations do not overlap • These are haploid organisms

Driftworm Activity1. Roll the die.2. This number represents which worm

survives (by chance)and can be colored in for the next generation.

3. Do this four more times.

Driftworm Activity4. Repeat this in as many generations as

possible• That is, until there is only one phenotype

“fixed” in the population.

What factors prevent an allele from becoming “fixed” in a population?

1. Mutation2. Migration (Immigration and Emigration)

this is sometimes called “gene flow”3. Meiosis and Sexual Reproduction4. A large population (to avoid the effects of

genetic drift)

Genetic variation (diversity) is

required for what micro-evolutionary

process?

Natural Selection

• A process in which some individuals have genetically-based traits that improve survival or reproduction

• They have more offspring surviving to reproductive age than other individuals.

Natural Selection

It is often called “survival of the _________.”“Fit” often refers to a phenotype’s success in reproducing.But… is this term accurate? Is it always the biggest, fastest, strongest?Let’s read a cartoon about natural selection to find out…

fittest

Types of Selection

• Artificial Selection: – People choose plants and animals with

desirable traits to reproduce.– Nature does NOT select.

Types of Selection

• Directional Selection: – One allele is favored over another, and the

population shifts in one direction

Types of Selection

• Stabilizing Selection: – Genetic Diversity decreases as a population

stabilizes on a particular genetic trait.–Extreme traits are

selected against.

Types of Selection

• Disruptive Selection: – Selection that favors the extremes of the

distribution

Overview of the types of selection

Which of type of selection is/can be caused by the following…

1. Human birth weight Answer: Stabilizing Selection

2. The finches Darwin observed on the Galapagos Islands

Answer: Disruptive Selection3. Lighter moths being selected by

predators after the trees became covered with soot (post Industrial Revolution)

Answer: Directional Selection

Natural Selection: Adaptations

• Adaptations: Come in many forms and help the organism survive. It could be:– Behavioral: Behaviors that an organism does

to survive– Functional: An adaptation in which one

aspect of the organism has increased function in an environment

– Structural: Physical features on an organism that enable it to survive.

Q: The following are examples of what type of adaptation?

1. Katydids blending in with their surroundings.

Structural2. A bird’s mating call

Behavioral3. A protein working at human body

temperature and denaturing at higher temperatures.

Functional

Q: The following are examples of what type of adaptation?

1. Echolocation in bats searching for food Behavioral

2. A bird’s beak Structural

3. The non-poisonous milk snake has a banding pattern similar, but not the same as a poisonous coral snake.

Structural: This is mimicry!

What about odd features that don’t seem to serve any selective

function?

Vestigial Structures– An inherited feature that is now functionless

and usually less elaborate than in the ancestor.

– Formed when a species experiences a different set of selective pressures than its ancestors, and selection to maintain the elaboration and function of the feature ends.

Misconceptions about Natural Selection

• It is not “Survival of the Fittest” - really it is “Survival of the ‘fit enough’”– Recessive alleles for disease can stay

unnoticed in a population• Organism does not choose• Perfection is not obtained

Coevolution

• A process in which two or more different species reciprocally effect each other’s evolution. – species A evolves, which causes species B to

evolve, which causes species A to evolve, which causes species B to evolve, etc.

– For ex. Co-Evolution of Computer – virus

B

Types of Coevolution

1. Predator/prey and parasite/host 2. Competitive species 3. Mutualistic species

• Like plants and pollinators

Macroevolution(Speciation)

• Form of large-scale evolution• Descent of different species from a

common ancestor

What evidence is there for

macroevolution?

1. Fossil Record

2. Geographic Distribution of Living Species

3. Homologous Body Structures

4. Vestigial Organs

5. Similarities in Early Development

Mechanism of MacroevolutionFor macroevolution to occur, two populations must be reproductively isolated (or separated) from one another

The separated population experience different selective pressures

Over time, they evolve in separate directions, into different species that cannot interbreed

Brown blends in with tree trunks

Green blends in with grass

What is a species?

Two populations that can mate and produce fertile offspring.

Ex. donkeys and horses are considered different species because they produce mules, that are not fertile.

But… all breeds of dogs are the same species“mutts” are still fertile

Does speciation occur gradually or suddenly?

Gradualism:

- Phenotypic changes due to evolution accumulate slowly over time

- Speciation occurs gradually

Punctuated Equilibrium:

- Species stay relatively unchanged over time

- Speciation is driven by major events that drastically change the environment, forcing rapid evolution

- Evolution occurs in rapid bursts, as supported by gaps in the fossil record.

• Speciation occurs both gradually and punctually

• There is evidence to support both ideologies

Does speciation occur gradually or suddenly?