Chapter 13Population Genetics

Question?

How did the diversity of life originate?

Through the process of Evolution.

Evolution

The processes that have transformed life on earth

Evolution: A change in a population’s gene pool over time.

Types of Evolution

Microevolution: Changes within a species (dozens or hundreds of generations)

Macroevolution: Origin of new species (much longer periods of time)

Galapagos Finches

Darwinian View

History of life is like a tree with branches over time from a common source.

Current diversity of life is caused by the forks from common ancestors.

Example

Darwin’s Voyage

Took a voyage on HMS Beagle

Used observations to make inferences about descent with modification

Observations:Observation 1 – Members of a

population often vary greatly in their traits.

.

Observation 2

Traits are inherited from parents to offspring.

Observation 3

All species are capable of producing more offspring than their environment can support.

Observation 4

Due to lack of food or other resources, many of these offspring do not survive.

Inference 1

Individuals whose inherited traits give them a higher probability of surviving and reproducing, tend to leave more offspring than other individuals.

Inference 2

This unequal ability of individuals to survive and reproduce will lead to the accumulation of favorable traits in the population over generations.

Natural selection

Nature determines which characteristics are favorable.

Natural Selection in action

Artificial Selection

When man determines the characteristics that are favorable

Result - the various breeds of animals and plants we’ve developed.

Ex - Mustard Plant

Original Cultivars

Evolution Success Measured By

Survival Reproduction Whoever lives long enough

and has the most kids is the “winner” in evolution.

Requirements

In order for Natural Selection to work, you must have: Variations within a population. Long periods of time (according

to Darwin).

Subtleties of Natural Selection

1. Populations are the units of Evolution.

2. Only inherited characteristics can evolve.

Changes in gene pools

Gene pool: all of the available genes in a population.

Allele frequency: How often an allele appears in a population

If evolution is not happening: allele frequency is not changing over time.

If evolution is happening: Allele frequency will change

Mechanisms for change

1. Natural selection: increased reproductive success leads to increased allele frequency and vice versa.

Mechanisms for change

2. Genetic drift: Random changes in small populations Founder effect- a small

founding population Bottleneck- only a small

population is left due to death

Mechanisms for change

3. Gene flow: change in gene pool resulting from migration of individuals between populations

Mechanisms for change

4. Mutation: direct conversion of one allele to another Some will be good Some will be bad Most will be neutral

Genetic Variation

Variation allows for populations to adapt

3 ways to get variation Mutations Crossing Over Independent Assortment

Inbreeding depression

Happens when variation is low because of small original population

Everyone is more closely related (more similar=less genetic variation)

Causes of Inbreeding Depression

Founder Effect Bottleneck Effect Both start with small

population and little genetic variation

Example

Cheetahs: Hunted to near extinction for hides

Small population that survived reproduced

Little genetic variation Prone to virus like HIV Prone to sterility

End of Part 1

Speciation

Species and Speciation

What is a species? A group of organisms

capable of breeding and producing fertile offspring under natural conditions

Keeping species separate

1. potential mates do not meet 2. potential mates meet but do

not breed 3. potential mates meet and

breed but do not produce fertile or viable offspring

Speciation

Speciation: the creation of new species

Happens when 2 populations of organisms become so different in their genetic makeup that they can no longer breed

Speciation

Mostly happens when a small population is isolated from the rest of the species

2 things that speed up speciation Bottleneck Founder

Types of Speciation

Allopatric- Speciation due to a physical barrier

Sympatric- Speciation in the same area (due to another type of barrier)

Reproductive Barriers

Any barrier that separates members of the same population into two different ones

Eventually these populations could become different species based on different selection factors

Isolating mechanisms

Prezygotic- isolation that happens before the zygote is formed. Geographical: Behavioral Temporal Gametic Mechanical Ecological

Prezygotic barriers

IsolationGeographical

Behavioral

Temporal

Mechanical

Gametic

Ecological

Separated by:Physical barriers

Courting rituals

Time -Day or Season

Anatomical featuresSperm and egg markers

Different niches

Isolating mechanisms

Postzygotic- after zygote is formed Hybrid inviability- won’t live Hybrid sterility- sterile Hybrid breakdown – hybrid’s

offspring won’t live

Adaptive Radiation

Rapid emergence of several species from a common ancestor ( often Allopatric speciation)

Common in island and mountain top populations or other “empty” environments.

Mechanism

Resources are temporarily infinite.

Most offspring survive. Result - little Natural

Selection and the gene pool can become very diverse.

When the Environment Saturates

Natural Selection resumes. New species form rapidly if

isolation mechanisms work.

Speed of speciation?

How long does it take a new species to form?

How many genes are involved?

Gradualism Evolution

Darwinian style evolution. Small gradual changes over

long periods time.

Gradualism Predicts:

Long periods of time are needed for evolution.

Fossils should show continuous links.

Punctuated Equilibrium

Evolution has two speeds of change:1. Gradualism or slow change

2. Rapid bursts of speciation

Possible Mechanism

Rapid: Adaptive Radiation, especially after mass extinction events

Gradual: Saturated environments favor gradual changes

End of Part 2