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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
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