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Evolution of populations

Chapter 16
by

nicole shiers

on 9 March 2014

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Transcript of Evolution of populations

Background photo by t.shigesa
Natural selection determines the different survival of groups of organisms. Diversity increase the chance that at least some individuals will survive. Reproductive and geographic isolation affect speciation.
New mutations are constantly being generated in the gene pool. Variations within speices increase the likelihood at least some individuals will survive changing environmental conditions.
Evolution as Genetic Change
Natural selection acts on phenotypes not genotypes.
Genetic drift in small populations can cause alleles to become more or less common just by random chance.

The process of Speciation
Genes and Variation
Evolution of Populations
Does sexual reproduction change genotype ratios?
Natural selection acts on phenotypes not genes, it only determines who lives to reproduce and who does not
pg. 404-410
BB
BR
RR
Vocabulary
gene pool
relative frequency
single-gene trait
polygenic trait
Key Concepts
What are the main sources of heritable variation in a population?

How is evolution defined in genetic terms?

What determines the number of phenotypes for a given trait?
How common is genetic variation?
Many genes have at least two forms or alleles.

Animals and plants often have several alleles for traits
ex body shape or flower color

Organisms can have slight biochemical differences " that are invisible"

Organisms can also be heterozygous for traits
ex 15 % of insects are heterozygous for their genes
4-8% in fish, reptiles and mammals
Variation and Gene Pool
In genetic terms, evolution is an change in the relative frequency of alleles in a population.
allele for black fur
allele for brown fur
Frequency of Alleles
Sample Population
Gene Pool for Fur Color in Mice
Variation and Gene Pools
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Vocabulary
directional selection
stabilizing selection
disruptive selection
genetic drift
founder effect
genetic equilibrium
Key concepts
How does natural selection affect single-genes and polygenic traits?

What is genetic drift?
Natural selection on single-gene traits
Natural Selection on Polygenic traits
Natural Selection on
Single-Gene Traits
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Directional Selection 
When individuals at one end of the curve have higher fitness than individuals in the middle or at the other end, directional selection takes place.
Natural Selection on
Polygenic Traits
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Stabilizing Selection 
When individuals near the center of the curve have higher fitness than individuals at either end of the curve, stabilizing selection takes place.
Natural Selection on
Polygenic Traits
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Disruptive Selection 
When individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle, disruptive selection takes place.
Natural Selection on
Polygenic Traits
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Sources of Genetic Variation
Two Sources:

1. Mutations - any change in DNA sequence

2. Gene shuffling - chromosome seggregation during meiosis (gamete formation) and crossing over. 8.4 million different combinations of genes are possible
Single-gene and Polygenic Traits
Single-gene ( 2 alleles) ex. widow's peak
Polygenic - 2 or more genes often with 2 or more alleles ex. height

A random change in allele frequency
Genetic Drift
What is genetic drift?

Genetic drift
may occur when a small group of individuals colonizes a new habitat.

Individuals may carry alleles in different relative frequencies than did the larger population from which they came.
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Genetic Drift
Genetic Drift
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Genetic Drift
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Genetic Drift
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When allele frequencies change due to migration of a small subgroup of a population it is known as the
founder effect
.
Population B
Population A
Descendants
Genetic Drift
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Evolution Versus Genetic Equilibrium

The
Hardy-Weinberg principle
states that allele frequencies in a population will remain constant unless one or more factors cause those frequencies to change.

When allele frequencies remain constant it is called
genetic equilibrium
.
Evolution Versus Genetic Equilibrium

Five conditions

are required to maintain

genetic equilibrium

from generation to generation:

(1) there must be random mating
(2) the population must be very large
(3) there can be no movement into or out of the population
(4) there can be no mutations
(5) there can be no natural selection
Evolution Versus Genetic Equilibrium
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Random Mating

Random mating ensures that each individual has an equal chance of passing on its alleles to offspring.
In natural populations, mating is rarely completely random. Many species select mates based on particular heritable traits.
Evolution Versus Genetic Equilibrium
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Large Population

Genetic drift has less effect on large populations than on small ones.

Allele frequencies of large populations are less likely to be changed through the process of genetic drift.
Evolution Versus Genetic Equilibrium
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No Movement Into or Out of the Population 

Because individuals may bring new alleles into a population, there must be no movement of individuals into or out of a population.

The population's gene pool must be kept together and kept separate from the gene pools of other populations.
Evolution Versus Genetic Equilibrium
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No Mutations

If genes mutate, new alleles may be introduced into the population, and allele frequencies will change.
Evolution Versus Genetic Equilibrium
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No Natural Selection

All genotypes in the population must have equal probabilities of survival and reproduction.

No phenotype can have a selective advantage over another.

There can be no natural selection operating on the population.
Evolution Versus Genetic Equilibrium
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Vocabulary
- speciation
- reproductive isolation
- behavioral isolation
- geographic isolation
- temporal isolation
pg. 404 in your TEXTBOOK
Key Concepts
- What factors are involved in the formation of new species?

- Describe the process of speciation in the Galapagos finches.
Natural selection
and
chance
events can change the relative frequencies of alleles in a population and lead to speciation.

Speciation
is the formation of new species.

A species is a group of organisms that breed with one another and produce fertile offspring.
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What factors are involved in the formation of new species?



The gene pools of two populations must become separated for them to become new species.
Isolating Mechanisms
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Isolating Mechanisms

As new species evolve, populations become reproductively isolated from each other.

When the members of two populations cannot interbreed and produce fertile offspring,
reproductive isolation
has occurred.
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Behavioral Isolation
 
Behavioral isolation occurs when two populations are capable of interbreeding but have differences in courtship rituals or other reproductive strategies that involve behavior.
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Geographic Isolation 

Geographic isolation occurs when two populations are separated by geographic barriers such as rivers or mountains.
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Temporal Isolation 
 
Temporal isolation occurs when two or more species reproduce at different times.
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Testing Natural Selection in Nature

Studies showing natural selection in action involve descendants of the finches that Darwin observed in the Galápagos Islands.

The finches Darwin saw were different, but he hypothesized that they had descended from a common ancestor.
Testing Natural Selection
in Nature
Example:
- different frog species live in and breed in same pond, but
they reproduce at different times of the year.
http://channel.nationalgeographic.com/wild/videos/lions-tigers-and-ligers/
Ex:
Horse + donkey = mule
- Remaining pops often geographically isolated
which can lead to inbreeding and a lack of genetic diversity.

-Therefore vulnerable species by the International Union for Conservation of Nature and Natural Resources (IUCN),

-the Asiatic subspecies is endangered

One group of lion populations in West Africa is isolated from the rest of the lion populatons of Central Africa. With little or no opportunity for exchange of breeding individuals.
** loss of habitat, prey decline,
pesticides, canine distemper, tuberculosis,
hunted
The eastern meadowlark (left) and western meadowlark (right) have overlapping ranges. They do not interbreed, however, because they have different mating songs.
http://naturebits.org/NoAmBirds/icteridae.htm
Testing Natural Selection
in Nature
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Testing Natural Selection
in Nature
Copyright Pearson Prentice Hall
Peter and Rosemary Grant tested Darwin’s hypothesis, which relied on two testable assumptions:

For beak size and shape to evolve, there must be enough heritable variation in those traits to provide raw material for natural selection.
Differences in beak size and shape must produce differences in fitness, causing natural selection to occur.
Testing Natural Selection
in Nature
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When food was scarce, individuals with large beaks were more likely to survive.
Testing Natural Selection
in Nature
Copyright Pearson Prentice Hall
Speciation in Darwin's Finches

Speciation in the Galápagos finches occurred by:
founding of a new population

geographic isolation
changes in new population's gene pool
reproductive isolation
ecological competition
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A few finches—species A—travel from South America to one of the Galápagos Islands.
There, they survive and reproduce.
Founders Arrive 
Speciation in Darwin's Finches
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A few finches—species A—travel from South America to one of the Galápagos Islands.
There, they survive and reproduce.
Founders Arrive 
Speciation in Darwin's Finches
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Some birds from species A cross to a second island.
The two populations no longer share a gene pool.
Geographic Isolation
Speciation in Darwin's Finches
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Seed sizes on the second island favor birds with large beaks.
The population on the second island evolves into population B, with larger beaks.
Changes in the Gene Pool
Speciation in Darwin's Finches
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Reproductive Isolation

If population B birds cross back to the first island, they will not mate with birds from population A.

Populations A and B are separate species.
Speciation in Darwin's Finches
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TB pg. 399-400
pg. 400-402
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