Transcript Population Evolution

Main Points of Darwin’s Theory of Natural Selection
1. Over production.
Most organisms produce more offspring than
can survive.
2. Competition.
Organisms compete for food and resources.
3. Variation.
There is variation among individuals of a
species.
4. Adaptation.
Individuals with traits best suited to the
environment will survive.
How Gene Frequencies Change
Sources of Variation
• Ways that new adaptations
and gene frequencies arise
– S – sexual reproduction
– C – crossing over during
meiosis (recombination of
genes)
– A – arrangement of
chromosomes
(alleles) during meiosis
– M – mutations of DNA
KEY CONCEPT
• Evolution by Natural Selection, causes
changes in POPULATIONS!
Population Evolution
Population genetics genetic
principles as they apply to entire
populations of organisms
Population group of organisms
of the same species living in the
same area
Genotype the representation
on the gene of an organism
Phenotype the physical trait
shown by a genotype
Allele  different form of a gene
Gene pool combined genetic
info. for all members of a
population
Population Genetics
• Natural selection acts
on individual’s
phenotypes not
genotypes.
• Populations evolve
– Individual’s genes
will stay the same
– Population’s gene
pool may change
over time due to
Natural Selection
Population Genetics
• Which trait was a better adaptation and
selected for?
• How were frequencies of different alleles affected?
Gene Traits:
A) Single gene trait: controlled by single
gene with two alleles
♦ Examples: widow’s peak, hitchhiker’s thumb,
tongue rolling
B) Polygenic trait: controlled
by 2 or more genes, each with 2
or more alleles
♦ Examples: height, hair color, skin
color, eye color
Most human traits are polygenic.
This type of variation can cause
different types of selection of
one phenotype over others
Directional, Disruptive, and
Stabilizing Selection
• Three modes of Natural Selection:
– Directional selection favors individuals at one
end of the phenotypic range
– Disruptive selection favors individuals at both
extremes of the phenotypic range
– Stabilizing selection favors intermediate
variants and acts against extreme phenotypes
© 2011 Pearson Education, Inc.
Three Types of Natural Selection
Stabilizing Selection = maintains an already existing
system.
> eliminates organisms that deviate from the norm.
> environment must remain unchanged.
> explains why there are “living fossils.”
> as long as environment doesn’t change,
organisms
Ginkgo Tree won’t change.
Horseshoe Crab
Stabilizing Selection
Stabilizing Selection
Key
Low mortality, high
fitness
High mortality, low
fitness
Birth Weight
Selection
against both
extremes keep
curve narrow
and in same
place.
Directional Selection = favors one extreme or the
other.
> eliminates organisms that are not in that
extreme.
> eventually leads to changes in the
population.
> occurs when organisms must adapt to a
change in their environment.
> may develop into a RESISTANCE (the ability
of an organism to withstand a harmful agent).
MALARIA
Directional Selection (page 398)
Key
Directional Selection
Low mortality,
high fitness
Food becomes scarce.
High mortality,
low fitness
Disruptive Selection = favors two extremes at
one time.
> eliminates organisms that are more
common.
> eventually leads to changes in the
population.
African butterflies
can range from red to
blue.
The red and blues
are foul-tasting to
predators.
The other colors are
eaten more often
resulting in a
selection in favor of
the extreme colors.
Disruptive Selection (pg 399)
Disruptive Selection
Low mortality,
high fitness
High mortality,
low fitness
Population splits
into two subgroups
specializing in
different seeds.
Beak Size
Number of Birds
in Population
Key
Number of Birds
in Population
Largest and smallest seeds become more common.
Beak Size
ORGIN OF SPECIES
What is a Species? A Group of interbreeding
organisms that can produce fertile offspring.
The Origin of Species
(Macroevolution)
Macroevolution
•
the formation of new species between
organisms (speciation) and
accompanying events
Microevolution
• refers to changes in allele frequencies in
a gene pool from generation to
generation. Represents a gradual
change in a population.
• Macroevolution
Microevolution
How Do New Species Form?
Speciation- formation of a new species
Causes of Speciation:
1. Geographic Isolation
2. Temporal Isolation
3. Behavioral Isolation
4. Ecological Isolation
Geographic Isolation
• Physical separation of members of
a population
(by formation of a canyon, mountain,
river, etc.)
– Leads to different adaptations on
each side of barrier, and eventually
new species (allopatric speciation)
Geographic Isolation = New
Species: An Example
Temporal Isolation
• Species isolated
because they
reproduce in different
seasons or times of
day
• Prevents species
from ever breeding
together – keeps two
species separate but
usually arises after
species have formed.
Behavioral Isolation
• Species Differ In
Their Mating Rituals
(e.g. different bird
songs, mating colors,
dances, pheromones,
etc.)
Ecological Isolation
– Species inhabit
the same area,
but different
habitats, so they
don’t encounter
each other
Other Factors Can Affect Genetic
Variation In A Population
• Other factors that increase
variation in the genetic material
(gene pool) of a population on
which natural selection acts:
– Random/nonrandom mating =
sexual selection
– Gene traits-single/polygenic
– Isolation
– Genetic Drift
– Fitness is the relative ability
of genotypes to survive and
reproduce
Microevolution
refers to changes in allele frequencies in a gene pool from
generation to generation. Represents a gradual change in
a population.
Causes of microevolution:
1) Genetic drift
2) Natural selection
3) Gene flow
(migration of genes from
one population to another)
2) Mutation
GENETIC DRIFT
DNA frequency in a population
changes simply by CHANCE not
fitness
Types of Genetic Drift
1. Founder’s Effect
some individuals of
a population
colonize a new
habitat
2. Bottleneck
Effect
few survivors are
left to reproduce
FOUNDER'S EFFECT:


some individuals of a population
colonize a new habitat
ex: Columbus bringing organisms to USA
Spain: DNA
Frequency
66% pink
33% red
S. America:
DNA
Frequency
25% pink
75% red
Bottleneck Effect:
Small representation of the
original population after a
catastrophic event. Alleles
in gene pool are not equally
represented and species
may evolve.
HOW DOES GENETIC DRIFT
AFFECT THE DIVERSITY OF A
POPULATION OVER TIME?
Genetic drift decreases diversity. In this case yellow
and pink DNA are gone, and red DNA is now the
majority.
Genetic drift = decrease in DNA diversity
Patterns of Evolution
• Convergent Evolution
• Divergent Evolution
• Coevolution
Convergent Evolution
• Organisms appear similar, because of similar habitat
and selection pressure. These organisms aren’t
closely related
• Why?
– Similar environments select for similar traits
Divergent Evolution
• accumulation of
differences between
groups which can
lead to the formation
of new species;
evolving from a
common ancestor
• Related populations
or species become
less and less alike
A Second Type Of Divergent
Evolution
• Artificial Selection
– Humans, rather than
nature, select traits
Coevolution
• Two or more species
in close association
w/one another
change together
– Predator/Prey
– Parasite/Host
– Plant/Pollinator