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.
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 individuals
phenotypes not
genotypes.
• Populations evolve
– Individual’s genes
will stay the same
– Population’s gene
pool may change
Population Genetics
• Which trait was a better adaptation and
selected for?
• How were frequencies of different alleles affected?
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
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
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
Do the following graphs show the distribution of
phenotypes for single-gene or polygenic traits? Explain.
type: single gene
type: polygenic
why? Only two
phenotypes possible
why? Multiple (many)
phenotypes possible
Example: tongue roller
or non-tongue roller
Example: height range
4feet to 9 feet all
Directional, Disruptive, and
Stabilizing Selection
• Three modes of 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.
a. Directional Selection: individuals at
one end of the curve have higher fitness
so evolution causes increase in
individuals with that trait
Key
Food becomes scarce.
Low
mortality,
High
high
fitness mortality,
low fitness
♦ Individuals with highest fitness: those at
one end of the curve
♦ Example: Galapagos finches – beak size
Directional Selection (page 398)
Key
Directional Selection
Low mortality,
high fitness
Food becomes scarce.
High mortality,
low fitness
b. Stabilizing Selection: individuals at the
center of the curve have highest fitness;
evolution keeps center in the same position
but narrows the curve
Stabilizing Selection
Key
Individuals with
highest fitness:
near the center of
the curve
(average
phenotype)
Example: human
Low
mortality,
High
high fitness
mortality, low
fitness
Birth Weight
Selection
against both
extremes
keep curve
narrow and
in same
place.
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.
c. Disruptive Selection: individuals at
both ends of the curve survive better
than the middle of the curve.
Disruptive Selection
Largest and smallest seeds become more common.
High mortality,
low fitness
Population splits
into two subgroups
specializing in
different seeds.
Beak Size
Number of Birds
in Population
Low mortality,
high fitness
Number of Birds
in Population
♦ Individuals with highest fitness: both
ends of curve
Key
Beak Size
♦ Example: birds where seeds are either
large or small
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)
Modes of Geographic
Isolation
Allopatric speciation
describes speciation that
takes place in populations
with geographically separate
ranges. Gene flow is
interrupted and new species
evolve.
Sympatric speciation
describes speciation that takes
place in geographically overlapping
populations.
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
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
How quickly do new species form?
Punctuated Equilibrium
Gradualism
Many transitional forms
between common ancestor
and modern organisms
Sudden shifts in form from an
ancestor to newer organism, so
not many transitional forms
between common ancestor and
modern organisms
Phylogeny
• Study of
evolutionary
relationships
among organisms;
Phylogenetic Tree- A
diagram of how
organisms are
related based on
the nucleotide
sequences in the
DNA.
Patterns of Evolution
• Convergent Evolution
• Divergent Evolution
– Adaptive Radiation
– Artificial Selection
• 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