Transcript Chapter 11: The Evolution of Populations

CHAPTER 11
The
Evolution of
Populations
11.1 Genetic Variation Within Population
KEY CONCEPT
A population shares a common gene pool.
11.1 Genetic Variation Within Population
Genetic variation in a population increases the
chance that some individuals will survive.
• Natural selection acts on phenotypes
• To have different phenotypes a population must
have genetic variation
• The more genetic variation the more likely there
is a wide range of phenotypes
• The more phenotypes the more likely some
individuals can survive in a changing
environment
• Example: short, round penguins vs. tall, slim
penguins
11.1 Genetic Variation Within Population
• Genetic variation is stored in a population’s gene
pool.
– made up of all alleles in a population
– allele combinations form when organisms have
offspring
• Allele frequencies measure genetic variation.
– measures how common allele is in population
– can be calculated for each allele in gene pool
11.1 Genetic Variation Within Population
11.1 Genetic Variation Within Population
Genetic variation comes from several sources.
• Mutation is a random change in the DNA of a gene.
– can form new allele
– can be passed on to
offspring if in
reproductive cells
– increases genetic
variation
• Recombination forms new combinations of alleles.
– usually occurs during meiosis
– parents’ alleles
arranged in new
ways in gametes
11.1 Genetic Variation Within Population
Genetic variation comes from several sources.
• Hybridization is the crossing of two different species.
– occurs when individuals can’t find mate of own
species
– topic of current scientific research
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11.2 Natural Selection in Populations
KEY CONCEPT
Populations, not individuals, evolve.
11.2 Natural Selection in Populations
Natural selection acts on distributions of traits.
• Traits where all phenotypes provide equal chance of
survival
• A normal distribution graphs as a bell-shaped curve.
– highest frequency near
mean value
– frequencies decrease toward
each extreme value
• Traits not undergoing natural
selection have a normal
distribution.
• Change in the environment
can cause a certain
phenotype to become an
advantage
11.2 Natural Selection in Populations
Natural selection can change the distribution of a trait
in one of three ways.
• Microevolution is evolution within a population.
– observable change in the allele frequencies
– occurs on a small scale
– can result from natural selection
11.2 Natural Selection in Populations
• Natural selection can take one of three
paths.
– Directional selection favors phenotypes at one
extreme.
– Example: drug resistant bacteria
11.2 Natural Selection in Populations
• Natural selection can take one of three
paths.
– Stabilizing selection favors the
intermediate phenotype.
– Example: gall flies
11.2 Natural Selection in Populations
• Natural selection can take one of three
paths.
– Disruptive selection favors both
extreme phenotypes.
– Example: buntings (birds)
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11.3 Other Mechanisms of Evolution
KEY CONCEPT
Natural selection is not the only mechanism through
which populations evolve.
11.3 Other Mechanisms of Evolution
Gene flow is the movement of alleles between
populations.
• Gene flow occurs when
individuals join new
populations and
reproduce.
• Gene flow keeps
neighboring populations
similar.
• Low gene flow increases
the chance that two
populations will evolve
into different species.
bald eagle migration
11.3 Other Mechanisms of Evolution
Genetic drift is a change in allele frequencies due to
chance.
• Genetic drift causes a loss of genetic diversity.
• It is most common in small populations.
• A population bottleneck can lead to genetic drift.
– It occurs when an event
drastically reduces
population size.
– The bottleneck effect is
genetic drift that occurs
after a bottleneck event.
– Example: northern elephant
seals
11.3 Other Mechanisms of Evolution
• The founding of a small population can lead to genetic drift.
– It occurs when a few individuals start a new population.
– The founder effect is genetic drift that occurs after start
of new population.
11.3 Other Mechanisms of Evolution
• Genetic drift has negative effects on a population.
– less likely to have some individuals that can adapt due to
a loss of genetic variation
– harmful alleles can become more common due to
chance (carried by heterozygotes)
11.3 Other Mechanisms of Evolution
Sexual selection occurs when certain traits increase
mating success.
• Sexual selection occurs
due to higher cost of
reproduction for females.
– males produce many
sperm continuously
– females are more
limited in potential
offspring each cycle
11.3 Other Mechanisms of Evolution
• There are two types of sexual selection.
– intrasexual selection: competition among males
– intersexual selection: males display certain traits to
females
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11.4 Hardy-Weinberg Equilibrium
KEY CONCEPT
Hardy-Weinberg equilibrium provides a framework for
understanding how populations evolve.
11.4 Hardy-Weinberg Equilibrium
Hardy-Weinberg equilibrium describes populations that
are not evolving.
• Biologists use models to study populations.
• Hardy-Weinberg equilibrium is a type of model.
• Developed by Godfrey Hardy and Wilhelm Weinberg
• Showed that genotype frequencies In a population stay the
same over time as long as certain conditions are met
11.4 Hardy-Weinberg Equilibrium
Hardy-Weinberg equilibrium describes populations that
are not evolving.
• Genotype frequencies stay the same if five conditions are
met.
– very large population: no genetic drift
– no emigration or immigration: no gene flow
– no mutations: no new alleles added to gene pool
– random mating:
no sexual selection
– no natural selection:
all traits aid equally
in survival
11.4 Hardy-Weinberg Equilibrium
Hardy-Weinberg equilibrium describes populations that
are not evolving.
• Real populations rarely meet all five conditions.
– Real population data is
compared to a model.
– Models are used to
studying how populations
evolve.
11.4 Hardy-Weinberg Equilibrium
The Hardy-Weinberg equation is used to predict genotype
frequencies in a population.
• Predicted genotype frequencies are compared with actual
frequencies.
– used for traits in simple dominant-recessive systems
– must know frequency of recessive homozygotes
– p2 + 2pq + q2 = 1
"The Hardy-Weinberg equation
is based on Mendelian
genetics. It is derived from a
simple Punnett square in which
p is the frequency of the
dominant allele and q is the
frequency of the recessive
allele."
11.4 Hardy-Weinberg Equilibrium
In a population of 1000 fish, 640 fish have forked
tail (dominant) and 360 fish have smooth tails.
Find the frequencies of the population.
q² = 360/1000 = .36
q² = .36
q = .6
p+q=1
p = 1 - .6 = .4
p² = .4² = .16
2pq = (2)(.4)(.6) = .48
11.4 Hardy-Weinberg Equilibrium
There are five factors that can lead to evolution.
• Genetic drift changes allele frequencies due to chance
alone.
11.4 Hardy-Weinberg Equilibrium
• Gene flow moves alleles from one population to another.
11.4 Hardy-Weinberg Equilibrium
• Mutations produce the genetic variation needed for
evolution.
11.4 Hardy-Weinberg Equilibrium
• Sexual selection selects for traits that improve mating
success.
11.4 Hardy-Weinberg Equilibrium
• Natural selection selects for traits advantageous for
survival.
11.4 Hardy-Weinberg Equilibrium
• In nature, populations evolve.
– expected in all populations
most of the time
– respond to changing
environments
*
11.5 Speciation Through Isolation
KEY CONCEPT
New species can arise when populations are isolated.
11.5 Speciation Through Isolation
The isolation of populations can lead to speciation.
• Populations become isolated when there is no gene flow.
– Isolated populations adapt to their own environments.
– Gene pools may change
– Random processes can also change gene pools
– Genetic differences can add up over generations.
– Individuals in one population may also begin to look and
behave differently from the other population
11.5 Speciation Through Isolation
• Reproductive isolation can occur between isolated
populations.
– members of different
populations cannot
mate successfully
– final step to
becoming separate
species
• Speciation is the rise of
two or more species
from one existing
species.
11.5 Speciation Through Isolation
Populations can become isolated in several ways.
• Behavioral barriers can cause isolation.
– called behavioral isolation
– includes differences in courtship or mating behaviors
– Example: fireflies
11.5 Speciation Through Isolation
• Geographic barriers can cause isolation.
– called geographic isolation
– physical barriers divide population; rivers, mountains,
dried lakebeds
– Example: snapping shrimp
11.5 Speciation Through Isolation
• Temporal barriers can cause isolation.
– called temporal isolation
– timing of reproductive periods prevents mating
– Example: 2 tree species on the Monterey Peninsula
Monterey Pine
Bishop Pine
Monterey pine
sheds its
pollen in
February and
Bishop pine
sheds its
pollen in April
*
11.6 Patterns in Evolution
KEY CONCEPT
Evolution occurs in patterns.
11.6 Patterns in Evolution
Evolution through natural selection is not random.
• Natural selection can have direction.
• The effects of natural selection add up over time.
11.6 Patterns in Evolution
• Convergent evolution describes
evolution toward similar traits in
unrelated species.
• Different species adapting to
similar environments
• Analogous structures are
common examples
11.6 Patterns in Evolution
• Divergent evolution describes
evolution toward different traits
in closely related species.
• Closely related species
become increasingly different
How do
convergent and
divergent
evolution illustrate
the directional
nature of natural
selection?
kit fox
red fox
ancestor
11.6 Patterns in Evolution
Species can shape each other over time.
• Two or more species can evolve together through
coevolution.
– evolutionary paths become connected
– species evolve in response to changes in each other
11.6 Patterns in Evolution
• Coevolution can occur in
beneficial relationships.
• Bull-thorn acacia and stinging
ants
• Acacia has hollow thorns in
which the ants nest and feed on
nectar
• The ants in turn protect the plant
from predation from small
herbivores that can maneuver
between the thorn and prey on
the leaves
11.6 Patterns in Evolution
• Coevolution can occur in competitive relationships,
sometimes called an evolutionary arms race
• Each species responds to pressure from the other through
better adaptations over many generations
11.6 Patterns in Evolution
Species can become extinct.
• Extinction is the elimination of a species from Earth.
– Often occurs when a
species as a whole is
unable to adapt to a change
in its environment
– Can be divided into two
categories: background
extinctions and mass
extinctions
11.6 Patterns in Evolution
• Background extinctions occur continuously at a very low
rate.
– occur at roughly the same rate as speciation
– usually affects a few species in a small area
– caused by local changes in environment
11.6 Patterns in Evolution
• Mass extinctions are rare but much more intense.
– destroy many species at global level
– thought to be caused by catastrophic events
– at least five mass extinctions in last 600 million years
11.6 Patterns in Evolution
Speciation often occurs in patterns.
• A pattern of punctuated equilibrium exists in the fossil
record.
– theory proposed by Eldredge and Gould in 1972
– episodes of speciation occur suddenly in geologic
time
– followed by long periods of little evolutionary change
– revised Darwin’s idea that species arose through
gradual transformations
11.6 Patterns in Evolution
• Many species evolve from one species during adaptive
radiation.
– ancestral species diversifies into many descendent
species
– descendent species
usually adapted to
wide range of
environments
11.6 Patterns in Evolution
• Mammal Radiation
– Mammals use to be tiny,
mostly insect eating, mostly
nocturnal
– Characteristics allowed
them to coexist with
dinosaurs
– Extinction of the dinosaurs
left environments full of
opportunities for other types
of animals
– In the first 10 million years
more than 4000 mammal
species had evolved