Transcript The Hardy-Weinberg equation can test whether a population is

Natural Selection affects populations
in a variety of ways
Stabilizing selection
Directional selection
Disruptive selection
Sexual selection
Natural Selection acts on
phenotypes present
Original
population
Stabilizing selection
Evolved
population
Directional selection
Pressure of
natural selection
Disruptive selection
Frequency of
individuals
Original population
Original
population
Evolved
population
Stabilizing selection
Phenotypes (fur color)
Directional selection
Disruptive selection
Sexual Selection
• Differential mating of males
• Competition among males for dominance
and mating privileges with the group.
• Female choice
Sources of Variation
Mutation
Sexual Reproduction
Diplody- 2 copies of each chromosome allows for
recessive alleles that maintain variation in a gene pool.
Polyplody- even more variation
Mutation and sexual reproduction
produce the genetic variation that makes
evolution possible
 Organisms typically show individual variation.
 However, in The Origin of Species, Darwin could
not explain
– the cause of variation among individuals or
– how variations were passed from parents to offspring.
 We now know that mutations are
– changes in the nucleotide sequence of DNA and
– the ultimate source of new alleles and new traits.
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Evolution occurs within populations
 A gene pool is the total collection of genes in a
population at any one time.
 Microevolution is a change in the relative
frequencies of alleles in a gene pool over time. A
change within a population.
 Population genetics studies how populations
change genetically over time
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The Hardy-Weinberg equation can test whether a
population is evolving
 To understand how microevolution works:
– Consider an imaginary population of iguanas with
individuals that differ in foot webbing.
– Let’s assume that foot webbing is controlled by a
single gene and the allele for non-webbed feet (W)
is completely dominant to the allele for webbed feet
(w).
The Hardy-Weinberg equation can test whether a
population is evolving
 The shuffling of alleles that accompanies sexual
reproduction does not alter the genetic makeup of
the population.
– No matter how many times alleles are segregated into
different gametes, and united in different combinations by
fertilization, the frequency of each allele in the gene pool
will remain constant unless other factors are operating.
– This equilibrium is the Hardy-Weinberg principle,
named for the two scientists who derived it independently
in 1908.
The Hardy-Weinberg equation can test whether a
population is evolving
 To test the Hardy-Weinberg principle, let’s look at two
generations:
 Figure 13.10B shows the frequencies of alleles in the gene
pool of the original population.
 From these genotype frequencies,
we can calculate the frequency
of each allele in the population.
The Hardy-Weinberg equation can test whether a
population is evolving
 If a population is in Hardy-Weinberg equilibrium,
allele and genotype frequencies will remain
constant, generation after generation.
 The Hardy-Weinberg principle tells us that
something other than the reshuffling processes of
sexual reproduction is required to change allele
frequencies in a population.
The Hardy-Weinberg equation can test whether a
population is evolving
 For a population to be in Hardy-Weinberg
equilibrium, it must satisfy five main conditions.
There must be
1. a very large population,
2. no gene flow between populations,
3. no mutations,
4. random mating, and
5. no natural selection.
Rarely are all five conditions met!!
CONNECTION: The Hardy-Weinberg equation is
useful in public health science
 Public health scientists use the Hardy-Weinberg
equation to estimate how many people carry alleles
for certain inherited diseases.
 One out of 10,000 babies born in the United States
has phenylketonuria (PKU), an inherited inability to
break down the amino acid phenylalanine.
 The health problems associated with PKU can be
prevented by strict adherence to a diet that limits the
intake of phenylalanine.
CONNECTION: The Hardy-Weinberg equation is
useful in public health science
 PKU is a recessive allele.
 The frequency of the recessive allele for PKU in the
population, q, equals the square root of 0.0001, or
0.01.
– The frequency of the dominant allele would equal
1 – q, or 0.99.
– The frequency of carriers = 2pq = 2  0.99  0.01 =
0.0198 = 1.98% of the U.S. population.
– Thus, the equation tells us that about 2% (actually 1.98%)
of the U.S. population are carriers of the PKU allele.
The Hardy-Weinberg equation
can test whether a population is evolving
 The Hardy-Weinberg principle states that
– within a sexually reproducing, diploid population,
– allele and genotype frequencies will remain in
equilibrium,
– unless outside forces act to change those frequencies.
 The Hardy- Weinburg equation can measure
microevolution.
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Stable Gene Pool
The frequency of an allele will not change IF:
1. There is no mutation.
2. The population is large.
3. The population is isolated. (no immigration or emigration)
4. Mating is random.
5. All individuals survive and produce the same
number of offspring. (no natural selection)
This is known as Hardy-Weinberg Equilibrium.
Hardy-Weinberg Equation
• Measures changes in allele frequency
• Uses a formula to track changes from
equilibrium.
p2 (AA) + 2pq(Aa)+ q2 (aa) = 1.0
(100%, whole population)
What factors will change allele
frequencies?
1. Mutation.
2. Small population
3. Emigration and immigration
4. Mate selection
5. Natural Selection
Alleles are changing = Species are evolving!
Small populations have BIG genetic problems…..
• Genetic Drift
• Genetic Bottleneck
• Founder Effect
…reduced genetic variation, allele frequency changes
Original
population
Bottlenecking
event
Surviving
population
Natural selection, genetic drift,
and gene flow can cause
microevolution
The origin of species is the source of
biological diversity
 Microevolution is the change in the gene pool of a
population from one generation to the next.
 Macroevolution is the rising of new species from
existing species.
 Speciation is the process by which one species
splits into two or more species.
– Every time speciation occurs, the diversity of life
increases.
– The many millions of species on Earth have all arisen
from an ancestral life form that lived around 3.5 billion
years ago.
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MICROEVOLUTION v. MACROEVOLUTION
The origin of species is the source of
biological diversity
 Over the course of 3.5 billion years,
– an ancestral species first gave rise to two or more
different species,
– which then branched to new lineages,
– which branched again,
– until we arrive at the millions of species that live, or once
lived, on Earth.
Adaptive Radiation-one species give rise to other species
Why? … To better fit their particular environment,
Often accompanied by reproductive isolation,
Often in response to competition
There are several ways to define a species
 The morphological species concept
 The ecological species concept
 The phylogenetic species concept
There are several ways to define a species
 The word species is from the Latin for “kind” or
“appearance.”
 Although the basic idea of species as distinct lifeforms seems intuitive, devising a more formal
definition is not easy and raises questions.
 In many cases, the differences between two species
are obvious. In other cases, the differences between
two species are not so obvious.
Reproductive barriers keep species separate
 Reproductive barriers
– serve to isolate the gene pools of species and
– prevent interbreeding.
 Depending on whether they function before or after
zygotes form, reproductive barriers are categorized
as
– prezygotic or
– postzygotic.
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Reproductive barriers keep species separate
 Five types of prezygotic barriers prevent mating or
fertilization between species.
1. Habitat isolation,
2. Temporal isolation
3. Behavioral isolation
4. Mechanical isolation.
5. Gametic isolation
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Habitat isolation
(lack of opportunities to encounter each other)
The garter snake Thamnophis
atratus lives mainly in water.
The garter snake
Thamnophis sirtalis
lives on land.
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Temporal isolation
(breeding at different times or seasons)
The eastern spotted skunk
(Spilogale putorius) breeds in
late winter.
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The western spotted skunk
(Spilogale gracilis) breeds in
the fall.
Behavioral Isolation
Blue-Footed Boobies Courtship Ritual
https://www.youtube.com/watch?v=z922by9_6Fw
The Dance of the Male Bowerbird
https://www.youtube.com/watch?v=wCzZj21Gs4U
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Mechanical Isolation
• Mechanical isolation in plants- two sage species, the
black sage and white sage. Even though they grow in
the same area, they are pollinated by different insects.
• Mechanical isolation in animals- incompatible
reproductive parts/genitalia
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Gametic Isolation
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Reproductive barriers keep species separate
 Three types of postzygotic barriers operate after
hybrid zygotes have formed.
1. In reduced hybrid viability, most hybrid offspring do not
survive.
2. In reduced hybrid fertility, hybrid offspring are vigorous
but sterile.
3. In hybrid breakdown,
– the first-generation hybrids are viable and fertile but
– the offspring of the hybrids are feeble or sterile.
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PREZYGOTIC BARRIERS
Habitat isolation
(different habitats)
Temporal isolation
(breeding at different times)
Mechanical isolation
(incompatible reproductive parts)
Behavioral isolation
(different courtship rituals)
Gametic isolation
(incompatible gametes)
POSTZYGOTIC BARRIERS
Reduced hybrid vitality
(short-lived hybrids)
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Reduced hybrid fertility
(sterile hybrids)
Hybrid breakdown
(fertile hybrids with
sterile offspring)
Zygote
Gametes
Prezygotic barriers
• Habitat isolation
• Temporal isolation
• Behavioral isolation
• Mechanical isolation
• Gametic isolation
Postzygotic barriers
• Reduced hybrid
viability
• Reduced hybrid
fertility
• Hybrid breakdown
Viable,
fertile
offspring
Allopatric speciation takes place due to
geographical isolation
Gene flow is interrupted when a population is divided into
geographically isolated subpopulations.
Sympatric speciation takes place
without geographic isolation
 Sympatric speciation occurs when a new species
arises within the same geographic area as its
parent species.
 Gene flow between populations may be reduced by
– polyploidy,
– habitat differentiation
– sexual selection.
Patterns of Evolution
 Divergent Evolution: two or more species that
become different over time. (HOW?)
 Convergent Evolution: unrelated species with similar
traits
 Parallel Evolution:
– Marsupital and placental mammals
 Coevolution:
– Predator and prey
– Plants and pollinators
The Darwin Orchid- green tube that drops down to the ground in
the picture is where the moth would stick its proboscis to get to
the nectar.
Speciation can occur rapidly or slowly
 There are two models for the tempo of speciation.
1. The punctuated equilibria model draws on the fossil
record, where species
– change most as they arise from an ancestral species and then
– experience relatively little change for the rest of their existence.
2. Other species appear to have evolved more gradually.
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Also known as phyletic gradualism
An organism’s evolutionary history
is documented in its genome
 The more recently two species have branched from
a common ancestor, the more similar their DNA
sequences should be.
 The longer two species have been on separate
evolutionary paths, the more their DNA should
have diverged.
 Phylogenetic trees and cladograms illustrate
evolutionary relationships.
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At the end of this unit-You should now be able to
1. Explain how the organisms Darwin saw while on
his voyage on the Beagle influenced his thinking.
2. Explain how the work of Thomas Malthus and
Charles Lyell influenced Darwin.
3. Explain why individuals cannot evolve and why
evolution does not lead to perfectly adapted
organisms.
4. Explain how the fossil record, comparative
morphology (anatomy), comparative embryology
and comparative biochemistry(molecular biology)
support evolution.
You should now be able to
5. Explain what an evolutionary trees illustrates.
6. Define the gene pool, population genetics, &
microevolution, macroevolution.
7. Explain why mutation is required for the emergence
of new traits in a population.
8. Describe the five conditions required for the HardyWeinberg equilibrium (no change/evolution in a
species).
9. Explain the purpose of the Hardy-Weinberg
principles.
You should now be able to
10.Define genetic drift, the bottleneck effect and the
founder effect.
11.Describe adaptive radiation.
12.Describe 5 types of prezygotic barriers & 3 types of
postzygotic barriers to species interbreeding.
13.Compare the gradual model and the punctuated
equilibrium model of evolution.
14.Compare and contrast phylogenetic trees &
cladograms.