The Process of
Evolution
Population
Genetics and
Speciation
Edited by L. Bridge
October 2015
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
1
The Process of Evolution
“Population Genetics”
• Evolution occurs at the population level
– Genetic changes occur within a population
– Microevolution: A change in gene frequencies
within a population over time.
Example: The Peppered Moth, a case of industrial melanism
2
The Process of Evolution
• Population Genetics
– Population- all members of a species occupying a
particular area at the same time
– Gene pool- the sum total of all alleles of all genes
in a population
– Hardy and Weinberg used the binomial equation
p2+2pq+q2 to calculate the genotype and allele
frequencies in a population
3
Calculating Gene Pool Frequencies
Using the Hardy-Weinberg Equation
4
5
The Process of Evolution
• The data from the previous graphic shows
that the next generation will have exactly the
same ratio of genotypes as before:
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The Process of Evolution
The Hardy-Weinberg Principle
– Allele frequencies in a gene pool will remain at equilibrium,
thus constant, in each generation of a large sexually
reproducing population as long as the following five
conditions are met
•
•
•
•
•
No mutations
No genetic drift (meaning: need a LARGE population)
No gene flow
Random mating
No selection
– In real life these conditions are rarely met
– Hardy-Weinberg law gives us a baseline by which to access
whether or not evolution has occurred
• Any change in allele frequencies indicates evolution
7
Microevolution
1963, Bernard Kettlewell, “Industrial Melanism”
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Microevolution
A change in gene pool frequencies within a population over time
.
Tutorial (uses Flash player) http://www.techapps.net/interactives/pepperMoths.swf
Video of Kettlewell’s study (YouTube) http://www.youtube.com/watch?v=LyRA807djLc
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Microevolution (other examples)
• MRSA
• Rats/ Warfarin
– Warfarin has its effect by inhibiting an enzyme involved in
the metabolism of Vitamin K (Vitamin K oxide reductase)
• mosquitoes/DDT
Pesticide resistance, herbicide resistance,
and antibiotic resistance are all examples of
microevolution by natural selection. The
enterococci bacteria, shown here, have
evolved a resistance to several kinds of
antibiotics.
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Agents of Evolutionary Change
(1) Mutations: The raw material for
evolutionary change
• Only source of new alleles in a population
• Brings about variation that leads to adaptation
– Ex: Daphnia thrives at 20C but there is a mutation
that requires Daphnia to live between 25 – 30 C.
20 Celsius
27 Celsius
Do you think
mutations will play
an important role in
global warming?
27-11
Mutations
Quaker Parrot Color Mutation
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Agents of Evolutionary Change
(2) Genetic Drift
• Change in allele frequencies due to chance
• Can cause certain alleles to be lost from a population
http://highered.mcgrawhill.com/sites/dl/free/0072835125/126997/animation45.html
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Bottleneck Effect:
14
Founder Effect
Founder Effect: a few
individuals found a
colony and their
collective genes
represent only a
fraction of the original
gene pool
However, over time
these previously rare
alleles become
overrepresented
15
Agents of evolution cont’d.
(3) Gene flow: Sharing of genes between two populations
• Keeps the gene pools of 2 or more populations similar
• Prevents close adaptation to a local environment
– Ex: Pollen & Plant populations
(4) Nonrandom mating: Occurs when individuals pair up
according to phenotype or genotype
– Nonrandom mating can indirectly serve as an agent of
evolution by shifting the frequencies of genotypes in a gene
pool
•
Inbreeding is an example of nonrandom – increases frequency of
recessive abnormalities (“inbreeding depression”)
– Sexual selection
– Intersexual –vs- Intrasexual selection
27-16
Process of evolution
• Agents of evolutionary change cont’d.
(5) Natural selection
• Process by which populations adapt to their environment
• Charles Darwin explained evolution through natural selection
• Evolution by natural selection requires the following
– Variation-members of a population differ (Mutations increase gene variety)
– Inheritance-differences are inheritable
– Differential adaptedness-some differences have a survival benefit
– Differential reproduction-better adapted individuals survive to reproduce more
offspring
27-17
Process of evolution cont’d.
• Maintenance of variation
– Sickle cell disease is good example of how variation is sometimes
maintained, despite the detrimental effect of the allele
• People homozygous for sickle cell trait die from sickle-cell disease
• People homozygous for normal RBC’s in malaria endemic areas
die from malaria
• People who are heterozygous are protected from both severe sickle
cell disease and from malaria
– Since these people have one normal allele and one sickle
allele, both are maintained in the gene pool
– The favored heterozygote keeps the two homozygotes equally
present in the population. “Heterozygote Advantage”
Why doesn’t
natural selection
completely
eliminate the
defective
hemoglobin
allele?
27-18
Maintenance of detrimental
alleles in populations
• Phenylketonuria (PKU) is a relatively rare inherited genetic
disorder. Both parents must pass on the defective gene in order
for a baby to have the condition. This is called an autosomal
recessive trait.
• Babies with PKU are missing an enzyme called phenylalanine
hydroxylase, which is needed to break down an essential amino
acid called phenylalanine. The substance is found in foods that
contain protein.
• Without the enzyme, levels of phenylalanine and two closelyrelated substances build up in the body. These substances are
harmful to the central nervous system and cause brain damage
• QUESTION: Why is such a harmful gene maintained in the
population? Why hasn’t natural selection eliminated it?
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The Process of Evolution
• Natural Selection
– “Fitness”: in Darwinian terms, is measured by the
number of fertile offspring produced by an
individual
• Variations that can contribute to fitness can arise from
– Mutation
– Crossing over
– Independent assortment
– Most traits on which natural selection acts are
controlled by polygenic inheritance
– Range of phenotypes which follows a bell-shaped
curve
20
The Process of Evolution
• Natural Selection in populations
– Three Main Types of Natural Selection
• Stabilizing Selection
• Directional Selection
• Disruptive Selection
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Normal Distribution Curve
AKA The Bell Curve
• Pick a trait that varies among individuals in a
population (phenotypic)
22
The Process of Evolution
• Stabilizing Selection
– Occurs when an intermediate, or average,
phenotype is favored
– Improves adaptation of population to a stable
environment
– Extreme phenotypes are selected against
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Stabilizing Selection
24
27.3 The Process of Evolution
• Directional Selection
– One extreme phenotype is favored
– Distribution curve shifts in that direction
– Can occur when population is adjusting to a
changing environment
25
Process of evolution cont’d.
• Directional Selection
• Ex: evolution of the horse
Forests
Equus, the modern day
horse, which is adapted
to grassland habitat,
Is much larger than its
ancestor Hyracotherium,
which was adapted to
a forest habitat.
Hiding,
low-crowned
teeth
Combat,
Speed,
Grinding
teeth
27-26
Grasslands
27.3 The Process of Evolution
• Disruptive Selection
– Two or more extreme phenotypes are selected
27
Process of evolution cont’d.
• Disruptive Selection
– Ex: 2 British land snails each More difficult to see by birds in the
forest areas and therefore survive and
adapted to a particular habitat reproduce!
Grass fields birds
feed on snails with
dark shells that
lack light bands.
Forest areas birds
feed mainly on
snails with lightbanded shells.
More difficult to see by birds in the grass fields and therefore survive and27-28
reproduce!
How Sexual Reproduction promotes
genetic variation in offspring:
• Crossing over and recombination during
meiosis
– Segregation of alleles
– Crossing over during synapsis of
meiosishttp://www.execulink.com/~ekimmel/crossing_over.htm
– Independent assortment of
chromosomeshttp://www.execulink.com/~ekimmel/independent_assort
ment.htm
– Result: no two gametes from an individual are likely to carry the exact
same combination of alleles for each gene
• Random fusion of Gametes from two parents
– Which sperm cell will fuse with which egg cell?
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Questions for review
• How does sexual reproduction maintain variation in
populations?
• Diploidy?
• What is a polymorphic trait? Example?
• What is a neutral variation? Example?
• Why are some harmful alleles not eliminated by
natural selection?
• What is “heterozygote advantage”? Example?
• Why doesn’t Natural Selection produce perfect
organisms?
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SPECIATION
• The evolutionary process by which new
biological species arise
• The splitting of lineages
• Campbell text Chapter 24
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27.4 Speciation
• How do you define a “species”?
Tutorials: http://evolution.berkeley.edu/evosite/evo101/VADefiningSpecies.shtml
http://www.askabiologist.org.uk/from_the_lab/species-concepts
• Biological Species Concept (BSC): a species is a a group of
subpopulations that have the same chromosome # and are therefore capable of
interbreeding and are isolated reproductively from other species.
• Morphological Species Concept (MSC): species recognition is based on
overall similarity. Individuals do not have to be exactly the same as each other,
because there is variation in morphology among most species (think how variable
people are).
• Ecological Species Concept (ESC): This is used to describe populations
that are adapted to certain ecological niches and because of their adaptations will form
discrete morphological clusters.
• Phylogenetic (Cladistic) Species Concept (PSC): considers the
evolutionary relationships among organisms and relies on common ancestry and
shared evolutionary history to define species. Think of this as a distinct branch on an
evolutionary tree.
• What are some of the problems with each of these?
32
Divergent Evolution
•
Same/related species become more and more dissimilar over time
•
This is most often driven by geographic isolation. Continued cutoff of gene
flow between populations can lead to eventual speciation. Example pic
•
Adaptive radiation describes divergence of a single lineage into different
groups (species) driven by natural selection of different traits, as a result of
having adapted to widely different ecological niches.
•
1) Example: Darwin's finches
•
2)Example: According to both fossil and DNA evidence, the polar bear
diverged from the brown bear, Ursus arctos, roughly 150,000 years ago.
33
Convergent Evolution
• Unrelated species develop more and more similar
features due to adaptation to similar environments.
• The same "selective pressures" result in similar
outcomes.
Ex:
euphorb (from Sahara desert)
cactus (from Mojave desert)
Ex:
34
Coevolution (AKA “parallel evolution)
• The joint changes that occur in two or more species in
close interaction.
• As one changes, natural selection forces the other to
adapt to it. Organisms must adapt to "fit" each other.
• Examples: symbiotic organisms (parasites & hosts,
flowers & pollinators)
Each exerts a
selective pressure on
the other to “keep
up” or perish
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27.4 Speciation
• The Process of Speciation
– Occurs when one species give rise to two species
• Occurs when reproductive isolation develops
– Allopatric Speciation: geographical barriers
separate a population into two groups
• Premating and then postmating isolating mechanisms
occur
– Sympatric Speciation: occurs without
geographical barriers
• Ex: Plants- multiplication of chromosome number in one
plant may prevent it from successfully reproducing with
others of its kind.
– Self-reproduction can maintain a new species
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27.4 Speciation
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An example of how speciation
could occur
• Fig. 27.18
27-38
Allopatric Speciation
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Prebles meadow jumping mouse
Zapus hudsonius preblei
Figure 3. Map of showing distribution and subspecies of Zapus
hudsonius (Krutzsch 1954; Hafner et al. 1981). (1) Z. h.
preblei, (2) Z. h. campestris, (3) Z. h. pallidus, (4) Z. h. luteus, (5)
Z. h. intermedius, (6) Z. h. americanus, (7) Z. h. acadicus, (8) Z.
h.ladas, (9) Z. h. canadensis, (10) Z. h. hudsonius, (11) Z. h.
tenellus and (12) Z. h. alascensis. (Source: Ramey et al. 2005)
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27.4 Speciation
• Adaptive Radiation
– A specific type of (divergent) speciation
which gives rise to many new species
– Ex: Galapagos Islands finches- studied by
Darwin
• Mainland finches migrated to one of the islands
– Reproduced and eventually spread to all the islands
– Subjected to different environmental selection
pressures
• Gave rise to many species of finches which
differ primarily in beak shape
– Adapted to allow use of different food sources
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The Galapagos finches
• Fig. 27.19
27-42
27.4 Speciation
• The Pace of Speciation
– Phyletic Gradualism
• Change is slow but steady before and after a divergence
– Explains why so few transitional fossils are found
– Reproductive isolation cannot be detected in fossils
– Punctuated Equilibrium
• Long periods of stasis followed by rapid speciation
– Occurs relatively rapidly
– Also can explain lack of transitional fossils
» Rapid development of changes does not result in
recognizable transitional links
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Phyletic Gradualism Compared to
Punctuated Equilibrium
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Summary:
What is microevolution?
•Microevolution involves evolutionary changes
within a population, defined as changes in gene
frequency.
What is macroevolution?
•The process by which new species are
produced from earlier ones. This is the process
of evolution at the species level or above.
27-45
27.5 Classification
– Assignment of species to a hierarchy of
categories
– From general to specific these are:
domain, kingdom, phylum, class, order, family, genus, species
• Should reflect phylogeny
– Species within a genus are more closely related than those in
different genera, for example
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Five Kingdom System of
Classification
47
• Six-kingdom system
– Placed into a kingdom based on mode of
nutrition, type of cell, level of organization
• Previously, Kingdom Monera - prokaryotes
• Has been split into TWO bacterial kingdoms:
Eubacteria and Archaebacteria
• Kingdom Protista-eukaryotic single-celled and
multi-celled plant-like, animal-like, and fungallike organisms
• Kingdom Fungi- multicellular heterotrophic
saprophytic organisms
• Kingdom Plantae- multicellular photosynthetic
organisms
• Kingdom Animalia- multicellular heterotrophic
27-48
animals
Three Domain System
Based Upon rRNA
49
Classification cont’d.
• Three-domain system
– Based on rRNA
– Domain Bacteria
• “normal” bacteria
– Domain Archae
• Archaebacteria that survive in very harsh
environments
– Domain Eukarya
• Eukaryotic organisms
27-50