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Evolution
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Darwin’s Theory of Natural Selection
Summary of Key Points
Examples of Evolution by Natural Selection
What influences Phenotype - Nature vs. Nurture?
- “Common Garden” Experiments
- Molecular Methods
Types of Selection
Genetic Drift
Not Covered Yet
Speciation
Readings: Chapter 4
Charles Darwin (1809-1882)
Published On the Origin of Species (1859)
(Full Title: On the Origin of Species by
Means of Natural Selection, or The
Preservation of Favoured Races in the
Struggle for Life)
Provided a mechanism for evolution
(“natural selection”):
• Individuals vary
• Not all survive
• Those with favored traits tend to survive
• Traits are inherited
• Population changes (species evolve)
Excerpt from Darwin’s Book
As many more individuals of each species are born
than can possibly survive; and as, consequently,
there is a frequently recurring struggle for existence,
it follows that any being, if it vary however slightly in
any manner profitable to itself, under the complex
and sometimes varying conditions of life, will have a
better chance of surviving, and thus be naturally
selected. From the strong principle of inheritance,
any selected variety will tend to propagate its new
and modified form.
– Charles Darwin
Darwin’s theory provided a coherent explanation
consistent with geological observations
The fossil record is
littered with “fossils” successes that later
became failures!
B.P. 17.9
http://evolution.berkeley.edu/evosite/evo101/
Science 27 February 2009:
Vol. 323. no. 5918, pp. 1197 - 1201
Darwin’s theory was also consistent
with observable biogeographic patterns
Voyage of the Beagle (27 Dec. 1831 to 2 Oct. 1836)
http://en.wikipedia.org/wiki/Second_voyage_of_HMS_Beagle
HMS Beagle near Tierra del Fuego
Painting of the Beagle by Conrad Martens - http://en.wikipedia.org/wiki/Hms_beagle
Darwin’s Finches
• 14 finch species found on the Galapagos
Islands today.
• All originated from ≈ 30 individuals of the
same species of seed-eating finch that
colonized the islands 2 to 3 million years
ago, and evolved into different species on
different islands (adaptive radiation).
• Each species has unique beak adaptations
that have allowed it to survive and
reproduce in its specific environment.
• Beak size reflects a wide variation in diet:
seeds, fruit, nectar, blood, eggs, insects,
etc.
http://www.geo.cornell.edu/geology/GalapagosWWW/GalapagosGeology.html
Nothing makes sense,
except in light of evolution
- Theodore Dobzhansky
• There is grandeur in this view of life, with its
several powers, having been originally
breathed into a few forms or into one; and
that, whilst this planet has gone cycling on
according to the fixed law of gravity, from so
simple a beginning endless forms most
beautiful and most wonderful have been, and
are being, evolved.
- Charles Darwin
Variation within species
• All species show
variation
• Variation is due to
– Genetic
characteristics
– Environment
• Genetic traits can
be inherited
http://www.okc.cc.ok.us/biologylabs/Documents/Evolution%20Tutorial/variation.htm
Competition – not all individuals
survive to reproduce
• In the wild, almost all
species produce
many more offspring
than survive to
adulthood
• The offspring
compete for food
resources, favourable
locations, safety from
predators etc.
• Many die before
maturity
Salmon fry: offspring of two parent salmon
http://www.robertclarkphoto.com/site/index.php?set_albumName=darwin&id=darwin_08&option=com_gallery&Itemid=83&include=view_photo.php
Natural selection
• Statistically, offspring that
survive to reproductive
age are those best suited
to their environment
• The environment selects
those variations that give
best survival chances
• Variations that increase
survival or reproductive
chances are most likely to
be passed on to next
generation
• This process is called
natural selection
http://evolution.berkeley.edu/evolibrary/article//bergstrom_02
Artificial selection
• Variation has long been exploited by farmers and breeders
• By breeding selected animals it has been possible to
produce diverse strains or breeds of crops and livestock
http://en.wikipedia.org/wiki/Image:IMG013biglittledogFX_wb.jpg
http://www.cz-pes.cz/really-irish-wolfhound/dovolena/foto_uvod.jpg
Darwin – a retrospective analysis
• Darwin’s basic mechanism (natural selection) has withstood
the test of time.
• Since Darwin’s time, our understanding of this mechanism
has expanded greatly, aided by:
– Mendelian genetics (“genes” as mechanism of inheritance;
“alleles” as alternate forms of the same gene)
– Molecular genetics (DNA, RNA, proteins…)
– Ecological theory and observations (island biogeography, etc.)
• Darwin’s ideas have also been supplemented:
– Genetic drift
– (Endo)symbiosis
Peppered moth – a visible example of evolution in action
Speckled form 
Natural selection in
response to
environmental change
Black form 
http://ukmoths.org.uk/show.php?bf=1931
Example of evolution: peppered moth
•
•
•
•
•
•
•
Peppered moth Biston betularia was well camouflaged on forest trees
Rare black variants were conspicuous: eaten by birds
Trees were blackened during industrial revolution
BP 17.7
Black variant became less conspicuous than the speckled form
Black variant became more dominant in industrial areas
Species has evolved in response to pressure from another species (Homo sapiens)
With recent improvements in air quality – speckled form has reappeared
http://ukmoths.org.uk/show.php?bf=1931
Phenotype – “the observable
manifestation of a species”
Adjustment to an environment can occur two ways:
1) During the life of an individual (e.g. physiological adjustment or “acclimation)”
2) Over several lifetimes, genetically, via natural selection (e.g. “adaptation” by evolution)
Key question in ecology:
What is the role of genetic vs. environmental
factors in determining “phenotype?”
(“Nature vs. Nurture”)
Two examples:
1) Clausen Keck & Heisey’s “Common Garden” studies
2) Ted Case’s desert chuckwalla studies
Key question in ecology:
What is the role of genetic vs. environmental
factors in determining “phenotype?”
Two examples:
1) Clausen Keck & Heisey’s “Common Garden” studies.
2) Ted Case’s desert chuckwalla studies
Common Garden Experiment
Clausen, Keck & Heisey (1940)
Timberline (3050 m)
Mather (1400 m)
Stanford (30 m)
Fig. 4.25 & 4.26, Molles & Cahill 2008
Fig. 4.26, Molles & Cahill 2008
Summary of Common Garden Results
Fig. 4.27, Molles & Cahill 2008
Take-home messages:
• The common garden studies of Clausen, Keck
and Heisey proved to be a powerful
experimental tool for isolating and quantifying
genetic and environmental influences on
phenotype.
• These experiments helped define the concept
of “ecotype” – a locally adapted population
(subspecies)
What is the role of genetic vs.
environmental factors in determining
“phenotype?”
Two examples:
1) Clausen Keck & Heisey’s “Common Garden” studies
2) Ted Case’s desert chuckwalla studies
Variation Within Chuckwalla Populations
• Natural selection works on variation in a
heritable trait associated with fitness.
Fig. 4.3, Molles & Cahill 2008
– The relative genetic contribution of individuals to future
generations.
• Phenotypic variation in a desert lizard
– Chuckwallahs are large herbivores lizards found in SW US
and NW Mexico
– Ted Case studied body size variation at 12 sites.
– Higher elevations = more rainfall
– More rainfall = more food.
– Lizards from higher elevations were
• 25% longer
• 2x heavier
Fig. 4.5
Molles & Cahill 2008
Variation Within Chuckwalla Populations
• Phenotypic variation in a desert lizard
– Do these size differences arise because food availability
differs or because populations are genetically distinct?
– Christopher Tracy’s experiments:
• Reared juvenile chuckwallahs from 6
different populations under identical
conditions = common laboratory
conditions (c.f. common garden
experiment).
• Individuals collected from higher
elevations consistently grew larger
(Size must also be genetically
determined).
Fig. 4.6, Molles & Cahill 2008
We now have tools to understand these principles at a molecular level
Fig. 4.28
Molles & Cahill
The current challenge is to link the emerging “molecular” understanding with our
“ecological” understanding – a question of putting information into context.
Types of Natural Selection
• Stabilizing Selection
• Directional Selection
• Disruptive Selection
Stabilizing Selection
• Stabilizing selection acts to impede changes in a
population by acting against extreme phenotypes
and favouring average phenotypes.
• Most common in a stable environment.
Stabilizing Selection
• The incidence of human infant mortality is
higher for very heavy as well as for very light
babies.
Directional Selection
• Directional selection leads to changes in
phenotypes by favoring an extreme phenotype
over other phenotypes in the population.
• Most common in a changing environment.
Directional Selection
• Boag and Grant (1981) studied the effect of drought
on beak size in a population of medium ground
finches on Daphne Major.
• Drought resistant plants produced large seeds that
were difficult to crack.
Sample
size
Beak length Beak depth
(mm)
(mm)
Before
642
10.68
9.42
After
85
11.07
9.96
Directional Selection due to Human Pressures
(pp. 97-99, Molles & Cahill 2008)
• Atlantic Cod
• Mountain sheep
Disruptive Selection
• Disruptive selection creates bimodal distributions by
favoring two or more extreme phenotypes over the
average phenotype in a population.
• Common in heterogeneous environments.
Disruptive Selection
• Black-bellied seedcracker in Cameroon.
[Stopped here]
Evolution
• Change in gene frequencies within a
population over time.
• Can occur through
– Natural selection
• Can favour, disfavour, or conserve the genetic make-up of a
population
• 3 types of selection
– Genetic drift
• Change in allele frequencies in a population due to chance or
random events
• Especially in small populations
Hardy-Weinberg Principle
• Stable populations tend to behave according
the Hardy-Weinberg principle (see pp. 92-95) –
maintain stable allele frequencies.
– Large population size
– Random mating
– No mutations
• What happens when these assumptions are
violated (e.g. small, isolated populations, or
non-random mating)?
“Genetic Drift”
• Affects small, isolated populations
• Random events cause change in allele
frequencies over time
What is a Species?
• A group of individuals that
– are morphologically &
reproductively more similar to one
another than to other populations,
– share a singular ancestordescendant heritage,
Hermit warbler
– and are more or less reproductively
isolated.
Townsend's warbler
Speciation
• The process in which one or more
new species evolve from a single
ancestral species, through the
acquisition of reproductive isolation.
• Example: Common Yellow-throats
• Seven distinct subspecies or races of
yellow-throats in the US.
• Reproductively isolated.
• Would merge into a single
reproducing population if they
occupied the same territory.
• Over time if they continue to be
isolated they may become separate
species.
What Causes Speciation?
• Allopatric speciation = a single population becomes
spatially subdivided into multiple subpopulations.
• Parapatric speciation = a population expands into a
new habitat within the range of its parent species.
• Sympatric speciation = a single population forms
genetically distinct subpopulations with no spatial
isolation.
– Can occur due to assortative mating.
• Individuals choose to mate with individuals that are similar to
themselves.
Types of Speciation:
Figure 4.21, p.105