Chapter 4

Photo: R. Sadek

Darwin 1820’s
Med SKI

↓
theology
↓
naturalist
friends
↓
The
Beagle
 * collect

Darwin 1831 - 1836 a lot

specimens
of

amazon
england
s

lnihabited
volcanic
island
• Islands contain unusual collection of
organisms:

tortoises
giant
e.g. giant tortoises
08_01.jpg

Large marine iguanas

1.
Feed
an
algae
 Influenced by:
Thomas Malthus (Rev.) 1798: An Essay on the Principle of
economist _ Population

as populations increase, their resources become scarcer 

food shortages, famine, diseases, war etc.
↓
competition
Populations
increase
geometrically

Resources •

increase
arithmetically

Darwin 1836 - 1858

• 1838: Read Malthus’s work, observed 19th

Century population changes. He theorized
some individuals would have a competitive
advantage conferred by favorable
characteristics.
• Noted successes in artificial selection
cattle )
[Paultry
_ . . .
 scared of
- naturalists
Darwin 1859: finding
gaps
in his
theory

“Origin of Species or The

Preservation of Favoured
Races in the Struggle for
Life.”
 bestseller, sold out in one
day.
" "

wall as → Letter
about theory

of evolution
 The theory generated
great controversy

Darwin’s Theory of Natural Selection

-

The problem that Darwin sawed

.

• Organisms produce like organisms.

• Chance variation between individuals. can
same
– Some are heritable. do better than
others

• More offspring are produced each generation than

can survive.
• Some individuals, because of physical or behavioral
traits, have a higher chance of surviving than others
in the same population.
•  Adaptation
 Mechanisms of
inheritance were missing
From darwin

Gregor Mendel, 1866

• Augustinian Monk
– Studied garden pea (Pisum sativum).
(work discovered in 1900)
– Discovered that characteristics pass from
parent to offspring in form of discrete
packets called genes.
• Exist in alternate forms - alleles.

7
• Some prevent expression of others.
• The development of genetics
 physiological, developmental, population
genetics)
• = the single most important contribution to
evolutionary thought. subject
to selection

Concepts
Phenotypic variations among individuals results
from the combined effects of genes and
environment.

The Hardy-Weinberg equilibrium model

indicates evolutionary forces that change gene
frequencies.

Natural Selection is the differential survival and

reproduction among phenotypes.

Genetic drift is the random change in gene

frequency in a population having more effect in
small populations
3/5/2013 18
 Variation Within Populations
due to enu ?
genetic or

• Variation in Plant Populations

– Many plant species differ dramatically in form
from one elevation to another.
• Clausen et.al. found evidence of adaptation by
ecotypes to local environmental conditions in
Potentilla glandulosa.
– Distinctive ecotypes.

*sire
* shape

• Plants from 3 elevations were

plant diff due to ?
grown in gardens at 3 elevations low
mid diff env
(“Common garden approach”)
high →
adapted to

#
env → diff

genes
• Significant differences between
the plants
grow
snaeternatiue
all sites (gardens) from diff locations
hypothesis
 Environmental effects.
^
→ diff exists
genetic to the same enu
3 prediction all Plants
grow

• Lowland ecotypes did best in -

^ Mia
low
nigh
lowland gardens, less in other
locations.
• Mid-elevation plants did best in
mid-elevation.
rejects
null hypothesis  genetic basis.
 Ecotypes (populations
adapted to local
environmental conditions)

phenotypes
are affected
env
by gene
-
 Variation Within Populations
• Variation in Plant Populations
– Phenotypic differences (growth and flower
production) within clones grown at the 3
elevations are the result of environmental
differences phenotypes
• Phenotypic plasticity. not fixed
height
&
*

* vision
7

cool
ice
become
adapted →
sheet →
endemic species
species in warm
shrinks
condition →
found in
ecological
island

Variation in Alpine Fish Populations

Lake Lucerne, Switzerland Whitefish, Coregonus

 Variation in Animal Populations
isolated population
\

Isolate
→ no
interbreeding
• Genetic Variation in Alpine Fish cakes
offish species
– Receding glaciation 12,000 years ago  Movement
of cold adapted aquatic species into the headwaters
of glacial valleys that lace the Alps created clusters of
geographically isolated populations.
• Douglas and Brunner used microsatellite DNA to
conclude Coregonus populations are highly
diverse and exhibit a high level of differentiation.

Hardy Weinberg

• Hardy Weinberg principle states that in a population

mating at random in the absence of evolutionary
forces, allele frequencies will remain constant.
• Given 2 alleles
 p = frequency of allele 1
 q = frequency of allele 2
 P+q=1
(p+q)2 = p2+2pq+q2
↓
↓ home
homo recessive
dominant
 Calculating Gene Frequencies
• SS (81%) SA (18%) AA (1%)
– Frequency of S allele ?
• SS + 1/2SA = .81 + ½(.18) = .90
• P=0.9, q=1
• (p+q)2 = p2+2pq+q2

(.90)2 + 2(.9x.1) + (.10)2 = 1.0

 SS (81%) SA (18%) AA (1%)

P q
0.90 0.10
p P2 pq
0.90 0.81 0.09
q pq q2
0.10 0.09 0.01
 Conditions Necessary for Hardy-Weinberg
Equilibrium
• Random Mating
• No Mutations
• Large Population Size
• No Immigration
• Equitable Fitness Between All Genotypes

Likely, at least one of these will not be met and

allele frequencies will change.
• Potential for evolutionary change in natural populations
is very great.

Evolution by Natural Selection

• Natural selection, which changes genotypic and
phenotypic frequencies in populations, can result
in adaptation to the environment.
• Some individuals in a population, because of their
phenotypic characteristics, produce more
offspring that themselves live to reproduce.
– Natural selection can favor, disfavor, or
conserve the genetic make-up of a population.
 Stabilizing Selection
• Stabilizing selection acts to impede changes in a
population by acting against extreme phenotypes
and favoring average phenotypes.

Stabilizing Selection
e.g. Egg size in Ural Owls:
• Intermediate egg sizes 
higher hatching rates and
higher lifetime production
of fledglings by females.
1. Stabilizing Selection

• Birth weight in humans

Directional Selection
• Leads to changes in phenotypes by favoring an
extreme phenotype over other phenotypes in the
population.
 Directional Selection

E.g. Rapid Adaptation by

Soapberry Bugs:
• Soapberry Bug (Jadera
haematoloma) feeds on seeds
from family Sapindaceae
• Slender beaks to pierce fruit
walls.
• Introduced species of family
Sapindaceae vary in fruit
radius

Rapid Adaptation by Soapberry Bugs

• Beak length should be under

selection.
• Found close relationship
between fruit radius and beak
length.
 Disruptive Selection
• Disruptive selection creates bimodal distributions by
favoring two or more extreme phenotypes over the
average phenotype in a population.

Disruptive Selection

• E.g. Galapagos ground

finches, Geospiza fortis.

• Possible Reasons:
– Food availability
– Competition reduction
– Selective mating (song
differences?).
 Genetic drift
• Change in gene frequency due to random
processes (chance)

Evidence of Genetic Drift in Chihuahua Spruce, Picea

chihuahuana
• Now restricted to peaks of Sierra
Madre Occidental in N. Mexico.
• Populations were examined to
determine if the species has lost
genetic diversity due to reduced
population size.
• Significant positive correlation
between population size and
genetic diversity of study
populations.
 Genetic Drift:
Change Due To Chance
• Random processes such as genetic drift can change gene
frequencies in populations, especially in small
populations.
• Major concern of habitat fragmentation is reducing
habitat availability to the point where genetic drift will
reduce genetic diversity within natural populations.
• In general, genetic variation is lower in isolated and
generally smaller, island populations.
– Reduced genetic variation indicates a lower potential
for a population to evolve.
 Phoenicolacerta laevis (Gray, 1838)
Common wall lizard, Palm Island

Mainland

Photo: R. Sadek
 Genetic Diversity and Butterfly Extinctions
• Frankham and Ralls point out
inbreeding may be a contributor to
higher extinction rates in small
populations.
– Reduced fecundity, depressed
juvenile survival, shortened life-
span.
• Saccheri conducted genetic studies on
populations of Glanville fritillary
butterflies (Melitacea cinxia).
– Populations with highest levels of
inbreeding had highest probabilities
of extinction.