|ZIMMER and EMLEN
||FREEMAN and HERRON
8.1 Evolution in a Bird's Beak p. 220
Peter and Rosemary Grant on Daphne Major in the Galapagos (Fig.
- Diversity in
- Medium ground finch
- Variation in
beak size influences efficiency at eating different types of seeds
- Beak size evolution
- Drought resulted in more hard, woody seeds
- Favored larger beaked birds
- Natural selection is variable over
time (Fig. 8.5)
- Key Concepts
- Beak size influences fitness and is heritable
–Natural selection can cause change
- Directional selection favors
increases or decreases in the mean of a trait
- Stabilizing selection
favors average values of a trait
- Long-term studies reveal
fluctuation in the direction and strength of natural selection
Female Galapagos medium ground finch.
Photograph taken at Mangle Point on Fernandino Island in the
Galapagos Islands by
3.4 THE EVOLUTION OF BEAK SHAPE IN GALAPAGOS FINCHES
- Galapagos Finches
- Charles Darwin and the Voyage of the Beagle, 1831-1836
- 13 species of Galapagos and one Cocos finches [Figs. 3.6, 3.7]
- Isla Daphne Major [Fig.3.8
- Geospiza fortis on Daphne Major is a seed eater.
- beak size and shape correlate with seed size
- size and shape of the beak has an important consequence for fitness.
Testing Postulate 1: Are Populations Variable?
- Beak depth (and many other characteristics) are variable
- 1976 measurements form a bell shaped curve [Fig. 3.9]
- Variation in beak size and shape correlates with the food that is eaten.
Testing Postulate 2: Is Some of the Variation Among Individuals Heritable?
heritability of beak depth in Geospiza fortis [Fig. 3.10].
- issues that complicate how heritabilities are measured [Box 3.1]
- is the variation caused by environment or genetics?
- Fig 3.11 Genetic basis for beak development in Darwin's finches
Left: Differences in beak size and shape among six
Middle and Right: cross sections of the upper beak bud
in embryos at two stages of development. The cross sections have been
treated with a probe that stains mRNA made from the gene for bone
morphogenic protein 4, or BMP4.
Ground finches with larger beaks make BMP4 mRNA
earlier and in larger quantities
Testing Postulate 3: Is There an Excess of Offspring So That Only
Some Individuals Survive to Reproduce?
A drought in 1977 produced a dramatic selection event [Fig. 3.12]
even in normal times--89% of Geospiza conirostris die
in stable populations, each parent leaves on average of one offspring
in spite of astonishing reproductive capacity (biotic potential)
Testing Postulate 4: Are Survival and Reproduction Nonrandom?
a distinct subset of the population survived better [Fig. 3.13]
change in average beak depth is correlated to change in seed hardness
following heavy rains selection
was for smaller body and beak size.
Did Evolution Occur
the course of 30 years (Fig 3.15)
- evolution is a change in traits between generations
- Changes in weight, wing length, bill width, bill depth
Over same 30-year period birds evolved more pointed
beaks and significantly smaller body size.
The direction of selection is not constant
8.2 Mice in Black and White p. 224
- Three conditions are necessary for evolution by natural
selection to occur: (1) individuals must differ in their
expression of a trait; (2) the differences must be at least
partially heritable; and (3) some individuals must survive and
reproduce more effectively than others because of these
color variation affects fitness
- Light coat color evolved
independently in different populations (Fig. 8.7)
- Key Concepts
- Evolution in
response to natural selection is inevitable if:
- There is variation
in a trait
- Variation is heritable
- Some variants reproduce more
- Specific features of the environment can generate
natural selection on a trait
8.3 The Geography of Fitness p. 226
- Natural selection can lead to
variation in space -across habitats or environments- just as
dramatically as it can lead to variation in a single habitat
- Natural selection can be variable
- Gene flow can bring alleles to new locations
may increase or decrease fitness
- Aposematism favored only in areas
where coral snakes co-occur
- Key Concepts
- Natural selection can
lead variation over the geographic range of a species
The harmless red
milk snake (left) mimics the bright colours of the venomous
coral snake (right); mimicry is more pronounced (milk snakes are
darker) where the two species overlap; an example of Batesian mimicry
Adding migration to the H-W analysis: Migration as an evolutionary force
Empirical research on Migration and Allele frequencies:
Migration as a Homogenizing Evolutionary Force across Populations
8.4 Predators versus Parasitoids: When
Agents of Selection Act in Opposing Directions p. 228
of selection, such as storms, droughts, predators, or parasites,
can act in opposition
- for example, one selecting for large
individuals, another for small.
- The net effect can be stabilizing
selection for an intermediate trait value.
- Gall flies
induce plants to produce galls
- Gall diameter is variable and
- Stabilizing selection on gall size
- Intermediate size
favored (Fig. 8.11)
- Key Concepts
- Stabilizing selection results when agents of
selection act in opposing directions
- selection acts against the extremes of the frequency distribution of the
- fitness peaks at intermediate values of the trait
- over time, the mean value of the trait stays the same
- over time, the variance in the trait will decline
- selection on gall-making fly Eurosta solidaginis (fig.
8.5 Replicated Natural Experiments p. 230
Some characteristics can incur costs or benefits to the organisms
that sport them, and these costs and benefits can be considered
agents of selection.
- Net selection will depend on whether the costs
of producing or having the structure outweigh the benefits, or vice
- Eda involved in production of
lateral plates in stickleback (Fig. 8.12)
- Marine ancestral form
- Freshwater Derived form
- Lateral plate reduction recorded in
fossil record (Fig. 8.13)
- Eda complete allele favored in
- Eda low allele favored in freshwater populations
- Production of armor energetically costly
- Little predation pressure
- Eda low allele present at low frequency in marine
- Favored when introduced to freshwater
- Key Concepts
- Populations that independently experience parallel environmental
changes represent replicated natural experiments
Threeespine Sticklebacks (Figs. 16.16-16.18)
- 1.(a) Stickleback populations that occupy marine
environments are large and have prominent spines along their
- 2.(b) Stickleback species that live in limnetic or
open-water lake habitats consist of individuals that are small
and slim with relatively large eyes and spines.
- 3.(c) Lake populations that occupy benthic habitats near the
shores of a lake consist of larger, deeper-bodied individuals.
The genetics and adaptations of the Alaskan stickleback fish;
Zina Deretsky, National Science Foundation
8.6 Drinking Milk: A Fingerprint of
Natural Selection p. 234
- Strong selection can 'sweep' a favorable allele to fixation
within a population so quickly that large stretches of DNA
flanking the favorable allele also become fixed.
- Ability to digest
lactose as adults found in certain populations
- Lactase expression persists into adulthood
- Correlates with domestication of cattle
- Pattern of genetic
linkage indicates history of selective sweep (Fig. 8.15)
- Strong evidence for
positive selection on lactase persistence alleles
8.7 Humans as Agents of Selection p 237
- Humans can be a powerful selective force, shaping the food we eat
and the pets we keep.
selection in crop production
- Artificial selection has produced the COLE family of
- Numerous different vegetables are derived from a
single common ancestor related to wild mustard.
- Artificial selection in crop
- Gradual increase in cob size documented by
- Domestic dog diversity created in last 15,000 years
3.1 ARTIFICIAL SELECTION
Identification of alleles responsible for traits selected by
Figure 3.1 Wild and domestic tomatoes
Wild tomatoes have tiny fruit, like that of the currant
tomato on the left.
Domestic tomatoes are descended from tiny-fruited ancestors,
but as a result of artificial selection have large fruit, like that of the
Red Giant on the right.
Figure 3.2 A genetically determined difference in fruit size
These tomatoes are from sibling plants.
The one on the left carries only domestic alleles of the
fw2.2 gene. The one on the right carries, in addition, copies of the
wild (common in nature) allele. The fw2.2 gene encodes a protein that
represses fruit growth.
By selecting seeds from larger tomatoes, tomato farmers
artificially selected for plants that lacked the wild fw2.2 allele.
Fig 3.3 Domestic and wild varieties of Brassica oleracea
Chemical Warfare p. 240
- Many of the weed and pest species that plague human society
are simply evolving resistance to control measures.
- An understanding of evolutionary biology can lead to novel management
practices and slow the evolution of resistance in pest populations.
- Pesticides and herbicides act as agents of selection
- Resistance to
pesticides in houseflies
- Rapid evolution of herbicide resistance
- Alteration in EPSPS enzyme leads to Roundup resistance
- Creation of
refuges can slow the evolution of resistance
- Bt crops select for
resistance in pests
- Comes at a cost when Bt is not present
- Creation of Bt-free refuges favors Bt-susceptible insects
evolution of resistance
- Refuges are now required by law
Altered Environments and Invasive Species p 244
- Humans have changed the landscape and changed where species
- Although most species have vanished as a result, a few have
persisted, spread, and even become invasive.
cane toads have led to evolution of black snake populations
Hunting and Fishing as Agents of Selection 246
- Hunting and fishing have had lasting effects on the traits
of harvested populations, including altered behaviors, reduced
weapon or body size, reduced growth rates, and earlier sexual
- Evolution of shorter male horns due to hunting
- Cod fishing has
influenced life-history evolution (Fig. 8.26)
- Traits that
experience selection due to Human Hunting and Fishing (Table
- Key Concepts
- The speed of evolution depends on amount of genetic
variation and strength of selection
- Leads to rapid resistance in pest populations
understanding of evolutionary biology can lead to novel management