Chapter 8 - Natural Selection: Empirical Studies


8.1 Evolution in a Bird's Beak p. 220

  • Long-term studies show how natural selection itself can vary, whether that variation is in the direction of selection or in how quickly selection acts

Peter and Rosemary Grant on Daphne Major in the Galapagos (Fig. 8.1)

  1. Diversity in Darwin's finches (Fig. 8.2)
  2. Medium ground finch (Fig. 8.3)
    1. Variation in beak size influences efficiency at eating different types of seeds
  3. Beak size evolution (Fig. 8.4)
    1. Drought resulted in more hard, woody seeds
      1. Favored larger beaked birds
  4. Natural selection is variable over time  (Fig. 8.5)
  5. Key Concepts
    1. Beak size influences fitness and is heritable –Natural selection can cause change
    2. Directional selection favors increases or decreases in the mean of a trait
    3. Stabilizing selection favors average values of a trait
    4. Long-term studies reveal fluctuation in the direction and strength of natural selection

 Female Galápagos medium ground finch

Female Galapagos medium ground finch. Photograph taken at Mangle Point on Fernandino Island in the Galapagos Islands by Charles Sharp


  1. Galapagos Finches
    1. Charles Darwin and the Voyage of the Beagle, 1831-1836
    2. 13 species of Galapagos and one Cocos finches [Figs. 3.6, 3.7]
    3. Isla Daphne Major [Fig.3.8
    4. Geospiza fortis on Daphne Major is a seed eater.
    5. beak size and shape correlate with seed size
    6. size and shape of the beak has an important consequence for fitness.

Testing Postulate 1: Are Populations Variable?

  1. Beak depth (and many other characteristics) are variable
  2. 1976 measurements form a bell shaped curve [Fig. 3.9]
  3. Variation in beak size and shape correlates with the food that is eaten.

Testing Postulate 2: Is Some of the Variation Among Individuals Heritable?

  1. heritability of beak depth in Geospiza fortis [Fig. 3.10].
  2. issues that complicate how heritabilities are measured [Box 3.1]
  3. is the variation caused by environment or genetics?
    1. Fig 3.11 Genetic basis for beak development in Darwin's finches
      1. Left: Differences in beak size and shape among six species
      2. 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.
      3. 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 before breeding
  • 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 [Fig. 3.12]
  • following heavy rains selection was for smaller body and beak size.

Did Evolution Occur

  1. Evolution over the course of 30 years (Fig 3.15)
    1. evolution is a change in traits between generations
    2. Changes in weight, wing length, bill width, bill depth
    3. Over same 30-year period birds evolved more pointed beaks and significantly smaller body size.
    4. 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 differences.
  1. Coat color variation affects fitness (Fig. 8.6)
  2. Light coat color evolved independently in different populations  (Fig. 8.7)
  3. Key Concepts
    1. Evolution in response to natural selection is inevitable if:
      1. There is variation in a trait
      2. Variation is heritable
      3. Some variants reproduce more than others
    2. 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 over time.
  1. Natural selection can be variable across space
    1. Gene flow can bring alleles to new locations
    2. Alleles may increase or decrease fitness
  2. Aposematism favored only in areas where coral snakes co-occur (Fig. 8.8)
  3. Key Concepts
    1. Natural selection can lead variation over the geographic range of a species

 Nilk snake     Coral snake
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
Photographs from Wikipedia




Adding migration to the H-W analysis: Migration as an evolutionary force

  • migration: movement of alleles between populations = gene flow
  • One-island model [Fig. 7.4]
    • most gene flow is from the continent to the island.
    • Migration into a small island population can change both genotypic and allelic frequencies [Fig. 7.5]

Empirical research on Migration and Allele frequencies: Migration-selection balance

  • color pattern variation in Lake Erie water snakes [Figs. 7.6, 7.7]
    1. snakes on the mainland tend to be banded; on islands, unbanded to intermediate [Fig. 7.7]
    2. selection favors unbanded snakes on islands
    3. migration maintains banded individuals

Migration as a Homogenizing Evolutionary Force across Populations

  • homogenizing effect of gene flow on allele frequencies of red bladder campion on islands of different ages [Fig. 7.9]
    1. younger populations have more variation (between islands) as a result of founder effect
    2. migration homogenizes variation in intermediate age populations
    3. remnant older populations have more variation as a result of drift.
8.4 Predators versus Parasitoids: When Agents of Selection Act in Opposing Directions p. 228
  • Agents 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.
  1. Gall flies induce plants to produce galls (Fig. 8.9)
  2. Gall diameter is variable and heritable (Fig. 8.10)
  3. Stabilizing selection on gall size
    1. Intermediate size favored (Fig. 8.11)
  4. Key Concepts
    1. Stabilizing selection results when agents of selection act in opposing directions
stabilizing selection
  1. selection acts against the extremes of the frequency distribution of the trait
  2. fitness peaks at intermediate values of the trait
  3. over time, the mean value of the trait stays the same
  4. over time, the variance in the trait will decline
  5. selection on gall-making fly Eurosta solidaginis (fig. 9.26)
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 versa.
  1. Eda involved in production of lateral plates in stickleback  (Fig. 8.12)
    1. Marine ancestral form
    2. Freshwater Derived form
  2. Lateral plate reduction recorded in fossil record (Fig. 8.13)
  3. Eda complete allele favored in marine populations
  4. Eda low allele favored in freshwater populations
    1. Production of armor energetically costly
      1. Little predation pressure to counterbalance
    2. Eda low allele present at low frequency in marine environment
      1. Favored when introduced to freshwater
  5. Key Concepts
    1. Populations that independently experience parallel environmental changes represent replicated natural experiments

Threeespine Sticklebacks (Figs. 16.16-16.18)

  1. 1.(a) Stickleback populations that occupy marine environments are large and have prominent spines along their dorsal side.
  2. 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. 3.(c) Lake populations that occupy benthic habitats near the shores of a lake consist of larger, deeper-bodied individuals.

three-spined stickle backs

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.
  1. Ability to digest lactose as adults found in certain populations (Fig. 8.14)
    1. Lactase expression persists into adulthood
    2. Correlates with domestication of cattle
  2. Pattern of genetic linkage indicates history of selective sweep (Fig. 8.15)
  3. Strong evidence for positive selection on lactase persistence alleles (Fig. 8.16)
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.
  1. Artificial selection in crop production (Fig. 8.17)
    1. Brassica
      1. Artificial selection has produced the COLE family of vegetables; 
      2. Numerous different vegetables are derived from a single common ancestor related to wild mustard.
  2. Artificial selection in crop production (Fig. 8.18)
    1. Maize
    2. Tomato
    3. Rice
    4. Sunflower
  3. Gradual increase in cob size documented by archaeologists (Fig. 8.19)
  4. Domestic dog diversity created in last 15,000 years (Fig. 8.20)
  1. Identification of alleles responsible for traits selected by breeders
    1. Figure 3.1 Wild and domestic tomatoes
    2. Wild tomatoes have tiny fruit, like that of the currant tomato on the left.
    3. 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.
  2. Figure 3.2 A genetically determined difference in fruit size
    1. These tomatoes are from sibling plants.
    2. 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.
    3. By selecting seeds from larger tomatoes, tomato farmers artificially selected for plants that lacked the wild fw2.2 allele.
  3. 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.  
  1. Pesticides and herbicides act as agents of selection (Fig. 8.21)
  2. Resistance to pesticides in houseflies (Fig. 8.22)
  3. Rapid evolution of herbicide resistance (Table 8.1)
  4. Alteration in EPSPS enzyme leads to Roundup resistance (Fig. 8.23)
  5. Creation of refuges can slow the evolution of resistance
    1. Bt crops select for resistance in pests
      1. Comes at a cost when Bt is not present
    2. Creation of Bt-free refuges favors Bt-susceptible insects
      1. Slows evolution of resistance
    3. Refuges are now required by law

Altered Environments and Invasive Species p 244

  • Humans have changed the landscape and changed where species occur.
  • Although most species have vanished as a result, a few have persisted, spread, and even become invasive.
  1. Introduced cane toads have led to evolution of black snake populations (Fig. 8.24)

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 maturation.
  1. Evolution of shorter male horns due to hunting  (Fig. 8.25)
  2. Cod fishing has influenced life-history evolution  (Fig. 8.26)
  3. Traits that experience selection due to Human Hunting and Fishing (Table 8.2 )
  4. Key Concepts
    1. The speed of evolution depends on amount of genetic variation and strength of selection
    2. Leads to rapid resistance in pest populations
  5. An understanding of evolutionary biology can lead to novel management practices



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