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Evolutionary Analysis

Fourth Edition

Chapter 10

Studying Adaptation:

Evolutionary Analysis of

Form and Function

Scott Freeman • Jon C. Herron

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Adaptation

A trait, or integrated suit of traits, that increases the fitness of its possessor is called an adaptation.
A plausible hypothesis about the adaptive value of a trait is the beginning of a careful study, not the end.

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10.1 All Hypotheses Must Be Tested: Oxpeckers Reconsidered

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Caveats for studying adaptation

1. Differences among populations or species are not always adaptive.
2. Not every trait of an organism, or every use of a trait by an organism, is an adaptation.
3. Not every adaptation is perfect.

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10.2 Experiments

Experiments are the most powerful method for testing hypotheses.
A good experiment restricts the difference between study groups to a single variable.

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Figure 10.7 Tephritid flies mimic jumping spiders to avoid predation

These graphs show how jumping spiders reacted to the fly types listed in Figure 10.6. The height of each bar represents the number of jumping spiders, out of 20, that showed each type of maximum response to each type of fly. Thus, for group A flies, 15 of the spiders retreated from their test flies [graph (a)], and five killed their test flies [graph (c)]. For group B flies, 15 of the spiders retreated from their flies, 2 stalked and attacked but did not kill their flies, and 3 killed their flies. From Greene et al. (1987). Most of the flies that waved marked wings (groups A and B) survived their encounters with spiders unscathed, whereas most of the flies that lacked markings or waving or both were attacked, and many were killed. The red bars at bottom identify treatment groups where the spider responses were statistically indistinguishable from one another. Groups A and B were indistinguishable from each other, but different from groups C, D, and E. Because each spider was presented with one fly of each type, the sample size in each treatment group was 20.

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Experimental Design

1. Defining and testing effective control groups is critical
2. All of the treatments (controls & experiments) must be handled exactly alike.
3. Randomization is a key technique for equalizing other, miscellaneous effects among control & experimental groups.

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10.3 Observational Studies

When an experiment is impractical, a careful observational study may be the next best method for evaluating a hypothesis.

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Behavioral Thermoregulation

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Physiological abilities of the desert iguana (Dipsosaurus dorsalis) as a function of body temperature

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Do Garter Snakes Make Adaptive Choices When Looking for a Nighttime Retreat?

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Body temperatures of garter snakes in nature

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10.4 Comparative Methods

The comparative method seeks to evaluate hypotheses by testing for patterns across species, such as correlations among traits, or correlations between traits and features of the environment.

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A graphical interpretation of the basic procedure in Felsenstein's method for evaluating phylogenetically independent contrasts

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Correlated evolution of group size and testes size in fruit bats and flying foxes

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10. 5 Phenotypic Plasticity

An individual’s phenotype is influenced by its environment is to say that its phenotype is plastic.
Genetically identical individuals reared in different environments may be different in form, physiology, or behavior. Such individuals demonstrate phenotypic plasticity.

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A water flea, Daphnia magna, asexual reproduction most of the time

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Variation in phototactic behavior in Daphnia magna

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Evolution of phototactic behavior in Daphnia magna

Oud Heverlee Pond

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10.6 Trade-Off & Constraints

It is impossible to build a perfect organism. Organismal design reflects a compromise among competing demands.

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Female Flower Size in a Begonia: A Trade-Off

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Begonia involucrata

(a) Male (left) (pollen reward) and female (right) flowers (no reward).

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Why Female Flowers Resemble the Male Flowers in Color, Shape and Size in a Begonia?

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Why Female Flowers Resemble the Male Flowers in Color, Shape and Size in a Begonia?

Hypothesis 1: Female flowers mimic typical male flowers.

Hypothesis 2: Female flowers mimic the most rewarding male flowers.

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Figure 10.21 An analysis of selection on female flower size in Begonia involucrata

(a) The two hypotheses investigated by Schemske and Ågren (1995). See text for more details. (b) Schemske and Ågren's three size classes of artificial flowers. The "mean" size class is the same size as the mean size of natural male flowers. (c) Pollinator preference as a function of flower size. The blue bars represent the number of bees that approached the artificial flowers; the brown bars represent the number of pollinators that actually visited the artificial flowers. Schemske and Ågren placed equal numbers of each size flower in the forest, but larger flowers attracted significantly more approaches and significantly more visits from bees. (d) Number of female flowers per inflorescence as a function of flower size. There is a statistically significant trade-off between flower size and flower number. From Schemske and Ågren (1995).

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An analysis of selection on female flower size in Begonia involucrata

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Flower Color Change in a Fuchsia: A Constraint

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Fuchsia excorticata

This bird-pollinated tree is native to New Zealand. Why do its flowers change color?

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Hypotheses for Why Fuchsia Keep the Flower & Color Change to from Green to Red?

1. Red flowers attract pollinators to the tree (no, experiments).

2. Physiological constraints prevent Fuchsia from dropping its flowers sooner than it does (yes, growth of pollen tubes).

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Host Shifts in an Herbivorous Beetle: Constrained by Lack of Genetic Variation

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Host Shifts in Feather Lice: Constrained by Dispersal Ability?

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10.7 Selection Operates on Different Levels

Selection on cells in populations versus selection on mitochondria in cells.
Among mitochondria within yeast cells, selection favors parasites, because they replicate faster.
Among yeast cells within populations, selection favors yeast containing normal mitochondria, because they can harvest energy by respiration as well as fermentation

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Parasitic mitochondria thrive in small yeast populations but fall to low frequency in large yeast populations

Five experimental populations started with yeast cells containing a mixture of normal versus parasitic mitochondria

Chloramphenicol resistance is selectively neutral at both levels of selection

Figure 10.27 Selection on cells in populations versus selection on mitochondria in cells

(a) Each bar represents the average of five experimental populations started with yeast cells containing a mixture of normal versus parasitic mitochondria. Among mitochondria within yeast cells, selection favors parasites, because they replicate faster. This selective advantage was constant across experiments. Among yeast cells within populations, selection favors yeast containing normal mitochondria, because they can harvest energy by respiration as well as fermentation. This selective advantage varies among yeast cultures maintained at different population sizes; it is weakest in small populations and strongest in large populations. Parasitic mitochondria thrive in small yeast populations but fall to low frequency in large yeast populations. (b) Each bar represents the average of four or five control populations started with yeast cells containing a mixture of chloramphenicol-resistant versus chloramphenicol-susceptible mitochondria. Chloramphenicol resistance is selectively neutral at both levels of selection. From Taylor et al. (2002).