Adaptation

Chapter 10: Studying Adaptation

Overview of Adaptation

  • Focus on the evolutionary analysis of form and function in natural populations.

Fundamental Questions

  • Is a trait an adaptation?
       - Not all traits increase fitness; for example, chucks in male túngara frogs are not favored by natural selection (Futuyma, Evolutionary Biology).

  • What is the adaptive function (adaptive significance) of a trait?
       - Sometimes the adaptive function is obvious, but often it is unclear.
       - What seems obvious may actually be incorrect; revealing the truth requires careful evolutionary analysis.

Current Utility vs. Historical Origin

  • Current utility should not be equated with the historical origin of a trait:
       - Demonstrating that a trait is functional does not explain how, when, or why it evolved.
       - A structure may appear useful today due to natural selection for its current function, but it could have originated for a different function or even for no purpose (exaptation).

Historical Misconceptions: Dr. Pangloss

  • Voltaire’s Dr. Pangloss:
       - Quote: “Things cannot be other than they are … Everything is made for the best purpose.”
       - This idea leads to false inferences across various fields trying to explain history from the present.

Case Study: Polar Bears

  • Question: Why are polar bears white?
        - Possible explanations: crypsis, thermoregulation, structural properties, protection from abrasion/bacterial degradation, or genetic drift (neutral).
        - Gloger’s Rule (1833): Heavily pigmented forms tend to be found in more humid environments, like near the equator.

  • Obvious Answer: Camouflage when hunting seals; however, observations indicate camouflage is not often used (Stirling, 1974; Anderson & Guravich, 1992).

  • Alternative Hypothesis:
       - Polar bears act as solar heat collectors.
       - Photographed under UV light, polar bears appear black as they absorb UV radiation.
       - Grojean et al. (1980) explained optical properties of polar bear fur:
         - Fur behaves like a “drawn quartz tube with an irregular inner surface,” reflecting UV towards the skin.

Synergy in Polar Bear Fur

  • The light-harvesting system in polar bear fur operates through cooperation among many hairs. Light enters a hair through scattering, travels a short distance, and may exit through subsequent scattering processes, eventually being either dissipated or absorbed by the black skin underneath.

Testing Hypotheses for Adaptations

  • Example: Giraffes with long necks thought to reach high leaves:
       - Initial Hypothesis: Long necks evolved to reach high foliage.
       - Research Findings: Badlangana et al. (2009) outlined cervical and thoracic dimensions indicating this may not be the case.
       - Alternative Hypothesis: Neck length resulted from sexual selection.
          - Male necks are 30-40 cm longer and 1.7 times heavier than females.
       - Longer-necked males reportedly win contests and are preferred by females (Pratt & Andersen, 1985; Simmons & Scheepers, 1996).

Requirements of Sexual Selection

  • A characteristic must:
       1. Be exaggerated in one sex.
       2. Be used in dominance contests.
       3. Have no immediate survival benefits.
       4. Incur survival costs.
       5. Show positive allometry.
       6. Exhibit historical increases in size not correlated with other body parts.

  • Current Findings: Mitchell et al. (2009) suggested that giraffes do not fit all criteria to support the sexual selection hypothesis, indicating that the high browse hypothesis is more likely.

Conflicting Research on Giraffes

  • Simmons & Altwegg (2010): Countered Mitchell’s claims about neck length and sexual dimorphism.

  • Mitchell et al. (2013) rebutted, stating that neck length and mass do not show sexual dimorphism adequately, concluding that the high browse hypothesis holds.

Conclusion on Adaptation Hypotheses

  • Charles Darwin: “The preservation of each species can rarely be determined by any one advantage, but by the union of all, great and small.”

  • Importance of testing hypotheses rigorously; not merely accepting plausible explanations.

Methods in Testing Adaptation Hypotheses

  • Experimental Approaches

  • Observational Studies

  • Comparative Method

Case Study: Zonosemata Flies

  • Investigated if wing markings and waving behavior of Zonosemata mimic jumping spider threats.
       - Hypotheses:
          1. Zonosemata is not a mimic, wing markings are for courtship.
          2. The flies mimic jumping spiders to deter other predators.
          3. The flies mimic jumping spider threat displays to deter jumping spiders.

Methodology for Testing Hypotheses

  • Treatment Group Analysis for Zonosemata:
       - Untreated vs. wing markings with waving.
       - Cut and re-glued wings for control.
       - Results indicated wing markings and wing-waving together deter jumping spider predation.

  • Mimicry does not deter other predators, as shown by various tests with arachnid and insect species.

Adaptation Study: Desert Iguanas

  • As ectotherms, desert iguanas maintain their internal temperature depending on the environment.
       - Function optimally within narrower temperature ranges (35°C - 43°C) than present in their natural habitat (15°C - 47°C).
       - Active choice of thermoregulation observed.

Observational Studies of Thermal Behavior

  • Behavioral thermoregulation is confirmed.

  • Two critical conditions must be shown to prove behavioral thermoregulation:
       1. Animals actively choose temperature ranges more than expected by random chance.
       2. Confirm the adaptive advantage of those choices.

Critical Study with Garter Snakes

  • Lab temperatures maintained at 28-32°C.

  • Field studies conducted with transmitters to monitor body temperature in Western terrestrial garter snakes (Thamnophis elegans).
       - Findings show active thermoregulation by selecting environments, demonstrating advanced ecological adaptation.

Phenotypic Plasticity Overview

  • Adaptations typically result from genotype-environment interactions.

  • Phenotypic plasticity allows individuals of the same genotype to display varied phenotypes in different environments.

  • To illustrate plasticity is adaptive, demonstrate its function and associated fitness benefits.

Daphnia Study on Phenotypic Plasticity

  • Phototactic behavior tested across clones under varied conditions.

  • Variability shown among genetically identical clones in response to light indicates strong plasticity in behavior influenced by environmental factors.

Findings on Plasticity in Daphnia

  1. Clonal behavioral variations in phototaxis.

  2. Plasticity is particularly pronounced in Lake Blankaart where fish presence influences behavior.

  3. Fish predation is proposed to select for plasticity; research indicates clones from heavy stocking periods display more adaptive plasticity.

Statistical Methods in Adaptation Studies

  • Comparative Method: Compares species that have evolved independently to understand constraints.

  • Uses statistical methods to analyze allometry and phylogenetic trees to track trait evolution.

Case Study: Kiwi Egg Size

  • Kiwi eggs are large relative to body size (25% of body weight).

  • Examining large egg benefits:
       - Potential for well-developed chicks requiring minimal parental care post-hatching.

Further Interpretations on Kiwi Evolution

  • Alternative perspective: Kiwis as phyletic dwarfs evolving from larger birds, not necessarily meaning larger eggs equate to direct advantages in ecological fitness.

Evolutionary Trade-offs: Begonia Flower Size

  • Begonia's monoecious nature dominates adaptability; hermaphroditic flowers face selective pressures for size (male vs. female).

  • Evolutionary trade-off leads to intermediate flower sizes.

Fuchsia Flower Color Adaptive Hypotheses

  • Flower color changes from green to red once pollination is complete; red dissuades pollinators from nectarless flowers.

  • Physiological constraints propose that color retention is necessary for pollination cues.

Conclusions from Fuchsia Studies

  • Data suggest Fuchsia is also limited physiologically; its adaptations meet pollinator signaling needs without excessive energetic costs during reproduction.

Final Thoughts

  • Understanding evolutionary adaptations requires complex analyses balancing historical, ecological, and physiological factors to determine how traits developed and their functions in contemporary settings.