8.3 Selected effects theory and its alternatives

Function and teleology in biology are complex topics. The selected effects theory offers a way to understand trait functions based on evolutionary history. It argues a trait's function is the effect it was selected for by natural selection.

This theory has strengths, like explaining apparent purposefulness in biology, but also faces criticisms. Alternative approaches, like causal role theory, focus on a trait's current contributions instead of its evolutionary past. Understanding these perspectives is crucial for grasping biological functions.

Selected Effects Theory

Main Tenets

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• Argues a trait's function is the effect for which it was selected by natural selection in the past
• Distinguishes between a trait's function and its incidental effects
  • Only the effect that contributed to the trait's selection is considered its function
• Relies on an etiological approach, defining function in terms of a trait's causal history rather than its current dispositions or capacities
• Suggests a trait's function remains stable even if the trait no longer performs that function or if the selective environment changes
• Allows for the possibility of malfunctioning traits
  • A trait can have a function even if it is currently unable to perform that function
• Determines a trait's function by its "proper" function, the effect for which it was naturally selected, rather than its actual causal role

Implications and Consequences

• Provides a way to identify and define the functions of biological traits based on their evolutionary history
• Offers a framework for understanding the purpose or goal-directedness of biological systems in terms of natural selection
• Suggests that the functions of traits are not necessarily tied to their current causal roles or capacities
• Implies that traits can have functions even if they are not currently contributing to the organism's fitness or survival
• Raises questions about how to determine the specific selective pressures that shaped a trait's evolution in the past
• Highlights the importance of considering the evolutionary context in which traits evolved to understand their functional significance

Strengths and Weaknesses

Strengths

• Provides a clear and intuitively appealing account of biological function grounded in evolutionary history
• Offers a principled way to distinguish between a trait's proper function and its mere effects, avoiding the problem of functional promiscuity
  • Functional promiscuity: the idea that a trait can have many different effects, making it difficult to identify its primary function
• Can accommodate the possibility of malfunctioning traits, which is consistent with biological and medical usage of the concept of function
• Gives a naturalistic explanation for the apparent goal-directedness or purposefulness of biological systems
• Provides a way to connect the concept of function to the process of natural selection, which is central to evolutionary biology

Weaknesses and Criticisms

• Reliance on the notion of "selection for," which some argue is problematic or difficult to define precisely
  • It can be challenging to determine the specific selective pressures that drove the evolution of a particular trait
• Focus on a trait's evolutionary history may seem to neglect the importance of a trait's current causal role in the organism's survival and reproduction
• May have difficulty accounting for the functions of recently evolved traits or traits that have not been subject to long periods of selection
• Some argue that the theory is too restrictive and cannot account for all legitimate uses of the concept of function in biology
• The theory's emphasis on evolutionary history may not always align with the explanatory interests of biologists studying proximate mechanisms or current adaptations

Selected Effects vs Alternatives

Causal Role Theory

• Defines a trait's function in terms of its current causal contribution to the organism's survival and reproduction
• Ahistorical, focusing on a trait's present dispositions and capacities rather than its evolutionary history
• May better account for the functions of recently evolved traits or traits that have not been subject to extensive selection
• Emphasizes the importance of a trait's current causal role in maintaining the organism's fitness

Organizational Theory

• Defines function in terms of a trait's contribution to the self-maintenance of the organism or biological system
• Emphasizes the interdependence of traits and their roles in maintaining the integrity and stability of the organism
• Focuses on the synchronic organization of biological systems rather than their evolutionary history
• May better capture the systemic and holistic nature of biological functions

Pluralistic Approaches

• Suggest that there may be multiple legitimate concepts of biological function, each useful for different explanatory purposes
• Acknowledge the complexity and diversity of functional explanations in biology
• Allow for the coexistence of different function concepts, such as selected effects functions, causal role functions, and organizational functions
• Emphasize the pragmatic and context-dependent nature of functional ascriptions in biology

Applying the Selected Effects Theory

Analyzing Organ Functions

• The human heart's function is pumping blood, as this is the effect for which the heart was selected by natural selection
  • Distinguishes the heart's function from incidental effects (producing rhythmic sounds) which did not contribute to its selection
• The human appendix may have no current function, as it was likely selected for a different role in the evolutionary past (digesting cellulose)

Analyzing Behavioral Functions

• The honey bee waggle dance's function is communicating the location of food sources to other bees, as this is the effect for which the behavior was selected
• The function of bird songs is attracting mates and defending territories, as these are the effects that contributed to the selection of song production and recognition mechanisms

Analyzing Molecular Functions

• The oxygen-binding properties of hemoglobin were selected for their contribution to efficient oxygen transport in the bloodstream
  • The function of hemoglobin is to bind and transport oxygen, even if it also has other incidental effects (buffering blood pH)
• The function of enzymes is to catalyze specific biochemical reactions, as this is the effect for which their specific structures and active sites were selected
  • Enzyme specificity and efficiency are the result of selection for improved catalytic performance over evolutionary time