Function and in biology are complex topics. The 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 .
This theory has strengths, like explaining apparent purposefulness in biology, but also faces criticisms. Alternative approaches, like , 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 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