Living things have certain characteristics. Organisms have physical boundaries and are only partially open to the flow of matter and energy. They are functional, possessing all of the energy capturing and processing capability needed for metabolism, and this keeps them far from thermodynamic equilibrium until they die, which they all eventually do. Living things capture not only energy, but also information from their surroundings, often responding to external conditions. Living things replicate themselves, either by sex or some other method.
Teleology, the idea of purposive behavior, has always been a major problem in biology. Jack and Cy liked to discuss function and purpose in terms of Aristotle's four causes of everything: What is it made of (material cause)? What is it that was made (formal cause)? Who or what made it (efficient cause)? Why, for what purpose, was it made (final cause)? In considering an organism such as an oak tree, the four questions can be asked in three different ways regarding form, function and evolution. In terms of form, an oak is made up of organic constituents. The thing that was made is an oak. We could say that the information coded in DNA made the tree. Asking why it was made is problematic - does it have a purpose?
In terms of function, a tree is made of molecules, cells, tissues and their interactions. The thing that was made is a living tree that functions. In asking what made the tree, we could again say DNA and other components made it. Finally, the tree is functional in order to survive - survival is the purpose of function.
The material cause of things that evolve is again the organic components of life. Oak trees and all other life forms are the things that are made through evolution. What causes evolution? Is it natural selection knocking out all the unfit mutants? This seems incomplete, but perhaps there are other efficient causes not yet clear to us. Does evolution have a purpose? Is its purpose to increase the complexity of life over time? Oak trees are distinctive, but are made up of the same components as other trees, so perhaps the material cause is organization and evolutionary history. And what of the material cause of evolution? What evolves? Is it DNA, individuals, populations, species or all of these?
Aristotle's final cause, purpose, is easy to apply to the activities of humans, but goal-directed behavior applied to biology can lead to unscientific explanations. On the other hand, goal-directed behaviors, teleological processes, are an essential part of biology - function is part of the definition of life. A leaf can be said to exist in order to catch light and house cells with chlorophyll. Chlorophyll exists in order to conduct photosynthesis. One function of individuals is reproduction, and the purpose of reproduction is survival of the species. Humans have intent - we do things in order to reach a desired future state, but intentions can't be assumed for other forms of life. Anthropomorphic explanations are valid only for humans, but teleology can be extended to other species. Humans eat and plants photosynthesize in order to survive.
Function and purpose are two aspects of teleology, and perhaps the difference between them is where they fit in a hierarchy. Purpose pertains to an individual doing things in a certain sequence to reach a desired outcome. Function is the means of fulfilling the purpose of maintenance and persistence. Purpose and function both result from bringing together potentially independent (often biochemical) events to produce an end result that would not otherwise exist. Functional explanations are unique to biology. Storms and organisms both capture energy and are self-organized and emergent, but storms can't reproduce, aren't self-contained and their boundaries are vague. They will disintegrate when the energy flow stops. The Gaia Hypothesis suggests that the Earth is like an organism. It has a boundary-like atmosphere and evidence suggests organisms have collectively altered its properties, especially with the release of molecular oxygen, a byproduct of photosynthesis. More recently, we humans are altering the planet by releasing heat-trapping gases, but to call the Earth an organism may be taking teleology too far. First, we need to explain bacteria.
Goals and purposes can't be made law-like or causal in an objective science. Functioning organs can be comfortably placed within the confines of the Covering Law Model when they are seen as part of the explanans and not the event to be explained. The initial condition could be an animal equipped with a heart and associated piping. The laws would be those that govern electrical currents and hydrostatics, and the event caused would be the circulation of blood. At the next level of organization, the initial conditions include all organisms with a functioning circulatory system. The relevant laws address the flow of matter and energy in these organisms, causing the event of survival. This second explanation deals with purpose, but it already includes function within its conditions.
The idea of function is central to Darwinian theory - the organisms with the most useful things will survive and breed. Yet, natural selection is non-teleological because it is external to organisms, providing neither function nor purpose, and random mutations are also non-teleological, as they have no identifiable function or goal. Teleology can't be eliminated from biology and adaptation is an initial condition for evolution - non-adapted organisms never lived.
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