Exploring the philosophical battle between purpose and mechanism in evolutionary biology
Why do eyes see, hearts pump, and wings fly? In our everyday language, we confidently assign purposes or functions to biological traits. This intuitive concept of "what something is for" becomes deeply puzzling when viewed through the lens of evolution.
If complex traits emerge through the blind, mechanical process of natural selection, can we truly say they have genuine functions?
Philosopher Robert Cummins accused selectionist accounts of smuggling purposeful design through the back door while claiming to adhere to strictly mechanistic evolutionary principles 1 .
Cummins' central argument contends that natural selection cannot possibly explain why traits have functions because 1 :
Proposes that complex biological systems can emerge without positive selection through neutral processes that create irreversible complexity 2 .
A neutral interaction exists that can buffer the effects of future mutations
A mutation occurs that would be deleterious without the pre-suppressor
The mutation spreads through genetic drift, making the system dependent on the pre-suppressor
Occurs when gene duplication creates redundant copies that accumulate mutations, occasionally leading to novel functions that can be selected for 1 .
Molecular evidence shows positive Darwinian selection in early stages after gene duplication
The evolution of trichromatic color vision in primates provides compelling evidence for understanding biological functions. The molecular basis involves visual pigment genes called opsins, with most mammals having two types but many primates possessing three 1 .
Scientists sequenced opsin genes from various primate species
Constructed evolutionary trees to determine gene relationships
Compared synonymous and nonsynonymous mutation rates
Measured pigment sensitivity to different light wavelengths
Correlated vision capabilities with feeding behaviors
The third opsin gene resulted from a gene duplication event approximately 35-40 million years ago 1 .
| Amino Acid Position | Effect on Spectral Sensitivity | Selection Signature |
|---|---|---|
| 277 | Tyrosine → Phenylalanine shift | Positive selection detected |
| 285 | Threonine → Alanine shift | Positive selection detected |
| 308 | Alanine → Serine shift | Convergent evolution |
| Primate Group | Color Vision Type | Number of Opsin Genes | Advantages |
|---|---|---|---|
| Most mammals | Dichromatic | 2 | Better night vision |
| New World monkeys | Polymorphic trichromacy | 2-3 (polymorphism) | Some females see three colors |
| Old World monkeys & Apes | Uniform trichromacy | 3 (fixed) | Superior fruit detection |
Amplify specific gene sequences for analysis across species
Express proteins in model systems to test functional properties
Reconstruct evolutionary relationships and detect selection
Detect positive/negative selection through substitution analysis
Modify genes in model organisms to test functional consequences
The "cheap tricks" debate has profoundly enriched our understanding of biological functions, pushing scientists and philosophers toward more nuanced positions that acknowledge both evolutionary history and current causal contributions.