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What is an individual organism? A multilevel selection perspective
Henri J Folse1, Joan Roughgarden
1Department of Biology, Stanford University, Stanford, California, 94305-5020, USA. HFOLSE@STANFORD.EDU
The Quarterly Review of Biology
|January 20, 2011
Summary
Biologists often define organisms by "one genome in one body," but this fails for colonial life. A new evolutionary definition based on fitness alignment and germ-soma specialization better defines biological individuality.
Area of Science:
- Evolutionary biology
- Developmental biology
- Genetics
Background:
- The common definition of an individual organism as "one genome in one body" faces challenges with colonial and social organisms.
- Physiological and genetic criteria are insufficient for defining individuality across diverse life forms.
Purpose of the Study:
- To propose and evaluate a new, evolutionarily based definition of biological individuality.
- To apply these criteria to various types of organisms, including colonial, eusocial, modular, and chimeric forms.
- To explore the concept of the "evolutionary individual" and the role of sexual reproduction.
Main Methods:
- Developing a definition of individuality based on three evolutionary criteria: alignment of fitness, export of fitness via germ-soma specialization, and adaptive functional organization.
- Applying these criteria to analyze different biological entities, from complex multicellular organisms to genetic chimeras and species.
- Reviewing the adaptive significance of internal genetic heterogeneity within individuals.
Main Results:
- Complex multicellular organisms and eusocial insect colonies meet the proposed evolutionary criteria for individuality.
- Modular colonies and genetic chimeras generally do not meet these criteria, with some exceptions.
- Species may qualify as "evolutionary individuals," and sexual reproduction can enhance species-level evolvability.
- High genetic relatedness is not essential for individuality, and genetic variability can offer adaptive benefits at the individual level.
Conclusions:
- The proposed evolutionary definition provides a more robust framework for understanding biological individuality, especially for colonial and social systems.
- This redefinition clarifies the status of various biological entities and highlights the adaptive role of genetic diversity within individuals.
- The concept of the "evolutionary individual" broadens our understanding of life's organization and evolution.
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