Multilevel group selection is an evolutionary theory focusing on how natural selection acts on multiple levels of organization, including both individuals and groups. It highlights that traits favored by selection can sometimes benefit a group, even if they might be costly to individuals within that group.
Multilevel selection theory focuses on the phenotype because it looks at the levels that selection directly acts upon. This means it examines the observable characteristics and behaviors of organisms and groups to understand how they are affected by evolutionary pressures.
Understanding the Levels of Selection
In biology, evolution by natural selection typically focuses on the individual organism. However, multilevel selection proposes that selection can simultaneously occur at different levels:
- Individual Level: Selection favors traits that benefit the individual's survival and reproduction compared to other individuals within the same group.
- Group Level: Selection favors traits that benefit the survival and reproduction of the group compared to other groups.
For a trait to evolve through group selection, groups with that trait must be more successful (e.g., grow faster, survive better, reproduce more) than groups lacking the trait.
The Role of Phenotype
According to the reference, multilevel selection theory emphasizes the phenotype. This is crucial because selection doesn't act directly on genes but on the observable traits (phenotypes) that genes produce.
- Individual Phenotype: The characteristics of an individual organism.
- Group Phenotype: The collective characteristics or properties of a group, which can emerge from the interactions of individuals within it (e.g., cooperation levels, organizational structure).
Selection pressures act on these phenotypes at their respective levels.
How Group Selection Can Emerge
Sometimes, individual-level selection might favor selfish traits (e.g., individuals who exploit resources or avoid contributing to group efforts). However, if groups composed of more cooperative or altruistic individuals are significantly more successful than groups of selfish individuals, selection can favor these group-beneficial traits even if they impose a cost on the individual.
The reference provides a key insight:
- For humans, social norms can be argued to reduce individual level variation and competition, thus shifting selection to the group level.
This suggests that cultural factors like social norms can play a significant role in shaping the selective landscape. By enforcing cooperation or regulating behavior, norms can level the playing field among individuals within a group, reducing the intensity of individual competition. When individual differences within a group become less significant for survival and reproduction, the differences between groups (in terms of their collective traits or norms) become more important targets for natural selection. This can lead to the evolution of traits that benefit the group as a whole.
Key Concepts
- Hierarchical Structure: Life exists in hierarchical levels (genes, individuals, groups, populations). Selection can potentially act at any or all of these levels simultaneously.
- Between-Group vs. Within-Group Selection: Multilevel selection considers both selection acting between different groups (favoring groups with certain traits) and selection acting within groups (favoring individuals with certain traits). These forces can sometimes be in opposition.
- Emergent Group Properties: Groups can have properties (phenotypes) that are not simply the sum of their individual members' properties (e.g., coordination, collective decision-making). These emergent properties can be targets of selection.
In summary, multilevel group selection is a framework that recognizes the potential for natural selection to operate on different levels of biological organization, particularly individuals and groups, with a focus on how phenotypes at these levels are favored or disfavored by evolutionary pressures.
