What Is Co-evolution?

Co-evolution occurs when evolutionary changes in one species drive reciprocal changes in another. This process is common in:

• Mutualism: Both species benefit and adapt in response to each other (e.g., flowering plants and their pollinators).

• Parasitism: The parasite evolves to exploit the host, while the host evolves defenses.

• Interspecific Competition: Competing species may evolve traits to minimize overlap or outcompete the other.

In all these cases, the relationship is reciprocal: changes in one species create selective pressures on the other, resulting in a cycle of adaptation and counter-adaptation.

Why Commensalism Does Not Force Co-evolution

Commensalism is a type of interaction where one species benefits while the other is neither helped nor harmed. Because the host species does not experience a significant change in fitness, there is little to no selective pressure for it to evolve in response to the commensal. As a result, the relationship is typically one-sided, and reciprocal evolutionary changes—the hallmark of co-evolution—do not occur.

Examples of Commensalism

• Barnacles on whales: Barnacles gain mobility and access to food, but whales are unaffected.

• Epiphytic plants: Orchids growing on trees benefit from better light, but the tree is not impacted.

In these cases, the host species does not adapt in response to the commensal, so co-evolution is not driven by the interaction.

Contrast with Other Interactions

• Mutualism: Both partners adapt to maximize benefits, often resulting in highly specialized relationships.

• Parasitism: The host evolves defenses, the parasite evolves counter-defenses, leading to an evolutionary arms race.

• Interspecific Competition: Species evolve strategies to minimize resource overlap, such as niche differentiation.

All these interactions create reciprocal selective pressures, driving co-evolution.

Conclusion

Among the major types of species interactions, commensalism does not force co-evolution because only one species benefits while the other remains unaffected. In contrast, mutualism, parasitism, and interspecific competition all involve reciprocal effects that drive evolutionary change in both partners.

Correct answer:
(1) Commensalisms