The Evolution of Derived Monomorphism From Sexual Dimorphism: A Case Study on Salamanders

Area of Science:

• Evolutionary biology
• Zoology

Background:

• Sexual dimorphism, the distinct appearance of males and females, is well-studied.
• Derived monomorphism, where both sexes share derived traits, is less understood.
• Historically, female "male-like" traits were attributed to genetic correlation; recent work highlights independent selection on both sexes.

Purpose of the Study:

• To examine sexual dimorphism and derived monomorphism in the salamander genus Aneides.
• To test predictions of social selection on these patterns.
• To propose and define "derived monomorphic traits" for traits shared by both sexes.

Main Methods:

• Comparative analysis of sexual dimorphism in Aneides species.
• Examination of snout-vent length and head width in relation to sexual dimorphism.
• Interpretation of findings within the framework of social and sexual selection theories.

Main Results:

• Aneides hardii exhibits the most significant sexual dimorphism in snout-vent length and head width.
• Other Aneides species show reduced sexual dimorphism.
• Reduced dimorphism in females reflects derived monomorphism, with females expressing traits previously limited to ancestral males.

Conclusions:

• The observed patterns in Aneides exemplify derived monomorphism, not a reversion to ancestral monomorphism.
• The term "derived monomorphic traits" is proposed for traits typical in both sexes.
• Further study of derived monomorphism is crucial for understanding selective forces, including sexual selection, on both sexes.