Evolution of the Two Sexes under Internal Fertilization and Alternative Evolutionary Pathways

Area of Science:

• Evolutionary biology
• Reproductive strategies
• Biophysics

Background:

• The transition from isogamy to anisogamy drives sexual selection and conflict.
• Gamete dynamics theory models fertilization based on gamete limitation, competition, and zygote survival, typically assuming broadcast spawning.

Purpose of the Study:

• To broaden gamete dynamics theory by incorporating internal fertilization scenarios.
• To test the robustness of existing predictions under different fertilization modes.

Main Methods:

• Comparative analysis of broadcast spawning with two internal fertilization models: gamete casting/brooding and packet casting/brooding.
• Mathematical modeling of gamete dynamics under varying conditions.

Main Results:

• Isogamy prevails with low gamete limitation/competition and similar gamete/zygote resource needs.
• Anisogamy emerges with increased gamete competition/limitation and higher zygote resource demands, correlating with multicellular complexity.
• Internal fertilization models yield robust predictions consistent with broadcast spawning.

Conclusions:

• Gamete dynamics theory provides a strong framework for understanding isogamy and anisogamy evolution.
• The theory's predictions are consistent across broadcast and internal fertilization, irrespective of evolutionary order.