Density cycles and an offspring quantity and quality game driven by natural selection

Area of Science:

• Evolutionary Biology
• Ecology
• Population Dynamics

Background:

• A hypothesis suggests natural selection favors distinct female reproductive strategies during population density cycles.
• At low densities, selection favors many small offspring (r-strategists); at high densities, selection favors fewer large offspring (K-strategists).

Purpose of the Study:

• To provide the first empirical evidence of a genetic r-vs-K selection game driving stable population cycles.
• To investigate how density-dependent selection influences reproductive strategies and population dynamics in lizards.

Main Methods:

• Conducted decade-long fitness studies on lizards with differing throat-color morphs.
• Employed game theory to analyze selection pressures varying with population density.
• Tested frequency-dependent selection on progeny size by introducing rare and common hatchlings.

Main Results:

• Identified two distinct female reproductive strategies linked to throat-color morphs (orange-throated as r-strategists, yellow-throated as K-strategists).
• Demonstrated that selection strength varied with population density, favoring orange morphs at low density and yellow morphs at high density.
• Confirmed negative frequency-dependent selection on progeny size, with larger hatchlings having a survival advantage when rare.

Conclusions:

• Intrinsic frequency- and density-dependent selection mechanisms drive an evolutionary game.
• This game promotes stable, two-generation population cycles in lizards through distinct r and K strategies.
• The findings support the hypothesis of density-dependent selection favoring divergent reproductive strategies.