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Egg Clutch Size

APBIO-EYVKCV

In the wild, female Red-Winged Blackbirds (Agelaius phoeniceus) lay egg clutches that vary in size from just two eggs per clutch to seven eggs per clutch. Clutch size is a genetic trait, so a single female will lay clutches of a set size throughout her reproductive life.

Prior to a seven-year-long drought, a population ecologist surveys a breeding population of birds in a two-mile radius and determines the average number of eggs per clutch laid by females in that population to be five.

The ecologist surveys the same breeding population of birds at the end of the drought and finds the average number of eggs per clutch laid by females in that population to be two.

Three years later, during which time the area receives normal amounts of precipitation, the average clutch size in the population of breeding females has returned to five.

Which of the following BEST explains these observations?

A

Directional selection shifted to favor females who laid fewer eggs during the drought and then shifted again after the drought to favor females who laid more eggs.

B

By the end of the seven-year-long drought, all of the females who laid fewer eggs starved to death, leaving behind the females who laid more eggs.

C

By the end of the seven-year-long drought, all the males who preferred females who laid fewer eggs starved to death, leaving only the males who preferred females who laid more eggs.

D

Female birds that laid more eggs starved to death during the drought, leaving only females who laid fewer eggs. When the drought ended, new niches were available which caused the remaining females to experience adaptive radiation.