Male peacocks have tail feathers that make up 60% of their body length. During the mating season they fan their tails and shake these feathers in front of females. Females evaluate the tail-shaking behavior as part of selecting their mate. Which behavior is most likely to have been the original behavior that, through natural selection, resulted in this mate choice behavior of tail shaking?
1. head movement to make eye contact
2. involuntary muscle contraction such as shivering
3. sound production with the larynx flight
4. grooming feathers with the beak

A single-celled zygote will undergo multiple rounds of cleavage, or cell division, in order to produced a ball of cells, called a blastula, with a fluid-filled cavity in its center, called a blastocoel.In animals with little yolk in the egg, the zygote undergoes holoblastic cleavage, in which the entire zygote is cleaved repeatedly; in animals with a lot of yolk in the egg, the zygote undergoes meroblastic cleavage, in which only part of the zygote is cleaved.The blastula eventually organizes itself into two layers: the inner cell mass (which will become the embryo) and the outer layer or trophoblast (which will become the placenta); the structure is now called a blastocyst.During gastrulation, the blastula folds in on itself to form three germ layers, the ectoderm, the mesoderm, and the endoderm, that will give rise to the internal structures of the organism.