One of the major elements of natural selection is that all species have genetic variation. Natural selection is a process when organisms adapt better to their environment. It is an essential key in evolution. There is a change in phenotype of a population over time.
If you turned a frog egg upside down, you would expect it to reorient relative to gravity because the dense yolk granules concentrated in the vegetal hemisphere of the egg respond to gravity.
The frog egg is very large as compared to the normal egg cell. It is unevenly distributed with a top dark colored pole called animal pole and a light yolky pole called a vegetal pole.
The vegetal pole is concentrated with the yolk that is present for providing the nourishment to the growing embryo. The animal pole is the one from where the sperm enters and the development of embryo take place.
As the vegetal pole is denser as compared to animal pole so it will respond to the gravity and reorient along the direction of gravity.
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Air that resists vertical movement is said to be stable. The vertical movement of air is referred to as convection, while advection involves the horizontal movement of air and heat energy transference, Temperature differences at the Erath's surface occur wherever there are differences in surfaces substances.
