Natural Selection lets them survive.
I think genetic variation, but i'm not completely sure
Answer:
1. Muscle Fibers
2. Bones
3. Joints
Explanation:
Muscle tissue is made of muscle fibers. The plasma membrane of a muscle fiber is called sarcolemma while its cytoplasm is known as sarcoplasm. Muscle cells are characterized by the presence of specialized endoplasmic reticulum which is called sarcoplasmic reticulum.
Muscle cells exhibit contractility and extensibility. The ability of muscle cells to shorten their length forcibly in response to a stimulus is their contractility. Muscle fibers have the ability to extend and shorten themselves.
Extension and contraction of muscle fibers are responsible for the movement of the human body and its parts.
Muscles are attached to bones via tendons which in turn are the fibrous connective tissues. Muscles are also part of our joints where they assist in the movement by pulling the bones as well as stabilize and strengthen the joints.
Answer: The organic molecules required for building cellular material and tissues must come from food. Carbohydrates or sugars are the primary source of organic carbons in the animal body. During digestion, digestible carbohydrates are ultimately broken down into glucose and used to provide energy through metabolic pathways.
Hope this helps...... Stay safe and have a Merry Christmas!!!!!!!! :D
