Unlike other ways of reproduction, sexual reproduction provides variation through inheriting various genes. This variation is essential for adaptation as it would not just create a copy of the original life form, but rather a different unique life form that could adapt to something, and then have it's own offspring with the adapted trait.
Mechanical stress applied to bone increases osteoblast activity in bone tissue, and the removal of mechanical stress decreases osteoblast activity.
Explanation:
Osteoblasts are cells , that are produced in the bone marrow and are responsible for formation of bones and bone matrix.
Whenever a mechanical stress is applied on the bones , a series of reactions are triggered. As a result the activities of osteoblast increases to synthesize more and more bone matrix to provide strength.
Again on the other hand a decrease in mechanical causes a decrease in the activities of the osteoblasts.
Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in heritable traits of a population over time. The term "natural selection" was popularised by Charles Darwin who compared it with artificial selection, now more commonly referred to as selective breeding.
Answer:
Certain fossils, called index fossils, help geologists match rock layers. To be useful as an index fossil, a fossil must be widely distributed and represent a type of organism that existed for a brief time period. Index fossils are useful because they tell the relative ages of the rock layers in which they occur.
