Answer:
An index fossil is a fossil used to determine the age of the sediments in which it is found. When quickly approximating the age of new finds, an archaeologist looks at the rocks in which it is buried, as those sediments would have been on the surface at the time of the death of the pre-historic creature.
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Natural selection act directly upon <u>Alleles</u>
Explanation:
Genes, such as that of fur color, have many variants called alleles. Natural selection act on the alleles and chooses the best fit for an environment and eliminates the disadvantageous ones from the population over generations. Mutations are a rare occurrence that increases the number of allelic variants of a genes on which natural selection can act upon.
Lymphoid organs in the immune system, includes the bone marrow
