The process in which pressure from the upper layers of sediment pushes down on the lower layers, causing the sediments to stick together and form solid rock.

The process is called as compaction.

Compaction is the process by which the sediment progressively losses its porosity due to the effect of loading. When the sediment is formed by its own it contains pores. The pore space is usually filled with water. As the sediment is deposited over time the upper layer of sediments pushes the lower layer and due to this reason the sediments deposits and stick together to form solid rock. The pore size of different sedimentary rock is different like mudstone has porosity of more than 60% and sandstone of porosity more than 40%. The porosity is reduced by compaction.

not necessarily. higher fitness requires the organism to be reproductive successful. for example, an organism at age 10 with 15 offspring is much more fit than an organism at age 15 with only 5 offspring.