Natural selection is the process by which individuals with characteristics that are advantageous for reproduction in a specific environment leave more offspring in the next generation, thereby increasing the proportion of their genes in the population gene pool over time. Natural selection is the principal mechanism of evolutionary change, and is the most important idea in all biology. Natural selection, the unifying concept of life, was first proposed by Charles Darwin, and represents his single greatest contribution to science.
Natural selection occurs in any reproducing population faced with a changing or variable environment. The environment includes not only physical factors such as climate or terrain, but also living factors such as predators, prey, and other members of a population.
Mechanism of Natural Selection
The mechanism of natural selection depends on several phenomena:
• Heredity: Offspring inherit their traits from their parents, in the form of genes.
• Heritable individual variation: Members of a population have slight differences among them, whether in height, eyesight acuity, beak shape, rate of egg production, or other traits that may affect survival and reproduction. If a trait has a genetic basis, it can be passed on to offspring.
• Overproduction of offspring: In any given generation, populations tend to create more progeny than can survive to reproductive age.
• Competition for resources: Because of excess population, individuals must compete for food, nesting sites, mates, or other resources that affect their ability to successfully reproduce.
Given all these factors, natural selection unavoidably occurs. Those members of a population that reproduce the most will, by definition, leave more offspring for the next generation. These offspring inherit their parents' traits, and are therefore also likely to succeed in competition for resources (assuming the environment continues to pose the same challenges as those faced by parents). Over several generations, the proportion of offspring in a population that are descended from the successful ancestor

Uloborid spider eggs and spiderlings. In any given generation, populations tend to create more offspring than can survive to reproductive age.
increases, and traits that made the ancestor successful therefore also increase in frequency. Natural selection leads to adaptation, in which an organism's traits conform to the environment's conditions for existence.
Unlike a steady pattern of growth, arthropods exhibit an external growth that proceeds in a sequence of step-like jumps, as the organism builds and replaces its exoskeleton. The arthropod does not grow bigger in size between these jumps. An aquatic arthropod gradually replaces body mass for water detained inside the tissues between molts.
The cell cycle has two phases the Interphase and mitosis. Interphase is the growth phase where the cell prepares itself for the cell division. It is divided into three phase G1 phase( Cell growth),S phase ( DNA synthesis) and G2 phase(cell growth).
Mitotic phase has a multiple steps this is where the duplicated chromosomes are aligned or arranged, separated, and move into two new, identical daughter cells(cytokinesis)
The first step in the mitotic phase is called karyokinesis or nuclear division where the chromosomes are separated and move in opposite direction and the second portion of the mitotic phase, called cytokinesis which is the physical separation of the chromatids into the two daughter cells.
