When Charles Darwin boarded the ship the HMS Beagle in December 1831, he never would have guessed that what he found during his voyage would revolutionize the scientific world. The nearly five-year voyage produced copious amounts of research, specimens and notes that Darwin would later compile into his theory of evolution by natural selection. Darwin joined the crew as the ship's naturalist, but his observations of finches and tortoises would yield one of the most fundamental theories in biology.
Competition for Resources
Resources such as food, space and light are limited in every community. Because organisms need these things to survive, individuals must compete with one another for these limited items. The individuals with traits that best exploit these resources will grow, thrive, mate and reproduce. By becoming bigger and stronger than others, the advantaged individuals live long, healthy lives filled with many opportunities for mating.
Variations Among Individuals
Each individual within a species is different; no two individuals have the same genes unless they are twins or clones. Individuals differ from one another in their appearance, their physiology and their behavior. Unless you are an identical twin, no one else on Earth shares your exact same characteristics and genes.
Differences in Survivability
Not all individuals in a population have the same amount of success in their environment. Those individuals whose characteristics best adapt them to a certain environment will have a better chance to survive and pass on their genes. In the distant past, those giraffes that possessed longer necks could reach the higher leaves of trees. By reaching these taller branches, these giraffes were better equipped to exploit a greater variety and number of food sources. These longer-necked giraffes would have an advantage in survivability over their shorter-necked friends and would produce more offspring. This concept is often referred to as "survival of the fittest," where fitness means reproductive success.
Variations Are Inherited
Because the differences in individuals within a species are present in the genes themselves, the differences are passed down from generation to generation. Individuals that possess characteristics, such as the giraffe's long neck, that give them a survival advantage over others in the population will reproduce more. A greater reproductive rate means that those individuals will pass on their genes to a greater percentage of the population. These advantageous genes will then represent a bigger portion of subsequent generations. Over time, the advantageous genes will be present in the majority of the population.
Many organisms invest a lot of time and effort into making themselves attractive to the opposite sex. The bottom line of such a large investment is that the more attractive an individual is to the opposite sex, the greater the opportunities for reproduction. More chances to reproduce means that an individual's genes will be well represented in future generations. In some animal societies such as elephant seal populations, certain males never get a chance to mate. Only the alpha male, the head of the herd, mates. The ultimate goal of an organism's struggle to find mates is reproductive success, which refers to the number of offspring an individual contributes to the next generation; so the more opportunities an individual has, the more offspring he or she most likely will contribute to the next generation. Darwin's theory of evolution by natural selection explains that organisms better adapted to their environment have greater reproductive success.
About the Author
Ho-Diep Dinh has been writing since 2005. She is a contributing writer on eHow and Answerbag, specializing in topics such as human health and the prevention and treatment of diseases. Dinh received her Bachelor of Science in physiology from the University of California at Davis.