When things in nature happen naturally (through processes in nature) that help an organism live and survive, it is called natural selection. Natural selection is a process when the necessary or needed traits (features of living things) of an organism are most likely to reproduce. Natural selection is often referred to as the 'survival of the fittest.'
The Earth is billions of years old and there have been many living organisms that have lived and died during those years. Some survived for millions of years, other species died out in a shorter time-period. Evolution is the process of living organisms changing and developing to adapt to their environment. All the species on Earth have evolved and will continue to evolve in the future. If an organism does not change with the environment, the organism will die. The Theory of Evolution and natural selection was first introduced by a naturalist, Charles Darwin.
The change that takes place by the organism is at the heart of natural selection. The organism does not purposely or consciously change, but it occurs naturally over a long period of time-based on the needs of the organism. Natural selection is the central concept of evolution.
Because of heredity or genetics, members of the same species do not have the same traits. For example, two humans are the same species, but one may be taller than another. Different species have variations of traits that allow them to have an advantage over other species. For example, a house fly has wings which allow it to escape and fly away from danger, giving it an advantage over an ant, which has limited options.
Traits are passed on from parent to offspring. Of course, not every trait will be passed on to the young of a species. The traits that give an organism a more favorable advantage will more likely be passed on to an offspring. The more favorable traits of an organism that are passed on, the more likely the organism will adapt and survive in the environment. The organisms that can adapt will have a better chance of survival because they are better 'fit' for their environment.
Imagine if over the next million years the earth became covered in water. The organisms that could adapt to the new environment would have a better chance of survival. Obviously, nearly all kinds of fish would survive, but what about squirrels or other small animals? It is possible that during the one-million years a squirrel could adapt by growing gills, which would help it breathe underwater. The change would occur naturally and over a long period of time. If the adaptation did not take place, the species would die off. If the squirrel (with gills) could survive, it would be due to natural selection. Organisms that cannot adapt to an environment are less likely to survive and reproduce.
Some examples of natural selection include red and green bugs. Birds prefer the taste of red bugs, so there will be less red bugs and more green bugs. The green bugs reproduce more green bugs, and eventually, the red bugs die out. Some giraffes have long necks and others have short necks. If the low-lying shrubs the short-neck giraffes rely on for food begin to die out, after a few generations, there would only be giraffes with long necks. Many insects become resistant to pesticides and then their offspring are resistant as well. Some insects may become immune to a chemical in just a few months.
An organism that can adapt better to its environment will survive and reproduce more than those that cannot adapt to its environment. Those that do adapt will be 'naturally selected', and the traits will be passed on to the offspring, ensuring survival.
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