How Life Changes Over Time
Why do giraffes have long necks? Why do polar bears have thick, white fur? The answer is adaptation. An adaptation is a trait that helps an organism to survive and reproduce in its specific environment. These traits don't appear overnight. They are the result of a long, slow process called evolution, which is driven by natural selection.
The Theory of Natural Selection
The theory of natural selection was famously proposed by Charles Darwin. It is the primary mechanism that explains how evolution happens. The theory is based on four key principles:
1.Overproduction: Organisms produce more offspring than can possibly survive. A frog might lay thousands of eggs, but only a few will grow up to be adult frogs. This creates a struggle for existence.
2.Variation: The individuals within any population are not all exactly the same. They have natural, inherited differences, or variations. Think about your classmates—you are all humans, but you have different heights, hair colors, and other traits.
3.Selection (or 'Survival of the Fittest'): In the struggle for existence, some individuals will have variations that make them better adapted to their environment. These individuals are more likely to survive and live long enough to reproduce. This is what's meant by 'survival of the fittest'—not necessarily the strongest, but the one that fits the environment the best. A polar bear with slightly thicker fur will be better able to survive the cold. A gazelle that can run slightly faster is more likely to escape a cheetah.
4.Adaptation: The organisms that survive and reproduce pass their advantageous traits on to their offspring. Over many, many generations, this beneficial trait becomes more and more common in the population. The population as a whole becomes better adapted to its environment. This is evolution in action.
An Example: The Peppered Moths
A classic example of natural selection is the peppered moth in England.
Before the Industrial Revolution: Most peppered moths were a light, speckled color. This was a great adaptation because they could blend in with the light-colored lichen on the trees, hiding them from birds. There was variation; a few moths were born with a mutation that made them dark. These dark moths were easily seen and eaten by birds.
During the Industrial Revolution: Factories produced a lot of soot that coated the trees, turning them dark. Now, the light-colored moths were easy for birds to see against the dark bark, and their population declined. The rare dark moths, however, were now perfectly camouflaged. They survived better, reproduced more, and passed on the gene for dark color.
The Result: Over generations, the dark form of the moth became the most common. The population had evolved and adapted to the changing environment.
It's crucial to remember that natural selection doesn't create the variation. The dark moth mutation was already there by chance. The changing environment just made that particular trait advantageous. Without variation, there's nothing for natural selection to select!