The first observation is overproduction. This means all species produce more offspring than will survive to become adults. Think of all the thousands of fish eggs produced each year. This means populations of species should be getting larger all the time, but they aren't, because there are mechanisms in place to curb population explosions, such as competition for food, predation, and disease. From this, Darwin was able to make a deduction: there is a struggle for existence. Many of those fish eggs are food for predators, who would not survive without them.
Darwin's second observation was variation. This means members of the same species show variation in characteristics. For example, zebras show variation in pattern and color of their stripes. Goldfish have gold scales, orange scales, or brown scales, or a mixture of all three.
Variation applies to all traits, even those essential for the survival of the individual organism. An eagle depends on excellent eyesight to locate prey, but it can still be born shortsighted. This means that variations are random and are not specific to any favorable adaptation.
The next observation, selection , means organisms that survive are more likely to reproduce and pass on favorable adaptations to their offspring than those with unfavorable adaptations. For example, height is an inheritable characteristic, and for the giraffe, having long necks (which added to their height) was reproductively advantageous. The taller giraffes were able to reach leaves in tall trees, which kept them alive longer, and able to reproduce, making them more reproductively fit than shorter giraffes who couldn't reach those leaves and died without reproducing.
The last observation Darwin made wasadaptation . This means that some traits are passed on to new generations and become common within the population, like the long neck of the giraffe. This doesn't happen overnight, it takes time for these traits to spread throughout a population.
So, what does this mean for natural selection and genetic diversity? Let's find out.
