Charles Darwin is credited with first introducing the idea of evolution by means of natural selection. In order for natural selection to take place, three important principles must be evident in a population.

1. Inherited variation must exist in the population
   If similar DNA is shared, why doesn’t every family member look identical? This picture is an example of variation within a family. While genetically similar, each possesses unique traits. This is known as variation.

A population requires variation in order for natural selection to occur. If all members of the population were identical, in theory, they would all survive and reproduce. The population would never change over time. It is important to note that these variations must be heritable, in other words, they can be passed on to offspring. Look at the picture of Darwin’s finches. What variation do you see among the four species? Record your answer in your notes.



The finches that Darwin observed in the Galapagos Islands were similar but had a variation in beak size and shape, depending on the available food sources. These variations allowed natural selection to occur, resulting in different species of finches.


2. Finite supply of environmental resources
   There is only so much food, water, and space available in any given habitat. Organisms compete for these resources in order to survive. Organisms that are better adapted to retrieve these resources are more likely to survive and reproduce. Organisms that do not gain resources or are out-competed will likely die before they can breed and pass traits on to their offspring. This competition allows natural selection to occur. This picture shows koi fish competing for food.

3. Organisms produce more offspring than can survive
   In nature, organisms have a great potential to reproduce. They may only have one offspring at a time, such as giraffes, or have over 100 at a time, like the koi fish pictured above. Ultimately, not all of the offspring live to adulthood and are able to reproduce. Many are lost to disease, predators, or other conditions. Overall, very few make it to the reproductive age. This is often referred to as survival of the fittest.

These three mechanisms of natural selection can contribute to or detract from reproductive success in a population. Organisms that are a better fit for their environment tend to reproduce and pass on their traits, resulting in a shift of the population that reflects the inherited trait or traits.