Organisms within an environment may compete for resources such as food or shelter. The strongest organisms would gain the resources and survive to reproduce. This could lead to wiping out an entire species. Charles Darwin called this the “Theory of Survival of the Fittest.” Certain new traits of a species will allow for survival.
Species can become extinct if the environment changes to quickly and they cannot make an adaptation. For example, one cold winter all the bunny rabbits with short fur did not make it through the winter. While the rabbits with the long fur survived and reproduced. Now all the rabbits have long fur. The rabbits with short fur no longer exist. Lamark had a “Theory of Use and Disuse.” The theory stated that if the organism did not use a certain part, this part would disappear in future generations. Organisms could also grow organ as they needed them, organ could also grow larger from repeated use. He looked to giraffes for answers. He noticed that the giraffes were having trouble finding food on the lower branches of trees. If the giraffe would stretch its neck it could get food off the higher branches. The more the giraffe would use their neck the larger/longer the neck would then become. Thus future generations of giraffes would then have longer necks. When in reality only the giraffes wit the long necks would be able to find food, survive and reproduce offspring with long necks. Thus over generations the giraffes with short necks would die out.
We know that other species once existed by looking at fossil records. We can tell a lot from fossil records. We can figure out how long ago an organism lived and make a prediction under what conditions. Looking at the sample of sedimentary rock we see layers that form. Each layer holds different information about a time past. The layers closer to the top are the most recent while the layers on the bottom are the oldest.
The changes that I have described here could take hundreds of years depending on the length of an organisms life cycle. Insects and bacteria have life cycles must shorter than your average organism. Insect can have new generations within 72 hours. The chances for mutation and adaptation increase. Certain bacteria are now resistant to some forms of antibiotics.