Evolution - change over time
Plants and animals have many adaptations that help them survive in challenging environments. For example:
- Polar bears have a layer of fat under their skin for insulation, white fur for camouflage, wide paws to walk on snow, sharp teeth and claws for catching prey
- Cheetahs have extra large lungs and heart to help them run fast, long tails to help them balance, and camouflage to blend in. These help them to catch their prey for food
- Saguaro cacti can absorb and hold large amounts of water. They have colorful flowers with strong smells and nectar to attract bats to pollinate them. They produce fruit which attracts animals to help spread their seeds.
Natural selection
An important mechanism that causes living things to evolve is natural selection. There are three important principles to natural selection:
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Evidences of Evolution
The diagram below shows how the horse has evolved over the last 60 million years. What evidence do scientists have that the horse evolved in this way? What evidence is there that evolution exists at all? There are a few evidences we will focus on:
- Fossil evidence
- Embryology
- Homologous structures
- Analogous structures
- Vestigial structures
Fossil evidence
Scientist compare animals that live today (such as the whale and horse above) to animals that lived in the past. They can see similarities between them. Based on these similarities, they conclude that animals in the past evolved to be like animals today. In other words, they are related to each other. The fossil evidence clearly shows that life is old and has changed over time.
Embryology
Embryos are the early stages of development of an organism. Embryos of many organisms resemble each other. What do the embryos on the right have in common? Gill slits and tails are found in fish, reptiles, birds and humans when they are embryos. Closely related organisms develop similarly in the their early stages of development. This suggests that these organisms are related to each other. As they develop more, they lose these similarities. Notice the progression of development in the embryos pictured below.
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Comparative Anatomy
Comparative anatomy uses similarities in structures of animals to figure out how they are related. There are three different types. See the pictures below for descriptions of each.
- Homologous structures (picture #1)
- Analogous structures (picture #2)
- Vestigial structures (pictures #3 and #4)
Click here for a slide show with a summary of all of these evidences of evolution with videos.