Sheri Amsel

Transcription

1 Here is a herd of caribou. All of the caribou in the herd make up a population of animals. A population is many of the same species living in one area. Though the caribou are all the same species, not every one is exactly alike. Not all the antlers are the same size or the coats the same color. Their traits vary. This is called genetic variability. Sometimes one of the herd is born with a trait that gives it an advantage. If the trait helps it survive longer, it is called an adaptive trait. If it can pass the trait onto its offspring it is an inheritable trait. Yet how does this new trait become an adaptation?

2 Let s look at an example. A caribou is born with bigger, hairier feet than the rest of the herd. This trait helps him run faster on top of the crusted snow. Bigfoot now has an advantage over the other caribou. When he runs across the snow, he doesn t break through the crust, so he can run faster using less energy and escaping the wolves that prey on the herd. He has more energy to stay warm and grow bigger and stronger than the other caribou.

3 When he grows up and it s time to compete for females during the mating season every fall, he has the size, strength and energy to father many offspring.

4 The following spring many calves are born with his big feet. They inherited his adaptive trait. They now have an advantage too. If most of Bigfoot s offspring are born with this adaptive trait, they may live longer and have more offspring than the other caribou in the herd. This is an example of natural selection. Natural selection can lead to the adaptive trait spreading throughout the whole population over time.

5 Over time more and more of Bigfoot s herd will have his big feet. It takes many, many generations for an adaptive trait to spread throughout a population, but eventually the whole herd will have bigger feet. It all starts with genetic variability giving rise to a new adaptive trait. The herd has adapted.

6 Summing up Adaptation There are three things that make natural selection and adaptation happen: 1) Genetic variability in a population, so that a trait appears that can give an advantage. 2) The trait must be inheritable so it can be passed on to the offspring. 3) The trait must lead to more offspring surviving so it will eventually spread throughout the population. This is called differential reproduction. Vocabulary A trait is a physical or behavioral feature of a living thing. An adaptive trait is a trait that helps a living thing survive longer. An inheritable trait is a trait that is passed down from parent to offspring. Differential reproduction is when an adaptive, inheritable trait spreads throughout a population because it leads to more offspring surviving. Genetic variability is when living things of the same species have traits that are not exactly the same. Natural selection is when an inheritable trait helps a living thing survive longer than others in its population without that trait. Population is many of the same species of living in one area.

7 Adaptation Discussion Questions: 1. Can you think of other examples of traits that would give the caribou an advantage, so would be considered adaptive traits? 2. All living things have adaptive traits. Name five living things, one adaptive trait that you can see and how it helps them survive. For example, a grasshopper is greenish-brown so it blends into the grass on which it feeds (adaptive trait). This helps it hide from birds that might prey on it Why does it take many, many generations for an adaptive trait to spread throughout the whole population?