World Builders™
World Builders™
Session Five  --  Seaweeds
Session Five  --  Seaweeds

How Evolution Works
How Evolution Works

    Simple Cells

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fish Descendants

This diagram shows how many different kinds of fish evolved from a common ancestor.

This is not really a true story. This is a simplified illustration of how evolution works.

At first there was only one kind of fish in the ocean, but there were many different environments: tide pools, rocky shores, quiet lagoons, and the deeper water. The one kind of fishes lived in all the different environments.

In each environment, some fishes survived and some were eaten by predators or were unable to find enough food. Over time the fishes best fitted to their environments passed on their characteristics, and their descendents began to develop into fishes that adapted to different environments. Eventually these fishes became different species. (Species are groups of life forms that have descended from a common ancestor but are unable to produce fertile young if they should happen to try to interbreed.)

Notice that groups of these fishes have changed in body form from generalists that could survive fairly well in a number of environments to specialist fishes who are experts at surviving in a particular environment.

    There are many kinds of living organisms on earth. Looking at their amazing and splendid variety, we ask many questions:

  • How did there get to be so many different kinds of living things?
  • Which organisms are closely related to each other?
  • Why are some animals and plants alive today and others extinct?

     Scientists have tools that help them to answer these questions. They believe that life appeared and flourished only once on earth, and that all living things are related to one another, and are possibly the descendants of a single cell.

    In 1859 Charles Darwin published his book The Origin of Species. In this book he proposed the theory of Evolution. This theory states that:

  • Living organisms compete for resources (energy, water, light, space.)
  • Individual organisms differ from one another.
  • The organisms that are well adapted to the environment have the most descendants.
                           (Natural Selection.)      (Survival of the fittest.)
  • Inherited adaptations lead to changes in the organisms over the generations. (Natural  Evolution)

Let's look at some examples to see how this works.

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Example One: Unicellular Organisms

The earliest life bearing oceans were full of all sorts of chemicals. The first cells were able to use the chemicals for their life processes. Over time, the chemicals were used up and became harder to find. Starvation loomed. Oh, no! The first crisis!

Some cells adapted by using chlorophyll so that they could make food from carbon dioxide, water. and the sun's energy. (Later these cells moved into larger cells and are now called chloroplasts.) This worked: they continue to live in plant tissues today.

Some cells adapted by ingesting (surrounding and digesting) other living cells. Some developed special features to help them to swim. These adaptations also worked. Their descendants are animals.

Some cells ingested dead cell material. This was also successful. These life forms became our decomposers.

Some cells did not adapt successfully. They have no descendants.

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Example Two: Grasses

Grasses are a huge family today, but they probably started out as just a simple little plant.

Grasses live where there is not enough rainfall to support forests. Over millions of years they have diversified into a huge family of varied plants, plants which support many of the life forms on earth. Corn is a grass. Wheat is a grass. Rye and barley and rice are grasses. There are pasture grasses, such as the tall, vigorous grasses that covered the prairies. There are shorter grasses for drier places, huge grasses like pampas grass, decorative grasses, alpine grasses, and so on. All of them have descended from a common ancestor.

Grasses have been successful because they have found niches, which are places where there are available resources. There is lots of light where there are no trees between a plant and the sun, so the dry areas offered a niche to plants that did not need so much water. Grasses have developed strategies for dealing with these drier habitats.

Special Adaptations:

Drought Resistance: Grasses grow rapidly when moisture is available. Think of how the hills turn green here in California when the rains come! The grasses grow quickly, and make seeds. Their dense stands allow for efficient wind pollination. When the hot summers come, the grass stems and blades die. Our hills turn golden brown. The grass remains dormant until the rains return. In some varieties the roots remain alive, in others, new plants will sprout from the seeds.

Growth from the Roots: Grasses are eaten by many animals. Most plants grow at the tips of their shoots and branches, but grasses have developed the ability to grow from the bottom of their blades, growing up right from the roots. In that way, their growth is not interrupted when animals eat the tops of the blades.

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Example Three: Birds

Birds have developed from land-dwelling animals, possibly from a species of dinosaur. There are many different kinds of birds: humming birds, eagles, pelicans, penguins, ducks, sparrows, flamingos -- each one adapted to a specific niche.

Special Adaptations:

Very light skeletons with struts inside the bones allow for flight.

Endothermic metabolism (warm bloodedness) allows rapid energy use in the body. This keeps the muscles working during sustained flight.

Feathers: light, strong feathers provide insulation and decoration. A number of different kinds of feathers protect the bird's body, provide protective coloration, and make flight possible.

Eggs and Nest Building It was the reptiles who developed the egg that could survive out of water -- a leathery shelled egg. Birds developed the hard-shelled egg. Live births are not very practical for animals that are trying to be lighter and lighter so that they can fly! (Bats do it, but with only one baby at a time.)

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Things to remember:

All living things are descendants of single celled organisms.

Animals and plants compete for energy sources, water, nutrients, and space.

Sexual reproduction leads to a variety of individuals in each generation, individuals who differ from each other.

The organisms which are most successful at capturing energy and escaping predators are the parents of the next generation.

Offspring tend to resemble their parents, so the species gradually becomes better and better fitted to the environment that it lives in.

The individuals who are less fit may have an advantage if circumstances change -- e.g., climate change, new predators.

Consider that you will be developing your plants and animals from generic starter species into a diversified group with adaptations for different niches. This can be a lot of fun!

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Windows to the Universe team. Temperature of Ocean Water. Boulder, CO: ©2000-04 University Corporation of Atmospheric Research (UCAR), ©1995-1999, 2000 The Regents of the University of Michigan, Aug 313, 2001.   Online. Available: . Nov. 22, 2003.
© 1996,1997, 1998, 1999, 2000, 2002, 2003.   Elizabeth Anne Viau. All rights reserved. This material may be used by individuals for instructional purposes but not sold. Please inform the author if you use it at .