Name: ______

Natural Selection Packet:

Guppies and Snakes

GUPPIES

Guppies live in streams and ponds in Caribbean islands.

Scientists have studied guppy populations in some of these islands.

They studied what happened to populations of guppies living in ponds before and after a new predator came to live in the ponds. The new predator was a fish called a killifish. Killifish are bigger than guppies, and they eat guppies.

The scientists examined how the population of guppies changed after the killifish moved into the guppies pond. The table below shows what they found.

Characteristics of guppies before killifish moved to the pond / Characteristics of guppies after killifish moved to the pond
Average length of guppies / 2.50 cm / 2.48 cm
Percent of guppies that have fins and tails / 100% / 100%
Percent of guppies that are colorful / 92% / 21%

Use the evidence to work out as much of the natural selection process as you can. Work together and write one answer for your group, after you discuss each answer. Then you will receive an additional page to help you. Discuss your answers completely, and come to an agreement with your partner:

What is the environmental change?

How does the population of guppies change after the environmental change?

Why does the population change? Explain how this happens. (Think about which guppies have more offspring and why.)


RED-BELLIED BLACK SNAKES

Red-bellied blacksnakes live in Australia. They eat many small animals, including frogs and toads. Snakes eat their prey by opening their mouths wide and swallowing the prey hole, like the picture on the right.

In 1935, a poisonous toad was introduced to Australia. From 1935 until now, the toad has slowly spread from the east coast of Australia into the areas farther from the coast.

When red-bellied snakes eat cane toads, the snakes often die, because the toads are poisonous.

Scientists wondered how the snakes might have changed because of the introduction of the poisonous cane toads.

The scientists compared 225 red bellied snakes in three parts of Australia, and they examined the percent of the snakes with heads big enough to eat the toads. They also examined the color of the snakes. This is what they found:

Snakes that live in areas with no cane toads. / Snakes that live in areas with many cane toads
Number of snakes examined / 75 / 75
Color of snakes / All were black with red bellies. / All were black with red bellies.
Percent of snakes with heads big enough to eat cane toads / 76% / 22%

Use the evidence to work out as much of the natural selection process as you can. Work together and write one answer for your group, after you discuss each answer. Then you will receive an additional page to help you. Discuss your answers completely, and come to an agreement with your partner:

What is the environmental change?

How does the population of snakes change after the environmental change?

Write your answers to this question after you have discussed the questions and agreed on the answers with your partner.

Why does the population change? Explain how this happens. (Think about which snakes have more offspring and why.)

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