Island biogeography and evolution

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gallotia galloti palmae

Gallotia galloti palma Gallotia atlantica Gallotia stehlini
Purpose: To explore evolutionary relationships among the lizard genus Gallotia based on comparative anatomy and island biogeography.
This activity is about the evolution of lizards on the Canary Islands. You will work with real data from real populations of the lizard genus Gallotia. You will use mapping and comparative anatomy to help determine a possible evolutionary history of the populations. The data include observations of lizard morphology (body form), geology, geographic distance, and molecular genetics. picture 3

Ever since Charles Darwin formulated his hypothesis on how the finches of the Galapagos Islands evolved into 13 species, islands have been a prime target for the study of evolution. By their very nature, islands are isolated and are essentially a living laboratory of evolution.
Africapicture 2

The Canary Islands form an archipelago of seven volcanic islands off of the NW coast of Africa. Geologists theorize that a geologic hot spot of upwelling magma has been drifting westward for the past 20 million years, gradually forming islands as it moves. Volcanic islands are particularly good for studying evolution because they can be dated accurately using radioactive decay and because they start out as bare rock when they emerge from the sea.


1) If the island chain is drifting westward, which island is the oldest? (Use the map on page 3 to better read the names) ________________

2) What is another island chain that has been formed by this same method? ___________
More Background:

The development of an ecosystem on a volcanic island is somewhat unpredictable. Ecological succession does occur first with pioneer organisms that gradually alter the environment until a stable climax community is established. What is unpredictable is which plant and animal species will colonize these new environments. Much of this is left to climate, proximity to other landmasses and, of course, chance.

Pioneer organisms can arrive to the islands in a variety of ways. One of these ways is “rafts” of vegetation washing out to sea during rainstorms. These rafts can carry plants and animals to islands. Other ways organisms arrive on islands are storms blowing insects and birds off course, and plant seeds floating on the ocean currents.

The arrival of the Gallotia lizards was probably by rafting. Rafts of natural vegetation may be washed out to sea when high river levels cause riverbanks to collapse. The rafts carry away both plants and animals clinging to them. Colonization by airborne organisms, such as insects and birds, usually occurs during storms.

Some general principles of island colonization are:

  1. The closer the island is to another landmass, the higher its probability of colonization.

  2. The older the island, the more likely it will be colonized.

  3. The larger the island, the more likely it will be colonized.

  4. Geographic isolation reduces gene flow between populations.

  5. Over time, colonial populations become genetically divergent from their parent population due to natural selection, mutation and/or genetic drift.


Predict which island is the most likely to have been colonized first and which last. Tell why you think so and what general principle(s) of island colonization you used.

Part I: Geological History

In this part of the activity, you will look at the ages of the different Canary Islands. The maximum age of each island was estimated by sampling volcanic rocks found on all islands. The ratio of radioactive potassium to its breakdown product argon was used to estimate the age of the rocks. This data will be used to determine if your CLADOGRAM from Part 2 is correct.


Using the data in Table 1, make a list of the islands in order of age with difference in age (in millions of years) between each island (ex: Hierro – 1.2 Palma – etc.).

Table 1: Maximum age of the Canary Islands in millions of years.

Lanzarote &

Gran Canaria












Part II: Morphology (body form)

In this part of the activity, you will look at morphological similarities and differences between the Gallotia lizards. Figure 2 on the next page shows the relative sizes of the lizards on each island.


  1. Measure each lizard from the tip of the snout to the longest part of the tail.

  2. Record measurements in a table, as shown below.

  3. On a separate piece of paper, draw a CLADOGRAM based on this new data.

Table 2: Gallotia Lizard Lengths

Lizard (Island)

Length (cm)

picture 7

On another sheet of paper, draw a CLADOGRAM using the data from Table 2 (the lengths of the lizards only).

3) What is a pioneer organism?

4) In your own words, what is a cladogram?

5) Explain your reasoning for your tree. What general principle(s) of island colonization did you use?
Part III: Geographic Distance

In this part of the activity, you will look at distances of each island from the mainland of Africa on the next page. This data will be used to prepare a CLADOGRAM based on geographic locations.


Measure the distances of each island from the closest land point of AFRICA according to the map (not to exact scale). Record measurements.

Table 3: Distances from Africa


Distance (cm)

3. The map below shows the distribution of the lizards. Using it and the data you collected from the map, draw a second cladogram of the six populations of Gallotia lizards, this time using this data and the distances from Table 3. Label your end branches with the following population names:

picture 1

G. atlantica

G. stehlini

G. galloti Gomera

G. galloti Tenerife

G. galloti Palma

G. galloti Hierro
Map 1.


6) How has your tree changed now that you have geographic data to help determine the evolutionary relationships of

the lizards on the Canary Islands?

7) Describe how your CLADOGRAMS are different.


On your other sheet of paper, write a conclusion that addresses your prediction. Was it supported or refuted? Why?

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