Cactus & Succulent Society
of New Zealand (CSSNZ)

Convergent Evolution of C&S

[Scene: A plant market/show somewhere in the world.]

Buyer: I'll have that cactus there, please (pointing to a Euphorbia species)
Seller: Certainly Sir, that's $10, thanks!!.

[Director: Cut, lets do that scene again, but differently.]
[Take 2:]

Buyer: I'll have that cactus there, please (pointing to a Euphorbia species)
Seller: Sir, that's actually not a cactus, but a Euphorbia. It's an example of convergent evolution, whereby totally unrelated genera of plants evolve similar morphology to cope with similar conditions in different areas or even different continents. Euphorbias come from Africa and Cacti from the Americas, they are totally unrelated, but evolved similar structures to cope with the lack of water. I can sell you this nice cactus AS WELL so you can observe convergent evolution in your home. That's $20 for the euphorbia and the cactus please!!

Introduction

Completely unrelated plants and animals have evolved in similar ways to cope with similar environments or harsh conditions. This is because there are only a limited number of ways to adapt to the environment, as dictated by physical laws and feasibility.

For Example:

• Fins on Dolphins(mammal), Sharks(fish) and Penguins(bird)

• Wings on Birds, Bats(mammal) and the ancient pterodactyl(reptile)

• The streamlined profile of the Shark and the whale

• The spiny ant-eater (Australia) is NOT related to the giant anteater (Latin America)

• and finally the succulent Euphorbias of Africa and the Cacti of the Americas.

Cacti and other Succulents

In the final example above the plants have adapted or evolved to their environment by their shape (morphology). These two unrelated plants (cacti and Euphorbias) in different parts of the world, but under similar conditions of water availability and temperature have evolved similar forms. To the "general public" the succulents of Africa can be confused with the cacti of the Americas, even though the plants are unrelated. This similarity of shape according to similarity of environment for unrelated plants is called Convergent Evolution.

The similar forms/shapes/features are:

• Round body shape

• Reduced or no leaves on plant

• Spines/thorns on the plant body

Each of these adaptations suits a plant in water limited environments:

A round body shape results in the least surface area for volume of plant, and less surface area means less exposure to the sun and drying winds while enabling the maximum storage of water within the body.

Reduced or no leaves once again reduces surface area and therefore potential exposure to drying winds.

Spines etc have been thought to act in a number of ways: Fend off animals who would otherwise see the plant as a good source of water for itself; act as a dew collection point and channelling the water down the body to the roots; some sort of mineral and waste deposit system for the salts the plant accumulates as it drags water out of the soil.

In summary totally unrelated families of plants have evolved similar morphology (body shape etc) to cope with similar environmental factors, even in completely different continents. Why reinvent the wheel?