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Biogeography

Biogeography

The Theme of Evolution in Biogeography

As you might have guessed, evolution plays a big role in biogeography. After all, evolution filters traits that are suited for survival, so distinct characteristics evolve in different places. Evolution depends on natural selection acting on variation in individuals, and closely related organisms share certain traits. For example, all mammals have fur or hair and all fish have gills. Species may share similarities because they are closely related. However, they may also look or act alike because of similar evolutionary pressures that Mother Nature keeps placing on them. 

At the beginning of the biogeography section, we talked about deserts, and showed where on Earth deserts are found. Even though deserts are scattered around the globe, they all share one thing: they are very dry. Dry climates = water shortage = living things have to adapt to dry conditions. Plants in different deserts have evolved similar solutions to conserve water: succulence. Succ-you-what? Succulence is the name for having thick, fleshy leaves or stems that store water, such as the leaves on the prickly pear cactus.

Yup, those thick green things are leaves. Cacti are not the only plants that evolved succulent stems and leaves. Desert plants in Africa, called the euphorbs (family Euphorbiaceae) are also succulent. Succulence in both types of desert plants, even though they are not closely related, is called convergent evolution. In other words, two different groups of organisms converge on the same traits because of similar environments rather than a shared evolutionary history.


The golden barrel cactus has a succulent stem. 

Another example of convergent evolution is pitcher plants. Pitcher plants are carnivorous plants that have leaves specialized for catching insects. Pitcher plants live in bogs, which are nutrient-poor wetlands. They can also be found in hit comedy rock horror musicals. Since the plants do not get enough nutrients through the soil, the insects they catch round out their diet.

Pitcher plants of the genus Nepenthes live in Asia and Africa, and pitcher plants of the genera Sarracenia, Darlingtonia, and Heliamphora live in North and South America. These are not closely related plants, but have evolved the same bizarre lifestyles and structures that make them look like they should be related. Each of them has evolved pitcher-like structures from their leaves, showing that similar conditions, such as nutrient-poor soil, can result in similar structures around the world in distantly related plants.


The North American pitcher plant Sarracenia rubra. Image from here


The Asian pitcher plant Nepenthes albomarginata.

Pretty crazy plants, right? Charles Darwin studied the biogeography of carnivorous plants, which he thought were the most wonderful plants in the world (Darwin 1898). We wonder if his wife agreed, or if Valentine's Day bouquets were a source of strife in the Darwin household. Darwin did experiments with carnivorous plants and showed that unrelated species had evolved similar mechanisms to capture and digest insects.

Another evolutionary aspect of biogeography is adaptive radiation. Adaptive radiation occurs when one taxon rapidly evolves into many different species because its descendents are able to specialize on different food sources, or habitats, or something else in the environment. Each species develops an adaptation that allows them to survive and differentiate itself from the others. 

The Galapagos finches are a famous example of an adaptive radiation. The Galapagos Islands, off the Pacific coast of South America, were one of the sites Charles Darwin visited when he was developing his theory of natural selection. When Darwin visited, he found 14 different species of finch, each with a different beak shape that is adapted for a specific diet. One finch, the green warbler finch (Certhidea olivacea), has a small thin beak that lets it catch insects. Yum. Another bird, the large ground finch Geospiza magnirostris, cracks seeds open with its big beak. Some finches eat cactus flowers, others eat seeds or other plant parts. All of these finches evolved from one ancestor species that colonized the Galapagos Islands a few million years ago. 

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