The plants in the images above look quite similar. Both have a rosette of thick, fleshly leaves with spines. The stems of both plants are short, with the base of the leaves only just above the soil surface. It is easy to think that they must be closely related. But they are not.
The plant on the left is Aloe broomii. Species in the Aloe genus are native to Africa and the Arabian Peninsula (1). Several Aloe species, such as Aloe vera are widely cultivated as ornamental plants, and to produce extracts for use in herbal medicines and cosmetic products. On the right is Agave Americana, also known as the century plant or American aloe. Agave species are native to countries on the opposite side of the Atlantic Ocean; Mexico, Southern and Western USA, and tropical South America (2).
The aloes and agaves both belong to the monocot clade of flowering plants (note the way that leaves wrap around the stem like in other monocots such as grasses, orchids and leeks). Even so, the aloes and agaves are only distantly related, with their last common ancestor thought to exist around 93 million years ago (3), when dinosaurs still walked the Earth.
Many of the similarities between the Aloe and Agave genera are due to independent evolution (known as convergent evolution) of adaptations to the dry climates they live in. Both aloes and agaves are known as leaf succulents because their leaves can store water, giving them the thick, fleshy appearance. However, the structure of aloe and agave leaves differ. Agave leaves are fibrous, whereas aloe leaves are what I would describe as “gooey”. The fibrous agave leaves have been traditionally used to make ropes and Agave sisalana (sisal plant) is now widely cultivated to make fibrous materials for ropes, cloth, paper and other products. The spines on the leaves protect the plant from unwanted attention from herbivores.
Both the aloes and the agaves have an alternative form of photosynthesis called CAM (Crassulacean Acid Metabolism) that helps them conserve water. CAM photosynthesis is found in many diverse families of plants, including cacti, and does not appear to have a single evolutionary origin. CAM photosynthesis is thought to have evolved independently in the aloes, agaves and cacti (4).
The flowers of the aloes and agaves also have some similarities. Both genera of plants produce flowers at the top of a long tube (1,2). However, while aloe plants will usually flower many times during their lifetime of around 100 years, agaves only flower once after about 10-30 years and die shortly after.
The aloes and the agaves are an example of how organisms can independently evolve similar traits in response to similar environmental challenges.
1) Wikipedia: Aloe (retrieved 29/05/14)
2) Wikipedia: Agave (retrieved 29/05/14)
3) Janssen and Bremer (2004). The age of major monocot groups inferred from 800+ rbcL sequences. Botanical Journal of the Linnean Society.
4) Borland et al (2011). The photosynthetic plasticity of crassulacean acid metabolism: an evolutionary innovation for sustainable productivity in a changing world. New Phytologist.