Convergent evolution

**Convergent evolution** is a process in biology. It occurs when two unrelated species evolve the same traits or features, separately and independently. That happens because they live in similar habitats and must develop solutions to the same kind of problems.

## Similarity in traits 
*  and *Astrophytum*, are only distantly related. They have converged on a very similar body form]]
 convergent evolution is the fossa, a Malagasy carnivore *Cryptoprocta ferox* This has evolved in appearance and behaviour to be so much like a large cat that it was originally classified in the Felidae, but in fact, it is closely related to the mongoose.]]

Similarity in traits can occur in two ways. 

Both species could have inherited the trait from a common ancestor. In that case, the structures are called *homologous*. An example is the tetrapod limb, which has been inherited from early tetrapods in the late Devonian/early Carboniferous periods, about 360 million years ago. 

On the other hand, both species might have independently evolved the same traits as adaptations to similar conditions in their habitats. In that case, the structures are *analogous''. Convergent evolution leads to analogous features.

## Examples 
Examples of convergent evolution are extremely numerous, and it is an important feature of evolution. Below are just a few examples.

### Wings 
The wings of insects, birds, bats and pterosaurs are similar to a certain degree. In particular, they are all thin and strong, with a wide surface area. They create lift by being mechanically moved in a regular way. 

In each case, the wings evolved separately, so their form reflects certain physical necessities. The three larger animals all have insulation and temperature regulation and hence a high rate of metabolism. That is also necessary for flight, which requires a great deal of energy.

### Eyes 
One of the most famous examples of convergent evolution is the camera eye of cephalopods (e.g. squid), vertebrates (e.g. mammals) and cnidaria (e.g. box jellies). Their last common ancestor had a simple photo-receptive spot, but through evolution, its structure was progressively refined into the advanced camera eye. 

The similarities of such a complex structure shows how certain challenges in nature might have an optimal solution, which unrelated organisms reach independently through evolution.

### Nectar-eaters 
Four groups of songbirds from different families in different countries specialise in nectar-eating. They are the hummingbirds (Trochilidae, which live in the Americas); the sunbirds (Nectariniidae; South Africa); the honeyeaters (Meliphagidae; Australia); and the honey-creepers (Drepanididae; Hawaii).p224 

These species have similar adaptations because all of them use their tongue to eat nectar from the center of flowers.

### Vultures 
Vultures of the Old and the New Worlds come from separate though related families. Old World vultures come from the family Accipitridae, which also includes eagles, kites, buzzards, and hawks. Old World vultures find carcasses exclusively by sight. New World vultures belong in the family Cathartidae and use scent as well as sight. 

Old and New World vultures evolved several traits independently. Both are large, soaring birds and are specialist feeders on dead carcasses. They have powerful beaks, long featherless necks, strong stomach acids, and an extensive crop to store the food while eating. These traits developed in both types of vultures because they helped the vultures survive. 

### Aquatic animals 
Large, fast-moving aquatic animals tend to be torpedo-shaped: tuna, sharks, dolphins, killer whales, and ichthyosaurs all have a similar shape. The streamlined shape reduces drag as they move through the water. In some large aquatic animals (like ichthyosaurs and sharks) have fins on the same places on the body. They have arrived at this shape from very different starting points.

### Saber-toothed cat 
The Saber-toothed cat lifestyle evolved independently at least five times in mammals.

### Small tree-dwelling animals 
Small animals that sleep in trees often hang on with their feet. These animals usually have a tendon-locking device, which keeps them hanging on even during sleep. Once the tendons are locked, the muscles in their legs and feet can relax. Even dead bats stay hanging. 

Those various examples have evolved independently of each other: this is known from the fine details of the mechanism. One advantage of a locking mechanism is that it allows attachment to slender branches on which predators might be unable to walk.

## Parallelophyly 
Parallelophyly is the special case of two or more lines with a close common ancestor getting the same characteristic independently. Cichlids, a fish in Lake Tanganyika, in East Africa, have developed the same feeding method in six different lines.

Stalked eyes occur irregularly and independently in acalypteran flies. They have clearly inherited the genetic capacity for such eyes. This capacity is selected only in some lines.p62, 225

## References