BioG 105/106 | Autotutorial Introductory Biology

Systematists attempt to make evolutionary connections by evaluating similarities among different organisms, but unfortunately not all similarities are inherited from a common ancestor. Species from different evolutionary branches may come to resemble one another if they have similar ecological roles and natural selection has shaped analogous adaptations. This is called convergent evolution and similarity due to convergence is termed analogy. A number of different examples are shown here and in the display case in room 205.

Convergence in Vertebrate Body Form

Convergence in overall body form is common in vertebrate evolution. The example below shows (on the left) a mammal, a reptile, and a fish that have all become oceanic predators, and (on the right), two desert rodents that have adopted the lifestyle of fast jumping insect predators.

Convergence in Birds

The lefthand members of the two pairs of birds below all belong to a closely related group of birds from North America while the birds on the right are all from a group of closely related genera that occur in Africa and India. These two groups (the North American genera and the African genera) have independently produced three species filling three particular ecological roles. The result is three remarkable instances of evolutionary convergence.

Of the first pair, the bird on the left is Sturnella magna, the meadowlark. This bird is an inhabitant of the grasslands of Texas. On the right is Macronyx croceus, an inhabitant of the savanna in Africa, a very similar habitat. These two species are in two different, unrelated families yet the resemblance between them is marked.

The second pair of birds are orioles. The first is a Northern Oriole (Icterus galbula) while the second is a Scarlet Minivet (Pericrocotus flammeus) from Assam, India.

Despite the superficial similarity in each case, the true relationships of he birds are indicated by differences in other traits: the birds on the right in each pair have a smaller overall body size, a different head shape, and a smaller, stouter beak. It is typical in instances of convergence that some superficial characteristics such as color and body shape show the convergence most strongly while more conservative characteristics such as skeletal or internal anatomy retain features indicative of the organisms' phylogenetic stock. Head and beak shape are probably intermediate in this regard.

Convergence in Plants

Here is a well-known example of convergent evolution among plants. Plants from the cactus family (Cactaceae), the euphorbia family (Euphorbiaceae), and the milkweed family (Asclepiadaceae) are shown here. The similarities of their various adaptations make them look so much alike that the average person normally calls all of them "cacti," yet they are really not closely related. Can you think of reasons why these plants, which live in the same sort of habitat, might have evolved the particular characters that they all have?

Convergent Evolution

Comparable selective forces, acting on plants growing in similar habitats but different parts of the world, often cause totally unrelated species to assume a similar appearance. The process by which this happens is known as convergent evolution.

Let us consider some of the adaptive characteristics of plants growing in desert environments - fleshy, columnar stems (that provide the capacity for water storage), protective spines, and reduced leaves. Three fundamentally different families of flowering plants - the euphorbia family, the cactus family, and the milkweed family - have members that have evolved in this direction. The cactus-like representatives of the euphorbia and milkweed families shown here evolved from leafy plants that look quite different from one another.

Euphorbia (left), Cactus (center), and one type of Milkweed (right).

The cacti occur (with one exception) exclusively in the New World. The comparably fleshy members of the euphorbia and milkweed families occur mainly in desert regions in Asia and especially Africa, where they play an ecological role similar to that of the New World cacti.