Evolution - Divergence and Convergence

When we look at the biosphere, there appears to be two distinct types of evolutionary outcomes, depending on the selective forces that are involved. These outcomes are generally referred to as divergent and convergent evolution.


Divergent evolution

As the name suggests, divergent evolution increases diversity. In fact, the usual outcome is the formation of new species from a single ancestral species.


Divergent

The figure shows an example of how divergent evolution may occur. The ancestral species (A)experiences a natural selection event and undergoes speciation (that is, twodistinct species, B and C emerge). At a later point in time, species B diverges to produce species D and E, after another natural selective event (2). D produces species F and G.

Although each population is unique, they all share certain traits.These shared traits will show specific adaptations, but are all derived from the same ancestral trait. Structures that show such patterns are called homologous structures. One example of this are thelimbs of the vertebrates. The fins of fish, limbs of the amphibians, reptiles, birds and mammals, as well as the wings of the birds and bats are homologous structures. They are all built on a common structural plan, usually referred to as the pentadactyl limb (they have the same overall layout of bones). Despite their shared ancestry, these limbs show specific adaptations to their hosts’ usual mode of life. Thus, fins and flippers show adaptations for swimming, wings for flying, while forelimbs andhindlimb for terrestrial movement.

Hence, homologous structures provide evidence for divergent evolution.

Evidence for evolution - pentadactyl lim

Homologous structures: the pentadactyl limbs of the vertebrates. Despite differences in function, they show a common anatomical plan.

Image from https://wikis.engrade.com/a121biology2012/evidenceofevolution



Convergent evolution

Convergent evolution does not give rise to new species. There are ways that convergent evolution manifests itself:

(i) changes within species: sometimes, the frequency of traits in a population will change as a result of a natural selection event (without the formation of a new species). For example, bacterial populations exposed to antibiotics may become resistant to that chemical, or insects may become resistant to insecticides used to controltheir populations.

(ii) analogous structures/features: another example of convergence is the development of similar traits by evolutionarily unrelated (or distantly related) species. For example, insects and birds show similar bodily adaptations for flight - wings. Insect and bird wings are not structurally similar and are actually derived from unrelated structures. Vertebrate and insect eyes are another example. Such features are called analogous structures.


analogous

Examples of analogous structures. Note that bones in the wings of birds and bats are homologous structures.

Image fromhttp://www.bio.miami.edu/dana/106/106F05_4.html



Sham Nair 2014