Ancestral and derived traits

Taxa evolve from common ancestors and then diversify. Scientists use the phrase “descent with modification” because even though related organisms have many of the same characteristics and genetic codes, changes occur. This pattern repeats as one goes through the phylogenetic tree of life:

1. A change in an organism’s genetic makeup leads to a new trait which becomes prevalent in the group.
2. Many organisms descend from this point and have this trait.
3. New variations continue to arise: some are adaptive and persist, leading to new traits.
4. With new traits, a new branch point is determined (go back to step 1 and repeat).

If a trait in a clade is found in the ancestor of a group, it is considered a shared ancestral trait, also called a symplesiomorphy, within that group because all of the organisms in the clade have that trait. The vertebrate in Figure 9.1 is a shared ancestral character. Now consider the amniotic egg characteristic in the same figure. Only some of the organisms in Figure 9.1 have this trait, and to those that do, it is called a shared derived trait, also called a synapomorphy, because this trait derived at some point but does not include all of the ancestors in the tree.

The tricky aspect to symplesiomorphies (shared ancestral characters) and synapomorphies (shared derived characters) is that these terms are relative. We can consider the same trait one or the other depending on the particular diagram that we use. Returning to Figure 9.1, note that in the Amniotes clade, the amniotic egg is a symplesiomorphy (shared ancestral character) for lizards, rabbits, and humans, while having hair is a synapomorphy (shared derived character) only for humans and rabbits. However, when considering all the taxa on this phylogeny, the amniotic egg is a synapomorphy (shared derived character) that is not seen in fish. These terms help scientists distinguish between clades in building phylogenetic trees.

Convergent Evolution

The evolution of species has resulted in enormous variation in form and function. Sometimes, evolution gives rise to groups of organisms that become tremendously different from each other. When two species evolve in diverse directions from a common point, it is called divergent evolution. Such divergent evolution can be seen in the forms of the reproductive organs of flowering plants which share the same basic anatomies; however, they can look very different as a result of selection in different physical environments and adaptation to different kinds of pollinators (Fig 9.2).

In other cases, similar phenotypes evolve independently in distantly related species due to those species facing similar selective pressures. For example, flight has evolved in both bats and insects, and they both have structures we refer to as wings, which are adaptations to flight (Fig 9.3). However, bat and insect wings have evolved from very different original structures. We call this phenomenon convergent evolution, where similar traits evolve independently in species that do not share a recent common ancestry. The trait in the two species came to be similar in structure and have the same function, flying, but did so separately from each other.

Homologous and analogous structures

Some organisms may be very closely related, even though a minor genetic change caused a major morphological difference to make them look quite different. Similarly, unrelated organisms may be distantly related, but appear very much alike. This usually happens because both organisms share common adaptations that evolved within similar environmental conditions, called convergent evolution. When similar characteristics occur because of environmental constraints or similar selective pressures, and not due to a close evolutionary relationship, it is an analogy or homoplasy. For example, insects use wings to fly like bats and birds, but the wing structure and embryonic origin is completely different. Thus, wings in bats and birds are a homoplasy (Fig 9.3).

Another example can be seen in arctic mammals such as foxes and snowshoe hares, which each grow white fur during the winter months. White fur allows these organisms to blend into the ice and snow that characterizes their polar home, and presumably protects them from predation. However, foxes and snowshoe hares do not share a common ancestor with white fur. Of course they ultimately share a common ancestor, as do all mammals, but the fox lineage is full of non-white animals, as is the group to which hares belong. The winter white of arctic foxes and snowshoe hares is thus a homoplasy, due to convergent evolution in a white, wintry landscape.

Similar traits can be either homologous or analogous. Homologous structures share a similar embryonic origin due to their deep evolutionary relationship. Analogous structures have a similar function, but often very different developmental pathways. For example, the bones in a whale’s front flipper are homologous to the bones in the human arm. These structures are not analogous. A butterfly or bird’s wings are analogous but not homologous. Scientists must determine which type of similarity a feature exhibits to decipher the organisms’ phylogeny.

We use developmental/embryonic origin as an indicator of homology because development is a very complex process. The more complex the feature, the more likely any kind of overlap is due to a common evolutionary past. Imagine two people from different countries both inventing a car with all the same parts and in exactly the same arrangement without any previous or shared knowledge. That outcome would be highly improbable. However, if two people both invented a hammer, we can reasonably conclude that both could have the original idea without the help of the other. The same relationship between complexity and shared evolutionary history is true for homologous structures in organisms.

References

Adapted from Clark, M.A., Douglas, M., and Choi, J. (2018). Biology 2e. OpenStax. Retrieved from https://openstax.org/books/biology-2e/pages/1-introduction