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).

Phylogenetic trees can show when certain traits evolved and which species have those traits. For example, Figure 10.1 shows a tree with several animal species. The labeled dashed lines show when different traits evolved. For example, lungs evolved within a species that was a common ancestor of antelopes, bald eagles, and alligators; this means present-day antelopes, bald eagles, and alligators have lungs. We also learn from that tick mark that lamprey and sea bass do not have lungs. Fur evolved within an ancestor of modern-day antelopes. Only antelopes have fur – all other species depicted on this phylogeny do not. We can also see that all species except lamprey have jaws. Note that tick marks may also show where a trait is lost – there aren’t any tick marks like that in Figure 10.1, but many trees do show trait losses, because traits can be both gained and lost over evolutionary time.

Ancestral and derived traits

Clades are defined as being all of the descendants of a particular ancestor. If an ancestor of a clade had a particular trait, then all of the descendants will have that trait (unless the phylogeny indicates it was lost through evolution). For this clade, this would be an ancestral trait because it evolved before the ancestor of the clade. In figure 10.2, the amniotic egg is ancestral to the Amniota clade. All of the members of the Amniota clade have an amniotic egg because the ancestor of the clade had this trait. However, the amniotic egg is not ancestral to the Vertebrata clade because it evolved more recently than (i.e., after) the ancestor of the Vertebrata clade. Thus, some of the members of the Vertebrata clade have amniotic eggs, but not all of them. This makes amniotic egg a derived trait with respect to the Vertebrata clade. Derived traits are traits that evolved after the ancestor of a clade, and thus are possessed by only some, not all, of the members of the clade.

When traits are unique to a particular clade, they are useful for identifying and distinguishing which organisms belong to that clade. This special type of trait is called a synapomorphy, sometimes called a shared derived trait. With respect to the Amniota clade, the amniotic egg is a synapomorphy because all of the members of the Amniota have this trait and no species outside of this clade have this trait. Thus, you can conclude that if a species has an amniotic egg, it belongs to the Amniota clade.

If a trait is ancestral to a clade and evolved long before the ancestor of the clade, it is called a symplesiomorphy, sometimes also called a shared ancestral trait. This is because all members of the clade have that trait, but so do members of other clades. With respect tot he Amniota clade, vertebrae would be a symplesiomorphy. The ancestor of the Amniota has a vertebra, and so all members of the Amniota have a vertebrae. However, other organisms that are not members of the Amniota clade, such as the fish and lampey, also have vertebrae. Therefore, while it is true that all Amniota have vertebrae, knowing that an organism has a vertebra does not tell you whether that species is in the Amniota clade or not.

It is important to remember that all these terms (derived, ancestral, synapomorphy, and symplesiomorphy) are relative to the clade being referenced. A trait that is a synapomorphy for one clade could be a symplesiomorphy for a different clade and a trait that is ancestral to one clade could be derived within a larger clade. For example, in Figure 10.2, the amniotic egg is a synapomorphy with respect to the Amniota clade, but derived with respect to the Vertebrata clade. These terms help scientists distinguish between clades in building phylogenetic trees.

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