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Phylogeny and Vertebrate Evolution

MUST KNOW

Kou often hear the statement “humans evolved from chimpanzees,” but what does that mean? When you say something like that, you’re referring to the fact that a long, long time ago, before there were any humans or chimpanzees, there existed a common ancestor to both humans and chimps. There was a population of mammals that most likely looked chimp-like, and this population split into two divergent evolutionary pathways: one led to modern-day chimps, the other led to Homo sapiens (humans). Speciation occurred (woohoo! we just learned about this in the previous chapter!), and in brief, it looks like this:

Phylogenetic tree of human and chimpanzee evolution

The common ancestor that existed ~5 million years ago is no longer present because speciation occurred and created the current species (such as humans and chimps). The phrase “we evolved from chimpanzees” is all sorts of wrong because it suggests: chimps → humans . . . and if that happened, there would be no chimps around, because they all evolved into people!

The branching diagram below is a phylogenetic tree and the basis of our must know for this chapter. Phylogenetic trees (and their close cousin, the cladogram) provide a visual representation of the relatedness of species. The point where two branches are connected indicates a common ancestor between the two separate species. When an event caused isolation of gene pools in that ancestral population, it led to speciation (as shown by the branches leading off of the common ancestor).

A phylogenetic tree is a hypothesis about the evolutionary relationships between groups of organisms; it’s a hypothesis because humans weren’t around to witness the actual evolutionary event occur. These branching diagrams allow us to study evolutionary history by grouping species by shared, heritable traits. These traits, or characters, include physical, behavioral, or molecular characteristics.

General phylogenetic tree with common ancestors (numbers) and species grouped by common derived characteristics (letters). Each species group can also be referred to as a taxon (or taxa, plural).

A branch point (or “node”) is a divergence of two lineages from a common ancestor. It shows where two groups separated due to some mutation and acquired trait. Node 1, for example, represents the common ancestor for all the species shown (A–E). Notice that species A didn’t change much from that original common ancestor. Species A is called the basal taxon, meaning it was the first to diverge from the ancestral species and has the fewest number of adaptations acquired since diverging. Branch point 2 represents the common ancestor of all the following taxa (B, D, E, and C).

A shared character is one found in all taxa under consideration. For example, in the figure below, a backbone is a character shared by fish, amphibians, reptiles, and mammals (but not the outlier, the invertebrate). The character of hair only applies to the mammal taxon.

Phylogenetic tree of vertebrate animals

A derived trait is a character present in a taxon or taxa, but not in the taxon’s common ancestor. A group that shares some derived trait is called a clade, and all species after a derived characteristic make up the clade. For example, amphibians, reptiles, and mammals are all of a clade defined by having four legs (the circled portion of the above figure). Again, as our must know concept suggested, phylogenetic trees help us visualize how groups of species are related, and how they share similar characteristics. For example, as animals evolved, species acquired traits that helped them better survive and reproduce on land. The large evolutionary events depicted in the previous phylogenic tree outline the momentous process of life moving from the oceans (where it began) onto the land. For example, if you are going to become mobile outside of an aquatic environment, you’re going to need legs.

Sometimes, a species’ next evolutionary step may involve losing characteristics that were acquired earlier in their evolutionary history. Snakes, for example, might have evolved from burrowing lizards. Whales are mammals that decided to head back to water and evolutionarily lost their rear legs (though they maintain a vestigial pelvis that acts as a reminder of their four-legged past).

As species colonized ecological niches that are farther removed from water, they gathered adaptations that helped them fight against constant water loss, either from their own bodies (amphibians lack the water-retaining scale layer that reptiles acquired), or from their eggs (amphibian eggs don’t have shells and must remain in water, whereas reptile eggs are shelled and can survive on land). If you look at that phylogenic tree, an “amnion” means a membranous sac that surrounds and protects the embryo. Mammals take it one step further and retain the amnion (and the embryo) inside their bodies during gestation (thanks, Mom).

REVIEW QUESTIONS

1. The saying “humans evolved from chimpanzees” is horribly wrong. Rephrase the statement so it correctly describes the relatedness between humans and chimps.

2. Answer the questions based on the following phylogenetic tree:

a. Which node is the common ancestor of species E and B?

b. Which species is most similar to the original common ancestor for species A–E?

3. Circle the node on the following cladogram that represents the common ancestor for species B and C.

4. Based on the following cladogram, which species is most closely related to species D?

5. Which derived trait(s) in the following phylogenetic tree is(are) only found in the taxa including amphibians, mammals, and reptiles?

6. Answer the questions based on the following phylogenetic tree:

a. Which of the following species is considered the basal taxon?

b. Which node—X or Y—indicates a common ancestor from earlier vertebrate evolution?

7. What can’t a phylogenetic tree show you?

a. Relatedness of different species

b. Time taken for an evolutionary event

c. Presence of common ancestors

d. Species grouped by shared traits