1. Introduction: Evolution’s Branching Pattern

In the previous tutorial, we focused on observations of evolutionary change within a single lineage of organisms. A lineage is a line of descent. Within that lineage, there will be an ancestral form (A), a most recent form (E), and one or more intermediates  (B through D). Whether or not these forms are varieties or species depends on the degree of differentiation (a topic we’ll take up later in this unit).

Evolution also proceeds in a branching pattern, with an ancestral species (A) giving rise to daughter species (E and H), as shown below.

When repeated branchings occur (see below), we wind up with one species giving rise to multiple descendants. That process is called adaptive radiation.  The result is descent with modification that runs in parallel lines. Tracing these parallel modifications can provide us with evidence for evolution.

2. Adaptive Radiation and Homologous structures

Here’s an example of adaptive radiation. The birds on the right side of the evolutionary tree live on the Galapagos islands, about 600 miles (1000 kilometers) off the coast of Ecuador. The bird on the left is their common ancestor.

Evolutionary tree of the Galapagos finches	The Galapagos Islands

Why is the process of one species splitting into multiple descendants called adaptive radiation? In this context, radiation means spreading out. In relation to the Galapagos finches, the basic idea is this. After the birds first arrived from Ecuador (probably blown there by a storm), they were subject to natural selection in their new island environment. This led to the evolution of new adaptations, especially in the form of their beaks. As the birds spread from island to island, the process of natural selection and adaptation repeated itself. Eventually, you have an array of descendant species, each with a beak that’s adapted to a different food source.

Take a moment to study the diagram below.

The beaks of the Galapagos finches are structures that have evolved different functions. In each species, there’s been descent with modification. But because of different types of natural selection, the modification is different in each case.

Structures that are inherited from a common ancestor are called homologous structures. The related noun is homology. Homology is a similarity in structure between parts of different species due to evolutionary differentiation from a corresponding part in a common ancestor (adapted from Merriam-Webster).

How are the beaks an example of a homology?

• The beaks have a similar underlying structure.
• They’re slightly different because natural selection caused each species to differentiate (which means “to become different.)
• They’re inherited from a corresponding part in a common ancestor.

Study the flashcards below.

3. Adaptive Radiation and Homologous Structures Flashcards

4. Homology as Evidence for Evolution

Let’s look at the bones that make up the forelimbs of humans, dogs, birds, and whales. These structures, which make up the arm, forelimb, wing, and flipper of each respective animal, have the same underlying structure. They’re composed of the same bones.

How do we know that they’re the same bones?

• First, because of their similar position and relationship with one another.
• Second, because they develop from the same tissue in the embryo of each animal.

You can see this is the diagram to the right. Early in embryonic development, a patch of tissue forms a limb bud, circled in red. If you examined embryos from related species that were at the same developmental phase, you’d see that this tissue is identical in form. However, as the animal develops into its adult form, the limb bud differentiates into the form characteristic of its species.

Why are all vertebrate forelimbs composed of the same bones, and why do they develop from the same embryonic tissue? Because all of these animals share a common ancestor. Through adaptive radiation, the ancestral species split into several lineages. In each lineage, natural selection led to the development of unique adaptations shared by that evolutionary group.

Look again at the diagram showing homology in the forearms of various vertebrates.

• In the group that led to whales, natural selection modified the forelimb into a flipper.
• In birds, natural selection modified the forelimb into a wing.
• In the group humans belong to, the forelimb evolved an opposable thumb and the ability to rotate our forearms and shoulders. This adaptation is found in all primates, the order of mammals that humans are a part of. Other primates are monkeys and apes. Primate forearms are an adaptation for life in trees.
• In dogs and other carnivores, natural selection led to a forelimb adapted for running and capturing prey.

What was this common ancestor of humans, birds, whales, and dogs like? It wasn’t some animal that mixed the features of humans, dogs, birds, and whales. Rather, the common ancestor of all of the four-limbed vertebrates (amphibians, reptiles, birds, and mammals) was a lobe-finned fish, similar to the extinct Eusthenopteron shown below.

Looking at vertebrate forelimbs (as well as the beaks of the Galapagos finches), you can see why homologous structures have been compared to the musical form of “theme and variations.” This form, popular in classical music, takes a melody (the theme), and then modifies it (producing the variations). In evolution, the theme is the structure in the common ancestor; the variations are the specific adaptations in the descendants.

Why are homologous structures evidence for evolution? Because they confirm the claim of descent with modification. Homologous structures only make sense if seen as having evolved from a shared ancestor which diverged, over evolutionary time to form several related descendant species.

Let’s consolidate our understanding with flashcards, and then a quiz.

5. Checking Understanding: Adaptive Radiation and Homologous Structures

5a. Adaptive Radiation and Homologous Structures Flashcards

5b. Adaptive Radiation and Homologous Traits Quiz

6. Vestigial traits are a special type of homology

6a. The Case of the Blind Cave Tetra

The Mexican tetra (Astyanax mexicanus) is a small, freshwater fish found in Eastern Mexico and Texas. In many areas, these fish live in water-filled caves. Some of these cave-dwelling populations have lost much of their vision. Some have lost all of their vision. Some have lost their eyes.

About 30 different cave populations have been identified. None have achieved the status of a distinct species. They can all interbreed with surface fish and produce fertile offspring.

Why would fish populations that live in caves lose their vision? There are a couple of reasons:

1. Fish that live in caves can have mutations in genes that control the eyes’ structure or function, and suffer no loss in fitness. Thus, alleles for these mutations never get removed from these populations’ gene pools. Over time, these mutations can accumulate, causing average vision to get worse and worse from generation to generation.
2. The eyes, and the nerve cells that support them, are complex structures. They require energy to build, maintain, and repair. In a situation where eyes are of little benefit, genes for smaller, less functional eyes can benefit survival. Instead of supporting eyes, energy can be devoted to other activities related to survival and reproduction.

So, instead of functioning eyes, cavefish often have just a trace of an eye left. Such a trace of an organ that once played a functional, adaptive role in an ancestor is called a vestigial trait. In more general terms, a vestigial trait is a structure that has lost its function and only remains in a reduced form.

Why don’t these vestigial structures go away altogether? Sometimes they do. But there are a couple of reasons why a vestigial structure might be maintained.

1. Once the structure becomes reduced, there’s little energy cost involved in maintaining it, and very little selective pressure to reduce it further.
2. The genes for the structure might be involved in other functions or processes that are still important to survival. It might be impossible to turn off these genes without having negative effects that would reduce the organism’s overall fitness.

How are vestigial traits evidence for evolution? They’re examples of descent with modification. The only reason why a vestigial trait would exist is that the ancestor of the species with the vestigial trait had a functioning version of the trait.

6b. Additional Examples of Vestigial Traits

You don’t have to look very far to find other examples of vestigial traits.

Our ears are wired up with muscles. But can you move your ears at will? Some people can wiggle their ears a bit, but most people can’t. And, even if you can wiggle your ears, you can’t move them in the way that other mammals like cats, dogs, rabbits, and bats do. These animals move their ears to focus the sound and to pinpoint its source. Ear movement helps predators be better at catching prey. It helps potential prey be better at avoiding being caught.

Human ear muscles are a vestigial trait. They’re evidence of descent with modification. In this case, we evolved from mammalian ancestors that had functioning ear muscles.

Here’s another human vestigial trait: the coccyx, or tailbone.

Humans belong to a group of mammals called primates. Most primates, such as monkeys and lemurs, have tails. Within the primates, we belong to the family Hominidae or great apes. This group includes chimpanzees, gorillas, and orangutans. All of the species within this family lost their tails during the course of evolution. But we still have a vestige of a tail: our coccyx (tailbone). The coccyx is shown at “2” in the image on your right. Why do we have a coccyx? Because of descent with modification.

 

6c. The Evolution of Vestigial Traits: Interactive Reading

7. Vestigial and Homologous Structures Flashcards

What’s next?

Proceed to Topics 7.6 – 7.8, Part 3: The Fossil Record (the next tutorial in AP Bio Unit 7).