Homologous and Vestigial Structures as Evidence for Evolution

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1. Adaptive Radiation and Homologous structures

1a. Adaptive radiation results in differentiation

Here’s a story about songbird evolution on an an island chain.

• Roman numerals I through V represent a time sequence that begins with I ends with V.
• Letters A through E represent different songbird species.
• Letter w represents a continent. Letters x, y, and z represent the islands in an island chain that’s at some distance away.
• Numbers 1 through 9 represent speciation events (events that form new species).

Note that we’ll cover how speciation works in the next module.

In diagram I, there’s only one songbird species, A. It lives on continent “w.” Islands  x, y, and z are beyond the normal migration range of species A. However, a storm blows a small population of A out to sea.  Instead of perishing, they land on island “x” (event 1).

In diagram II, the different environment on island x places selective pressure on the new arrivals from species A. Over time, they evolve enough genetic differences to qualify as a new species, distinct from species A. This is event 2, and it creates species B. In addition, some members of species B migrate over to island y (event 3)

In diagram III, different selective pressures on island y cause the birds there to evolve into species C (event 4). These birds have become so different from their ancestors that when some migrate back to island x, they no longer interbreed with species B (event 6). At the same time, some members of species C migrate over to island z (event 5).

In diagrams IV and V, additional migrations followed by differentiation lead to the production of four descendant species (B, C, D, and E). Species A continues to exist on continent “w,” for a total of 5 species.

Continue with the interactive reading below.

1b. Differentiation from a common ancestor results in homologous structures

Structures that are inherited from a common ancestor and which have evolved different functions are called homologous structures. The related noun is homology, defined by Merriam Webster as a “likeness in structure between parts of different organisms …. due to evolutionary differentiation from a corresponding part in a common ancestor.”

Following this definition, the beaks of the Galapagos finches are homologous. They’re structures that evolved from a related structure in a common ancestor, and differentiated into different forms as each population adapted to specific environments. Each beak became specifically adapted to enable each evolving species to fill a specific ecological niche. An ecological niche is the way that a species makes its living.

Here are two examples.

• The large ground finch evolved into a seed-eating niche.
• The woodpecker finch evolved into an insect-eating niche.

As each population evolved into these specific niches, selective pressure modified their beaks into a shape that was optimized for the type of food they eat.

This type of adaptive radiation giving rise to homologous structures can best be represented by a phylogenetic tree. A phylogenetic tree shows evolutionary relationships among various species. Here’s a phylogenetic tree for six of the Galapagos finch species.

Adaptive radiation leading to homologous structures in related species is one of the most fundamental patterns in living things. It shows up everywhere: in physical structures (morphology); in genes, in metabolic pathways.

Homology is perhaps the most powerful piece of 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 to one another. Second, because they develop from the same tissue in the embryonic form of each animal.

You can see this is the diagram to the right. Early in embryonic development, a patch of tissue differentiates to form a limb bud, circled in red. In embryos at the same developmental phase, this tissue is identical in form. However, as the animal develops into its adult form, the limb further 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, selective pressure led to the development of unique adaptations shared by that evolutionary group. In the group humans belong to, selection was for an opposable thumb and the high degree of forearm rotation found in all primates. That’s an adaptation for life in trees. In the group that led to whales, selective pressure modified the forelimb into a flipper. In birds, different selective pressures modified this form into a wing. In dogs and other carnivores, they became the forelimb characteristic of dogs, cats, bears etc.

What was this common ancestor 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.

In what follows, we’ll look at other categories of evidence for evolution. But first, let’s consolidate our understanding with a quiz.

1c. Checking Understanding: Homologous Traits

2. Vestigial Traits are a special type of homology

2a. The Case of the Blind Cave Tetra

The Mexican tetra (Astyanax mexicanus) is a small, freshwater fish found in Eastern Mexico, with a range that extends north into 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. 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, cave fish 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 organ. In more general terms, a vestigial, structure is one that has lost its function, and only remains in a reduced or rudimentary form.

Why don’t these 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 process 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.

2b. 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, but most can’t. And, even if you can move your ears, you can’t move them in the adaptive way that other mammals can. The muscles are a vestigial trait. They’re a vestige or remnant of our mammalian past, when our ancient, non-human ancestors were able to move their ears to focus sounds.

Here’s another human vestigial trait: Humans belong to group of mammals called primates. Most primates, such as monkeys and lemurs, have tails. Within the primates, we belong to the 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. It’s our coccyx, or tailbone. The coccyx is shown at “2” in the image to your right.

2c. Vestigial Traits: Checking Understanding

Links

1. The Fossil Record (the next tutorial in this module)
2. Evidence for Evolution Menu