1. Introduction

Let’s start by reviewing two key types of evidence for evolution: homologous and vestigial structures.

2. Embryology as Evidence for Evolution

As stated in one of the cards above, one way that we know that a structure is homologous is because it originates in a similar patch of tissue in the embryo of each animal. In the diagram to the left, the same patch of tissue develops into the wing of a bird, and the arm of a human being. As the animal develops into its adult form, the structure becomes modified to serve a different function.

Let’s look at a few more embryos. The images below show embryos from different types of vertebrates — animals with backbones — at equivalent phases of development. The top row consists of photographs with some modifications (a yolk sac removed in the salamander). The bottom row shows drawings from Ernst Haeckel, a famous 19th-century biologist.

The similarity in embryonic form provides two additional pieces of evidence for evolution. First, the similarity of form is a homology. The pattern of development shows that these animals are descended from a common ancestor.

Second, comparing embryos also provides us with additional evidence for evolution related to vestigial traits.

• Gill slits appear in humans, rabbits, and chickens. Gill slits are openings from the back of the head into the mouth. Fish use them to pass water over their gills through which they absorb oxygen from water. These disappear later in development.
• Tails appear in humans and chickens and disappear later in development.

Why do animals such as humans, rabbits, and chickens, during development, expend the energy to create gill slits, only to break them down and absorb them later? It’s because having gill slits is a part of a developmental pattern inherited from a vertebrate ancestor. In early embryos, the genes for this plan are still expressed.

The same is true of the tail. All vertebrate embryos have a pronounced tail. That includes animals like humans and birds, which as adults don’t have tails. Why do these embryos have tails? Because having a tail is a vestigial trait. It’s a vestige of a developmental pattern inherited from a common ancestor.

What’s the takeaway? Embryological development allows us to more clearly see the homologies that come about through adaptive radiation.

• All vertebrates descend from a common ancestor.
• As vertebrates develop, their body plan unfolds in a way common to all vertebrates. That embryonic pattern is itself a homology.
• As development continues, the form of each lineage diverges. The same patch of tissue will develop into a wing of a bird or the forelimb of a lizard, rabbit, or frog.
• The homologous structure of the forelimb in the adult form of each animal reflects adaptive radiation from a common ancestor.

Finally, note that this same pattern of embryonic similarity followed by development into diverse forms can be found in any other clade (a group of organisms descended from a common ancestor). You can learn about the differentiation of insect mouthparts in this article on Wikipedia.

3. Flashcards: Embryological Evidence for Evolution

4. Molecular Homologies

Above and in previous tutorials, we’ve discussed how homologous structures serve as evidence for evolution. Molecules can be homologous too, establishing another line of evidence for evolution.

The table below shows two proteins that are widely shared among many species.

Cytochrome c is a protein that’s used in cellular respiration.	Hemoglobin is a protein used to carry oxygen in red blood cells.

Proteins are one of the key molecules in living things. If you want to review protein structure in depth, you can do so at this Learn-Biology.com tutorial. But for what’s below, you just need to know that proteins are built from subunits called amino acids. A chain of amino acids is shown at right. The type of amino acids making up a protein, and the order in which these amino acids are linked together, determine the protein’s three-dimensional structure.

Cytochrome c and hemoglobin are found in many species with either an identical or highly similar sequence of amino acids and a correspondingly similar three-dimensional shape. Why do so many different species share these molecules? Because they inherited the genes for making these molecules from a common ancestor.

To learn more, complete the following interactive reading.

What’s the takeaway?  The sharing of common proteins like cytochrome c and hemoglobin among animals isn’t unusual. Most of the proteins found in human beings are also found in other animals. The sharing of these proteins is a homology, indicating descent from a common ancestor.

What’s true of proteins is also true of genes. Based on how the term “gene” is defined, humans have between 20,000 and 25,000 genes. Almost all of these genes are also found in other animals. These shared genes are, like the proteins they code for, homologies. We share these genes with other animals because we inherited them from a common ancestor.

Here are two examples:

• Genes coding for eye development: A gene called eyeless codes for eye formation in fruit flies (and other insects). This insect gene is homologous to a gene called small eye in mice (and other mammals). How do we know that the genes are homologous?  First, because of the shared sequence of nucleotide bases in each gene. Second, because of a series of experiments in which these genes were transferred from fruit flies to mice, and vice versa.  The eyeless gene has been experimentally inserted into mutant mice embryos that are from a strain of mice that don’t develop eyes. With the fruit fly gene, these mice produce eyes. The reverse is also true. Insert the mouse gene into embryos of mutant fruit flies that don’t develop eyes, and eyes will develop.
• Genes coding for body segments: Almost all animals are built using a similar developmental plan. The plan involves the development of a head, a middle region, and a hind region. In many animals, these regions can be subdivided into segments. The genes that control how each segment develops are called Hox genes, and they’re shared between insects and mammals. How do we know they’re homologous? Because of shared sequence, and because of gene transfer experiments. In other words, the same gene that directs the formation of structures on the head in an insect also directs the formation of a head in mammals (including you). These genes are shown in red in the diagram to the right. The same is true of the structures on the rest of the body, from head to tail (even if your tail is just a vestige).

What do these genetic homologies tell us? That mice and fruit flies share a common ancestor from which they both inherited the same genes for eye development and body segment identity. That common ancestor existed 500 million years ago when the line leading to vertebrates split apart from the line leading to arthropods (insects, crabs, spiders, etc.) And just to make this personal, that means that insects are our very ancient cousins. Think of that the next time you swat a fly.

5. Flashcards: Molecular Evidence for Evolution

6. Checking Understanding Quiz: Embryonic and Molecular Evidence for Evolution

7. The Deepest Homologies

7a. Homologous features found in all eukaryotes

Our domain, the eukaryotes, arose about 1.5 billion years ago. Despite their incredible diversity, eukaryotes share a single common ancestor. How do we know? Because of the homologous features possessed by all eukaryotes.

Here’s a short list of the homologous features that all eukaryotes share.

1. Mitochondria
2. Complex cells with internal compartments such as the rough and smooth endoplasmic reticulum, the Golgi apparatus, and lysosomes.
3. A membrane-bound nucleus encloses the cell’s chromosomes, separating the chromosomes from the cytoplasm.
4. Linear chromosomes (as opposed to the circular chromosomes in bacteria and archaea).

Note that multicellularity is not a shared eukaryotic feature (because many eukaryotes are unicellular).

 

7b. Commonalities for all life

We can go one level deeper, from traits that unify eukaryotes to traits found in all living things.

All living things possess molecular and genetic homologies that point to one of the most profound insights from the study of biology: that all living things are related. We’re all cousins. “We,” in this context, doesn’t just mean all human beings (though that’s true). We’re also cousins to sparrows, leaf-cutter ants, oyster mushrooms, rhododendrons, kelp, Archaeans that live in hot springs, the photosynthetic bacteria that live in the oceans, and the E. coli bacteria that live by the billions in our guts.

To see what these homologies are, and to distinguish the conserved features of all life from the conserved features that bind together the eukaryotes, complete the quiz below.

8. Flashcards: The Deepest Homologies

Links

To finish this high school-level module on evidence for evolution, click the following link for a cumulative flashcard deck and quiz.

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