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Evidence of Evolution: Homology
Sometimes a discussion of evolution needs to consider how evolution does NOT happen.
A question often posed by folks who do not understand evolutionary mechanisms is:
"If humans evolved from monkeys, then why are there still monkeys?"
1. Humans did not evolve from monkeys.
2. Rather, humans and monkeys share a common ancestor.
3. There are still monkeys because, like humans, they are descended from successful ancestors.
4. And if you go back far enough, all species are derived from a common ancestor.
All living things give birth to things that are--more or less--fairly similar to themselves.
- Mushrooms don't evolve into pine trees.
- Fish don't evolve into amphibians.
- Frogs don't evolve into reptiles.
- Reptiles don't evolve into birds.
- Birds don't evolve into mammals.
Evolution proceeds by incremental changes to the raw material that's already present in any given population of organisms.
It is a process of gradual "remodeling" of a body plan by both random events and natural selection.
How do we know?
What observable evidence exists to support the idea that life on earth has
evolved its diversity through a long process of descent with modification?
One of the most powerful forms of evidence is homology.
In biology, a homology is a characteristic shared by two species (or other
taxa) that is similar because of common ancestry.
- morphological homology - species (correctly) placed in the same
taxonomic category show anatomical similarities.
- ontogenetic homology - species placed in the same
taxonomic category show developmental (embryological) similarities.
- molecular homology - species placed in the same
taxonomic category show similarities in DNA, RNA and protein.
Structures derived from a common ancestral structure (that may or may not
be used for the same function in the species in which it occurs) are called
A classic example of homology is seen in the skeletal components of
Evolution can be considered a process of "remodeling" a population over
the course of many generations, with one of the main the driving forces being the natural
selection factors that favor one form over another in specific
Scientists have several criteria for determining whether structures are homologous.
Let's have a look at some of our own homologies in The Zoo that is YOU.
Shared homologies allow us to devise hypothetical "family trees" of living organisms called phylogenies. These can be represented as a branching diagram called an phylogenetic tree or evolutionary tree.
The three purple hashmarks on the tree represent evolutionary innovations shared by all the groups above it (i.e., to the right) on the tree. Such shared characters allow evolutionary biologists to determine common ancestry.
Shared Characters: Clues to Shared Ancestry
Just as physical similarities among siblings provide evidence that they are related to each other,
physical similarities between species tell us that they are related.
Just as humans change across generations, so, too, do natural populations of other species.
By comparing the traits of species that are both closely and distantly related to each other, we can begin to understand their evolutionary history.
Primitive and Derived Characters
We can learn about the process of evolution from studies of both extant and extinct species, and comparing their physical (and sometimes chemical) characteristics. In any organism you're studying...
A primitive character is one that is relatively unchanged from its
original, ancestral form.
Example: All vertebrates have a bony tail posterior to the anus.
It was present in the ancestral vertebrate, and though different vertebrates have evolved differently shaped tails, they are all evolved from the common ancestral tail. The PRESENCE of the tail is primitive.
A derived character is one that is relatively modified from its
original, ancestral form.
Example: the modified shapes of various vertebrate tails. An extreme example: The loss of the external tail in great apes (including humans). The baboon shows the primitive condition, whereas the gorilla and Very Handsome Man show the derived condition of the post-anal tail.
We great apes still have a tail; it's just very reduced and tucked where it doesn't show.
Though there is the occasional atavistic reminder of our ancestry.
Note that these are relative terms.
You can't call something
"primitive" or "derived" without comparing it to something else.
In other words, when you invoke the "primitive" or "derived" state of a particular character,
you are examining homology between two or more species derived from a common ancestor.
Systematists often use the existence of homologous, shared characters among
taxa to help reveal their common ancestry.
The more recently two species diverged from a common ancestor, the more
synapomorphies they will share.
- a symplesiomorphy is a shared, primitive character
- a synapomorphy is a shared, derived character
Unique synapomorphies found only in a single group provide strong evidence for the common ancestry of members of that group.
The synapomorphic astragalus ankle bones of whales (Cetaceans) and cloven-hooved mammals (Artiodactyls) helped biologists determine that these two groups share a relatively recent common ancestor.
Synapomorphies are more informative than symplesiomorphies when you're trying to determine common ancestry. Why? Let's consider our own species and some of our closest vertebrate relatives.
(See what you missed if you didn't come to class?)
Analogous Structures and Convergent Evolution
Of course, structures can look the same for reasons other than common ancestry.
An analogous structure found in two different species
- serves the same function in two species
- but is not derived from a common ancestral structure
Example: The wing of a butterfly and the wing of a bat.
How do we know the common ancestor of the butterfly and the bat didn't pass on the wings to both species?
The last stage shown in the diagram is known as a gastrula. It's the first point in animal development where there is a MOUTH. At the gastrula stage, the mouth is known as the blastopore.
First, a quick overview of the earliest ontogeny gone through by ALL animals:
But at some point in our evolutionary history, a HUGE genetic change took place. In one ancestral gastrula lineage, the blastopore didn't become a mouth. It became an anus.
And Kingdom Animalia was turned on its head.
Protostomes vs. Deuterostomes Animals whose blastopore becomes the mouth are known as protostomes (from the Greek proto, meaning "first" and stoma, meaning "mouth")
Animals whose blastopore becomes the anus (with a secondary opening becoming the mouth) are known as deuterostomes (from the Greek deutero, meaning "second" and stoma, meaning "mouth")
This divergence took place millions of years ago.
Most animals are protostomes:
But one lineage, including vertebrates, became Deuterostomia:
- flatworms (e.g., Platyhelminthes)
- roundworms (Nematoda)
- segmented worms (Annelida)
- mollusks (Molluska)
- arachnids, insects, crustaceans (Arthropoda)
- starfish, sea urchins, etc. (Echinodermata)
- tunicates (Urochordata)
- lancelets (Cephalochordata)
- vertebrates (Vertebrara)
Butterflies (Arthropoda) are protostomes.
Bats (Vertebrata) are deuterostomes.
The most recent common ancestor of a butterfly and a bat existed billions of years ago
...and possibly looked a bit like today's gastrula
(A hypothetical beast called a gastrea.)
No eyes. No head. No wings. As evidenced by their most recent common ancestor, the wings of the butterfly and the wings of the bat evolved independently, long after their ancestral lineages diverged.
More examples of analogous structures:
- wings of various species (though the bones supporting the wing may be homologous, in some cases)
- walking limbs of insects and vertebrates
- cranium of vertebrates and exoskeleton head of insects
- fusiform shape of fish and cetaceans (whales and dolphins)
Stephen Jay Gould first coined the term exaptation:
The pre-existence of a character--
initially with no known adaptive significance--that under changed environment (and hence, changed natural selection pressures) "suddenly" conferred a selective advantage to those individuals exhibiting it.
Vestigial Characters: Highly Derived and Mostly Non-functional
A vestigial structure is one that has marginal, if any use to the
organism in which it occurs.
These are some of the most interesting examples of homology.
Why do structures become vestigial? Is it, as Lamarck suggested, a case of "use vs. disuse"?
Modern hypothesis: It's all about natural selection.
The study of embryo development is one of the hottest areas in evolutionary biology, the study of the evolution of development (a.k.a., evo-devo).
Ever heard the phrase, "Ontogeny Recapitulates Phylogeny"?
ontogeny is the process of embryo development (from the Greek
onto, meaning "being" or "existence" and genesis meaning
phylogeny is a species' evolutionary history (from the Greek
phyla, meaning "tribe" and genesis meaning "origin")
The phrase above was coined by German physician Ernst von Haeckel, who quit his medical practice to become an apologist for evolution after reading Darwin's On the Origin of Species by Means of Natural Selection.
Haeckel was the first to notice that the embryonic forms of different species had amazing physical similarities.
He suggested that every living thing passes through its evolutionary history during its embryo development.
We now know he didn't quite get that right.
But it is true that the similarities of embryo development of different species are remnants of the embryonic development features of those species' common ancestor.
Differences in ontogeny among taxa are not trivial.
Such changes reflect the
evolutionary relationships of those taxa, and the modified development that resulted in one species becoming two.
Developmental innovations may have provided the evolutionary "raw material" for the origin of entirely new lineages.
The claws of the baby hoatzin we saw previously also fall into this category.
Consider members of the Phylum Chordata.
At some point in their development, all members of Chordata show these traits, which are unique to Chordates:
In early chordates (Phylum Cephalochordata - the
lancelets), the notochord persists throughout the life of the organism:
- a tail posterior to the anus
- dorsal, hollow nerve cord
- muscles arranged in bundles (sarcomeres)
- cartilaginous notochord
In the somewhat more derived chordates (Phylum Urochordata), the
notochord is seen in the larval form, but lost in the adult:
And in the most derived chordates (Phylum Vertebrata), the notochord
is found only in early embryos, and is completely lost in the adult.
Only by studying the embryological development of vertebrates can one can
find this shared chordate character, where it is present in
all three taxa.
This is one of the most accurate ways to determine whether a
structure appearing similar in adult organisms is homologous or
If it is present in the embryo of all taxa being considered, it
is most likely a homology.
Ontogenetic changes in a developing organism, whether due to mutations or to other heritable changes in the DNA, ultimately are reflected in the adult organism.
Ontogenetic similarities offer one of the most profound ways to determine whether a structure is homologous in different species.
Is the lancelet a "baby form" tunicate? Or is the tunicate a more complicated lancelet?
In embryo development,Timing is Everything!
Bottom line: More complex adult forms are the result of changes in embryo development. These can be
Changes in the timing of ontogenetic events (between related species) is known as heterochrony.
- changes at various stages of development
- reduction of certain embryonic developmental traits
- addition of greater complexity towards the end of development
The deepest level of homology: The molecules that carry the instructions for making and running our bodies, encoded in our DNA.
The evolutionary history of a species can be seen in its DNA sequences. The more closely related two species are, the more similar their DNA sequences.
The very existence in of DNA every living thing on earth is, in itself, strong evidence of common ancestry. It would be highly unlikely for DNA to have evolved independently so many times, over and over.
We can see common ancestry in the chromosomes, themselves:
Molecular homologies also can be seen at the level of the finished protein
product encoded by the DNA, as shown in this comparison of amino acid sequence in
hemoglobin (the protein in your red blood cells that carries oxygen) of different species:
The study of molecular homology is the Evolutionary Biology of the Future.
And the future is now.