Learning Goals
By the end of this reading you should be able to:
- Distinguish between analogous and homologous traits
- Discuss the purpose of cladistics
- Explain ancestral and derived characters
- Describe maximum parsimony
Introduction
In general, organisms that share similar physical features and genomes tend to be more closely related than those that do not. Such features that overlap both morphologically (in form) and genetically are referred to as homologous structures; they stem from developmental similarities that are based on evolution. For example, the bones in the wings of bats and birds have homologous structures (Fig. 1).
Notice it is not simply a single bone, but rather a grouping of several bones arranged in a similar way. The more complex the feature, the more likely any kind of overlap is due to a common evolutionary past. Imagine two people from different countries both inventing the car with all the same parts and in exactly the same arrangement without any previous or shared knowledge. That outcome would be highly improbable. However, if two people both invented a hammer, it would be reasonable to conclude that both could have the original idea without the help of the other. The same relationship between complexity and shared evolutionary history is true for homologous structures in organisms.
Misleading Appearances
Some organisms may be very closely related, even though a minor genetic change caused a major morphological difference to make them look quite different. Similarly, unrelated organisms may appear very much alike. This can happen when both organisms were in common environmental conditions which lead to the development of similar adaptations. When characteristics bearing similar functions occur because of environmental constraints, it is called an analogy. For example, insects use wings to fly like bats and birds, but the wing structure and embryonic origin is completely different (Fig. 2). Moreover, if the characteristics simply resemble each other, those structures are called a homoplasy.
Some structures are both analogous and homologous such as the wings of a bird and the wings of a bat (Fig. 2). Bats and birds share a common evolutionary pathway in their bone structure of the wings. However, the actual mechanics of the wing are different. Bat wings are composed of tissues stretched between the bones. Bird wings contain feathers that create the flight surfaces. Scientists must determine which type of similarity a feature exhibits to decipher the phylogeny of the organisms being studied. It should be noted that structures between two individuals may be any combination of homologous, analogous, and homoplastic, to include all three.
Molecular Comparisons
DNA evolves by mutations being incorporated in the DNA and fixed in populations. This can lead to a divergence of DNA sequences in different species. With the advancement of DNA technology, the area of molecular systematics which describes the use of molecular information including DNA analysis has blossomed. New computer programs can be used to confirm the relationships of earlier classified organisms and to uncover new relationships between organisms. As with physical characteristics, even the DNA sequence can be tricky to read in some cases. For some situations, two very closely related organisms can appear unrelated if a mutation occurred that caused a shift in the genetic code (a frameshift). An insertion or deletion mutation would move each nucleotide base over one place, causing two similar codes to appear unrelated.
Sometimes two segments of DNA code in distantly related organisms randomly share a high percentage of bases in the same locations, causing these organisms to appear closely related when they are not. For both of these situations, computer technologies have been developed to help identify the actual relationships, and, ultimately, the coupled use of both morphologic and molecular information is more effective in determining phylogeny.
Building Phylogenetic Trees
How do scientists construct phylogenetic trees? After the homologous and analogous traits are sorted, scientists often organize the homologous traits using a system called cladistics. This system sorts organisms into clades: groups of organisms that descended from a single ancestor.
For example, all of the organisms in the yellow region in Figure 3 evolved from a single ancestor that had amniotic eggs. Consequently, all of these organisms also have amniotic eggs and make a single clade, also called a monophyletic group.
Review Question:
Clades can vary in size depending on which branch point is being referenced. The important factor is that all of the organisms in the clade or monophyletic group stem from a single point on the tree. Monophyletic breaks down into “mono,” meaning one, and “phyletic,” meaning evolutionary relationship. Thus each clade comes from a single point, whereas the non-clade groups show branches that do not share a single point.
Review Question:
Shared Characteristics
1. A change in the genetic makeup of an organism leads to a new trait that enhances fitness and as a result becomes more prevalent in the group.
2. Many organisms descend from this point and have this trait.
3. New variations may arise after that branch point: some are adaptive and persist, leading to new traits.
4. With new traits, a new branch point is determined (go back to step 1 and repeat).
If a characteristic is found in the ancestor of a group, it is considered a shared ancestral character because all of the organisms in the taxon or clade will likely have that trait.
In the phylogeny shown in Figure 3, the possession of vertebrae is a shared ancestral character. Now consider the amniotic egg characteristic in the same phylogeny. Only some of the organisms have this trait, and in those that do, it is a shared derived character because this trait derived at some point but does not include all of the ancestors in the tree.
The tricky aspect of shared ancestral and shared derived characters is the fact that these terms are relative. The same trait can be considered one or the other depending on the particular diagram being used. Returning to the phylogeny shown, note that the amniotic egg is a shared ancestral character for the Amniota clade while having hair is a shared derived character for some organisms in this group (lions and primates but not turtles). Shared ancestral and shared derived characteristics help scientists distinguish between clades in the building of phylogenetic trees.
Review Question:
To aid in the tremendous task of describing phylogenies accurately, scientists often use a concept called maximum parsimony. This concept predicts that the pathway with the least number of events that could have occurred is the most likely one. For example, if a group of people entered a forest preserve to go hiking, based on the principle of maximum parsimony, one could predict that most of the people would hike on established trails rather than forge new ones. For scientists deciphering evolutionary pathways, the same idea is used: the pathway of evolution probably includes the fewest major events that coincide with the evidence at hand. Starting with all of the homologous traits in a group of organisms, scientists look for the most obvious and simple order of evolutionary events that led to the occurrence of those traits.
Review Question:
Summary
Using phylogenetic tools and concepts scientists can begin to tackle the task of revealing the evolutionary history of life on Earth. Similar traits can be either homologous or analogous. Homologous structures share a similar embryonic origin; analogous organs have a similar function. Within phylogenetic trees, organisms can be clustered into clades that show common ancestry. Recently, newer technologies have uncovered surprising discoveries with unexpected relationships, such as the fact that people seem to be more closely related to fungi than fungi are to plants. Sound unbelievable? As the information about DNA sequences grows, scientists will become closer to mapping the evolutionary history of all life on Earth.
End of Section Review Questions:
1) If a characteristic is found in an ancestral group it is considered a shared _______ character, while if only some of the organisms within a tree share a characteristic it is considered a shared _______ character.
2)
The green boxes in image _______ and image _______ show groupings that are NOT clades.
Evolution Connection: Why Does Phylogeny Matter?
Evolutionary biologists could list many reasons why understanding phylogeny is important to everyday life in human society. For botanists, phylogeny acts as a guide to discovering new plants that can be used to benefit people. Think of all the ways humans use plants—food, medicine, and clothing are a few examples. If a plant contains a compound that is effective in treating cancer, scientists might want to examine all of the relatives of that plant for other useful drugs.
A research team in China identified a segment of DNA thought to be common to some medicinal plants in the family Fabaceae (the legume family) and worked to identify which species had this segment. After testing plant species in this family, the team found a DNA marker (a known location on a chromosome that enabled them to identify the species) present. Then, using the DNA to uncover phylogenetic relationships, the team could identify whether a newly discovered plant was in this family and assess its potential medicinal properties.
Attributions
Figure 1. Image courtesy of CNX OpenStax [CC BY 4.0 (https://creativecommons.org/licenses/by/4.0)], via Wikimedia Commons
Figure 2. Image courtesy of CNX OpenStax [CC BY 4.0 (https://creativecommons.org/licenses/by/4.0)], via Wikimedia Commons
Figure 3. Image created and provided by D. Jennings
Figure 4. Image in Public Domain.
Text adapted from OpenStax, Biology. OpenStax CNX. Nov 7, 2018 http://cnx.org/contents/185cbf87-c72e-48f5-b51e-f14f21b5eabd@11.6.
homologous
analogous OR homoplastic
A, E, F
A,C
Secondarily lost traits
It is possible for a shared ancestral character to be lost in certain lineages, but not others. We say that the trait was secondarily lost. For instance, snakes descend from 4-legged ancestors (tetrapods)... in fact we still call them tetrapods even though they no longer have legs. Their legs were secondarily lost at some point in their ancestry.
The amniotic egg evolved before hair did. Turtles, which do not have hair, share the amniotic egg characteristic with lions and primates which do have hair.
A
ancestral
derived
B, D (either order)
