2
Learning Goals
-
Describe how comparative anatomy and embryology support the theory of evolution
-
Explain how information can be gathered from the fossil record
-
Discuss the contributions of biogeography and vestigial structures to evolutionary theory
-
Define and give examples of homologous traits
Introduction
Comparative Anatomy and Embryology
In addition to the homology that is seen in the limbs of some animals, there is also evidence of homology in the embryological development across a widely diverse group of vertebrate animals. In vertebrate embryos, including humans, there is a stage in which gill slits and tails are present. These structures are absent in the adults of terrestrial groups but adult forms of aquatic groups such as fish and some amphibians maintain them. This commonality of structures at some stage of embryo development suggests a shared common ancestry among these groups.
It is possible to make a connection between the homologous features that occur later in the development of organisms with homologous groups of cells that are present in their embryos. Thus, the similarity that is seen later in development is one that happens as a result of shared developmental pathways. Why do some organisms share the same pathways then? Because they have a shared common ancestor. We now know that with only a few minor exceptions, all organisms use the same codons (triplets of mRNA nucleotides) to specify the same amino acid carrying tRNAs and that all organisms use the same nucleotides (DNA) to store the genetic information within each cell. Thus, not only are the similar structures found within groups of organisms the result of genetic programs within the organism, they are often the result of similar genes.
Evidence of change over time
Fossils: Fossils provide evidence that organisms from the past are not the same as those today, the vast majority of fossils that have been discovered are unlike the species that are present today. It was through the study of available fossils that George Cuvier first proposed that some organisms that once existed had become extinct. While this may not seem like earth-shattering news to you, at the time he proposed this theory it was. Through the use of radiometric dating, scientists can now determine the relative age of fossils. This allows them to determine where organisms lived in the past relative to other organisms of the same age as well as the place changes in groups of similar fossils in chronological sequence. These records can show the evolution of form over millions of years. In addition, some fossils show transitional forms that show characteristics with an ancestral organism as well as a current species. Lastly, fossils demonstrate changes in the environment over time, for example, the presence of fossils of aquatic organisms in areas where there is currently no water.
Geology: Some of the information that we currently have about the relative age of fossils comes from work done in geology. Geologists can determine the relative age of strata (layers of rocks) based on their relative position. The lower strata were typically formed first and thus based on the principle of superposition determined to be older.
Biogeography: Biogeography is the study of the geographic distribution of organisms both living and fossil on the planet. It reveals patterns of distribution that can be explained by evolution in conjunction with tectonic plate movement over geological time. Some groups of organisms are broadly distributed across the current continents, and thus are likely to have evolved before the supercontinent Pangaea broke up (about 200 million years ago).
Review Question:
Summary
Comparative anatomy and embryology revealed to scientists that there are shared structural features in some groups of organisms. We now know that these features are often the result of shared development pathways and ultimately similar sequences of nucleotides in genes. Thus, homologous structures are indicators of a shared common ancestry. Vestigial structures can also be an indicator of shared ancestry. Changes in organisms over time is sometimes captured within the fossil record, and the dating of the rock strata can give a timeline of the appearance, disappearance, and sometimes the transitional states of groups of organisms. Finally, the geographical distribution of organisms and fossils can provide an indication of shared ancestry as well as the changes that have occurred environmentally over time.
End of Section Review Questions:
1) The wing of the bat and the fore-limb of the dog are said to be homologous structures. What does this mean?
Review: Shared Ancestry
2) Which of these provides support for shared ancestry?
Review: Explaining the evidence
3) How do vestigial structures support the theory of evolution?
References
Image Attribution
Learning Goals
-
Discuss the need for a comprehensive classification system
-
List the different levels of the taxonomic classification system
-
Describe how systematics and taxonomy relate to phylogeny
-
Discuss the components and purpose of a phylogenetic tree
Introduction
Phylogenetic Trees
A phylogenetic tree is a diagram used to reflect evolutionary relationships among organisms or groups of organisms. Scientists consider phylogenetic trees to be a hypothesis of the evolutionary past since one cannot go back to confirm the proposed relationships. In other words, a “tree of life” can be constructed to illustrate when different organisms evolved and to show the relationships among different organisms. Unlike a taxonomic classification diagram, a phylogenetic tree can be read like a map of evolutionary history. Many phylogenetic trees have a single lineage at the base representing a common ancestor.
The point where a split occurs called a branch point represents where a single lineage evolved into a distinctly new one (Fig. 3). A lineage that evolved early from the root and remains unbranched is called a basal taxon. When two lineages stem from the same branch point, they are called sister taxa. A branch with more than two lineages is called a polytomy and serves to illustrate where scientists have not definitively determined all of the relationships. It is important to note that although sister taxa and polytomy do share an ancestor, it does not mean that the groups of organisms split or evolved from each other. Organisms in two taxa may have split apart at a specific branch point, but neither taxa gave rise to the other.
Diagrams can serve to show a pathway and aid our understanding of evolutionary history. The pathway can be traced from the origin of life to any individual species by navigating through the evolutionary branches between the two points. Also, by starting with a single species and tracing back towards the "trunk" of the tree, one can discover that species' ancestors, as well as where lineages share a common ancestry. In addition, the tree can be used to study entire groups of organisms.
Many disciplines within the study of biology contribute to understanding how past and present life evolved over time; these disciplines together contribute to building, updating, and maintaining the “tree of life.” Information is used to organize and classify organisms based on evolutionary relationships in a scientific field called systematics. Data may be collected from fossils, from studying the structure of body parts or molecules used by an organism, and by DNA analysis. By combining data from many sources, scientists can put together the phylogeny of an organism; since phylogenetic trees are hypotheses, they will continue to change as new types of life are discovered and new information is learned.
Review Question:
Match the term with the appropriate definition.
| 1) polytomy | A) groups of organisms that are more closely related to each other than to any other groups |
| 2) rooted | B) a diagram in which all of the included organisms are thought to have arisen from a common ancestor |
| 3) basal taxon | C) branch on a phylogenetic tree that has not diverged significantly from the root ancestor |
| 4) branch point | D) a point in the phylogenetic tree that indicates the last common ancestor of different groups |
| 5) sister taxa | E) multiple lineages that arise from a common branch point |
Limitations of Phylogenetic Trees
Another aspect of phylogenetic trees is that, unless otherwise indicated, the branches do not account for the length of time, only the evolutionary order. In other words, the length of a branch does not typically mean more time passed, nor does a short branch mean less time passed— unless specified on the diagram. Figure 5 does not indicate how much time passed between the evolution of amniotic eggs and hair. What the tree does show is the order in which things took place. This particular tree shows that the oldest trait is the vertebral column, followed by hinged jaws, and so forth. Remember that any phylogenetic tree is a part of the greater whole, and like a real tree, it does not grow in only one direction after a new branch develops. So, for the organisms in this tree, just because a vertebral column evolved does not mean that invertebrate evolution ceased, it only means that a new branch formed. Also, groups that are not closely related, but evolve under similar conditions, may appear more phenotypically similar to each other than to a close relative.
Review Question:
What are somethings that you cannot determine from looking at a phylogenetic tree
A) the actual evolutionary ages of the species in the tree
The Levels of Classification
The taxonomic classification system (also called the Linnaean system after its inventor, Carl Linnaeus, a Swedish botanist, zoologist, and physician) uses a hierarchical model. Moving from the point of origin, the groups become more specific, until one branch ends as a single species. For example, after the common beginning of all life, scientists divide organisms into three large categories called a domain: Bacteria, Archaea, and Eukarya. Within each domain is a second category called a kingdom. After kingdoms, the subsequent categories of increasing specificity are: phylum, class, order, family, genus, and species.
Review Question:
Link to Learning: Visit this Nova website (Classifying Life) to try your hand at classifying three organisms—bear, orchid, and sea cucumber—from kingdom to species. To launch the game, under Classifying Life, click the picture of the bear or the Launch Interactive button.
Summary
End of Section Review Questions:
Attribution:
Text: Modified from OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX. License: CC BY: Attribution
Figure 1. credit: modification of work by John Beetham
Figure 7. (credit “plant”: modification of work by "berduchwal"/Flickr; credit “insect”: modification of work by Jon Sullivan; credit “fish”: modification of work by Christian Mehlführer; credit “rabbit”: modification of work by Aidan Wojtas; credit “cat”: modification of work by Jonathan Lidbeck; credit “fox”: modification of work by Kevin Bacher, NPS; credit “jackal”: modification of work by Thomas A. Hermann, NBII, USGS; credit “wolf”: modification of work by Robert Dewar; credit “dog”:
Learning Goals
-
Discuss the need for a comprehensive classification system
-
List the different levels of the taxonomic classification system
-
Describe how systematics and taxonomy relate to phylogeny
-
Discuss the components and purpose of a phylogenetic tree
Introduction
Phylogenetic Trees
A phylogenetic tree is a diagram used to reflect evolutionary relationships among organisms or groups of organisms. Scientists consider phylogenetic trees to be a hypothesis of the evolutionary past since one cannot go back to confirm the proposed relationships. In other words, a “tree of life” can be constructed to illustrate when different organisms evolved and to show the relationships among different organisms. Unlike a taxonomic classification diagram, a phylogenetic tree can be read like a map of evolutionary history. Many phylogenetic trees have a single lineage at the base representing a common ancestor.
The point where a split occurs called a branch point represents where a single lineage evolved into a distinctly new one (Fig. 3). A lineage that evolved early from the root and remains unbranched is called a basal taxon. When two lineages stem from the same branch point, they are called sister taxa. A branch with more than two lineages is called a polytomy and serves to illustrate where scientists have not definitively determined all of the relationships. It is important to note that although sister taxa and polytomy do share an ancestor, it does not mean that the groups of organisms split or evolved from each other. Organisms in two taxa may have split apart at a specific branch point, but neither taxa gave rise to the other.
Diagrams can serve to show a pathway and aid our understanding of evolutionary history. The pathway can be traced from the origin of life to any individual species by navigating through the evolutionary branches between the two points. Also, by starting with a single species and tracing back towards the "trunk" of the tree, one can discover that species' ancestors, as well as where lineages share a common ancestry. In addition, the tree can be used to study entire groups of organisms.
Many disciplines within the study of biology contribute to understanding how past and present life evolved over time; these disciplines together contribute to building, updating, and maintaining the “tree of life.” Information is used to organize and classify organisms based on evolutionary relationships in a scientific field called systematics. Data may be collected from fossils, from studying the structure of body parts or molecules used by an organism, and by DNA analysis. By combining data from many sources, scientists can put together the phylogeny of an organism; since phylogenetic trees are hypotheses, they will continue to change as new types of life are discovered and new information is learned.
Review Question:
Match the term with the appropriate definition.
| 1) polytomy | A) groups of organisms that are more closely related to each other than to any other groups |
| 2) rooted | B) a diagram in which all of the included organisms are thought to have arisen from a common ancestor |
| 3) basal taxon | C) branch on a phylogenetic tree that has not diverged significantly from the root ancestor |
| 4) branch point | D) a point in the phylogenetic tree that indicates the last common ancestor of different groups |
| 5) sister taxa | E) multiple lineages that arise from a common branch point |
Limitations of Phylogenetic Trees
Another aspect of phylogenetic trees is that, unless otherwise indicated, the branches do not account for the length of time, only the evolutionary order. In other words, the length of a branch does not typically mean more time passed, nor does a short branch mean less time passed— unless specified on the diagram. Figure 5 does not indicate how much time passed between the evolution of amniotic eggs and hair. What the tree does show is the order in which things took place. This particular tree shows that the oldest trait is the vertebral column, followed by hinged jaws, and so forth. Remember that any phylogenetic tree is a part of the greater whole, and like a real tree, it does not grow in only one direction after a new branch develops. So, for the organisms in this tree, just because a vertebral column evolved does not mean that invertebrate evolution ceased, it only means that a new branch formed. Also, groups that are not closely related, but evolve under similar conditions, may appear more phenotypically similar to each other than to a close relative.
Review Question:
What are somethings that you cannot determine from looking at a phylogenetic tree
A) the actual evolutionary ages of the species in the tree
The Levels of Classification
The taxonomic classification system (also called the Linnaean system after its inventor, Carl Linnaeus, a Swedish botanist, zoologist, and physician) uses a hierarchical model. Moving from the point of origin, the groups become more specific, until one branch ends as a single species. For example, after the common beginning of all life, scientists divide organisms into three large categories called a domain: Bacteria, Archaea, and Eukarya. Within each domain is a second category called a kingdom. After kingdoms, the subsequent categories of increasing specificity are: phylum, class, order, family, genus, and species.
Review Question:
Link to Learning: Visit this Nova website (Classifying Life) to try your hand at classifying three organisms—bear, orchid, and sea cucumber—from kingdom to species. To launch the game, under Classifying Life, click the picture of the bear or the Launch Interactive button.
Summary
End of Section Review Questions:
Attribution:
Text: Modified from OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX. License: CC BY: Attribution
Figure 1. credit: modification of work by John Beetham
Figure 7. (credit “plant”: modification of work by "berduchwal"/Flickr; credit “insect”: modification of work by Jon Sullivan; credit “fish”: modification of work by Christian Mehlführer; credit “rabbit”: modification of work by Aidan Wojtas; credit “cat”: modification of work by Jonathan Lidbeck; credit “fox”: modification of work by Kevin Bacher, NPS; credit “jackal”: modification of work by Thomas A. Hermann, NBII, USGS; credit “wolf”: modification of work by Robert Dewar; credit “dog”:
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.