Determining Evolutionary Relationships

Distinguishing between Similar Traits

Similar traits can be either homologous structures that share an embryonic origin or analogous structures that share a function.

Learning Objectives

Explain the difference between homologous and analogous structures

Key Takeaways

Key Points

  • Organisms may be very closely related, even though they look quite different, due to a minor genetic change that caused a major morphological difference.
  • Unrelated organisms may appear very similar because both organisms developed common adaptations that evolved within similar environmental conditions.
  • To determine the phylogeny of an organism, scientists must determine whether a similarity is homologous or analogous.
  • The advancement of DNA technology, the area of molecular systematics, describes the use of information on the molecular level, including DNA analysis.

Key Terms

  • analogous: when similar similar physical features occur in organisms because of environmental constraints and not due to a close evolutionary relationship
  • homologous: when similar physical features and genomes stem from developmental similarities that are based on evolution
  • phylogeny: the evolutionary history of an organism
  • molecular systematics: molecular phylogenetics is the analysis of hereditary molecular differences, mainly in DNA sequences, to gain information on an organism’s evolutionary relationships

Two Options for Similarities

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.

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Homologous structures: Bat and bird wings are homologous structures, indicating that bats and birds share a common evolutionary past.

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 probable that any overlap is due to a common evolutionary past. Imagine two people from different countries both inventing a 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 be distantly related, but appear very similar. This usually happens because both organisms developed common adaptations that evolved within similar environmental conditions. When similar characteristics occur because of environmental constraints and not due to a close evolutionary relationship, it is called an analogy or homoplasy. For example, insects use wings to fly like bats and birds, but the wing structure and embryonic origin is completely different. These are called analogous structures.

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Analogous structures: The (c) wing of a honeybee is similar in shape to a (b) bird wing and (a) bat wing, and it serves the same function. However, the honeybee wing is not composed of bones and has a distinctly-different structure and embryonic origin. These wing types (insect versus bat and bird) illustrate an analogy: similar structures that do not share an evolutionary history.

Similar traits can be either homologous or analogous. Homologous structures share a similar embryonic origin; analogous organs have a similar function. For example, the bones in the front flipper of a whale are homologous to the bones in the human arm. These structures are not analogous. The wings of a butterfly and the wings of a bird are analogous, but not homologous. Some structures are both analogous and homologous: the wings of a bird and the wings of a bat are both homologous and analogous. Scientists must determine which type of similarity a feature exhibits to decipher the phylogeny of the organisms being studied.

Molecular Comparisons

With the advancement of DNA technology, the area of molecular systematics, which describes the use of information on the molecular level including DNA analysis, has blossomed. New computer programs not only confirm many earlier classified organisms, but also uncover previously-made errors. 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. 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. Ultimately, the coupled use of both morphologic and molecular information is more effective in determining phylogeny.

Building Phylogenetic Trees

A phylogenetic tree sorts organisms into clades or groups of organisms that descended from a single ancestor using maximum parsimony.

Learning Objectives

Describe the cladistics as a method used to create phylogenetic trees

Key Takeaways

Key Points

  • Phylogenetic trees sort organisms into clades: groups of organisms that descended from a single ancestor.
  • Organisms of a single clade are called a monophyletic group.
  • Scientists use the phrase “descent with modification” because genetic changes occur even though related organisms have many of the same characteristics and genetic codes.
  • A characteristic is considered a shared-ancestral character if it is found in the ancestor of a group and all of the organisms in the taxon or clade have that trait.
  • If only some of the organisms have a certain trait, it is called a shared- derived character because this trait derived at some point, but does not include all of the ancestors in the clade.
  • Scientists often use a concept called maximum parsimony, which means that events occurred in the simplest, most obvious way, to aid in the tremendous task of describing phylogenies accurately.

Key Terms

  • monophyletic: of, pertaining to, or affecting a single phylum (or other taxon) of organisms
  • derived: of, or pertaining to, conditions unique to the descendant species of a clade, and not found in earlier ancestral species
  • clades: groups of organisms that descended from a single ancestor
  • ancestral: of, pertaining to, derived from, or possessed by, an ancestor or ancestors; as, an ancestral estate
  • maximum parsimony: the preferred phylogenetic tree is the tree that requires the least evolutionary change to explain some observed data

Building 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 orange region 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. Clades must include all of the descendants from a branch point.

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Common ancestors: Lizards, rabbits, and humans all descend from a common ancestor that had an amniotic egg. Thus, lizards, rabbits, and humans all belong to the clade Amniota. Vertebrata is a larger clade that also includes fish and lamprey.

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. This can be remembered because monophyletic breaks down into “mono,” meaning one, and “phyletic,” meaning evolutionary relationship. Notice in the various examples of clades how each clade comes from a single point, whereas the non-clade groups show branches that do not share a single point.

Shared Characteristics

Organisms evolve from common ancestors and then diversify. Scientists use the phrase “descent with modification” because even though related organisms have many of the same characteristics and genetic codes, changes occur. This pattern repeats as one goes through the phylogenetic tree of life:

  1. A change in the genetic makeup of an organism leads to a new trait which becomes prevalent in the group.
  2. Many organisms descend from this point and have this trait.
  3. New variations continue to arise: 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).
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Examples of clades: All the organisms within a clade stem from a single point on the tree. A clade may contain multiple groups, as in the case of animals, fungi, and plants, or a single group, as in the case of flagellates. Groups that diverge at a different branch point, or that do not include all groups in a single branch point, are not considered clades.

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 have that trait. Now, consider the amniotic egg characteristic in the same figure. Only some of the organisms have this trait; to those that do, it is called 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 to 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. These terms help scientists distinguish between clades in the building of phylogenetic trees.

Choosing the Right Relationships

Imagine being the person responsible for organizing all of the items in a department store properly; an overwhelming task. Organizing the evolutionary relationships of all life on earth proves much more difficult: scientists must span enormous blocks of time and work with information from long-extinct organisms. Trying to decipher the proper connections, especially given the presence of homologies and analogies, makes the task of building an accurate tree of life extraordinarily difficult. Add to that the advancement of DNA technology, which now provides large quantities of genetic sequences to be used and analyzed. Taxonomy is a subjective discipline: many organisms have more than one connection to each other, so each taxonomist will decide the order of connections.

To aid in the tremendous task of describing phylogenies accurately, scientists often use a concept called maximum parsimony, which means that events occurred in the simplest, most obvious way. 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.