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, we can reasonably 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.
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 much alike. This usually happens because both organisms were in 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 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 analogous structures Figure 2. Similar traits can be either homologous or analogous. Homologous structures share a similar embryonic origin. Analogous organs have a similar function. These structures are not analogous. 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. A phylogenetic tree sorts organisms into clades or groups of organisms that descended from a single ancestor using maximum parsimony. 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.
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.
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. Organisms evolve from common ancestors and then diversify. This pattern repeats as one goes through the phylogenetic tree of life:. 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. Bats and bees do not share common ancestry, so the structures cannot be homologous. Both bat wings and bee wings serve a common purpose - helping bats and bees fly!
The structures look similar on the outside, too. However, their internal structures are very different - bat wings have a bony structure with muscles, while bee wings are membranous extensions. Other animals have body parts that look totally different, but have a shared background. Currently taking Biology and need help with other bio topics? Learn about photosynthesis , enzymes , cell theory , and what and how the cell membrane and endoplasmic reticulum work. Are you considering taking the SAT subject tests in a subject like biology?
Make sure you get the inside information on the test before you make your decision. Want to know the fastest and easiest ways to convert between Fahrenheit and Celsius? We've got you covered! Check out our guide to the best ways to convert Celsius to Fahrenheit or vice versa. Hayley Milliman is a former teacher turned writer who blogs about education, history, and technology. When she was a teacher, Hayley's students regularly scored in the 99th percentile thanks to her passion for making topics digestible and accessible.
Our new student and parent forum, at ExpertHub. See how other students and parents are navigating high school, college, and the college admissions process. Ask questions; get answers. How to Get a Perfect , by a Perfect Scorer. Score on SAT Math.
0コメント