How are whales and other mammals related? What similarities do they share? What traits set them apart? Morphological homology - the similarities in structure and form of organisms due to common ancestry - helps us understand the evolutionary relationships between different organisms.
- We will define morphological homology.
- We will distinguish it from two other types of homology: molecular and developmental homology.
- We will then discuss the differences between homologous and analogous features, and why it is important to distinguish between the two.
Definition of Morphological Homology
Some species have similar traits and features as they have common ancestors, or have been subjected to similar selection pressures.
Homology refers to similarities in traits and features due to common ancestry. On the other hand, homologous traits are similar traits found in different organisms that can be traced back to a common ancestor.
By studying the homology and homologous traits of organisms, we can infer how they are related. In general, the more similarities shared by organisms, the more closely related they likely are.
Definition of Morphological Homology and Morphological Homologous Structures
Let's dig down and see what these terms actually mean.
Morphology: the study of the structure and form of organisms. Morphological homology: when different species have similar structures with the same basic form due to common ancestry.
Morphological homologous structures: these are structures that appear similar and can be traced back to common ancestors, but serve different functions due to evolution.
Examples of Morphological Homology and Morphological Homologous Structures
Vertebrates like pigs, birds, and whales have forelimbs with the same basic components which can be traced back to a common ancestor. Due to evolution, the forelimbs of these vertebrates have changed over time to serve different purposes that suit their present environment. The forelimbs of vertebrates are examples of morphological homologous structures.
Another example of morphological homology can be observed in the classification of mammals. Mammals can be classified as monotremes, marsupials, and placentals.
Monotremes, like platypuses, are mammals that lay eggs.
Placentals, like rodents, dogs, and whales, are mammals with a placenta, which is a temporary organ that connects the embryo to the mother's uterus.
Marsupials, like kangaroos, wombats, and koalas, use external pouches to raise their newborn offspring.
Organisms under each group - monotremes, placentals, and marsupials - are classified as such because they have morphological homologous structures and can be traced back to a common evolutionary ancestor.
Have you ever wondered why there are flightless birds? Wings on flightless birds are an example of vestigial structures. Vestigial structures are anatomical parts of a species that have lost their original function in the course of evolution.
Vestigial structures like wings on flightless birds are often homologous and used as evidence of evolution, as they show how species with common ancestry can change over time. Other examples of vestigial structures are the pelvic bone of snakes, the appendix of humans, and the eyes of blind cave-dwelling animals.
Other Types of Homology
Besides structure and form, similarities also occur in the genetic code and in the developmental stages of organisms. In this section, we will briefly discuss two other types of homology: molecular homology, and developmental homology.
Definition of Molecular Homology
Molecular homology is when different species have similar nucleotide sequences in DNA or RNA that were inherited from a common ancestor. Both DNA and RNA are organic molecules that contain genetic information.
They contain nucleotide bases that pair up as follows:
Nucleotide base pairings in DNA | Nucleotide base pairings in RNA |
Adenine (A) pairs with thymine (T) | Adenine (A) pairs with uracil (U) |
Cytosine (C) pairs with guanine (G) | Cytosine (C) pairs with guanine (G) |
The sequence of nucleotide bases determines the genetic information contained in the DNA. For example, a DNA sequence CGATGG might transmit genetic information for black hair, while CGATCG might transmit genetic information for brown hair.
To determine molecular homology, researchers align the DNA or RNA sequences of the organisms being studied.
DNA or RNA sequencing is the process of determining the order of nucleotide bases. Scientists use DNA or RNA sequencing to determine the evolutionary relationship of organisms by aligning comparable sequences.
If the sequences are different at only one or a few sites, the species are likely to be very closely related.
If there are different bases of various lengths at many sites, the species are likely to be distantly related.
It is important to note that closely related organisms can look very different but have similar genes. Likewise, organisms that are not closely related can look very similar.
Hawaiian silverswords are different species of plants (ranging from low bushes to tall shrubs) that are genetically similar to each other despite looking very different. The Dubautia linearis and Argyroxiphium sandwicense below are examples.
Evidence shows that all the silverswords of Hawaii descended from a common ancestor: the tarweed (Carlquistia muirii) from North America. When the tarweed colonized the islands of uninhabited and geologically diverse islands of Hawaii, it spread into different ecological niches and formed adaptations specific to these niches.
Because they are adapted to their unique ecological niches, the descendant species became very different from each other. Over time, many species of Hawaiian silverswords were formed.
Figures 2-3. Dubautia linearis (left) and Argyroxiphium sandwicense (right) are two species of Hawaiian silversword plants that look morphologically different but are genetically similar.
Definition of Developmental Homology
Developmental homology is when different species have similar structures in particular stages of development. These structures tend to appear only during the embryonic stage and disappear when they reach their juvenile or adult stage. For this reason, it is also known as embryonic homology.
For example, all vertebrate embryos (even humans!) have gill slits and tails. For terrestrial animals, these disappear by the time of birth, while for fish and other aquatic groups, they maintain these structures even in adulthood.
The image below shows an image of a 5-week-old human embryo where we can clearly see the tail that will disappear by the time it reaches 8 weeks.
Figure 4. We can clearly see a tail in this photo of a 5-week-old human embryo.
What is the Difference between Morphological, Molecular, and Developmental Homology?
The main difference between morphological, molecular, and developmental homology is where the similarities occur.
In morphological homology, similarities appear in the structure and form of the species.
In molecular homology, similarities appear in the genes or DNA sequence of the species.
In developmental homology, similarities appear in particular developmental stages of the species.
“Analogous” vs. “Homologous” in Biology
In Biology, homologous means having similar traits due to common ancestry. Homologous traits can help us determine the evolutionary relationship among different organisms.
On the other hand, analogous means having similar structures, not because of common ancestry, but rather convergent evolution. Convergent evolution is when species that are not closely related have evolved to have similar features due to common selection pressures.
For example, insects and birds share features that serve the same function: both have wings that give them the ability to fly.
But the anatomical structures and embryonic origins of their wings are completely different from each other. This shows us that insects and birds are not closely related. Similar structures that are not due to common ancestry are called analogous structures.
Figures 5-6. Close up of wing of an insect (left) and a bird (right).
Selection pressures: these are external factors - including environmental conditions - that affect an organism’s chances of surviving in its environment.
Why Is it Important to Distinguish Between Analogous and Homologous Traits?
The evolutionary history and relationship of organisms are usually presented in branching diagrams called phylogenetic trees. When reconstructing phylogenetic trees, scientists focus on homologous traits, because only these show the evolutionary relationships among organisms. This is because analogous traits are caused by selection pressures, not by common ancestry.
The image below is a phylogenetic tree of Rodentia. By studying the molecular homology of these organisms through the genome alignment of protein-coding and non-coding sequences, researchers were able to determine how these organisms are related.
Apart from molecular homology, rodents also share specific characteristics such as having an upper pair and a lower pair of rootless incisor teeth that grow continually. Such characteristics help researchers distinguish them from organisms that appear similar or have similar traits (for example, hedgehogs and shrews) but are not actually closely related.
Figure 7. A
phylogenetic tree of Rodentia based on homologous genes. Source: M ralser, CC BY-SA 4.0, via Wikimedia Commons.
Morphological Homology - Key takeaways
- Homology refers to similarities in traits and features due to common ancestry. There are three major types of homology:
In morphological homology, similarities appear in the structure and form of the species.
In molecular homology, similarities appear in the genes or DNA sequence of the species.
In developmental homology, similarities appear in particular developmental stages of the species.
By studying the homology of organisms, we can infer how they are related. In general, the more similarities shared by organisms, the more closely related they likely are.
- Analogous means having similar structures due to similar selection pressures, but not due to common ancestry.
- Homologous traits and features can help us determine the evolutionary relationship among different organisms.
References
- Figure 2: Dubautia linearis (https://upload.wikimedia.org/wikipedia/commons/3/3b/Dubautia_linearis_Kalopa.jpg) by Karl Magnacca. Licensed by CC BY-SA 2.5 (https://creativecommons.org/licenses/by-sa/2.5/deed.en).
- Figure 3: Argyroxiphium sandwicense (https://commons.wikimedia.org/wiki/File:Argyroxiphium_sandwicense_Haleakala.jpg) by Karl Magnacca. Licensed by CC BY-SA 2.5 (https://creativecommons.org/licenses/by-sa/2.5/deed.en).
- Figure 4: Human embryo with visible tail (https://commons.wikimedia.org/wiki/File:Tubal_Pregnancy_with_embryo.jpg) by Ed Uthman, MD (https://www.flickr.com/photos/euthman/). Public domain.
- Figure 5: Insect Wing (https://commons.wikimedia.org/wiki/File:Variety_of_different_insect_wings,_details_of_wings_of_a_wasp,_a_series_of_photos,_3rd_of_3.jpg) by Retro Lenses. Licensed by CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/deed.en).
- Figure 6: Bird wing (https://commons.wikimedia.org/wiki/File:Sarcoramphus_papa_-Berlin_Zoo_-wing-8b.jpg) by s shepherd (https://www.flickr.com/people/84175980@N00). Licensed by CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/deed.en).
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