Morphological Homology

Definition of Morphological Homology

Some species have similar traits and features as they have common ancestors, or have been subjected to similar selection pressures.

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.

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.

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:

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.

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

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.

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.

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.