Why Study the Anatomy of Other Organisms?
Homologous and analogous structures are often confusing topics to grasp in biology class. However, understanding these key concepts is of great importance in understanding the similarities and differences between various organisms. This knowledge allows scientists to better utilize other animals to study human biological processes and drug treatments without extensive and potentially dangerous experimentation on humans. There are many, very distinct differences between animals such as cats and fish: for example, fish have scales while cats have fur. Structural characteristics of organisms make up their morphology, which includes both external features such as shape and color as well as internal features such as bones and muscles. The study of those internal features of animals or plants is known as anatomy.
Image Source : Wikimedia Commons
Homology vs. Analogy
In comparing and contrasting certain traits in organisms, biologists often look at similarities in structure, function, and evolutionary ancestry. Features of animals that have similar structure are classified as homologous if they have a common evolutionary origin, even if they have different functions in different animals. A classic example of homologous structures is that of the wing of a bat and the arm of a human. Both have similar internal bone structure, and as mammals, the evolutionary origin is clearly similar. However, the wing of a bat is utilized for flying, whereas the arm of a person is used to carry items or to perform other tasks and is not suited for flight. Similarly, the leg of a dog, wing of a bird, and fin of a whale are also homologous to the human arm. Such structures are said to have diverged over time, indicating that at one time, they may have had the same function in the common ancestral organism.
Image Source : Wikimedia Commons
Alternatively, many structures that are clearly similar have entirely different evolutionary ancestors. These have converged to have the same or comparable function despite differing origins and are known as analogous or homoplastic structures. Consider bird wings and insect wings; both features of these organisms allow for flight, but the development of wings on the bird and the insect evolved differently and subsequently converged to allow both to function similarly. By comparing gene sequences in these organisms, it can be shown that birds and insects have most certainly evolved from different parent species, yet gained a similar ability to fly. Observation of these two varying sets of wings would also show that, although they have similar functions, they are structurally different and thus physiologically divergent. In the same way, bird wings (3) are homologous to bat (2) and dinosaur wings (1), as shown in the diagram above.
Epigenetic Changes Promote Evolution of Structures
To understand how these features have developed in animals, and then later converged or diverged to alter structure and function, one must consider how the organism develops. In biology, all features and functions of organisms are determined by gene expression. Tissues are made up of cells, which are further divided into compartments called organelles. One of these organelles is the nucleus, which contains DNA. That DNA must be transcribed into RNA, which is subsequently translated into protein in order to allow for the cellular signaling necessary for growth, development, and maintenance of an organism. Interestingly, although each individual has a unique DNA sequence, that sequence is utilized differently by different cells of the body. It is this regulation that makes a skin cell act in another way than an eye cell, which is an important feature of gene expression that allows for variation in cells and tissues.
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How could this differential gene expression promote the emergence of homologous or analogous structures? In fact, when DNA is in the nucleus, it is bound to proteins called histones, which can either be tightly or loosely attached to various gene segments. Together, histones and DNA are referred to as chromatin, which is shown in the image above (histone proteins are blue, DNA brown). Loosely bound segments of DNA and histones are euchromatin, whereas genes tightly bound to histones are known as heterochromatin. Histones are carefully modified by the addition of various chemical groups that can promote or inhibit closer DNA binding and thus regulate gene expression in cells above the level of typical gene regulation. Genes that are tightly bound to the histone proteins are less accessible and therefore less able to be expressed. The opposite is true for genes in the euchromatin state, in which transcription factors are more able to gain access to the necessary regions of the genes to promote their expression. The study of these differences in expression of genes without any direct changes to DNA is epigenetics and includes other modifications of DNA such as gene or promoter methylation.
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Such differences in gene expression can be responsible for structure homology or analogy. Over time, epigenetic regulation of gene expression can occur due to the inheritance of these histone modifications but also from environmental factors such as diet, activities, stresses, or exposure to toxins. Histone modifications can change over time throughout an organism, or merely in a single cell or tissue, which can result in the variation needed to allow for changes in structure or function of a limb, for example. Alternatively, there may be similar genes expressed in these tissues, but a variation on gene function may occur, thus causing differential protein signaling. In other words, a protein might interact with a specific binding partner in the cells of one tissue, but in the absence of this binding partner in the cells of another tissue could promote its binding to a completely different protein, resulting in very different downstream signal transduction in cells. For example, although a particular gene may be expressed in both human arms and bird wings, gene expression between species varies in response to the protein product. This variation is primarily due to mutations in the protein-coding sequence of the gene that alter its function.
Genetic Mutations over Time Allow for Anatomical Evolution
Although epigenetic alterations are incredibly important in controlling gene expression throughout the body, the more apparent changes are seen in the basic sequences of genes such as the gene in both arms and wings. Mutation of the DNA sequence itself clearly has implications for changing the structures of organisms over time. The DNA sequence included in a gene (intragenic region) is crucial for determining the function of its protein product, in the same way as a recipe must be written down correctly to achieve the proper combination of ingredients in order to produce a loaf of bread. Changes in the DNA sequences of organisms throughout time has allowed for variations in the gene pool in populations, which promotes the natural selection of the best fit genes to be passed to offspring, and thus the evolution of species based on their abilities to adapt to a particular environment.
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Additionally, mutations in the non-coding regions of genes (intergenic regions), such as promotors or other regulatory sequences, can differentially influence gene expression to promote these differences in structure between species. Some genes have been well-conserved in the protein coding region, but due to changes in other regulatory regions of the DNA serve the same signaling function in cells, albeit in a different tissue. Mutations in a promoter region, which is a necessary sequence for the binding of proteins called transcription factors, as well as for the RNA polymerase enzyme that actively transcribes the DNA to RNA, can either enhance or inhibit the process of RNA synthesis. Since RNA is necessary for the synthesis of protein, the amount of that particular protein in cells may be changed at a level that influences cellular function and ultimately, the functions of an organ.
Such mutations can thereby result in homologous or analogous structures in organisms over time since a gene that has previously been present in the genome but suppressed could become activated to promote a new tissue function or structure analogous to another. In contrast, the promoter region of a gene could also gain a suppressive mutation that inhibits the expression of that gene, which was previously important for tissue function. Thus, this mutation would create a homologous structure that may look similar anatomically, but which now serves a very different role in the organism in comparison to that of its ancestor.
Homologous and Analogous Structures Are Derived from Molecular Changes
To conclude, anatomical structures in animals or plants frequently diverge in function due to DNA mutations or epigenetic regulation, resulting in homologous structures in future offspring if the change is favorable for the survival of that organism. Alternatively, changes in gene expression of two completely unrelated species can eventually result in very similar anatomy and physiology in certain tissues. Such structures are analogous, or homoplastic, since they are derived from differing ancestors, but have very similar functions. In order to determine the closeness of a species to a particular ancestor, mutations at the DNA level are typically compared between organisms, where those with fewer changes in the DNA sequences are considered to be more closely related. Importantly, these changes in DNA sequences were only passed to offspring if they were favorable enough to allow the organism to survive long enough to be able to reproduce, a concept known as natural selection.
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