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Learning Goals
By the end of this reading you should be able to:
Describe how comparative anatomy and embryology support the theory of evolution
Explain how information can be gathered from the fossil record
Discuss the contributions of biogeography and vestigial structures to evolutionary theory
Define and give examples of homologous traits
Introduction
When Charles Darwin finally published his pinnacle work “On The Origin of Species”, it was the culmination of years of observations on his part and the works of other scientists that came before. People had observed changes in populations of organisms over time before Charles Darwin and his contemporary Alfred Russell Wallace put pen to paper. What the works of both Darwin and Wallace did was bring together ideas in a new way to formulate a theory that provides an explanation of how organisms over time became more adapted to their environments. Like other scientific theories, the theory of evolution identifies a pattern that exists in the natural world and a process that is responsible for that pattern. The questions that evolutionary biology addresses are where do living things come from, how and why do living things change over time, and, ultimately, what is the source of the diversity that is seen in living organisms. The theory of evolution contends that organisms are related by common ancestry and that species have changed over time, what Darwin referred to as “common descent with modification”. Like other scientific theories, the theory of evolution is based on evidence and is supported by research.
Comparative Anatomy and Embryology
As humans studied the anatomy of the organisms around them they noticed that there were some key structural similarities across groups of organisms. The term homology was used to refer to having the same or similar relation, relative position, and/or structure. Let’s examine at some of the evidence of evolution that comparative anatomy provides. A comparison of the fore and hind limbs of animals reveals that there is significant homology in structure (Fig. 1), specifically the arrangement and some cases the number of bones in the limbs.
Figure 1. Comparative anatomy of the forelimbs of different mammals
For example, the forelimb of the dolphin and the bat, two limbs that are used for very different purposes – flying in the bat and swimming in the dolphin. However, if you look at the organization and structure of the limbs you can see that each has one bone in the upper part of the limb, then two bones, and finally five digits. The same comparison can be made between other vertebrate animals.
In addition to the homology that is seen in the limbs of some animals, there is also evidence of homology in the embryological development across a widely diverse group of vertebrate animals. In vertebrate embryos, including humans, there is a stage in which gill slits and tails are present. These structures are absent in the adults of terrestrial groups but adult forms of aquatic groups such as fish and some amphibians maintain them. This commonality of structures at some stage of embryo development suggests a shared common ancestry among these groups.
Figure 2. Comparative anatomy of floral structures
We can expand this idea of homology of structure to other groups of organisms as well. For example, flowers in plants are all composed of the same set of parts with different modifications. While each flower may not have all of the exact same structures, there are some parts that are found in common in all flowers (Fig. 2). Thus, there is a similarity in the structural design of each type of flower but the appearance of each flower is uniquely different.
It is possible to make a connection between the homologous features that occur later in the development of organisms with homologous groups of cells that are present in their embryos. Thus, the similarity that is seen later in development is one that happens as a result of shared developmental pathways. Why do some organisms share the same pathways then? Because they have a shared common ancestor. We now know that with only a few minor exceptions, all organisms use the same codons (triplets of mRNA nucleotides) to specify the same amino acid carrying tRNAs and that all organisms use the same nucleotides (DNA) to store the genetic information within each cell. Thus, not only are the similar structures found within groups of organisms the result of genetic programs within the organism, they are often the result of similar genes.
Vestigial structures: Some structures exist in organisms that have no apparent function at all, and appear to be residual parts from a past common ancestor. For example, the presence of eye sockets in blind cave salamanders reduced wings on flightless birds (penguins, ostriches), and in some snakes the presence of highly reduced hip and rear leg bones. Thus, vestigial traits are ones that appear to be functionless or rudimentary structural, developmental, or genetic features in one group of organisms that have a function in a closely related species.
Fossils: Fossils provide evidence that organisms from the past are not the same as those today, the vast majority of fossils that have been discovered are unlike the species that are present today. It was through the study of available fossils that George Cuvier first proposed that some organisms that once existed had become extinct. While this may not seem like earth-shattering news to you, at the time he proposed this theory it was. Through the use of radiometric dating, scientists can now determine the relative age of fossils. This allows them to determine where organisms lived in the past relative to other organisms of the same age as well as the place changes in groups of similar fossils in chronological sequence. These records can show the evolution of form over millions of years. In addition, some fossils show transitional forms that show characteristics with an ancestral organism as well as a current species. Lastly, fossils demonstrate changes in the environment over time, for example, the presence of fossils of aquatic organisms in areas where there is currently no water.
Geology: Some of the information that we currently have about the relative age of fossils comes from work done in geology. Geologists can determine the relative age of strata (layers of rocks) based on their relative position. The lower strata were typically formed first and thus based on the principle of superposition determined to be older.
Figure 3. Relative dating of strata based on the location of the layer in the rock profile
Thus, fossils found in lower strata are older than those in the upper layers (Fig. 3). In some areas, however, the rocks were either not deposited in horizontal layers or have been somehow overturned. In these cases, geologists can use the decay of radioactive elements, such as uranium, potassium, rubidium, and carbon to help determine the relative ages of the layers and the fossils within them.
Biogeography: Biogeography is the study of the geographic distribution of organisms both living and fossil on the planet. It reveals patterns of distribution that can be explained by evolution in conjunction with tectonic plate movement over geological time. Some groups of organisms are broadly distributed across the current continents, and thus are likely to have evolved before the supercontinent Pangaea broke up (about 200 million years ago).
Figure 4. Biogeographic distributions of non-human primates and Pinus species. Patterns of distribution can be used to determine the evolutionary relationships of species.
Other groups of organisms are unique to specific regions of the planet. Non- human primates are found in the southern hemisphere and Pine tree species are distributed in the northern hemisphere (Fig. 4). As another example, Australia has an abundance of endemic marsupial species—species found nowhere else— and no other native mammals. This type of distribution is typical of islands that are isolated by expanses of water. This isolation creates a barrier that prevents most species from migrating to or from the island. In Australia, over time, these ancestral marsupial species though evolutionary processes diverged into new species that look very different from marsupial species found anywhere else in the world.
A) There are patterns in the fossil record that suggest other species have diverged from a single ancestor species.
B) Anatomical structures in different groups appear to be modified versions of structures that might have been present in a common ancestor
C) There are biogeographic patterns in the distribution of species that suggest a common ancestor.
Summary
Comparative anatomy and embryology revealed to scientists that there are shared structural features in some groups of organisms. We now know that these features are often the result of shared development pathways and ultimately similar sequences of nucleotides in genes. Thus, homologous structures are indicators of a shared common ancestry. Vestigial structures can also be an indicator of shared ancestry. Changes in organisms over time is sometimes captured within the fossil record, and the dating of the rock strata can give a timeline of the appearance, disappearance, and sometimes the transitional states of groups of organisms. Finally, the geographical distribution of organisms and fossils can provide an indication of shared ancestry as well as the changes that have occurred environmentally over time.
Figure 1. Image (vertebrate limb homology) courtesy of Mcy jerry at the English language Wikipedia / CC BY-SAFigure 2. Left Image (floral structures) courtesy of Noah Elhardt; remade to SVG by Petr Dlouhý [Public domain]. Right Image (flower variation) courtesy of Alvesgaspar CC BY-SAFigure 3. Image (fossil strata)courtesy of Jillcurie [CC BY-SA]Figure 4. Top Image (non-human primate range) courtesy of Jackhynes [Public domain]. Bottom Image (Pinus distribution) courtesy of Nova CC BY-SA
