Unit 7.3 (Evidence of Evolution)

Theory of Biological Evolution

Evolution is the theory that describes how groups of organisms develop and change over large spans of time becoming different from earlier forms of life. The evolutionary process is driven by a process described earlier called “Natural Selection”, organisms that are less adapted to their environment are more likely to die before they can pass their traits to the next generation. Through the process of evolution new species arise. A “species” is a group of organisms that are capable of producing “viable” offspring. “Viable” refers to an organisms ability to have children. For two organisms to be considered the same species they must be able to have offspring and those offspring must be able to have offspring of their own.

Lines of Evidence Supporting Theory of Evolution

Fossil Record

A fossil is created when the organic material of an organism is replaced by inorganic minerals and materials. This can leave behind a mold or cast of the organism. This impression of the once living organism can give scientists great insight, showing us the hard structures of the organism like its skeletal structure.

By looking at the entirety of the world’s fossil collection scientists can compare the structures present between the organisms. When the fossil record is examined in its entirety we see a trend showing that organisms have moved from being simplistic to more complex.

When examining fossils found in sedimentary rock the more simplistic and primitive organisms are always found at the lowest layers with more complex advanced life forms being found at higher layers of rock.

Transition Species

Throughout the fossil record we find some peculiar fossils of organisms that have traits that are distinctly from what we would consider to distinct and unique groups. These “Transition Species” possess characteristics of two groups because they represent a missing link that bridges the gap between two groups of organisms. A famous example of this is a dinosaur called “Archeopteryx”, a small bird like dinosaur. It has the traits of both reptiles and modern day birds.

Like reptiles its bones were not hollow, the fossils indicate it had feathers like modern day birds. It had a beak like a bird but had reptilian teeth and fingers/claws on its wings. The fossil record is littered with examples of transition species, whales are another popular example.

Comparative Anatomy

By looking for similarities and differences between the anatomies (bodily structure) of various groups of organisms we can develop a better picture of how closely related these groups may be. The more closely related a group of organisms are the more similarities their anatomy would share, similarly siblings share similar anatomy because they are closely related.

The skeletons and bonds of primates are a good example of the usefulness of comparative anatomy. When looking at the skeleton of a primate like a gorilla one can see that many of the bonds are similarly structured and arranged/connected in a similar pattern.

When closely comparing the anatomy of organisms that appear wildly different similarities can still be observed. Most terrestrial organisms are quadrupeds with 5 digits meaning they have 4 limbs and each limb has the bones of 5 fingers/toes. This can be observed when looking at the internal anatomy of many terrestrial (land) animals.

The forelimbs of birds, bats, frogs, whales, humans, cats and horses all have the same bones arranged in the same order. The same bones have become modified for different purposes such as running, climbing, flying or swimming. These structures are “homologous” because they are modified forms of the same structure and demonstrate a common ancestor. Sometimes organisms will develop similar traits even though they do not share a recent common, this is due to both organisms having to overcome a similar challenge or obstacle in their environment. These structures that have a similar function but different origin are called “Analogous Structures”. The wings of birds are modified forelimbs while the wings of insects are modified pieces of skin, these would be analogous structures.

Sometimes structures that no longer serve a function are inherited from the ancestors of a species. These inherited structures that no longer serve a vital function are called “Vestigial Structures“. These vestigial structures are further evidence that supports the theory of evolution, that groups of organisms change over time.

One example of a vestigial structure in modern day humans is the “coccyx“, a structure inherited from our primate ancestors. The coccyx is commonly referred to as the “tail bone”, and serves no purpose for modern day humans. Another vestigial structure found on the bodies of modern humans is the “appendix“. The appendix is a small section of large intestine that no longer serves an important role in digestion, conversely in other primates the appendix is enlarged and is heavily involved in the digestion of plant matter.

Comparative Embryology

Comparative embryology compares and contrasts the embryological development of various species to draw conclusions about species relatedness. Various species go through similar stages of embryological development, baring similar or homologous structures as they progress. As organisms move through their development process certain common structures arise such as; pharyngeal gill slits, limb buds, and axial symmetry.

Even terrestrial organisms like snakes develop small vestigial limb buds form during embryological development. As snake embryos continue to grow they lose their limb buds and develop their signature legless bodies. The more similar the structures and development of different species’ embryos the more closely related they are.

Biogeography

The study of geography and how it relates to biological organisms and their adaptations to their environment is called “Biogeography“. The geographic distribution (spread) of organisms can tell us much about the evolutionary past of many groups. For example, the only places with wild populations of marsupial are Australia and the Americas. This can be explained by taking into account a concept called plate tectonics.

“Plate tectonics” describes a theory that explains that the movements of the continents across the surface of the Earth. This movement occurs over periods of millions of years and has resulted in the geographically distinct image of the Earth we have today. Animal and plant populations that are on these continents also get dragged with them, causing populations of organisms to become separated by vast distances of land or water. With enough time the environments the organisms are living in become drastically different and the forces of natural selection will lead to new or different traits being advantageous to their respective groups. This acquisition of new traits eventually becomes so numerous that the organisms will become two distinct and unique “species”, this event is called “Speciation“.

Molecular Biology

Molecular biology is a specialized niche of biology that uses advanced biochemistry to draw conclusions about how closely or distantly related a group of organisms are. Similarities and differences in the nucleic acid sequences of two species can be compared in order to get an idea of how closely related those species may be. If scientist have some information regarding the error rate of an organisms DNA polymerase, and the number of nucleotide differences between various organisms then they can use that information to estimate how long ago the ancestors of two groups “diverged” or speciation occurred.

Scientists can also compare two protein sequences of multiple organisms for amino acid differences. The fewer differences in the protein structure then the more closely related organisms are, while greater differences in protein structure would indicate a more distant relationship.

Organism	Number of Amino Acid Differences of protein MZDE3 Compared to Humans
Human	0
Chimpanzee	12
Cow	58
Snake	90
Tuna	124

To illustrate this purpose we have created a table above that shows the number of amino acid differences for the fictitious protein MZDE3 of various organisms compared to humans. According to this table the most closely related organism to humans would be chimps while the most distantly or least related to humans would the Tuna fish. Similar analysis can be done with enzyme structure and many other molecular hallmarks of the various organisms.

Observable Changes

If we pay close attention we can actually spot examples of evolution happening right before our eyes. Organisms respond to pressures in their environment, if they posses mutations or traits that allow them to survive they are more likely to pass these characteristics down to the next generation. We have observed this happening with plants, animals, fungi, bacteria and most recently viruses.

A historical example of observable changes happening in a population of animals is the story of the peppered moth. More information on the “Peppered Moth” can be found in our article on the peppered moth here.

Antibiotics are chemicals that inhibit the growth of or kill bacterial cells. This class of drug usually targets the cell walls of prokaryotes which disrupts the bacteria’s ability to regulate osmosis (the movement of water into and out of the cell).

In the diagram above we can see that the population of bacteria started out with some genetic diversity, resulting in some of them being more resistant to the antibiotic than other bacteria. Over time the resistant bacteria replenished the population. With the passage of time the bacteria with the resistant traits have been selected to survive and pass on their genes, leading to members of the population to develop greater resistance.

The Sars-Cov-2 Virus also known as Covid-19 is a pathogenic respiratory virus first appearing in China in 2019. The months and years went on the Covid-19 virus continued to acquire new mutations and expressed slightly different protein markers in order to escape the immunity of hosts or increase infectivity. The genetic profile of each subsequent variant was different enough to be identified via PCR analysis.

Evolution can be observed on both the macro (large) scale and the micro (small) scale. The theory of evolution is a particularly strong theory because as shown above it is supported by a variety of unrelated fields of study/ disciplines. Evolution has incredible predictive power, this means that you can make accurate predictions based solely off the theory of evolution and the prediction will be likely to occur. For these reasons the theory of evolution is an important foundation block for the field of biology.