Evograms are diagrams that carry information about how a group of creatures and their particular characteristics evolved. The figure below is an example of an evogram.
Evograms contain a lot of information, so they are not easily understood in a few seconds. However, they are worth understanding because they present information from different lines of evolution and are particularly useful in showing students the logic, power, and testability of evolutionary hypotheses.
Evogram of the Quadrupeds
You will see different types of information pictured in the diagram. The most important part of this diagram is a series of redrawings of an extinct group of vertebrates (from Eusthenopteron to Tulerpeton) that served as a bridge between water and land during the Devonian period, about 380 million years ago. The blue bars indicate the geological time periods in which the taxa fossils were found. These time zones are adjusted according to the time scale at the top of the evogram.
On the right side of the figure, some of these creatures have limbs drawn using different colors. Although each limb differs in shape from the others, they have corresponding bones as indicated by different colours. From this, students can grasp that similar bones have continuity across evolutionary time scales and change in shape and number over time.
To the left of the colored drawings are a series of branching lines; that is, an evolutionary tree, or phylogeny, showing the relationships among these animals. Among the figures shown here, Tulerpeton is shown to be the closest animal to modern-day quadrupeds. Ichthyostega is equally related to Tulerpeton and the living quadrupeds. Acanthostega, on the other hand, is the next creature most closely related to these three groups. And it continues like this. It is worth remembering that none of these animals are direct ancestors of each other; they're just the closest relatives we've just discovered in the fossil record. It's a bit like comparing you, your sibling, your first-born cousin, and your second-born cousin: none of these are direct ancestors of you or anyone else, but as you move in line, they become less close to you. You are the closest to your sibling because you only have to go back a generation to find a common ancestor; whereas you have to go back two generations to find the common ancestor that linked you to your cousin. Similarly, Tulerpeton is most closely related to living forms because it shares the most recent common ancestor with these forms compared to other living things in the phylogeny.
Evolutionary changes found in various branches in the phylogeny are marked with pink lines. These changes are inherited by all descendants of the line from which traits originally evolved. If you examine the diagram, for example, you will see that at some point after the Panderichthys separated from our ancestors, the lineage that went to the quadrupeds evolved weight-bearing elbows, the ankles were curved, and a separation between the head and the body, a neck, was formed. This lineage gave rise to Tiktaalik, Acanthostega, Ichthyostega, Tulerpeton, and the living quadrupeds, all of which inherited these key traits.
Phylogenies are not guesswork; these are testable hypotheses about evolutionary relationships based on many lines of evidence. To construct a phylogeny, biologists first select the group of organisms they want to take into account. Then, they study the characteristics of these creatures, namely their anatomy, their genetic sequence, if available, and / or the periods in which they lived. These phylogenies may be largely based on anatomy, as evograms may also include extinct groups. The characteristics of these creatures are encoded in a matrix. For example, one of the attributes could be "number of fingers" and each animal is coded as 8, 7, 6, or 5 based on the number of fingers it has. Another feature may be "elasticity of the skull joints" and is coded as "yes" or "no" for each animal. Biologists try to identify as many of these features as possible. Phylogenies are often based on hundreds of traits.
Biologists use "outgroups" to help decide which conditions of these traits are ancestral and which evolved later. These are one or more groups thought to be less close to one another than to any of the other living things on the evolutionary tree. For example, in this evogram, ray-finned fish are used as an outgroup; because we know that they did not belong to the ear-finned fish group and to the first quadrupedal ancestors. Rayfinder fish do not have split toes, and neither collecanths nor lungfish have split toes. This means that the ancestor of this whole group should not have any finger.
Phylogeny is formed by taking living things and their feature matrix and entering this data into a computer software. These software use different methods to calculate the most likely sequence of relationships between living things. The software is mostly designed to generate a tree that requires minimal evolutionary change and groups of organisms based on the number and sequence of newly evolved traits. In this case, Tulerpeton and the living forms share the highest amount of these recently evolved traits, so they are probably each other's closest relatives. This is how the phylogenetic tree is built. Although very few types of information are provided in evograms, the phylogenetic tree is the most necessary type of information; because without a reliable tree of evolutionary relationships we cannot show evolutionary changes in structures, functions, behaviors, physiology, natural environments, and many other traits.
Why Are Evograms Important ?
Evograms show how major evolutionary transitions occurred. The above example illustrates the evolution of fingers and other limb elements. However, using the same phylogenetic tree, we can map different traits and ask different questions. We can also use evograms to test evolutionary hypotheses. For example, consider the question of how animals have movable necks. Most ray-finned fish have no neck (their head is directly adjacent to their shoulders), and most primitive necks do not move at all. These early necks simply connected the animal's head to the rest of the body (as, for example, in Tiktaalik and Acanthostega). However, in vertebrates with movable necks, the first vertebra allows the creature to move its head up and down, and the second vertebra allows it to move its head left and right. We can form several hypotheses about how vertebrates evolved this neck. We can assume that the ability to look left and right evolved first, the ability to look up and down first evolved, or both evolved. Similarly, we can form many possible hypotheses about the functions these traits served when they evolved; For example, looking left and right might have helped the creatures see possible prey and predators, and looking up and down might have made these new land animals lower their noses to gather food on the ground.
The phylogeny in the evogram helps us test hypotheses about the evolutionary order of traits. For each animal, we can code whether they have a movable first vertebra, a movable second vertebra, two or neither. We then plot these features in phylogeny and calculate which evolved first. As it turned out, first the first vertebra gained its function, followed by the second. Of course, this doesn't say what functions these traits served when they first evolved, but sometimes phylogenetic analyzes can help us narrow down the possibilities.
This approach helps us understand how feathers evolved, how mammals gained their unique ears, how whales returned to the ocean and evolved their magnificent harmony, and many other changes. Using this approach, we can test hypotheses about evolution; just as we did with the neck vertebra question. One of the most important lessons we learn from testing hypotheses in this way is that structures can evolve new functions over time. For example, the fin-like limbs of animals such as Eusthenopteron and Panderichthys did not provide support on land; they were aquatic animals, and their chubby limbs helped them, in addition to helping them swim, perhaps also climb rocks and vegetation in the shallow waters where they lived. When we come to Tulerpeton and Greererpeton, we find limbs with fewer special bones, larger and stronger, and these are joined by joints in a way that can propel the animal forward on land. Thus, quadrupedal limbs evolved to assist movement in aquatic animals and later evolved to function only for locomotion on land. In evolutionary biology, we use the word exaptation to describe a structure that has evolved a new function. Exaptations are common and appear to be the major evolutionary pathway of new functions.
Feathers are a great example of exaptation. The first feathers evolved in small carnivorous dinosaurs of the dinosaur era. These were simple, hair-like, threadlike structures that formed like a veil over the animal's entire body. They were branched, feathery, not particularly broad. They didn't make an animal fly. But they may have provided warmth, more like the bristle feathers on a modern bird, and probably originally served as insulation. In later carnivorous dinosaurs, these feathers evolved more elaborate features. They had become branched and eventually evolved a central stem with loose threads around it. They were colored in a way that could help with concealment or appearance. Eventually, one of these groups of feathered dinosaurs evolved feathers that formed an effective wingtip and a wing. We call this group the birds. Flight feathers of today's birds are flight exaptations and insulating adaptations. So, to answer a question about the origin of an adaptation (eg, "What good is a half wing?"), We need to look beyond the current function of the feature, what its original function might have been.
Evolutionists sometimes accuse scientists of not having fossils that show transitions between major evolutionary traits and new adaptations, but this is a wrong interpretation. We do not have fossils of every extinct creature, but we do have fossils of many extinct creatures; enough to piece together the stories of some very important evolutionary transitions. Moreover, we learn more every day. Not all of the examples we're going to show you here could have been explained in such detail twenty years ago. Only the fossils we've discovered in the last few decades have illuminated these transitions. The more we search for and study fossils, the more detailed our knowledge of life's past becomes and the more certain we can be of that knowledge.