When scientists study how different species are related to each other, they use a number of lines of evidence to understand their evolutionary history. These types of data include the geographic distribution of species, fossil evidence, and shared anatomical characteristics (comparative anatomy). More recently, the use of DNA has helped scientists better understand the details of evolutionary histories.
Fossils are preserved remains of ancient life, which means they can give direct evidence of an evolutionary history. Fossils can show that a certain species once lived in a different region than its present range or provide physical evidence of features possessed by a common ancestor of two lineages.
Before genetic evidence was available, scientists often used the shared physical characteristics of groups of organisms to determine how they might be related. For example, the different groups of animals (mammals, fish, amphibians, etc.) each share a set of features unique to the group. While anatomy often suggests the relationship of organisms, it may sometimes mislead. For example, some features that seem quite similar, such as the spines on cacti and other unrelated succulents, may have evolved independently (convergent evolution).
When scientists want to study how different species are related to each other, they sometimes use genetic clues to find out more about these relationships. Because DNA mutates (changes) over time, if two species have very similar sequences at a particular gene, then they are likely to be closely related. Therefore, the more similar the sequence, the more closely related the two species are.
Examples from the Academy of how fossils, comparative anatomy, and DNA show the evolution of species
Marsupials in the Earthquake exhibit
Marsupials are a distinct line of mammals that diverged from the placental mammals at least 125 million years ago. Unlike placental mammals, marsupials have a very short gestation period, after which they spend a long developmental period feeding off of milk from their mother (most often in a pouch). Newborn marsupials have well developed forelimbs to help them crawl to their mothers’ pouch, but otherwise they are not fully developed and lack the ability to regulate their temperature.
Though Australia is associated with the diversity of marsupial species, the marsupial linage actually started much further north. The oldest fossil of any known marsupial was found in China and is estimated to be 125 million years old. Because there was still a connection between the continents in the North and South America, this marsupial line soon spread throughout the northern continents and into South America.
As marsupials went extinct in other parts of the world, those in South America survived and migrated across Antarctica into Australia, since South America was still connected to Antarctica and Australia long after other Gondwanan continents split off. DNA shows that Australian marsupials are related to those in the Americas. According to the DNA data, the South American monito del monte shares a common ancestor with the marsupials that now live in Australia. Fossil evidence of teeth found in Antartica also links the monito del monte to the lineage of marsupials that spread from Antactica to Australia. This extinct marsupial lived in Antarctica 70 to 55 million years ago.
Plants of Gondwana in the Earthquake exhibit
Proteaceae is a family of flowering plants that has its origins in the Gondwanan supercontinent. The most distinguishing feature of many plants in the family is its “flower,” which is actually made of many small flowers densely packed together. The exhibit features three different examples from the Proteaceae family: the king protea from Africa, the red silky-oak from Australia, and the Chilean fire bush from South America.
DNA evidence links the African genus Protea to Gondwana and fossil pollen also shows that this genus was on the southern supercontinent. Today, Protea is native only to Africa, one of the first continents to break away from Gondwana. So, of the three flowers on display, the example from Africa is the most distantly related.
Flowers of South American plants in the family Proteaceae, such as the Chilean fire bush, are much like those of Australian species, providing an anatomical clue to their relationship. DNA confirms that they shared a common ancestor before South America, Antarctica and Australia separated. Fossil pollen from Antarctica also suggests that ancestors of Australian plants crossed Antarctica and spread to South America, when all three continents were joined.
Flightless Birds throughout the Academy
The flightless birds highlighted in the Earthquake exhibit are all ratites. Ratites, which include ostriches, rheas, emus and kiwis, are a group of birds that share flightless features. These features include: a smooth breastbone, which lacks the keel that anchors the wing muscles in flighted birds; no wishbone, since this structure is usually needed to strengthen the ribcage during flight; a large, heavy-boned body; fewer and smaller wing bones; and soft, plume-like feathers.
Flightless ostriches, rheas, emus and kiwis live on different continents, but they’re related through an ancestor that lived when the continents were one. Was that ancestor also flightless? Based only on physical evidence, one might expect that flightlessness arose in an ancestor common to all the ratites that share the anatomical features listed above. However, DNA evidence indicates that flightlessness evolved a number of different times as the ratite lineage diverged and were carried apart on the different Gondwanan continents.
The exact nature of the evolutionary relationships of the different ratites is still actively being studied. DNA evidence shows ostriches are more distant cousins to others in the group, making it, potentially, the oldest line of ratites. These genetic data also fit with the order that continents broke away from the supercontinent Gondwana - Africa was the first. Genetic data also place the South American tinamou into the ratite group. The tinamou birds are able to fly, further indicating that the common ancestor of the ratites had the ability to fly.
While ratites are one large group of flightless birds, the Academy features two others: the Galapagos cormorant (found in Islands of Evolution) and South African penguins (in African Hall). Their anatomical features resemble some of those found in the ratites but are an example of convergent evolution. Much like the ratites, the Galapagos cormorant has a smooth breastbone that lacks a large keel and also has less developed wing bones. On the other hand, a penguin still has a well developed breastbone as it uses the wing muscles to swim.
Finches in the Islands of Evolution exhibit
The Galápagos Islands are an archipelago consisting of sixteen volcanic islands located 600 miles west of Ecuador in the Pacific Ocean. They formed about 4 million years ago when a series of underwater volcanoes erupted, spewing up magma that cooled to form the cone-shaped islands. When the islands first formed they were devoid of life, but over time animal and plant species colonized them, allowing them to be a unique place to study the dispersal and evolution of species. A classic example of how species colonized and diversified once they arrive in the Galápagos is the finch. The geographic distribution, anatomical characteristics and, now, DNA all inform the understanding of how the different finch species of the islands evolved.
New York Times, Antarctica yields first land mammal fossil
Carnegie Museum of Natural History, Sinodelphys szalayi
Evolution and the Nature of Science Institutes
Genetic Science Learning Center