Evolution - the evidence

There is a vast volume of data that provide strong support for the theory of evolution. Evolution, as with all scientific theories, are based on a large volume of data that has been collected by many researchers over a long period of time. For the HSC course, you must demonstrate an understanding of the different lines of evidence, including how that evidence is interpreted.


Evidence #1: Palaeontology

Palaeontology is the study of fossils. Indeed, fossils were one of the things that Charles Darwin studied extensively. Well before Darwin had embarked on his famous voyage, he was already aware that the Earth was very old. Indeed, geologists before Darwin had come to the conclusion that the Earth was hundreds ofthousands years old, if not more. When people examined fossils, they realised that fossils were similar to the bones of many animals. Some fossils also resembled insect exoskeletons. They concluded that fossils were the bodily remains of animals that had existed in the past. One thing was obvious: some of the animal looked very different to the bones of present-day animals. The same was true of plant fossils. So, they came to the conclusion that the fossil record suggested that life on Earth was very ancient, and they many of the organisms that lived in the distant past looked very different to present-day ones.

Anotherinteresting thing about the fossil record was that some fossils had the combined features of different kinds of organisms. These unusual fossils are called transitional fossils.There are some famous transitional fossils:


Crossoptergerian fossils are the remains of fish that show adaptation to life on land. Their fins are lobed and contain bones. This indicates that the lobed fins of this animal could have functioned as limbs for movement (perhaps on the sea shore). It is though that the Crossoptergerian fish is an intermediate between true fish and amphibians.

Therapsids are vertebrates that show features of both reptiles and mammals. The skeleton of the reptilian skull contains many distinct bone plates, but in the mammals, many of those bone plates are fused. The jaw bones of the Therapsid fossil are similar to that of the mammals, while the rest of the structure are similar to those of the reptiles. The therapsid skull shows an intermediate arrangement of skull bones.

One of the most famous transitional fossil is that of the Archaeopteryx. Thisfossil shows both reptilian and avian (bird) features. It has reptile-like teeth, but bird-like beak. It has a wishbone which, (in birds) are the attachment site for flight muscles. The Archeopteryx also (probably) had features on its skin (unlike the scales of other reptiles).


Why are transitional fossils important? Well, they indicate that one group of organism can develop from another. This is an important aspect of evolution. Evolutionary theory indicates that, as a result of natural selection and adaption, populations can change substantially. This can happen to such a degree that populations can look different or be able to perform different activities. Transition fossils provide evidence for that. While it is unlike that the Archaeopteryx ever flew, it possible to see how its descendants (the birds) have acquired the ability to fly.


Evidence #2: Biogeography

Biogeography is the study of the distribution of population. When you look at how different groups of organisms are distributed around the world, some interestingotters appear. For example, while most of the marsupial mammals are located in Australia, one type of marsupial (the Opossum) is found in North America? Why is this so? When a long time ago, the Australian continent was physically connected to the American continent. At that time, animals were able to freely cross between those continents. This was the case with the Opossum. However, after the Australian and American continents separated, marsupials evolved only in Australia. The ocean that separates the two continents function as a boundary. There are some other boundaries around the world, such as the Wallace line in Papua New Guniea.


Evidence #3: Comparative anatomy

Comparative anatomy is the study of the anatomical features of animals and plants. Some anatomical features are found in many, seemingly unrelated organisms. For example, fish, amphibians, reptiles, birds and mammals have a bony covering in their backbone. This is called the vertebral column (or the spine). It suggests that all of these groups of animal are closely related and probably descended from a common ancestor (indeed, all of these animal groups are classified as vertebrates).

Another classic example of comparative anatomy that supports evolution is the vertebrate limb. The limbs of all vertebrates have a similar arrangement of bones: a single bone for the upper part of the limb (e.g. upper arm), two bones for the lower part of the limb (e.g. lower arm), followed by bones that form the five digits (e.g. fingers). This same structure is preserved in the fins of fish and whales, the limbs of the amphibians, birds and mammals and the wings of birds and bat.

All land plants appear to use vessels called xylem to conduct water from their roots to their leaves. This suggests that land plants shared an ancient common ancestor.

What does comparative anatomy contribute to our understanding of evolution? It indicates that the body parts of organisms can change when populations adapt to environmental change. Whales, for examples, are closely related to land animals, such as cows. However the limbs of cows are not suited to life in a watery environment. Thus, over a period of time, as theancestors of whales moved into a watery domain, those with limb adaptations that allowed them to swim. The evidence from studies of comparative anatomy clearly shows that. Thus, this supports the concepts of evolutionary theory - as a result of natural selection, population adapt and become better suited to theenvironments in which they live.


Evidence #4: Comparative embryology

Embryology is thestudy of embryos. The study of vertebrate embryos reveals something quite fascinating - in their early stages of development, all vertebrate embryos look very similar - so much so that it is not possible to tell them apart. It is only later in embryonic development that each vertebrate group (i.e. fish, amphibian, reptile, bird and mammal) embryo expresses features that are characteristic for that group and begin to look different to the others. A famous biologist by the name of ErnstHaeckel called this phenomenon‘ontogeny recapitulates phylogeny’.

All early vertebrate embryos possess‘gill pouches’. Later in development, these structures become gills in fish and tadpoles, but become unrelated structures (called the pharynx) in terrestrial vertebrates.

Another example of comparative embryology is the circulatory system of the vertebrates. In the fish, this is a‘simple’ system consisting of a pump (heart) that pumps blood to the gills. Blood from the gills is delivered to the rest of the body. This simple system becomes the three-chambered heart in the amphibians that is coupled to the pulmonary system (lungs). In the birds and mammals, the heart is a four-chambered structure.

As with comparative anatomy, comparative embryology indicates that many groups of animals have a common, shared ancestry. During the course of evolution, many systems present in the ancestral organisms have become modified into more complex systems or adapted to suit life in different environments.


Evidence #5: Molecular data

One technique that is used to establish the relationship between different organisms isDNA:DNA hybridisation. Hybridisation means‘to mix’. DNA consists of two intertwined strands of nucleotides. When a solution of DNA is heated, the two strands fall apart. When that solution is cooled,then the two strand come back together. This is because the sequence of nucleotides are perfectly complementary to each other. If theDNA from two different organisms are put together, heated and then cooled, theDNA strands from the twoorganisms will join up with each other (technically, this is called annealing or hybridisation). If there are differences in the sequence of theDNAs from the two organisms, then parts of the two strands will not hybridise. Indeed, the more distantly-related two organisms are, the greater will be the differences in the sequences of theirDNAs and the degree of hybridisation between them will lower. Such studies have shown that humans are closely related to the other primates.

Another technique that is used in evolutionary studies is DNA sequencing and protein sequencing. This technique is more accurate than DNA:DNA hybridisation. However, the principle behind the interpretation of sequencing studies is the same as that of DNA:DNA hybridisation. The more closely related two organisms are, the more similar their DNA and protein sequences will be.


In summary, these five lines of evidence provide powerful proof that evolution by natural selection is a genuine process that drives changes in the biosphere. There are many more lines of evidence, but for the HSC, it is importantthat you demonstrate a good understanding of the information that is obtained from these evidence and how they help us construct the different aspects of evolutionary theory

Sham Nair 2014