Evolution - The Evidence

A theory is a scientist's explanation of a natural phenomenon that must be strongly supported by many different lines of evidence. Established scientific theories, such as the Theory of Evolution by Natural Selection, have withstood rigorous scrutiny over the years. This theory's strength is its ability to explain modern evidence, evidence that was not gathered by Darwin and Wallace.

Evolution has been occurring since the very first life form to create modern species

Humans and chimpanzees

Mapping allele frequencies to geographical distribution can show the origin of alleles

Biochemical Evidence

Darwin and Wallace argued that all living things shared a common ancestor. It was not until recently that biochemical evidence has shown that all living things share the same macromolecules such as proteins and DNA, and biochemical processes such as cellular respiration.

Biochemical evidence: Evidence drawn from the study of biochemical processes in organisms.

Biochemistry is the study of chemicals found in cells, and is an integral part of molecular biology and genetics. Recent and detailed evidence involves comparing the sequence of the basic units that make up these chemicals in species that may share evolutionary relationships.

Amino acids create chains, which in turn make proteins

Amino acid sequencing

Proteins are found in every living cell as part of the cell membrane and as enzymes in the cytoplasm. Proteins are made up of sub-units called amino acids. Living things contain a combination of about 20 different genetically determined amino acids. It is the number, type and sequence of these amino acids that determine the type of protein. A protein that is found in a wide range of organisms is usually studied to examine amino acid sequence and evolutionary relatedness.

The sequence of amino acids in the protein is analysed, and similarities and differences between organisms are identified. Similarities imply that the organisms may have shared a common ancestor. Differences imply that the organisms have evolved.

The number of differences is proportional to the length of time since the organisms separated. This information is used to construct evolutionary trees. Also known as phylogenetic trees, these are branching diagrams showing inferred evolutionary relationships.

For example, humans and chimpanzees have an identical sequence of amino acids in their haemoglobin and so they are more closely related than humans and gibbons, which have three differences.

DNA Hybridisation involves the merging DNA from at 2 species, the more compatible the DNA, the closer the two species are related

DNA-DNA Hybridisation

DNA is the genetic material found in our cells. It has been inherited from our parents. DNA is made up of sub-units called nucleotide bases. It is the number, type and order of these bases that determines our genes.

DNA-DNA Hybridisation is based on the assumption that DNA molecules of closely related species have a similar nucleotide base order. This involves splitting the double-stranded DNA molecule to expose the nucleotide bases on each individual strand. Separated segments of DNA from the two species that are going to be compared are mixed and the two strands combine to form a hybrid double strand of DNA. The more closely matched the base pairs are, the stronger the binding of the strands. Closely related species have a very similar order of nucleotide bases and so their DNA strands combine more strongly than species that are distantly related.

Process of DNA hybridisation

Section of a DNA Sequence

DNA Sequencing

In DNA sequencing, the exact order of nucleotide bases in the DNA of one species is compared with the sequence in a similar DNA fragment of a second species. The more closely related the species, the closer the order of nucleotide bases in the DNA.

A piece of DNA is isolated from each organism to be compared.
Multiple copies of each gene are made, using fluorescent dyes to distinguish between the four bases in DNA.
Computer-linked equipment called a DNA sequencer is used to graph and print out the entire sequence of bases, which are then compared.

Biochemical evidence suggests that organisms that share a common ancestor have fewer differences in the DNA base sequences. DNA sequencing is a more advanced technique that reveals more detailed information that other biochemical techniques. The limitations of biochemical evidence include that some changes in DNA or amino acid sequences may not be identified if a particular change that occurred in the past has reverted to its original form in a more recent organisms. The technique is also expensive and complicated.

Adenosine triphosphate molecule (ATP)

Similar molecules

The presence of similar molecules, enzymes, and biochemical processes between organisms indicates a common ancestry. Molecules essential to the functioning of all life are thought to have arisen early during the evolution of life on Earth, and therefore have been conserved across all kingdoms of life (archaea, bacteria, and eukaryotes).

ATP is a widespread molecule, which is critically important to life, as it powers cellular processes. It is found in all organisms on Earth, as a product of cellular respiration, and is essential for cell metabolism.
Nucleic acids (DNA and RNA) are also synonymous across organisms as the carriers of genetic information, which encodes the structure and function of an organism.
Cytochrome C is an essential protein for respiration, forming part of the electron transport chain which allows ATP production. Studies of amino acid composition across different animal species has indicated no difference between the structure of human and rhesus monkey cytochrome C composition reveals a difference of 1 amino acid, indicating that they are closely related, however less so than humans and chimpanzees.

DNA and RNA have a consistent structure

Human and chimpanzee skeletons

Comparative Anatomy

Comparative anatomy provides evidence for evolution, as the presence of homologous structures across different organisms may indicate that they evolved from common ancestors.

More similarities in the structure of organisms implies that they must have separated from a common ancestor more recently.

One, two, many comparison of forelimbs

Comparative anatomy

Comparative anatomy: Similarities and differences in anatomy of organisms.

The Pentadactyl limb is an example of a homologous structure present across many different animals. It is composed of the same base structure of five digits but is used in different ways depending on the organism's environment. This indicates that a common ancestor with some form of the pentadactyl limb was then isolated in different environments, and therefore new variations of the limb, and organisms, began to evolve.

Some body parts of organisms appear to be similar at first, but in-depth studies of their anatomy show that they are really vastly different in their basic structure. For example the wings of a bird (containing muscles and bones) and the wings of a grasshopper (made of a thin membrane of exoskeleton). Since these organs differ greatly in their basic plan, they are said to be analogous. This is convergent evolution, where changes in structure are adaptations that favour organisms with similar selection pressures.

The Australian echidna and the European hedgehog have both developed protective spines to discourage predation but, in terms of most other structures and their reproduction, they are dissimilar. The presence of analogous features does not provide evidence for evolutionary relatedness, but rather for evolution of structures to serve a common purpose in a common environment.

Echidna and Hedgehog

Vestigial structures are thought to be evolutionary remnants of body parts that no longer serve a useful function within that population. The presence of vestigial structures provides evidence of common ancestry. For example, the tail bone in humans and the pelvic bones in snakes and whales, are difficult to explain unless they are structures that have become reduced because they no longer carry out a useful function in the animal's life.

Embryos in early develop of different organisms

Comparative embryology

Comparative embryology: Study of similarities between embryos of different species.

Comparative embryology is the comparison of the developmental stages of different species. Similarities can be used to infer relationships between organisms.

By studying the development of structures in embryos of different species, it can be seen that significant similarities exist across different animals at the early stages of embryonic development. For example, human embryos develop fish-like gill arches during the fourth week of development. This indicates that common structures exist across organisms as a result of common ancestry.

The prediction is that related species show similarities in their embryonic development. This is best explained by common ancestry where they are all descendants from a common form. Today technology is used to track the migration of specific cells in embryos to show commonalities.

Similarities in embryos from different species

The greater the number of similarities in structure of organisms being compared, the more closely related the organisms appear to be. Numerous features need to be taken into account for this. Comparative anatomy is therefore used to reinforce inferences about common descent derived from the fossil record and therefore shares similar limitations.

Vestigal organs in the snake and the human

Fossil locations across continents

Comparative bio-geography

Changes in continental geography, as a result of continental drift, can have impacts upon the populations residing on different continents. Species can be isolated or merged together as land masses separate or collide. By tracing evolutionary relationships between animals on different continents, it can be determined that they may have evolved from a common ancestor when their contents were joined.

Animal diversity around the world

Comparative biogeography

Biogeography: The study of the distribution of life.

The Darwin-Wallace theory of evolution proposes that for a new species to arise a group of individuals must become isolated from the rest. A new species is one where the individuals cannot produce fertile offspring with the pre-existing species.

If isolation is a criterion necessary for new species to arise from an original species, the new species should resemble species with which they shared a habitat. That is they will be more similar to:

Species that lived close by than to species found far away
Or species that lived in a common area before it split up.

Darwin's finches are an example of biogeography.

An example of this is the ratites, a group of large flightless birds, which are found across different continents. This group includes emus in Australia, Ostriches in Africa, Rhea in South America, and Kiwi in New Zealand. It is thought that they each diverged from a common ancestor in the former supercontinent of Gondwana. As the continents separated, and were each subjected to different environmental changes, the ratites evolved slightly different characteristics. This provides evidence for divergent evolution.

Monkey species from different areas of South America

Ratites from around the world

Fossil evidence of human evolution

Fossil Evidence for Evolution

Fossils provide direct evidence of the existence of an organism in the past. Fossils may be mineralised remains in rock or the actual remains of the organism preserved in rock, ice, amber, tar, peat or volcanic ash.

Archaeoptryx fossil with evidence of feathers

Archaeoptryx reimagined

Fossil evidence

Palaeontology: The study of fossils.

Even before Darwin's proposal, scholars recognised that the idea of change in organisms over time was supported by evidence in undisturbed rock formations. The sequence in which fossils are laid down in rock reflects the order in which they were formed, with the oldest towards the bottom. This is called the law of superposition.

By examining the fossil record, and determining what types of organisms have existed in what particular order in the past, species can be shown to change from generation to generation over time. Most importantly, the existence of transitional forms, fossils which appear to bridge current animal classes, suggests that all animals have evolved sequentially from a common ancestor.

Darwin predicted that the fossil record should have intermediate forms - organisms that show transitions from one group to another, that is 'missing links' between groups. Today this is supported by thousands of known fossils that appear to have features common to two known groups, suggesting that a transition occurred in the past from one group to another. These fossils are called transitional forms and they represent successive change in organisms over a long period.

The Archaeoptryx is an example of a transitional form, which may have been an intermediary organism as birds evolved from reptiles. It was small in size (about the size of a magpie), with broad wings, feathers, and gliding ability, characteristics found in modern birds. However, it also shared features with dinosaurs of the period, such as jaws with sharp teeth, clawed fingers, a bony tail, and a hyperextensible "Killing Jaw".

A limitation of the fossil record is that it is incomplete and so it is not a random sample of past life. There is a bias towards organisms who's structure or environment makes them better suited to become fossilised. There is a lack of fossils representing the majority of early and soft-bodied organisms and there is unequal representation of transitional forms.

Relative dating relies comparison of samples

Radioactive dating relies on radioactive decay and half life to determine age

Relative dating is less percise that absolute dating

Techniques for dating fossils

To correctly determine the sequence of fossil forms in the fossil record, a number of dating techniques are utilised to accurately find the age of fossils. This is important as it aids in our understanding of evolutionary relationships.

Fossils can be dated in two ways, using either absolute or relative dating methods. Absolute methods give the actual age of the fossil, whereas relative methods can indicate whether a fossil is older or younger than another.

Absolute dating methods:

Fission track: uranium, which is present in many rocks and minerals, can split as it decays overtime. This is called fission. When fission occurs, atoms moving at a high speed through the substances can leave marks called 'fission tracks'. The number of fission tracks will increase as a fossil ages. Therefore, age of a fossil can be calculated by measuring how many fission tracks and how much uranium is present. This can be used to date the age of fossilised teeth, and is most effective for fossils between 5000-100 million years old.

Radioactive dating: using known rates of decay of radioactive elements (such as the isotope, Carbon-14) and comparing the amount of carbon to its decay product, an estimation of how long that element has been present in a fossil can be determined. This method is useful for fossils less than 60,000 years old, which are composed of organic materials.

Potassium-argon dating: a technique used to determine the age of a rock by measuring the ratio of radioactive argon to radioactive potassium in the rock sample. This method is based upon the decay of potassium-40 to radioactive argon-40. potassium/argon and argon/argon are useful to date volcanic rocks and minerals to 200 000 to four bill years. The age of volcanic ash can be determine by measuring the amount of argon-40 and radioactive potassium within the sample. If fossils are buried between layers of volcanic ash, the age of the ash can indirectly indicate the age of the fossil.

Relative dating methods:

Chemical analysis: used to provide relative dates for fossils found at the same site, chemical analysis is the method of analysing how much of a chemical (such as uranium and fluorine) has been absorbed into an object. The amount of nitrogen within an object may also be an indicator of age, as nitrogen levels will gradually decrease as the fossil decays. Due to external factors such as temperature, moisture and microbes, these methods are not exact, but still useful for relative dating. This method is effective for analysing fossilised bone, and can be effective for fossils as old as 4 billion years.

Stratigraphy: by analysing the order in which fossils have been deposited in sedimentary rock layers, their relative age can e determined. The older fossils will be found below the younger fossils deposited on top.

Biostratigraphy: some fossils are known to have existed during specific periods of history, and are therefore used to match isolated rock layers to a certain age. This information can be used to determine the age of surrounding rock layers and fossils, placing their age relative to known fossils. An example of this is the E. pseudoplanus which existed during the Middle Ordovician period. Any traces of this organism in rock layers will implicate a mid-Ordovician age. This method can be used for fossils up to 3 billion years old.

Palaeomagnetism: the study of the record of Earth's changing magnetic field in rocks. Some magnetic minerals in rocks lock in a record of the direction and intensity of Earth's magnetic field when the rock was formed. When scientists determine Earth's past magnetic field, they look for traces of iron oxide in the rocks. Iron oxide is magnetic and therefore when the rock formed the minerals will orientate in the direction of the magnetic field. Approximate dates can be determined from previous magnetic reversals.

Antibiotic resistence in bacteria

Modern day examples of evolution

Although evolution is a process which takes a long period of time, as change requires many generations to affect a species, there are some examples of modern organisms which have been seen to evolve very rapidly. This is facilitated by rapid breeding and increasing selection pressures.

While Darwin's work on evolution has been important to our understanding of evolution, modern evolutionary ideas have been further developed by scientists observing successive generations over a short period. Organisms that can produce a large number of offspring over a short period have been invaluable in our study of evolution

Cane toad distribution over time

Cane toads

The cane toad was introduced to Australia in 1935 in an attempt to control the grey-backed cane beetle and frenchi beetle populations. Since the event the toad has proliferated throughout the Queensland region, causing wide-spread environmental impacts and spreading diseases which impact native biodiversity. Their spread has been partially due to their rapid evolution, increasing their mobility and ability to thrive in the Australian environment.

3D imaging has revealed significant differences in the bodies of invasion-front toads (left in the image below) and long colonized populations. Compared to toads from long colonized populations, invasion-front toads have narrower pelvises; slimmer, higher-set skulls and longer/wider forearms.

The leg bones of the Cane toad have changed in their size and thickness to allow for a more powerful jump and increased stamina

Process of antibiotic resistance

Antibiotic resistance

You've probably heard about 'superbugs' on the news. They're a big problem in medicine, especially in hospitals, because they are strains of bacteria which do not respond to antibiotic treatment, and that means they are really hard to get rid of.

Antibiotic-resistant strains of bacteria evolved using the same fundamental principles of evolution by natural selection. Within every population of bacteria, a few are antibiotic-resistant due to natural variation and mutation. Whenever someone uses antibiotics, most of the bacteria are killed, but the few resistant ones survive. These are then able to reproduce, with antibiotic resistance becoming the dominant trait in the population, eventually leading to a new 'superbug' strain. Additionally, we know that bacteria are able to pass genetic information to each other using plasmids, circular pieces of DNA which bacteria can incorporate into their genomes. This process speeds up the evolution of species, as those antibiotic-resistant individuals can pass their genes for resistance onto others.

The process of bacterial evolution has also been 'sped-up' by a number of human factors. Firstly, we have been overusing antibiotics where it is not always necessary. Each time antibiotics are used, a selection pressure is applied to the bacteria, and more antibiotic-resistant individuals are selected for. By treating non-serious infections with antibiotics, instead of waiting for them to pass, humans have been continually adding to evolutionary process. Secondly, antibiotics are often also used to treat the wrong infections, for example to treat colds, which are viral infections. Viruses cannot be killed using antibiotics, so using antibiotics in this situation means that even non-threatening populations of bacteria which exist in symbiosis with humans can become pathogenic. If we combine these factors with the fact that bacteria reproduce at a very rapid rate (E.coli reproduce approximately every 30 mins), we can see how bacterium have evolved so quickly into these superbugs.

Evolution - the Evidence

Inquiry question: What is the evidence that supports the Theory of Evolution by Natural Selection?

Investigate, using secondary sources, evidence in support of Darwin and Wallace's Theory of Evolution by Natural Selection, including but not limited to:
- Biochemical evidence, comparative anatomy, comparative embryology and biogeography
- Techniques used to date fossils and the evidence produced
Explain modern-day examples that demonstrate evolutionary change, for example:
- The Cane toad
- Antibiotic-resistant strains of bacteria

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