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What is evolution?
The theory that describes the way in which organisms evolve, or change, over many years as a result of natural selection.
What did Darwin realise?
That organisms best suited to their environment are more likely to survive and reproduce, passing on their characteristics to their offspring. Gradually, a species changes over time to have a more advantageous phenotype for the environment in which it lives. We now know that the advantageous characteristics are passed on from one generation to the next by genes in DNA molecules.
When Charles Darwin was born, what did most people believe?
That God directly creates all life on earth, including human beings. The Bible doesn’t state how far in the past this occurred - in Darwin’s day the common belief was that this creation had occurred only a few thousand years before.
What did Lyell popularise the principle of?
Uniformitarianism, the idea that in the past, the earth was shaped by forces you can still see in action today, such as sedimentation in rivers, wind erosion and the deposition of ash and lava from volcanic eruptions. This challenged earlier geologists who claimed the Biblical explanation.
What did Lyell’s concept prompt Darwin to think of evolution as?
A slow process, one in which small changes gradually accumulate over very long periods of time.
Where did Darwin carry out some of his most famous research, and what did he notice?
The Galápagos Islands. He noticed that different islands had different finches. The birds were similar in many ways, but their beaks and caws were different shapes and sizes.
Through his observations, what did Darwin realise?
That the design of the finches’ beaks was linked to the foods available on each island. He concluded that a bird born with a beak more suited to the food available would survive longer than a bird whose beak was less suited. Therefore, it would have more offspring, passing on its characteristic beak. Over time, the finch population on that island would all share this characteristic.
What did Darwin do throughout his trip?
He sent specimens of organisms back to the UK for other scientists to preserve and classify, enabling scientists not only to see specimens firsthand, but also enabled them to spot characteristics and links between organisms that Darwin had not.
While Darwin was developing his ideas, what was another scientists doing?
Wallace was working on his own theory of evolution. As their ideas were so similar, they proposed the theory of evolution through a joint presentation of two scientific papers.
Why was Darwin’s book extremely controversial at the time?
Because his theory of evolution conflicted with the religious view that God had created all of the animals and plants on earth about six thousand years ago. A further implication of Darwin’s theory is that humans are simply a type of animal evolved from apes, which conflicted with the widely held Christian belief that God created ‘man‘ in His own image.
What are the different sources scientists use to study evolution?
Palaeontology.
Comparative anatomy.
Comparative biochemistry.
What is palaeontology?
The study of fossils and the fossil record.
What is comparative anatomy?
The study of similarities and differences between organisms’ anatomy.
What is comparative biochemistry?
Similarities and differences between the chemical makeup of organisms.
When are fossils formed?
When animal and plant remains are preserved in rocks. Over long periods of time, sediment is deposited on the earth to form layers (strata) of rock. Different layers correspond to different geological areas, the most recent layer being found on top. Within the different rock strata, the fossils found are quite different, forming a sequence from oldest to youngest, showing that organisms have gradually changed over time - the fossil record.
What evidence is provided by the fossil record?
Fossils of the simplest organisms such as bacteria and simple algae are found in the oldest rocks, whilst fossils of more complex organisms such as vertebrates are found in more recent rocks. This supports the evolutionary theory that simple life forms gradually evolved over an extremely long time period into more complex ones.
The sequence in which the organisms are found matches their ecological links to each other. For example, plant fossils appear before animal fossils. This is consistent with the fact that animals require plants to survive.
By studying similarities in the anatomy of fossil organisms, scientists can show how closely related organisms have evolved from the same ancestor. For example zebras and horses, members of the genus Equus, are closely related to the rhinoceros of the family Rhinocerotidae. An extensive fossil record of these organisms exists, which spans over 60 million years and links them to the common ancestor Hyracotherium. This lineage has been based on structural similarities between their skull (including teeth) and skeleton, in particular the feet (Figure 4).
Fossils allow relationships between extinct and living (extant) organisms to be investigated.
Is the fossil record complete?
No, for example, many organisms are soft bodied and decompose quickly before they have a chance to fossilise. The conditions needed for fossils to form are not often present. Many other fossils have been destroyed by the earth’s movements such as volcanoes or still lie undiscovered.
As the fossil record is incomplete, what do scientists look for?
Other sources of evidence to determine evolutionary relationships.
What is a homologous structure?
A structure that appears superficially different (and may perform different functions) in different organisms, but has the same underlying structure.
What does the presence of homologous structures provide evidence for?
Divergent evolution.
What does divergent evolution describe?
How, from a common ancestor, different species have evolved, each with a different set of adaptive features,
When will divergent evolution occur?
When closely related species diversify to adapt to new habitats as a result of migration or loss of habitat,
What is comparative biochemistry?
The study of similarities and differences in the proteins and other molecules that control life processes. Although these molecules change over time, some important molecules are highly conserved (remain almost unchanged). Slight changes that occur in these molecules can help identify evolutionary lnks.
What are two of the most commons studied molecules in comparative biochemistry?
Proteins and nucelotides.
What does the hypothesis of neutral evolution state?
That most of the variability in the structure of a molecule does not affect its function. This is because most of the variability occurs outside of the molecule’s functional regions.
What are changes that do not affect a molecule’s function called?
‘Neutral‘, since they have no effect on function, their accumulation is not affected by natural selection. As a result, neutral substitutions occur at a fairly regular rate, although that rate is different for different molecules.
What is compared in order to discover how two closely two species are related?
The molecular sequence of a particular molecule (looking at the order of DNA bases or at the order of amino acids in a protein).The number of differences that exist are plotted against the rate the molecule undergoes neutral base pair substitutions (which has been determined through studies). From is information scientists can estimate the point at which the two species last shared a common ancestor. Species that are closely related have the more similar DNA and proteins, whereas those that are distantly related have far fewer similarities. Ribosomal RNA has a very slow rate of substitution, so it is commonly used together with fossil information to determine relationships between ancient species.