evidence for evolution Flashcards
developing the theory of evolution
When Charles Darwin was born in 1809, most people in Europe believed, in a literal sense, in the Christian Bible. They believed Bood directly stated ow far in the past this occurred- i Darwin’s day God directly created all life on Barth, including human beings. The
the common belief was that this creation had occurred only a few thousand years before.
In 1831 aboard the HMS Beagle, Darwin read ‘Principles of Geology.
This book was written by his friend Charles Lyell, a Scottish geologist.
He suggested that fossils were actually evidence of animals that had lived millions of years ago. We now have scientific evidence that supports this.
In it Lyell also popularised the principle of uniformitarianism (the concept itself was originally proposed by another Scottish geologist, James Hutton). This is the idea that in the past, the Earth was shaped by forces that you can still see in action today, such as sedimentation in rivers, wind erosion, and deposition of ash and lava from volcanic eruptions. In emphasising these natural processes, he challenged the claims of earlier geologists who had tried to explain geological formations as a result of biblical events such as floods. This concept prompted Darwin to think of evolution as a slow process, one in which small changes gradually accumulate over very long periods of time.
Darwin carried out some of his most famous observations on finches in the Galapagos Islands. He noticed that different islands had differen! finches. The birds were similar in many ways and thus must be closely related, but their beaks and claws were different shapes and sizes.
Through these observations Darwin realised 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 moratipring, passing on its characteristic beak. Over time the finch population on that island would all share this characteristic.
Throughout his trip Darwin sent specimens of organisms back to the
UK for other scientists to preserve and classify. This enabled scientists not only to see specimens first hand but also enabled them to spot characteristics and links between organisms that Darwin had not.
For example, Darwin did not notice that the tortoises (which the Galapagos islands are named after) present on different islands were different subspecies. Before this was pointed out to him he had simply stacked their shells randomly in the hold.
Upon his return to England, Darwin spent many years developing ideas. He also carried out experimental breeding of pigeons to gain direct evidence that his ideas might work.
At the same time as Darwin was developing his ideas, another scientist, Alfred Wallace, was working on his own theory of evolution in Borneo. In 1858 he sent his ideas to Darwin for peer review before its publication. As Wallace’s ideas were so similar to Darwin’s, they proposed the theory of evolution through a joint presentation of two scientific papers to the Linnean Society of London on 1st July 1858.
A year later in 1859, Darwin published ‘On the Origin of Species’. It was in this book that he named the theory that he and Wallace had presented independently as the theory of evolution by natural selection (see Topic 10.7, Adaptations).
The book was extremely controversial at the time. The theory of evolution conflicted with the religious view that God had created all of the animals and plants on Earth in their current form, and only 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.
Darwin’s theory split the scientific community before his idea became generally agreed. Darwin’s theory of evolution is now widely accepted, however, even today, debate with religious groups continues.
evidence for evolution
Scientists use a number of sources to study the process of evolution.
These include:
• palaeontology - the study of fossils and the fossil record
- comparative anatomy - the study of similarities and differences between organisms’ anatomy
• comparative biochemistry - similarities and differences between the chemical makeup of organisms.
palaeontology
Fossils are 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 eras, the most recent layer being found on the top. Within the different rock strata the fossils found are quite different, forming a sequence from oldest to youngest, which shows that organisms have gradually changed over time. This is known as the fossil record.
Evidence 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.
The fossil record is, however, not complete. 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.
comparative anatomy
As the fossil record is incomplete, scientists look for other sources of evidence to determine evolutionary relationships. Comparative anatomy is the study of similarities and differences in the anatomy of different living species.
homologous structures
A homologous structure is a structure that appears superficially different (and may perform different functions) in different organisms, but has the same underlying structure. An example is the pentadactyl limb of vertebrates.
Vertebrate limbs are used for a wide variety of functions such as running, jumping, and flying. You would expect the bone structure of these limbs in a flying vertebrate to be very different from that in a walking vertebrate or a swimming vertebrate. However, the basic structures of all vertebrate limbs are actually very similar (Figure 6) - the same bones are adapted to carry out the whole range of different functions. An explanation is that all vertebrates have evolved from a common ancestor, therefore vertebrate limbs have all evolved from the same structure.
The presence of homologous structures provides evidence for divergent evolution. This describes how, from a common ancestor, different species have evolved, each with a different set of adaptive features. This type of evolution will occur when closely related species diversify to adapt to new
habitats as a result of migration or loss of habitat.
comparative biochemistry
Comparative biochemistry is the study of similarities and differences in the proteins and other molecules that control life processes. Although these molecules can change over time, some important molecules are highly conserved (remain almost unchanged) among species. Slight changes that occur in these molecules can help identity evolutionary links. Two of the most common molecules studied are cytochrome c, a protein involved in respiration, and ribosomal RNA.
The hypothesis of neutral evolution states 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. Changes that do not affect a molecule’s function are 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.
To discover how closely two species are related, the molecular sequence of a particular molecule is compared. (Scientists do this by 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 this 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.
