5 Evolution: On The Origin of Species
5 Evolution: On The Origin of Species
5 Evolution: On The Origin of Species
5 Evolution
Evolution is the process of change in all forms of life over generations, and evolutionary biology is
the study of how evolution occurs. Biological populations evolve through genetic changes that
correspond to changes in the organisms' observable traits. Genetic changes include mutations,
which are caused by damage or replication errors in organisms' DNA. As the genetic variation of
a population drifts randomly over generations, natural selection gradually leads traits to become
more or less common based on the relative reproductive success of organisms with those traits.
The age of the Earth is about 4.54 billion years. The earliest undisputed evidence of life on Earth
dates at least from 3.5 billion years ago at the beginning of the Archean Eon after geological
crust started to solidify, following the earlier molten Hadean Eon.
It is estimated that more than 99 percent of all species, amounting to over five billion species
that ever lived on Earth are extinct. Estimates on the number of Earth's current species range
from 10 million to 14 million, of which about 1.2 million have been documented and over 86
percent have not yet been described.
Evolution does not attempt to explain the origin of life (covered instead by abiogenesis), but it
does explain how early life-forms evolved into the complex ecosystem that we see today. Based
on the similarities between all present-day organisms, all life on Earth is assumed to have
originated through common descent from a last universal ancestor from which all known
species have diverged through the process of evolution.
The modern understanding of evolution began with the 1859 publication of Charles Darwin's
“On the Origin of Species”. In addition, Gregor Mendel's work with plants helped to explain the
hereditary patterns of genetics. Fossil discoveries in palaeontology, advances in population
genetics and a global network of scientific research have provided further details into the
mechanisms of evolution. Scientists now have a good understanding of the origin of new species
(speciation) and have observed the speciation process in the laboratory and in the wild.
Evolution is the principal scientific theory that biologists use to understand life and is used in
many disciplines, including medicine, psychology, conservation biology, anthropology, forensics,
agriculture and other social-cultural applications.
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Fifth Year Natural Science Evolution
The need to understand the patterns is a practical necessity, to be able to pass on knowledge of
safe and unsafe foods and medicines for example. Starting in ancient Greece philosophers tried
to organise and explain the patterns of diversity. Many put them on a scale of lowest to highest
forms, with plants at the bottom, able only to grow, animals in the middle, being able to move
and humans at the top, being able to reason (well most of them anyway). This was later called
the Great Chain of Being. Later it was adapted to the Christian view of the cosmos, with God at
the highest level and as the creator.
The Great chain was not very useful at classifying the diversity of life though, for example all
plants are on the lowest step. In the 1600’s naturalists started to develop systematic ways to
classify species. These were refined by Carolus Linneus (1707 – 1778) who developed a system
(Linnean classification) of nested groups or taxa. In this system:
Group Humans
Class mammals
Order primate
Family hominidae
Genus homo
Species sapiens
This is still used by biologists today, along with the Cladistic system.
The second question, how did the patterns develop was studied by other naturalists by looking
back over life’s history. They realised that animals and plants could leave a trace in rocks as
fossils. Nicolaus Steno (1638 – 86), a Catholic bishop, studied anatomy of animals and fossils of
marine life he found on a mountain, and concluded that the rock must have once been covered
by ocean, and sediment had covered the bodies of dead animals. Thus he produced the idea of
stratigraphy, with the oldest rocks underneath the younger rocks. Although he was a believer in
the biblical earth he was able to introduce the idea that life and the planet had a history filled
with change, and a record of that could be found in the Earth itself.
These ideas were built on by the Scotsman James Hutton who developed the principle of
Uniformism, stating that natural processes that operated in the past are the same as the ones
that operate today. This implied that the creation of mountains, for example, required processes
to act over a very long period of time, called deep time. This deep time was of a different scale to
the thousands of years calculated from the bible, a scale that is difficult to fully comprehend.
Hutton’s idea was developed further by Charles Lyell, a geologist and a teacher of Darwin, to the
idea of Gradualism where the enormous changes required were the result of many many tiny
changes and not large catastrophic events.
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Fifth Year Natural Science Evolution
The study of anatomy in the 1600’s William Patey (1743 – 1805) wrote a book “Natural
suggested that bodies were like Theology” where he asked his readers to imagine
living machines, made of pumps, walking in the countryside and finding a stone, they
engines, filters and channels would consider it as a natural part of the surroundings.
exquisitely adapted to its particular But instead, finding a watch they would conclude that it
way of life. The complexity of the could not form by accident, it would require a designer.
animal matched its position on the This book was read by Darwin and he was impressed by
Great Chain of Being, and was its arguments. He, however, went on to find another
strongly suggestive of a designer. atheistic mechanism for anatomical complexity.
According to fixism living species did not change and could be classified from simple to
using the Great Chain of Being.
One of the earliest evolutionary thinkers was Georges-Louis LeClerc de Buffon. Although he did
not accept the hypothesis of evolution he studied the natural world and developed the fields of
comparative anatomy and biogeography. With this he recognized that populations could change
over time.
Steno’s earlier work that fossils are the remains of living things opened up the science of
palaeontology. It became clear that some fossil species were different from living species, leading
to the idea of extinction. Since this was difficult to reconcile with The Great Chain of Being it met
a lot of resistance. The discovery by Mary Anning (1799 – 1847) of Ichthyosaurs and
Plesiosaurus closed the debate, and Hutton’s ideas of uniformism and deep time became
mainstream, however why species disappeared and emerged was fiercely debated.
Georges Cuvier (1769-1834) , who studied the anatomy of animals, opposed Buffon’s idea that
life evolved and proposed the idea of Catastrophism.
The first real evolutionist was Jean-Baptiste de Lamarck, an expert on plants and animals. He
concluded that the diversity of life was the product of evolution and produced a detailed
hypothesis of how it happened.
He argued that life was driven from simplicity to complexity and that humans descended from
microbes. In this way Lamarck held on to the concept of the Great Chain of Being, where lower
forms become higher forms. He thought that microbes were being spontaneously generated all
the time, and bacteria were just the newest arrivals.
Lamarck also believed that animals and plants could adapt to their environment. This is known
as the theory of use and disuse – parts of the body will vary in size and efficiency in proportion
to how much they are used by the organism concerned. Most importantly these changes would
be passed down to its offspring.
Lamarck is best known for his Theory of Inheritance of Acquired Characteristics, first
presented in 1801: If an organism changes during life in order to adapt to its environment,
those changes are passed on to its offspring.
Unfortunately for Lamarck, Cuvier’s study of animals seemed to show that there large groups
with no intermediates to join them and Lamarck died in obscurity, his ideas rejected.
Exercises:
1. What is meant by deep time?
2. What is comparative anatomy and why might it be important?
3. What is the main principle behind catastrophism?
4. Choose one of the concepts and one of the people in bold in the text. Do some research
and write a brief paragraph about the topic / person.
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Fifth Year Natural Science Evolution
The Beagle travelled from England to South America in 1831. During his voyage, Darwin
collected fossils of extinct animals. He trapped birds and collected barnacles. He observed the
ecological complexity of the jungles of Brazil. Darwin also learnt a great deal about geology in
South America. He recognised the layers of rock that has gradually formed and were then
reworked into mountains and Valleys. He experienced an earthquake in Chile, and he observed
that the shoreline had been lifted a few metres as a result. When Darwin set out on the Beagle
one of the books he took with him was the first volume of the “principles of geology” written by
Charles Lyell. Lyell made the provocative argument that the Earth's landscape had been created
not by gigantic catastrophies but by a series of many small changes (uniformitarianism). During
the earthquake in Chile, Darwin saw first-hand one of the small changes taking place, and
through his travels Darwin became a passionate “Lyellian”.
Darwin noticed the variety and uniqueness of the plants and animals he studied on the
Galapagos Islands, and these proved to be very important in his early evolutionary thoughts. The
Galapagos Islands consist of over 20 islands that lie in the Pacific Ocean about 1000 km to the
west of Ecuador, and in these islands are almost as synonymous with evolution in the public
psyche as the name of Charles Darwin.
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Fifth Year Natural Science Evolution
Darwin did not realise the importance of his observations until he returned to England in 1836.
He had collected a number of birds that had dramatically different beaks. Some had massive
beaks that suited crushing seeds, others had slender needle-like beaks for feeding on cactus
plants. Darwin assumed that he he found species of blackbirds, wrens and finches. He was very
surprised when, after his return to England, an ornithologist recognized tham all as finches., now
known as Darwin's finches. There are 13 species of finch on the Galapagos Islands and these are
unique to the islands and are not found anywhere else in the world. Furthermore, some of these
features are only found on one island within the Galapagos group. What could have caused this?
Darwin was unconvinced by Lamark’s vague mechanism and wanted a simpler, testable process
and so the distribution of finches (and that of other organisms unique to the islands, including
the giant tortoises) was essential in allowing him to propose theory of evolution by natural
selection. The explanation of the Finches distribution will be described later.
His published work in 1839 (Journal of Researches) outlined the travels of the Beagle and its
discoveries, but mentioned very little about his evolutionary theory. From the early to mid
1840s, the geological data built up during the peak of expedition was published as part of an
official record, but yet again nothing of evolutionary theory entered the wider domain.
Darwin's research and original thinking was stimulated during this time by his friendship with
several eminent scientists that included Charles Lyell, an outstanding and far thinking geologist,
and the Botanist Joseph hooker. Darwin knew that Lyell had a very low opinion of Lamark and
the theory of evolution and so he worked to answer all the possible objections that he might
have about his own theory.
Darwin spent much of the late 1840s and 1850s researching barnacles, small marine
crustaceans commonly found on rocky Shores in the intertidal zone, work that was published
during the of the 1850s and established him as a respected researcher.
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Initially Darwin’s theory was supported strongly by some people, Thomas Huxley, or “Darwin’s
bulldog” for one, but rejected fiercely by others. Darwin himself did not debate the issue and
continued his work as a naturalist. Slowly the thery was accepted, and when Darwin’s second
book on Man came out it generated less controversy.
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Homolgous Characteristics are similar in two or more species because they are inherited
from a common ancestor.
What accounts for this combination of differences and similarities? Some anatomists in the mid
1800s argued that each species were created according to an archetype, a fundamental plan to
which some variations could be added. Darwin prefer a simpler, less transcendental explanation:
seals, bats, and humans all shared a common ancestor but had limbs with wrists and digits. That
ancestor gave rise to many lineages. In each of them the limbs evolved, yet the underlying legacy
of our common ancestor survived.
Darwin's case for descent with modification was strengthened by the fact that many homologies
are found together in the same groups of species. Bats, humans and seals don't just share limbs
for example. They also have hair and the females of each species secrete milk to nurture their
young. Linnaeus had use these traits to classify humans that and seals as members of the same
category: all three species are mammals. Darwin argued that the very fact that we can classify
species in this way is consistent with the idea that they evolved from a common ancestor.
Although new traits can evolve in different lineages each species descends from an ancestral
mammal. That mammal, Darwin argued, shared an even older ancestry with other animals. For
example, we humans share many characteristics with fishes. We have eyes with the same
arrangement of lenses, retinas, and nerves. We have skulls, livers, and many other organs in
common.
Of course, we are different in some important ways. Just about all vertebrate on land have lungs.
so do vertebrates that have gone back to the ocean, such as whales and seals. some fishes have
lung like structures for breathing. but all fish have gills, which let them draw in dissolved oxygen
from water.
In some cases the homologies are clear when the animals are still embryos, not when they are
adults. A human embryo at first develops blood vessels in the same pattern as seen in fish gills,
later they are modified. Darwin argued that these are homologies from a common ancestor.
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Fifth Year Natural Science Evolution
Darwin was influenced by the writings of Maltus “An essay on the Principle of Population” where
he said that helping the poor is pointless as they will always eat more than a nation can grow.
Luckily this does not seem to be true for humans, but Darwin realized that this must be true in
the natural world. Also who survives is not just down to chance.
He also realised that individuals are different – selective breeding by choosing an individual with
a specific trait and breeding from that animal is a common way of producing domestic animals.
Darwin’s theory had no mechanism to explain how traits could be inherited, although Mendel
made his discoveries in the 1860’s Darwin did not know of his work. Only in the 1950‘s did
scientists understand how the two theories worked together as molecular biology.
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Fifth Year Natural Science Evolution
In the Linnean classification system a species is the most detailed level of classification and is
based on physical characteristics.
A species is giving a taxonomic name when the type specimen is described formally in a
publication that assigns it a unique scientific name, there is usually also a type specimen
showing the differences from other known species. But what is a species?
Most modern textbooks make use of Ernest Myers definition known as the biological species
concept . This defines a species as a group all groups of actually or potentially interbreeding
natural populations which are reproductively isolated from other such groups.
However there are problems with Myers approach, for example when scientists do not know
whether to morphologically similar groups of organisms are capable of interbreeding, this is the
case in all extinct life forms as breeding experiments are not possible, or when hybridization
permits substantial gene flow between species.
One example of the finch is the ground finch which is very similar to the finches found on the
mainland of South America. It has a typical finch like bill used for crushing seeds. Other finches
are unique to the Galapagos Islands and include the insectivorous finch that has a curved bill
and feeds on insects, and the cactus finch which has a long straight bill for getting nectar from
the numerous prickly pear cactus plants growing on the islands. Other species include the
Woodpecker Finch and the Warbler Finch that feed in similar ways to true woodpeckers and
Warblers respectively.
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Fifth Year Natural Science Evolution
There are two factors that have been critical in allowing the finches to evolve as they have into a
number of different species on the island:
1. There are very few other birds on the islands. consequently as the Finches evolved into a
range of types that eventually became species, they did not have other competitors. for
example, if there had been a large number of other species of insectivorous Birds the
Finches that slowly developed suitable beats for this type of food would have been
unlikely to be able to compete successfully with them, particularly in the early stages of
adaption. Accordingly, evolution in this Direction would have been stamped out.
2. Finches are generally not very good flyers. as a result, there was not a continuous
stream of new arrivals from the mainland South America, and this allowed the
Galapagos finches to evolve in isolation.
The evolution of the finches on Galapagos is an excellent example of adaptive radiation, the
process of a range of species rapidly evolving to fill the available ecological niches available.
Adaptive radiation is most likely to occur when there is relatively little competition from similar
species and or when there is a range of habitats available to be exploited. The Galapagos Islands
provided both these criteria in abundance.
This example also helps to explain why it is Darwin who gains most of the credit for the
development of each of evolutionary theory as opposed to Wallace, Darwin provided much more
evidence to support his ideas.
Within each species some organisms may be better adapted than others to prosper, even though
within a species that differences between individuals will be relatively small. For example, not
all the young starlings that hatch in the nests that exists in the eaves of many houses will survive
very long. The young of most birds do not exhibit much in the way of altruistic responses
towards their siblings, most act only for themselves in the battle for survival. Accordingly, the
young that hatch slightly earlier and hence have size and developmental advantages over some
of their siblings tend to be more effective in taking food from the foraging parents. As a result of
this competition, the stronger individuals are more likely to survive, often at the expense of the
weaker ones. This competition for survival, with the result that the better-equipped individuals
survive, is the cornerstone of Charles Darwin's theory of natural selection.
This preservation of favorable variations and the rejection of injurious variations, I call natural
selection.
Charles Darwin, on the Origin of Species by means of natural selection, 1859
Darwin described examples of the end product of natural selection, such as the Galapagos
finches and other unique species like the giant turtles found on the islands, as evidence for his
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theory. However he could not show natural selection happening in real time, nor could he
explain the mechanism by which it worked.
Exercise
1) Research one of the following examples of natural selection in action and explain how
and why it is an example of natural selection:
Antibiotic resistance in bacteria
The peppered moth
Directional selection
A more obvious form of selection is when it is apparent that there is a change in the species
involved full stop this type of selection is referred to as directional selection. the key underlying
feature with directional selection is that average individuals are not favoured or selected for but
the best adapted are closer to one of the extremes of variation. this invariably happens because
of a change in the environment full stop insect resistance to pesticides is an interesting example
of considerable economic importance. Over 500 species of insects a population that are resistant
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Fifth Year Natural Science Evolution
to at least one pesticide full stop when and where pesticides are used, the frequency of pesticide
resistant insects in a particular population rapidly increase until a very high proportion of all the
insects in a particular population are resistant full stop this is due to the resistant form having a
very strong selective advantage over the non-resistant form, the latter being quickly eliminated
when the pesticides is applied. However, in these populations of the insect concerned where the
pesticides are not applied, the resistant forms remain at a very low frequency in the population
as a whole as they are up to 10% less fit than the normal non resistant form in this environment.
Traits that provide fitness, and reproductive success, in some settings invariably are costly in
other environments.
Disruptive selection
The third type of natural selection, disruptive selection, can be explained by the following
theoretical example. In a particular environment the average or mean size of a species of rabbit,
or other herbivore, ceases to be the most favoured in terms of natural selection; for some
reason both extremes now have a selective advantage. This change may be related to a new
species of predators migrating into the area. If the largest hard herbivores, also being the
quickest, were more able to escape the new predator, then being large would be an obvious
advantage. Similarly, if the smallest herbivores, although being slow on the move, were clumps
of grass, or hide in small crevices, and escape predation by this method, this would also be an
advantage. However, if the medium sized herbivores were not particularly quick, or small
enough to hide, the effect would be that these animals are more likely to be killed. This example
shows the main features of disruptive selection; it is the extremes of the characteristic
concerned that are favoured and not the average or median values. In the example used above,
the resultant population becomes bimodal, it has two peaks, which summarise summarises the
three types of natural selection discussed.
The example of Darwin's finches is one of disruptive selection in action. The finches the first
reach the Galapagos Islands from South America were typical of the finches that lived in South
America, that is finches with short straight bills that are used for crushing seeds. As the finch
population expanded on the islands, the competition for one type of food that could be eaten
seeds almost certainly became quite intense. Accordingly as variations appeared naturally in
terms of bill size and shape, these variations have an advantage as the finches involved would
use the food resource that other finches couldn't. Therefore, in this particular environment, at
that particular time, the extremes in bill size and shape are favoured more than the average.
In summary, while stabilising selection is a very common biological feature that surrounds us in
everyday life, directional and disruptive selection are less frequent, and are associated normally
with environmental change, and are much more important in driving the process of evolution
and in the formation of new species.
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Fifth Year Natural Science Evolution
the differences between the two plants is due to difference in the plants environment and most
pacifically due to the amount of light available. To make this I totally scientifically valid
experiment, we could have used to geranium plants that were genetically identical, that is
produced from cuttings, to ensure that any differences produced could only have been
environmental. Darwin understood the importance of variation and he was aware that variation
provided the raw material of which natural selection would have its effects, but I stated earlier
he did not know the nature of it- he did not know how an organism’s characteristics will control
nor did he know how the blueprint for the development of these characteristics passed from one
generation to the next. To gain a greater understanding it is important to introduce the molecule
read itself DNA.
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