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EvolutionSarah Jones

The processes that have transformed life on Earth from its earliest forms to the vast diversity that characterises it today.

Evolution

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“As many more individuals of each species are born than can possible survive, and as, consequently, there is a frequently recurring struggle for existence, it follows that any being, if it vary however slightly in any manner profitable to itself, under the complex and sometimes varying conditions of life, will have a better chance of surviving and thus be naturally selected. From the strong principle of inheritance any selected variety will tend to propagate its new and modified form.” Charles Darwin

Charles Darwin• Wrote in 1859 “On the Origin of Species

by Means of Natural Selection”

• Two main points:

– Species were not created in their present form, but evolved from ancestral species.

– Proposed a mechanism for

evolution - Natural Selection.

Darwin’s Five Points1. Population has variations.

2. Some variations are favourable.

3. More offspring are produced than survive.

4. Those that survive have favourable traits.

5. A population will change over time.

Natural SelectionIndividuals with favourable traits are more likely to leave more offspring better suited for their environment.

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Variation• Natural selection happens because there is

variation i.e. natural differences in populations.

• Variation is caused by differences in genes.

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Speciation• When one species splits into two or more

separate species.• Speciation is why we see biodiversity on Earth.• Steps of speciation

1. Variation2. Isolation3. Selection

• Variation – within a populations is required before speciation can occur.

• Isolation – required for a new species to occur e.g. Grand Canyon ground squirrel.

• Selection – natural selection affects genotype. Changes prevent groups from breeding in the future.

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Evidence For Evolution

Palaeontology• The study of fossils — any form of preserved

remains thought to be derived from a living organism.

• Fossils are an important source of evidence for evolution.

Geophysical evidence suggests that geographical regions and climatic conditions have varied throughout the earth’s history, and these changes would have favoured a mechanism for evolutionary change.

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Ecological considerations also support this. Plants appeared on land before animals, and insects before insect-pollinated plants.

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Evidence from fossils• The lowest rock layers are usually the oldest

• The oldest rock layers contain the oldest fossils.

• The rock layers that formed later contain more complex kinds of organisms.

• The variety of fossils increases in the upper, more recent layers of rock.

• No fossil records exist of modern, living plants and animals.

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Relative age - Relative means that we can determine if something is younger than or older than something else. Relative time does not tell us how old something is, all we know is the sequence of events. For example: the sandstone in this area is older than the limestone.

Absolute age - Absolute age means that we can more or less precisely assign a number (in years, minutes or seconds) to the amount of time that has passed. We can say how old something is. For example: The sandstone is 300 million years old.

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CorrelationEarly fossil examiners were able to correlate or match layers of sedimentary rock merely by looking at the fossils they contained (fossils in a particular rock were different from rocks above and below).

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Some plant and animals only lived a short time in geological history, yet had a wide spread distribution. Known as index fossils – used to correlate layers of rock and therefore determine the age of the layer.

Biogeography – patterns of distribution• Distribution gives clues to the evolutionary

history of organisms and of the Earth itself.

• A major cause of speciation in most groups is thought to be geographic fragmentation of ancestral species e.g. continental drift and consequent isolation of populations.

Comparative Anatomy• Comparative anatomy is the study of similarities

and differences in the anatomy of organisms.

• Two major concepts of comparative anatomy are:

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Homologous and Analogous Structures

Homologous Structures• Homologous structures are those that are

similar in structure and development but which may have different functions.

• The wing of a bird, the flipper of a whale and the leg of a dog all possess the structural plan of the pentadactyl (5-digit) limb although their functions differ.

• Homology, therefore, implies common ancestral origins and suggests that differences in structure have evolved in response to different environmental conditions.

• This is called divergent evolution.

Analogous Structures• Analogous structures have a similar function but

no structural relationship. The wing of an insect and the wing of a bird serve the same function — flight—but are structurally dissimilar.

• This suggests that these two groups have different ancestral origins.

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These structures are regarded as examples of convergent evolution, whereby structures with different origins have become adapted to a common function.

• Parallel evolution occurs when related species evolve similar features independently.

• For example, within eucalyptus, a number of species have evolved a white, waxy coating on their leaves, which protects them from frost damage at high altitudes or from water loss in dry conditions.

Vestigial OrgansAn organ that was once useful in an animal’s evolutionary past, but now has no apparent nor predictable function e.g. rudiments of pelvis and hind limbs in snakes, wings on many flightless birds.

Comparative Embryology

Limitations of fossils• There are many gaps in the fossil record. This

is because:

• dead organisms decompose rapidly.

• dead organisms are eaten by scavengers.

• soft-bodied organisms do not fossilise easily.

• only a small fraction of organisms die in conditions favourable to fossilisation.

• only a fraction of the fossils have been unearthed.

Human Evolution

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