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There is overwhelming evidence for the evolution of life on earth.
Objectives
Describe what is meant by evolution.
Analyse the evidence for evolution.
Explain how one species can diverge to become two.
Key words: evolution / speciation / fossil / homologous structures / pentadactyl / gradual divergence / melanism
Understandings
Evolution occurs when heritable characteristics of a species change.
The fossil record provides evidence for evolution.
Selective breeding of domesticated animals shows that artificial selection can cause evolution.
Evolution of homologous structures by adaptive radiation explains similarities in structure when there are
differences in function.
Populations of a species can gradually diverge into separate species by evolution.
Continuous variation across the geographical range of related populations matches the concept of
gradual divergence.
Applications
Development of melanistic insects in polluted areas.
Comparison of the pentadactyl limb of mammals, birds, amphibians and reptiles with different methods of
locomotion.
EVOLUTION
Evolution occurs when heritable characteristics of a species change.
Evolution at its most fundamental level simply describes a change over time.
in living organisms, refers to the heritable characteristics of a species (biological evolution).
genes encode for heritable characteristics which may be transferred between generations as alleles.
,Biological evolution describes cumulative changes that occur within a population between generations.
defined as a change in the allele frequency of a population’s gene pool over successive generations.
Most biologists accept the theory of evolution based on evidence from:
1. Fossils – sequence of preserved organisms matches expectations. Patterns can be seen.
2. Selective breeding – artificial selection can occur.
3. Homologous structures – similar structures (maybe different functions) from common ancestor.
4. Patterns of variation – examples of gradual divergence across a geographical range.
FOSSIL RECORD
The fossil record provides evidence for evolution.
Evidence for evolution demonstrates a change in characteristics from an ancestral form.
the totality of fossils, both discovered and undiscovered, is referred to as the fossil record.
the fossil record reveals the features of an ancestor for comparison against living descendants.
shows that over time changes have occurred in the features of living organisms (evolution).
now supported by radioisotope dating.
Fossils
A fossil is the preserved remains or traces of any organism from the remote past.
preserved remains (body fossils) provide direct evidence of ancestral forms (bones, teeth, shells, etc.).
traces provide indirect evidence of ancestral forms (footprints, tooth marks, burrows and faeces).
Law of Fossil Succession
Fossils can be dated by determining the age of the rock layer (strata) in which the fossil is found.
sedimentary rock layers develop in a chronological order: older layers bottom, new layers on top.
each strata represents a variable length of time that is classified according to a geological time scale (eons,
eras, periods).
Different kinds of organisms are found in rocks of particular ages in a consistent order:
prokaryotes appear in the fossil record before eukaryotes.
ferns appear in the fossil record before flowering plants.
invertebrates appear in the fossil record before vertebrate species.
This ordered fossil succession suggests that newer species evolved as a result of changes to ancestral species.
, Transitional fossils
Fossils provide clues for evolutionary relationships, but the fossil record is incomplete.
fossilisation requires an unusual set of specific circumstances in order to occur, meaning very few organisms
become fossils.
only the hard parts of an organism are preserved, so only fragments of remains are discovered.
with limited fossil data, it can be difficult to discern the evolutionary patterns that result from ancestral forms
(‘missing links’).
Transitional fossils demonstrate the intermediary forms that occurred over the evolutionary pathway taken by a
single genus.
establish the links between species by exhibiting traits common to both ancestors and descendants.
e.g. archaeopteryx, which links the evolution of dinosaurs (jaws, claws) to birds (feathers).
discovery of new fossils show new evolutionary patterns and challenge old assumptions.
SELECTIVE BREEDING
Selective breeding of domesticated animals shows that artificial selection can cause evolution.
Selective breeding is a form of artificial selection by man in a species to produce desired traits in offspring.
breeding members of a species with a desired trait.
the trait’s frequency becomes more common in successive generations.
provides evidence of evolution as targeted breeds can show significant variation in a short period.
Selective breeding of plant crops allowed for new types of foods coming from the same ancestral plant source.
plants of the genus Brassica have been bred to produce different foods by modifying plant sections.
includes broccoli (modified flower buds), cabbage (modified leaf buds) and kale (modified leaves).
Selective breeding of domesticated animals has also resulted in the generation of diverse breeds of offspring.
e.g. horse, cows and dogs.
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