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IB Biology: Evolution Summary
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Evolution (Unit 5) Summary
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5.1 EVIDENCE FOR EVOLUTIONEvolution at its most fundamental level simply describes a change over time. In living
organisms this change refers to the heritable characteristics of a species (biological
evolution).
Heritable characteristics are encoded for by genes and may be transferred between
generations as alleles. Hence biological evolution describes cumulative changes that
occur within a population between one generation and the next.
A concise definition for biological evolution is: a change in the allele frequency of a
population’s gene pool over successive generations
The fossil record provides evidence by revealing the features of an ancestor for
comparison against living descendants. 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 and include bones, teeth, shells, leaves, etc. Traces
provide indirect evidence of ancestral forms and include footprints, tooth marks,
burrows and faeces (coprolite)
The totality of fossils, both discovered and undiscovered, is referred to as the fossil
record. The fossil record shows that over time changes have occurred in the features
of living organisms (evolution).
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, such that
lower layers are older and newer strata form 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, indicating a sequence of development
★ 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 chronological sequence of complexity by which characteristics appear to
develop is known as the law of fossil succession. This ordered succession of fossils
suggests that newer species likely evolved as a result of changes to ancestral
species.
While fossils may provide clues as to evolutionary relationships, it is important to
realise that 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 typically preserved, meaning usually
, 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. They establish the links between
species by exhibiting traits common to both an ancestor and its predicted
descendents. An example of a transitional fossil is archaeopteryx, which links the
evolution of dinosaurs (jaws, claws) to birds (feathers). As new fossils are
discovered, new evolutionary patterns are emerging and old assumptions are
challenged.
Selective breeding is a form of artificial selection, whereby man intervenes in the
breeding of species to produce desired traits in offspring. By breeding members of a
species with a desired trait, the trait’s frequency becomes more common in
successive generations. Selective breeding provides evidence of evolution as
targeted breeds can show significant variation in a (relatively) short period.
Selective breeding of plant crops has allowed for the generation of new types of
foods from the same ancestral plant source. Plants of the genus Brassica have been
bred to produce different foods by modifying plant sections through artificial
selection. This includes broccoli (modified flower buds), cabbage (modified leaf
buds) and kale (modified leaves).
Comparative anatomy of groups of organisms may show certain structural features
that are similar, implying common ancestry. Anatomical features that are similar in
basic structure despite being used in different ways are called homologous
structures. The more similar the homologous structures between two species are,
the more closely related they are likely to be.
Homologous structures illustrate adaptive radiation, whereby several new species
rapidly diversify from an ancestral source, with each new species adapted to utilise a
specific unoccupied niche.
Within a population of any given species there will be genetic variation (i.e. variation
which is inheritable). Typically this variation will be continuous and follow a normal
distribution curve as the rate of change is gradual and cumulative.
If two populations of a species become geographically separated then they will likely
experience different ecological conditions. Over time, the two populations will adapt
to the different environmental conditions and gradually diverge from one another.
The degree of divergence will depend on the extent of geographical separation and
the amount of time since separation occurred. Populations located in close proximity
that separated recently will show less variation (less divergence).Distant populations
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