Download Patterns of Evolution

Patterns of Evolution
Year 13 Biology
Revision of Yr 12 Work
 Darwin’s Theory
 Sources of Variation
 Agents that change gene frequencies
Darwin’s Theories
 Believed that species evolved from
ancient forms
 Species did not remain unchanged
throughout time
Darwin’s theories
 REASONING
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Species have high reproductive rates but
selection pressures act to create a struggle
for existence
There is variation in offspring, some better
adapted than other.
Those best adapted will survive longer and
therefore have an increased chance of
passing on their favourable characteristics
Fittest will survive
Natural Selection
Sources of Variation
 Three main sources of variation
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Crossing over
Sexual reproduction/meiosis
Mutations
 The actions of these sources create
variation by ‘mixing’ alleles into new
combination or creating new alleles
Agents that change allele
frequencies
 Genetic Drift
 Mutations
 Gene flow
 Founder Effect
 Natural Selection
Genetic Drift
 A change in allele frequencies of a
population as a result of chance –
random events
 This happens in small populations where
chance alone may play a considerable
role
 Bottleneck and founder effects are two
situations where genetic drift can be
significant.
Mutations
 The ultimate source of variation
 Can change equilibrium in populations
by adding or taking away alleles
therefore changing possible
combinations
 Also provides new material for Natural
Selection
Gene Flow
 Immigration/emigration
 Introduce (or take away) alleles into a
gene pool
 When immigrants arrive from another
population possessing a different gene
pool, new alleles are introduced.
 Can both increase or decrease genetic
variation
Natural Selection
 Sorts genetic variability and accumulates
and maintains favourable genotypes in a
population
 Reduces genetic diversity within the
gene pool but increases differences
between populations
 There are three types of Natural
Selection
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Stabilising, directional and disruptive
Types of Selection
 Stabilising selection - favours the
average
 Directional selection – one extreme
favoured over the other and average
 Disruptive selection – favours both
extremes
The Species Concept
 “A species is a group of actually or
potentially interbreeding natural
populations that is reproductively
isolated from other such groups”
 Boundaries between a species gene
pool can be somewhat unclear. This
leads to a continuous increase or
decrease in characteristics as you move
through a range of a species.
Cline
 A cline is the pattern of variation of
characteristics between adjacent
populations.
 Example:
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The NZ Tomtit shows a clinal variation in
the length of the tail and wing. North
Island tomtits have shorter tails and wings
than the southern tomtits
Ring Species
 When two apparently different species
are joined by a series of geographical
and structural intermediate types.
 Circular cline that results in two ends of
the cline overlapping.

E.g. herring gull and lesser black-backed
gull.
 Adjacent populations can interbreed but
not where the arms of the loop overlap.
Speciation
 The formation of a new species is called
speciation.

Species definition – a group of organisms
that normally interbreed in nature to
produce fertile offspring, and belong to the
same gene pool.
 An important part of forming a new
species is separation of the gene pool of
a species from the gene pool of the
parent population.
Speciation
 Speciation can be separated into two
types:
1. Allopatric
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Is when a species population is separated
by a geographical barrier.
Populations may experience geographic
isolation due to such events as mountain
building, changes in sea level, changes in
river courses.
 Complete pg 251
Speciation
2. Sympatric
 Occurs when a sub-population
becomes reproductively isolated in the
midst of the parent population.
Populations are said to be sympatric if
their ranges overlap.

This can arise from a change in niche or
lifestyle, where mating can be only
between those who have adopted a new
lifestyle or by the sudden appearance of
a new species
 Complete pg 253
HOMEWORK
 Complete Biozone pages
 245- 246 “The species concept”
Reproductive Isolating
Mechanisms
 These prevent populations of related
species in the same area from
interbreeding.
 They may operate before fertilisation
(prezygotic) or after fertilisation
(postzygotic).
Prezygotic (before fertilisation)
 Temporal
 Behavioural
 Structural
 Spatial
 Ecological
 Gamete mortality
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See page 243 of Biozone for details
Postzygotic (after fertilisation)
 Although the sperm and egg may fuse,
there may be other problems along the
path to the development of a fertile adult.
 Hybrid Inviability
 Hybrid Sterility
 Hybrid Disadvantage
Stages in Species
Development
 See page 250 Biozone
 Possible sequence of events in the
development of two new species.
 As time passes the amount of genetic variation
increases and each group becomes more
isolated from the other.
 Geographical barriers, prezygotic and
postzygotic mechanisms act to keep the two
species isolated
Patterns of Evolution
 Evolution may cause related species to
become different (diverge) or unrelated
species to become similar (converge)
 Divergent Evolution – when one
species evolves into two species
 Convergent Evolution – occurs when
largely unrelated species evolve similar
features
Divergent Evolution
 When an ancestral species diverges into
two or more species that eventually
occupy different ecological niches
New species
Ancestral species
New Species
 When many new species are formed it is
called Adaptive Radiation
Adaptive Radiation
 Like divergent evolution but is RAPID
and gives rise to a large number of
species that occupy different niches
 Examples
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Galapagos Finches, NZ Hebe
Homologous Structures
 Structural similarities that suggest
organisms descended from a common
ancestor.
 Used to indicate evolutionary
relationships
 Examples
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Pentadactyl limb: bones of forelimb of air
breathing vertebrates have similar bones
arranged in a comparable pattern eg, bird’s
wings, bat’s wings, dog’s leg, human arm
Convergent Evolution
 Ancestors are different but different
species come to resemble each other
due to similar ecological niches and
natural selection
Analogous Structures
 Arise as a result of convergent evolution.
 Structures are used for the same
purpose in unrelated species, but have
evolved from different origins.
 Examples
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Streamlined body shape in the unrelated
vertebrate groups: fish (sharks), mammal
(dolphin) and bird (penguin)
The eye in humans and the octupus
Other Ideas
 Punctuated Equilibrium
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Instead of a gradual change, species stay
the same for periods of time – most of a
species existence is spent in stasis and
little time is spent in active evolutionary
change.
Other Ideas
 Gradualism
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Assumes populations slowly diverge from
one another by accumulating adaptive
characteristics in response to different
selective pressures.
Gradualism vs. Punctuated
Co-evolution
 Each party in a co-evolutionary
relationship exerts selection pressures
on the other and, over time, the species
become dependent on each other.
 Examples
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Predator-prey relationships, parasite-host
relationships, plants and their pollinators