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Darwin’s Theories
Believed that species evolved from ancient forms
Species did not remain unchanged throughout time
Darwin’s theories
REASONING 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 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 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: 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 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
Speciation2. 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
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
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 speciesAncestral 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 Galapagos Finches, NZ Hebe
Homologous Structures
Structural similarities that suggest organisms descended from a common ancestor.
Used to indicate evolutionary relationships
Examples 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 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 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 Assumes populations slowly diverge from
one another by accumulating adaptive characteristics in response to different selective pressures.