Download Chapter 32

Chapter 32: Mechanisms of
evolution
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PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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Populations and their gene pools
•
Population
– group of individuals of the same species, usually
occupying a defined habitat
– over one or more generations, genes can be shared
through entire range of population
– asexual populations more difficult to define

•
characterised by similarities in phenotype
Gene pool
– sum of all genes in a population at a given time
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Species
•
Species
– many concepts proposed to define a species
•
Biological species concept
– groups of actually or potentially interbreeding natural
populations which, under natural conditions, are
reproductively isolated from other such groups (definition
proposed by Mayr and others)
•
Other species concepts emphasise different
aspects
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Evolutionary change
•
Microevolution
– change in gene pools
– natural selection

change due to impact of environment
– genetic drift

•
random change
Macroevolution
– change at or above the level of species

speciation
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Genetic variation
•
•
Genetic variation within populations drives
evolution
Variation arises from
– mutation
– recombination
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Mutation
•
Spontaneous or induced change in DNA sequence
– minor (e.g. nucleotide substitutions, deletions)
– major (e.g. chromosome inversions, translocations)
•
Effect of mutation is expressed in phenotype
– neutral

no effect
– disadvantageous

negative effect (reduces fitness)
– advantageous

positive effect (increases fitness)
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Measuring genetic variation
•
Methods of detecting and measuring genetic
variations
– phenotypic frequency
– genotypic frequency
– allele frequency
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Phenotypic frequency
•
Some phenotypic traits allow a population to be
characterised genetically
– variation in phenotype is directly related to genotype
– genetic markers
•
Variations (polymorphisms) in phenotypic trait are
controlled by different alleles
– example: Rhesus (Rh) blood groups in humans

Rh+ (dominant)
 Rh- (recessive)
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Genotypic frequency
•
Where dominance exists, phenotypic frequency
gives incomplete information about allele
frequency
– recessive allele gives rise to phenotype when individuals
are homozygous
– dominant allele gives rise to same phenotype whether
individuals are homozygous or heterozygous
•
Immunological tests identify allele combinations
– distinguish between homozygous and heterozygous
individuals
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Allele frequency
•
Calculate frequencies with which certain alleles
occur
– proportion of total alleles
– does not indicate combinations
p+q=1
where p and q are frequencies of each allele
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Hardy–Weinberg principle
•
Model of relationship between allele and genotypic
frequencies
• Phenotypic frequencies in a population tend to
remain constant at equilibrium values that can be
estimated from allele frequencies
• Hypothetical ideal population
– equilibrium established after one generation
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Hardy–Weinberg equation
•
Allows genotypic frequencies to be calculated from
phenotypic frequencies
– where dominance exists
p2 + 2pq + q2 = 1
– calculate frequencies from q2 (homozygous recessive)
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Assumptions of H–W
•
Individuals mate at random
• The population is so large that it is not affected by
genetic drift
• No mutation
• No migration
• No natural selection
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Microevolution
•
H–W assumption: Individuals mate at random
• Random mating
– trait has no effect on mate choice
•
Assortative mating
– trait has an effect on mate choice
– phenotypically similar mates

positive assortative mating
– phenotypically dissimilar mates

negative assortative mating
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Assumptions of H–W
• H–W assumption: The population is so large that it
is not affected by genetic drift
• Chance of microevolutionary change in a
population’s gene pool
– some alleles are lost
– other alleles become fixed
•
In small populations, the chance of genetic drift is
high
(cont.)
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Assumptions of H–W (cont.)
• H–W assumption: No mutation
• Mutation introduces novel genetic variation and
new alleles
• H–W assumption: No migration
• Migration can change composition of gene pools if
different groups exhibit different allele frequencies
(cont.)
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Assumptions of H–W (cont.)
•
H–W assumption: No natural selection
• Natural selection acts on phenotypes
• Changes frequencies of genotypes that give rise to
those phenotypes
– fitter genotypes appear in greater proportion to less fit
genotypes
•
Moves allele frequencies away from equilibrium
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Natural selection
1. More individuals are produced each generation
than can survive to have offspring themselves
– some individuals die before they reach breeding age
– what determines which die and which survive?
2. Variation exists between individuals in a population
and some of this variation involves differences in
fitness
– fitness is an organism’s ability to survive (viability) and
produce the next generation (fertility)
– some individuals have greater fitness than others
(cont.)
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Natural selection (cont.)
3. Fitter individuals make a relatively greater
contribution to the next generation than the less fit
individuals
– fitter individuals produce more offspring than others
4. Differences in fitness between individuals are
inherited
– reproducing individuals pass on their characteristics to
the next generation
(cont.)
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Natural selection (cont.)
•
•
•
Fitter individuals reproduce more successfully than
less fit individuals
Contribute proportionately more to the next
generation
Cumulative effect over generations
– results in change in gene pool
(cont.)
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32-20
Speciation and species’ concepts
•
•
Speciation is the process by which new species
are formed
Defining the concept of species is complex and no
single species’ concept is universally accepted
–
–
–
–
–
biological species’ concept
taxonomic or morphological species’ concept
recognition species’ concept
evolutionary species’ concept
cohesion species’ concept
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Species’ concepts
•
Biological species’ concept
– ‘groups of actually or potentially interbreeding natural
populations, which are reproductively isolated from other
such groups’
– does not consider morphologically different species that
can interbreed to produce hybrids or asexuallyreproducing species
•
Taxonomic species concept
– species is defined by phenotypic distinctiveness
– members of a species are morphologically alike
– problems with convergence and mimicry
(cont.)
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Species’ concepts (cont.)
•
Recognition species’ concept
– species are groups sharing a common mate recognition
system
– does not consider asexually reproducing species
•
Evolutionary species’ concept
– a species is a lineage of populations delineated by
common ancestry and able to remain separate from other
species
•
Cohesion species’ concept
– species have mechanisms for maintaining phenotypic
similarity, including gene flow and developmental
constraints
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Reproductive isolation
•
•
All species concepts consider reproductive
isolation (prevention of gene flow between
species) to be an important factor in maintaining a
species’ integrity
Reproductive isolating mechanisms inhibit or
prevent gene flow between species
–
–
–
–
–
–
ecological isolation
temporal isolation
ethological isolation
mechanical isolation
gametic isolation
postzygotic isolation
(cont.)
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Reproductive isolation (cont.)
•
Ecological isolation
– species do not hybridise because they occupy different
habitats
•
Temporal isolation
– species do not hybridise because they are not ready to
mate at the same time
– example: two plant species produce flowers at different
times
•
Ethological isolation
– species do not recognise each other as potential mates
because the courtship patterns differ between species
– example: frogs of different species have different mating
calls
(cont.)
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Reproductive isolation (cont.)
•
Mechanical isolation
– species do not hybridise because reproductive structures
differ
– example: differences in pedipalps of male spiders
•
Gametic isolation
– species do not hybridise because sperm are inviable in
female reproductive tract, do not recognise egg of other
species or cannot enter egg
•
Postzygotic isolation
– species may produce hybrids but hybrids are inviable or
are sterile
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Allopatric speciation
•
Populations of ancestral species are split by
geographical barrier
– inhibits migration and disrupts gene flow between
populations
•
•
Divergence of populations due to natural selection
and genetic drift
Reproductive isolation may develop, so if
populations were to be reunited, gene flow would
not be re-established
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Sympatric speciation
•
Sympatric speciation takes place without
geographical separation of populations
• Disruption of gene flow occurs when groups of
individuals become reproductively isolated from
other members of the population
• Polyploidy is a mechanism by which this occurs
– multiple sets of chromosomes
– common in plants
– also found in some animals
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Parapatric speciation
•
•
•
Parapatric speciation occurs in adjacent
populations
Geographical ranges are in contact, but selection
exerts different pressures on populations
Eventually gene flow is interrupted and populations
become reproductively isolated
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Fig. 32.15: Models of speciation
(a)
(cont.)
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Fig. 32.15: Models of speciation (cont.)
(b)
(cont.)
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Fig. 32.15: Models of speciation (cont.)
(c)
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Hybridisation
•
Not all hybrids are inviable or sterile
• Hybrids between species may become
parthenogenetic
– produce young from eggs without fertilisation
•
Avoids problems of chromosome pairing with
mismatched sets of chromosomes
– example: parthenogenetic triploid gecko Heteronotia
binoei formed by two hybridisation events
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Fig. 32.19: Origin of Heteronotia binoei
Copyright © Craig Moritz, University of Queensland
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(cont.)
32-34
Fig. 32.19: Origin of Heteronotia binoei (cont.)
Copyright © Craig Moritz, University of Queensland
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32-35
Molecular evolution
•
•
•
Molecular sequences have diverged from a
common ancestral sequence
Gene duplication and sequence divergence
produces gene families
Homologous genes are derived from a common
ancestral gene
– orthologous genes arise when a species with the
ancestral gene splits into two species
– paralogous genes arise by gene duplication in a line of
descent
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32-36