32-1 copyright 2005 mcgraw-hill australia pty ltd ppts t/a biology: an australian focus 3e by knox,...
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32-1Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
Chapter 32: Mechanisms of evolution
32-2Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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
32-3Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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
32-4Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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
32-5Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
Genetic variation
• Genetic variation within populations drives evolution
• Variation arises from – mutation– recombination
32-6Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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)
32-7Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
Measuring genetic variation
• Methods of detecting and measuring genetic variations
– phenotypic frequency– genotypic frequency– allele frequency
32-8Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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)
32-9Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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
32-10Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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
32-11Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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
32-12Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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)
32-13Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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
32-14Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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
32-15Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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.)
32-16Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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.)
32-17Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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
32-18Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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.)
32-19Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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.)
32-20Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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.)
32-21Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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
32-22Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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 asexually-reproducing species
• Taxonomic species concept– species is defined by phenotypic distinctiveness– members of a species are morphologically alike– problems with convergence and mimicry
(cont.)
32-23Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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
32-24Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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.)
32-25Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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.)
32-26Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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
32-27Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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
32-28Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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
32-29Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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
32-30Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
Fig. 32.15: Models of speciation
(cont.)
(a)
32-31Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
Fig. 32.15: Models of speciation (cont.)
(cont.)
(b)
32-32Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
Fig. 32.15: Models of speciation (cont.)
(c)
32-33Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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
32-34Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
Fig. 32.19: Origin of Heteronotia binoei
Copyright © Craig Moritz, University of Queensland(cont.)
32-35Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
Fig. 32.19: Origin of Heteronotia binoei (cont.)
Copyright © Craig Moritz, University of Queensland
32-36Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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