33-1 copyright 2010 mcgraw-hill australia pty ltd powerpoint slides to accompany biology: an...

33-1Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and SaintSlides prepared by Karen Burke da Silva, Flinders University

Chapter 33: Mechanisms of evolution


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


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


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


Genetic variation• Genetic variation within populations drives

evolution• Variation arises from

– mutation– recombination


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)


Measuring genetic variation• Methods of detecting and measuring genetic

variations– phenotypic frequency– genotypic frequency– allele frequency


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)


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


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


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


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)


Hardy–Weinberg assumptions• Individuals mate at random• The population is so large that it is not affected by

genetic drift• No mutation• No migration• No natural selection


Microevolution

Hardy–Weinberg 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


Genetic drift

Hardy–Weinberg 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


Mutation and migration

Hardy–Weinberg assumption: No mutation

• Mutation introduces novel genetic variation and new alleles

Hardy–Weinberg assumption: No migration

• Migration can change composition of gene pools if different groups exhibit different allele frequencies


Natural selection

Hardy–Weinberg 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

Question 1:

For a trait controlled by a single locus with two alleles which shows incomplete dominance, how many phenotypes are possible?

a) One

b) Two

c) Three

d) Four

e) Five



Natural selection (cont.)

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


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


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


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


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


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


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


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 (behavioural) 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


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

Question 2

Which of the following is not an example of a pre-mating reproductive isolating mechanism?

a) Temporal isolation

b) Mechanical isolation

c) Use of different sex attractant pheromones in butterflies

d) Ecological isolation

e) Incompatible sperm and ova



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


Fig. 33.17: Models of speciation

(a)


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


Fig. 33.17: Models of speciation (cont.)

(b)


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


Fig. 33.17: Models of speciation (cont.)

(c)


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


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

Summary• Evolution is the process of genetic change in a

population over time • Evolution results in individuals who are able to

survive and reproduce in their environment well enough to produce viable offspring

• Biological evolution requires heritable variation and a force to act on this variation

• Natural selection occurs when individuals with heritable differences show differences in survival and reproduction.


Summary (cont.)• Heritable traits from fitter individuals become more

numerous in a population over time• Natural selection may act differently on males and

females in the same population• Co-operation between individuals in a population

can increase their inclusive fitness• Speciation usually involves physical, ecological or

temporal isolation of populations, which diverge over time


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