BIO1M03 - Week 4 Lecture Notes

Lecture 1 Content:

Male-Male competition

  • Primary cause of sexual selection instead of mate choice

    • Eg. Elephant Seals - Compete to defend beaches w/ gentle slopes for females

    • Establish territories that they defend (fight w/ other males to win more matings w/ females)

    • Larger - Favoured —> more offspring (allele increases in freq)

    • Variation in reproductive success higher in males, lower in females —> sexual selection more intense in males

  • Sexual Dimorphism: Trait that differs between males and females of same species

Speciation

  • Gene flow stops, allele freq isolated

  • Populations diverge

    • Evolve independently

    • Mutation, natural selection, genetic drift

  • Genetic Divergence —> Speciation

  • Splitting event, creates 2 or more distinct species from ancestral species

    • Gradual or abrupt

Species

  • Species: evolutionary independent population/group of populations

    1. Biological Species Concept

      • Reproductive Isolation

      • Lack gene flow between populations

      • Cannot interbreed or produce fertilel offspring

        1. Prezygotic Isolation: prevented from mating

        2. Postzygotic Isolation: hybrid offspring dont survive

      • Disadvantages:

        • Fossils, asexual reproduction,geographically separated

    2. Morphological Species Concept

      • differ in morphological features

      • independent & isolated from gene flow

      • Widely applicable

      • Disadvantages:

        • Polymorphic species classified as more than one species

        • cannot identify cryptic species that differ in non-morphological traits

        • Subjective

    3. Ecological Species Concept

      • Exploit single set of resources

      • same range of environmental tolerances

      • same predators & parasites

      • natural selection & adapations

      • more useful for asexual

      • Disadvantages:

        • Cannot be defined by reproductive isolation

        • maintained by different ecological conditions

    4. Phylogenetic Species Concept

      • based on evolutionary history

      • Common ancestry

      • Monophyletic group —> ancestral population + all descendants (clade/lineage)

        • Identified by synapomorphies: shared traits from common ancestor & descendants

          • Missing in distant ancestor

      • Species = smallest monophyletic group

      • Advantages:

        • Any population

        • logical - different synapomorphies due to lack of gene flow

      • Disadvantages:

        • available for only tiny set of populations

        • would lead to recognition of many more species other than species concepts

Lecture 2 Content:

SPECIATION

  • Subspecies: populations

    • Live in discrete geographicareas

    • distinguishing features

    • not distinct enough to be considered a separate species

    • can interbreed if geographical barriers are removed

  • Dusky seaside sparrow:

    • 6 subspecies physically isolated

    • 6 males remailed in one sub species

    • breed males w/ females of nearby species

      • Hybrid offspring used to reestablish population

      • Preserving as much genetic diversity as possible

    • Phylogenetic analysis: 2 monophyletic groups exist

      • Genetically indistinguishable from other Atlantic Coast Sparrows, diff from Gulf Coast lineage

Isolation and Divergence in Allopatry

  • Gene flow between population reduced/eliminated —> genetic isolation

  • Geographically separated (Allopatry)

    • Allopatric Speciation

  1. Dispersal - population moves to new habitat (founder effect - genetic drift)

  2. Vicariance - physical splitting of habitat

  • Biogeography:

    • How species & populations distributed geographically

    • how dispersal/vicariance events occur

Allopatric Speciation by Dispersal

  • Colonization events

    • Colonists = newallopatric population

    • Traits differ between migrants & colonists

    • Natural selection favours specific alleles w.r.t environment

    • Natural selection + genetic drift

      • New population not separate species until there is no gene flow

Allopatric Speciation by Vicariance

  • physical barrier splitting geographic range of species


Isolation and Divergence in Sympatry:

  • Populations/species live in same geographic area (close enough to interbreed)

    • Gene flow would overwhelmd differences caused by genetic drift & natural selection

  • Can occur:

    • speciation among populations in same geographical area

  1. External Events - Disruptive selection (ecological niches/mate preferences)

  2. Internal Events - Chromosomal Mutations


Sympatric Speciation by Disruptive Selection

  • Niche: Range of ecological resources that species can use/range of conditions it can tolerate

  • not geographically isolated, reproductively isolated by adapting to different egological niches (disruptive selection)

    • Eg. Apple Maggot flies and Hawthorn Maggot flies (Apple maggot flies originated from hawthorn flies after apples introduced)

    • Species avoid scent of other fruit - Genetic bases (alleles)

    • Hybrids not as strongly oriented as parents (lower reproductive success)

    • Potential to cause rapid divergence among sympatric species


Sympatric Speciation by Changes in Ploidy

  • Polyploidy: Error in meiosis/mitosis = more than 2 sets of chromosomes

  • unlikely for populations to become isolated by mutation alone (low rates)

    • Can lead to reproductive isolation

    • Chromosome level mutations

    • Instantaneous

    • High genetic diversity = rapid diversification of plants

  • Polyploids:

    • Autopolyploid (within):

      • mutation results in doubling of chromosome number

      • Chromosomes all from same species

    • Allopolyploid (different species):

      • Hybridization: Different species mate + error in mitosis = viable, nonsterile offspring (fertile hybrids)

      • Offspring w/ two different sets of chromosomes

Autopolyploidy

  • Less common

  • two copies each of one set of chromosomes from diploid parent (instead of one)

    • Eg. Tetraploid from Diploid

  • Reproductively isolated from original population

  • tetraploids can successfully breed w/ other tetraploids, not diploids (hybrids are sterile)

    • According to BSC: speciation in a single generation

Allopolyploidy

  • new tetraploid species created when 2 diploid species hybridize

  • usually offspring sterile (infertile)

    • Error during mitosis —> diploid gametes in adults

    • meiosis proceeds —> gametes (self fertilization) fuse = tetraploid individual

    • 2 copies EACH of 2 sets of chromosomes

      • Can form new species

Speciation by Polyploidy Common in Plants

  • Diploid plant species have closely related polyploid species

    • Evidence for Speciation by polyploidy

  • Polyploids = higher levels of heterozygosity than diploids

    • Can toerate higher levels of self-fertilization

      • Not as affected by inbreeding depression as diploids

    • Genes on duplicated chromosomes can diverge independently (increases genetic variation)

Lecture 3 Content:

When Isolated Populations Come into Contact

  • Divergence + Prezygotic isolation = mating between populations rare

    • Gene flow minimal = Populations continue to diverge

  • No prezygotic isolation = populations successfully interbreed

    • Gene flow = erase distinctions between populations

1. Reinforcement

  • Populations diverged + genetically distinct = fitness of hybrid lower

  • If populations well adapted to respective habitats —> hybrid offspring not well adapted to either

  • Postzygotic Isolation: Strong natural selection against interbreeding

    • Preventing interbreeding = Reinforcement

    • Natural selection can act to reduce mating between species only when their ranges overlap

2. Hybrid Zones

  • when hybrids can mate + produce offspring

  • Hybrid Zone: geographic area where interbreeding occurs & hybrid offspring common

  • depends on hybrid offspring fitness & extent of breeding between parent species

    • Hybrid offspring can have intermediate characteristics

3. Speciation by Hybridization

  • When two species interbreed + produce hybrid offspring (fertile)

    1. New species has unique combination of alleles —> different characteristics

Secondary contact of two populations outcomes:

  • Fusion of populations (no prezygotic isolation - gene flow)

  • Reinforcement of divergence (postzygotic isolation - hybrid infertile)

  • Founding of hybrid zones (hybrids fertile - live in distinct geographic area)

  • Extinction of one population (one species better competition than other)

  • Form new species (hybridization - speciation)