Macroevolution - How Species Evolve

Defining Species: Biological Species Concept

  • Learning Outcomes:
    • Define species according to the biological species concept.
    • Discuss the importance of reproductive isolation.
    • Describe speciation types and causes.
  • Defining a Species
    • Intuitive understanding of species.
    • Example: Owls
      • Image shows potentially three types of owls.
      • Owls A and B are morphs of the same species due to interbreeding.
      • Broods can have both colors.
      • Owl C is a different species based on call and mitochondrial DNA.
      • Separate gene pools indicate no interbreeding.
  • Early Taxonomy vs. Biological Characteristics
    • Early taxonomists distinguished species by morphology.
    • Challenges:
      • Species look alike but are different.
      • Same species looks different.
      • Example: Owls A and B are the same species with different morphologies.
    • Species concepts developed based on biological characteristics beyond morphology.
  • Biological Species Concept
    • Species are groups of actually or potentially interbreeding populations.
    • Reproductively isolated from other groups.
    • Core: Ability to mate and produce viable offspring.
    • Interbreeding indicates gene pools are not separated.
    • Does not require 100% reproductive isolation; allows some genetic leakage through hybridization.
    • Applicable to most sexually reproducing species.
  • Drawbacks of Biological Species Concept
    • Slow evolution of reproductive isolation.
    • Continuum from interbreeding to complete isolation.
    • Example: Butterflies
      • Populations at extremes are isolated.
      • Overlapping distributions lead to interbreeding and gene flow.
    • Testing reproductive isolation in the lab is impractical for large species.
    • Lab results may not represent natural conditions.
    • Doesn't work for asexually reproducing species.
    • Example: Bacteria can exchange genes through horizontal gene transfer (HGT), challenging the concept.
  • Alternative Species Concepts
    • Depend on:
      • Organism type (bacteria, viruses, algae, etc.).
      • Reproductive mode (sexual vs. asexual).
      • Ecology.
      • Genetic diversity.
      • Other features.
  • Phylogenetic Species Concept
    • Emphasizes species as the outcome of evolutionary divergence.
    • Definition: An irreducible or most basal cluster of organisms diagnosed a bit different from other such clusters.
    • Example: Domestic cat and leopard are irreducible clusters.
      • Leopard populations.
      • Domestic cat breeds still interbreed.
    • Parental pattern of ancestry and descent.
      • Lineage splits into two from a common ancestor.
      • One evolves into leopard, another into domestic cat.
        • Shared genetic history due to common ancestor.
        • Unique genetic history within each lineage.
    • Used for classification and systematics.
  • Morphological Species Concept
    • Emphasizes physical similarities.
    • Problem: Convergent evolution.
    • Marine mammals and sharks look alike due to convergent evolution.
  • Hybridization and Species Definition
    • Cannot unambiguously assign species due to hybridization.
    • Hybrid zones occur in areas of overlapping distributions.
    • Introgression: Genes from one species become incorporated into the gene pool of another.
    • Example: Bombina toads
      • Morphologically different species that interbreed in a narrow area.
      • Transect studies show a clinal pattern in allele frequency and morphology.
      • Steep cline indicates strong selection against hybrids.
    • Rarely test for interbreeding ability when defining new species.
    • Rely on morphological and phenotypic characters.
    • Genetic markers provide a clearer picture of gene flow.
  • Importance of Accurate Species Definition
    • Conservation: Correctly identify species (Kea, Kaka, Kakapo).
    • Biosecurity: Identify invasive species.
      • Brown marmorated stink bug vs. rough stink bug.

Evolution of Reproductive Isolation

  • Example: Monkey Flowers (M. lewisii and M. cardinalis)
    • Never produce hybrids in the wild.
    • Easily crossed experimentally in the lab.
    • Ecological Factors: M. lewisii at higher elevations, pollinated by bees; M. cardinalis at lower elevations, pollinated by hummingbirds.
      • Low chance of pollen contact in the wild.
  • Reproductive Barriers
    • Prezygotic barriers: Prevent formation of zygote.
      • Reduce likelihood of hybrid formation.
      • Examples:
        • Habitat separation.
        • Pollination vectors.
        • Different mating times.
        • Mating preferences.
        • Failure of gamete union.
      • Usually asymmetrical.
    • Postzygotic barriers: Formed after the hybrid is produced.
      • Reduce hybrid viability and fertility.
    • Monkey flowers show both pre- and postzygotic isolation.
      • Elevation and pollinator contribute most.
      • Pre- and postzygotic barriers are influenced differently by selection.
  • Examples of Zygotic Isolation
    • Ecological Prezygotic:
      • Petrels mating at different times.
      • Cicadas emerging at different times.
      • Ladybirds feeding and mating on different hosts.
    • Sexual Prezygotic Isolation:
      • Female frogs preferring calls of their own species.
      • Signaling in moths/butterflies.
      • Male butterflies using visual cues.
    • Postzygotic Barriers:
      • Reduce hybrid vigor or fertility.
      • Extrinsic factors: Hybrid less fit to environment.
      • Intrinsic factors: Incompatible genes from parents (Dobzhansky-Muller incompatibilities).
    • Dobzhansky-Muller Incompatibilities Example:
      • Arabidopsis
      • StrainA:<br/><br/>alpha=functional,β=non−functionalStrain A : <br />\normalsize <br />\newline alpha = functional, \beta = non-functional
      • StrainB:<br/><br/>alpha=non−functional,β=functionalStrain B : <br />\normalsize <br />\newline alpha = non-functional, \beta = functional
      • F1 hybrids viable (at least one functional copy).
      • F2 hybrids can be homozygous for non-functional loci (lethal).
  • Speed of Reproductive Isolation Evolution
    • Fast in model insects (Drosophila) and cichlid fishes.
    • Polyploidy in plants can create new species in one or two generations.
    • Often slow (200,000 to 3 million years for wild populations).
  • Experiment on Pre- vs. Postzygotic Barrier Evolution in Drosophila
    • Small genetic distance, means shorter time since split.
    • Large genetic distance, means longer time since split.
    • Prezygotic isolation evolves quickly.
    • Postzygotic isolation evolves slowly (requires longer time).

Speciation

  • Cichlid Fishes in African Rift Valley Lakes

    • High diversity (250-430 species in one lake).
    • Evolved from a small number of ancestral species.
      • Example: Lake Victoria: 450 species evolved from 5 ancestors in 15,000 years.

  • Speciation Defined

    • Evolution of reproductive isolation.
    • Reproductive barriers decreases in chance of mating or offspring survival.
    • Speciation occurs when populations become reproductively isolated.
    • These barriers drive survival and species diversification rather than extinction.

  • Process of Reproductive Isolation

    • Starts with geographic barriers.
    • Barrier separates populations, preventing interbreeding and gene flow.
    • Each population undergoes evolutionary adaptation or random genetic drift.
    • Allele frequency and morphological features change.
    • If the barrier is removed, the populations may no longer interbreed (different species).

  • Types/Modes of Speciation

    • Allopatric Speciation:

      • Populations isolated due to geographic changes.
      • Example: River formation.
      • Involves ecological speciation.

    • Ecological Speciation:

      • Geographically isolated populations adapt to different ecological conditions.
      • Can occur without geographic isolation (insects adapting to different host plants).
      • Monkey flowers: Ancestral species (M. lewisii) colonizes lower altitudes and evolves into M. cardinalis.

  • Models of Allopatric Speciation

    • Vicariance:
      • Geographic barrier separates populations.
      • Each population evolves independently.
    • Founder Event:
      • Small peripheral population becomes isolated.
      • Random genetic drift has a stronger impact due to the small population size.
      • The smaller population evolves into a different species.
    • Study shows allopatric speciation is more likely with:
      • Larger islands.
      • Species with low dispersal abilities.
    • Geographic distance also matters; sexual isolation increases with distance.

  • New Zealand Example: Kakapo, Kea, and Kaka Complex

    • Australian ancestor undergoes vicariance due to the breakup of Gondwanaland (100 million years ago) that resulted in the evolution of Protococcacapo.

    • Protocacapo splits again (60-80 million years ago).

    • One lineage evolves into Kakapo.

    • The other lineage diverges ecologically into Proto-Kaka.

      • Proto-Kaka evolves into Kea (adapted to alpine environment) and Kaka.
      • Kaka has subspecies evolved through migration to different islands or sea-level changes.

    • Sympatric Speciation:

      • A single population in sympatry evolves into two reproductively isolated populations.
      • Example: Lord Howe Island palms.
        • Live in close proximity, wind-pollinated.
        • Separated by the peak of their flowering time.

    • Sympatric speciation requires barriers to gene flow but still allows some gene flow.

      • Less common than allopatric speciation.
      • Requires assortative mating.

    • Parapatric Speciation:

      • Intermediate between allopatric and sympatric.
      • Speciation with gene flow (difficult to identify).
      • Neighboring populations exchange genes but diverge.

    • Example: Grass

      • One species grows on mine waste (heavy metal tolerance).
      • The other species grow on uncontaminated soil that cannot use the first soils.
      • Gene flow happens, but different flowering times separate gene pools.

  • Mechanisms Causing Speciation

    • Ecological Mechanisms:

      • Reproductive isolation evolves as a side effect of ecological adaptation.
      • Example: Monkey flowers adapted to different pollinators in different geographic areas.

    • Genetic Conflict:

      • Selfish genetic elements spread by manipulating reproduction.

    • Example:

      • Females of species A have selfish genomic elements that kill Y bearing sperms.
      • Males in this species have evolved suppressors to counteract this.
      • Females in species B don't have this element. If they mate with males of the first species, we see only females in that next generation.

    • Sexual Selection:

      • Females impose strong sexual selection.
      • Drives rapid evolution of secondary sexual traits in males.
      • Can cause prezygotic isolation.
      • Example: Hawaiian crickets (females only respond to calls of their own species).

    • Polyploidy:

      • Results from unreduced gametes (improper meiosis).

      • Increases the number of gene copies, that results in changes in gene expression.

      • Common in plants.

      • New species in one or two generations.

      • Two types:

        • Autopolyploidy (unreduced gametes from the same species).
        • Allopolyploidy (unreduced gametes from different species).

    • Hybrid Speciation:

      • Rare cases of hybrid speciation
      • Hybrids may be better adapted through new gene combinations.

    • Random Genetic Drift and Founder Effects:

      • Founder population size small.

      • Random evolutionary forces are stronger.

      • Genetic drift drives speciation in the founder population.

        • Paradise kingfisher:
          • The main island or the larger island population started to colonize these two smaller islands.
          • Mainly through random genetic drifts and some natural selection the smaller populations evolve into different species.

    • Reinforcement:

      • Selection favors the evolution of prezygotic isolation.

      • Selection for discrimination alleles (increases the probability of mating within populations rather than interbreeding between populations).

      • Assortative mating.

      • Example: Phlox flowers.
        * Pollinators tend to stick to same color flowers.
        * This reinforces the red color-morph of P. dromondii in the overlapping areas and prevents P. dromondii to interbreed or hybridize with the other species.