Macroevolution: The Origin of Species Over generations, a population can undergo a great deal of change from its original state. But all members of that population are still members of the same species unless some members become reproductively isolated from one another. Speciation is the separation of two previously interbreeding populations into two populations that can no longer mate to produce fertile, viable offpring. The Species Concept: Depends on whom you ask As you can see, species don't always adhere to our definitions. In fact, there are many different species concepts, each one appropriate when addressing species questions from a particular perspective.

  • typological (morphological) species concept: a species is a group of organisms conforming to a common morphological plan, emphasizing the species as an essentially static, non-variable assemblage. (as old as Plato)

  • biological species concept: a species is a group of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups (Mayr, 1940)

  • evolutionary species concept: a species is a lineage (an ancestral-descendant sequence of populations) evolving separately from others and with its own unitary evolutionary roles and tendencies (Simpson, 1961)

  • ecological species concept - a set of organisms occupying the same ecological niche (Ridley, 1993)

  • cladistic/phylogenetic species concept - the smallest group of individuals descended from a common ancestor (Ridley, 1993) list just a few.

    "A population does not lose its species status when an individual belonging to it makes a mistake." --Ernst Mayr

    Macroevolution: The Genesis of Reproductively Isolated Populations from an Ancestral Population

  • Speciation is a temporal process.
  • Populations exist in various stages of speciation at any given time
  • Extant populations are even now undergoing microevolutionary changes that may eventually give rise to new species.
  • Species on the verge of becoming separated are called incipient species.

    How might one species become two?

    Repeated Cladogenesis and Adaptive Radiation An ancestral species can give rise to a variety of diverse species through repeated cladogenesis if each of its descendant species "radiates" into a new ecological niche. When this occurs, the group of related species is said to have undergone adaptive radiation.

    This diversification can be driven by mutation, migration, assortative mating, genetic isolation, and/or natural selection. Character Displacement and Adaptive Radiation The ecological niche of a species describes all that species' ecological requirements: what it eats, where it lives, where it nests, its interactions with other species and with its environment.

    Gause's Law (aka Competitive Exclusion Principle): In a stable environment, two species cannot coexist if they use exactly the same resources.

    To avoid competition, species must evolutionarily adapt, specialize and move towards resource partitioning: dividing a common resource so that each competing species uses only a portion of that resource.

    Physical changes associated with this resource partioning are called character displacement.

    In 1958, MacArthur reported how five very similar (and undoubtedly closely related) warblers were able to co-exist by utilizing different parts of an evergreen forest for foraging.

    On the Hawaiian islands, a single, finchlike ancestor gave rise to about 40 different species of Honey Creepers. Each is specialized in bill shape and size, as selected by its particular microhabitat and diet. Colors may have evolved in response to sexual selection.

    And let's not forget our classic Galapagos finches

    Anoline lizards...

    Poison Dart Frogs... where mate choice and ecology collide

    How does it happen?

    Modes of Speciation Now that we have considered that one species can change into a new species, we must consider the physical and genetic factors that interact to produce speciation.

  • ALLOPATRIC SPECIATION - A single population is divided into two by a physical, geographic barrier.

    The physically separated population may not always undergo complete reproductive isolation: re-establishment of physical contact may simply result in resumed mating.

    However, if the two separated populations become so differentiated during their isolation that they can no longer interbreed when they meet again, allopatric speciation has occurred.

  • PERIPATRIC SPECIATION - By entering a new ecological niche, a small subset of a large population becomes isolated at the periphery of the original population's range. Over generations, the small group becomes reproductively isolated from the original population.
    This is sometimes considered a special case of allopatric speciation.

    example: Polar bears (Ursus maritimus are now known (from DNA evidence) to share a most recent common ancestor with an extinct Eurasian Brown Bear (Ursus arctos) population that once lived in Ireland.

    (Think: Whale ancestors...)

  • PARAPATRIC SPECIATION - This occurs on a larger scale than parapatric speciation, with large numbers of a population gradually becoming differentiated (due to genetic drift and/or selection) along the range of the population. Adjacent demes may be able to interbreed to some degree, but widely separated demes cannot. In such isolation and under different selective pressures, the more widely separated populations undergo reproductive isolation/speciation.

    Ring Species: Products of Parapatric Speciation

  • Salamanders in the genus Ensatina clearly have a common ancestor.

  • Siberian Greenish Warblers

  • Ring Species: how microevolution can lead to macroevoluion

  • SYMPATRIC SPECIATION - speciation occurs without physical separation, within the range of the ancestral population. (This is often due to a sudden genetic event that causes very rapid reproductive isolation of a subset of the original population.)

    Sympatric speciation is well known in plants, which can speciate quickly via polyploidy, either

    The modes of speciation can be visualized this way.

    Here's an overview.

    Hybrids: Evolution in Flux Recall that a hybrid is the offspring of genetically dissimilar parents.

    One can consider hybridization within a species (as Mendel did with his Sweet Peas and agriculturists do with everything from apples to cattle to wheat) or between species:

    Hybridization between related species can sometimes result in reproductive isolation of a fertile, hybrid offspring.

    If a polyploid is self-fertile, it can give rise to a population that is partly or completely reproductively isolated from the original parent population.

    A famous example is that of Goat's Beard. Wild Tragopogon dubius, Tragopogon porrifolius, and Tragopogon pratensis were imported from Europe and became established in the Western U.S. (Washington and Idaho).

    Promiscuous and fertile, the three species interbred, producing infertile interspecies that went through allopolyploid reproduction to become self-fertile.

    The European Ancestors (from left to right, T. dubius, T. porrifolius, and T. pratensis.

    The crosses and doublings:

    Tragopogon mirus and T. miscellus are tetraploid, self-fertile hybrids derived from older species of Tragopogon.
    Two new species are born of hybridization.

  • A hybrid zone is an area of secondary contact between two related species, where limited hybridization is taking place between them.

  • Introgression is the introduction of alleles from one species' gene pool into that of another (closely related) species, due to limited hybridization.

    One well-known example is hybridization between the White-tailed Deer (Odocoileus virginianus, left) and the Mule, or Black-tailed Deer (Odocoileus hemionus, right). Which brings us to The Tale of Bambi and That Other Guy:

    Hybrid Speciation There is a growing body of evidence that hybridization between related species may result in reproductive isolation that can then lead to speciation, as in the case of Tragopogon and wild sunflowers (Helianthus spp.) (Helianthus annuus x Helianthus petiolaris --> Helianthus anomalus)

    Speciation via Hybridization in an Animal
    Hybridization between two related species can result in speciation if the hybrid offspring...

    Example: Hybridization of two species of tephritid fruit flies results in a new hybrid species.

    Once removed from interaction with parent species, the hybrid individuals may breed among themselves and evolve into a new species.

    The Pace of Evolution How long does it take for a species to evolve? It depends.

    Phyletic Gradualism Darwin's understanding of evolution was that species gradually changed into new species in a relatively gradual, stepwise manner.

    Species, he thought, accumulated small changes over generations, and eventually a new species came into being.

    A classic example seen in many natural history museums is the evolution of the modern horse.

    Punctuated Equilibrium Darwin's classical idea of gradual change was upended in 1972 when Niles Eldredge and Stephen Jay Gould proposed a new idea, punctuated equilibrium: major changes can occur relatively suddenly, and "punctuate" long periods of relatively little change (stasis).

    The term "sudden" is relative, geologically speaking. Punctuations in species diversity can happen over thousands of generations (quick!) instead of over millions (not so quick!)

    Punctuated equilibrium could explain how awkward intermediate forms needed to transform a land reptile into a flying bird

    or a terrestrial tetrapod into a marine whale...

    (Click on pic!)

    ...might have transitioned relatively quickly into more adaptive forms.

    In the case of whales, consider the contribution of assortative mating:

  • Individuals genetically predisposed to be better adapted for intertidal life would likely hang together.
  • This might mean they would also be more likely to mate with each other
  • This would generate a smaller effective population where inbreeding could magnify aquatic adaptations relatively quickly.

    Another scenario: A major genetic event could produce a phenotype that was drastically different from the original. If it were adaptive, the new phenotype could quickly displace the old wild type.

    Examples of punctuated equilibrium in action: