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.
...to list just a few.
Macroevolution: The Genesis of Reproductively Isolated Populations from an Ancestral Population
How might one species become two?
cladogenesis (= diversifying evolution) - the divergence of two new species from a single ancestral species. (Net increase in species diversity).
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
Poison Dart Frogs... where mate choice and ecology collide
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.
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.
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...)
Ring Species: Products of Parapatric Speciation
Sympatric speciation is well known in plants, which can speciate quickly via polyploidy, either
or allopolyploidy (chromosomes in the new species come from two different (but related) ancestral species)
The modes of speciation can be visualized this way.
Here's an overview.
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.
One well-known example is hybridization between the White-tailed Deer (Odocoileus
virginianus, left) and the Mule, or Black-tailed Deer (Odocoileus
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...
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...
...might have transitioned relatively quickly into more adaptive forms.
In the case of whales, consider the contribution of assortative mating:
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: