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The Origin of Species

One species can give rise to a different species if there is

The variety of living species that has resulted from millennia of speciation is known as earth's biodiversity.

The Pace of Evolution

Biodiversity has changed not only in space, as the continents drifted apart, but also across time. How long does it take for a species to evolve? It depends.

Phyletic Gradualism

(Click on pic)

Punctuated Equilibrium

Examples of punctuated equilibrium in action:

But what are the mechanisms of those changes?
The ultimate "raw material" of evolution is mutation.
But how are those mutations incorporated into evolving populations?

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

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.

  • 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 related species are said to have undergone adaptive radiation.

    This diversification is driven by one or more of the five factors that can alter allele frequencies:

    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.

    And let's not forget our classic Galapagos finches

    Anoline lizards...

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

    Colors may have evolved in response to differences in sexual selection.

    How does it happen?

    Modes of 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
    Ring species are products of parapatric speciation.

    Their name comes from the way their geographic distribution often describes a ring around some type of physical barrier, such as a mountain range or other habitat not conducive to their colonization.


  • 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 a grand overview with more details.

    Natural Selection Can Drive Speciation
    At the start of a "selection cycle" a population may be comprised of individuals expressing a particular trait along a continuum (bell-shaped curve).

    stabilizing selection: selective forces at work on a population favor greatest reproduction by individuals exhibiting the average state of a particular character. In this instance, the composition of the population doesn't change.

    directional selection: the individuals at one extreme or the other of the bell shaped curve have a reproductive advantage over the rest.

    disruptive (= diversifying) selection: individuals at the average point on the curve are at a selective disadvantage; individuals with either extreme have a reproductive advantage.

    A classic example of diversifying selection is personified by the Rock Pocket Mouse in New Mexico's Valley of Fire.

    Horizontal Gene Transfer
    You already have seen the phylogeny of the three Domains of life.

    Organisms in all three Domains give rise to others like themselves via vertical gene transfer: the passing of genes from parent to offspring.

    As we now know, small changes can occur from generation to generation.
    Over time, such changes can accumulate to produce new species.

    The length of the branches between the taxa (represented as the nodes and the tips of the branches) represent the overall difference in DNA between them.

    The longer the branch, the more different the DNA.

    The diversity of Bacteria and Archaea is far greater than that of Eukaryota.

    Microbes (bacteria and archaea) undergo horizontal gene transfer so readily that an average of more than 80% of their DNA is a product of this process.

    The phylogenetic tree sometimes looks more like a web!

    So are bacteria all one big species?


    Several studies have demonstrated that bacteria in a given ecosystem occur in genetic "clusters" and ecological "clusters", too.
    Genetic makeup determines ecological niche, and that can drive reproductive isolation.

    Dr. Frederick Cohan proposes that bacterial strains (genetic variants of a species) be given the code "ecovar" (for "ecological variant") to distinguish it from genetically similar but ecologically distinct bacteria.

    The ability to recognize and characterize ecological strains of bacteria will be vital for epidemiologists and other health care workers.

    What About Humans?
    Paleoanthropology is the study of human origins and evolution. Because humans and chimpanzees have existed as separate species for only a few million years, this branch of science examines only a very small, recent portion of the fossil record.

    Humans, apes, and monkeys all diverged from a common anthropoid ancestor that shared the characteristics common to all primates. Natural selection has driven the specialization of each primate species to be what it is today.

    The earliest fossil members of our genus (Homo) range in age from about 2.5 to 1.6 million years, and are currently classified as Homo habilis:

    Homo erectus shared common ancestry with H. habilis, and shows up in the fossil record from about 1.8 million to 0.5 million years ago:

    Homo sapiens neanderthalensis, named for the Neander Valley in Germany where its fossils were first found, may have arisen from an H. erectus-like ancestor, as H. erectus is known to have migrated into both Europe and Asia (timing uncertain) from Africa.
    If your ancient ancestors left Africa and evolved into today's Europeans, there's a good chance you may share some DNA also found in ancient Neanderthals.

    Neanderthal DNA is absent in modern humans descended from Africans who never left the continent.

    There are multiple, competing hypotheses about human origins.

    As more DNA data become available, incorrect hypotheses may be rejected.
    The oldest hominoid fossils are found in Africa. It is clear that the common ancestor of all modern humans originated there.

    What makes humans different from the other great apes?