[Paleopsych] SW: Selection and the Origin of Species

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Evolutionary Biology: Selection and the Origin of Species

    The following points are made by A.Y. Albert and D. Schluter (Current
    Biology 2005 15:R283):
    1) Why are there so many species on earth? Answering this question
    requires an understanding of how species form. An obvious place to
    start looking for answers is Darwin's ON THE ORIGIN OF SPECIES BY
    MEANS OF NATURAL SELECTION (1859). But his title is deceptive:
    Darwin's book is about adaptation and the origin of varieties and has
    surprisingly little to say about selection and the origin of species
    -- that mystery of mysteries
    2) To be fair to Darwin, it was not for another 80 years or so that
    the modern view of the species was developed. The "biological species
    concept" defines a species as one or more populations of potentially
    interbreeding organisms that are reproductively isolated from other
    such groups. Humans and chimps are today separate species not only
    because we are genetically and phenotypically distinct, but because we
    are reproductively isolated. Neither finds the other attractive when
    choosing a mate ("premating isolation") and very likely, hybrids are
    inviable or sterile ("postmating isolation"). Reproductive isolation
    is therefore the most salient evolved feature of a species, at least
    in sexual organisms. Even "good" species may hybridize once in a
    while, but to meet the species criterion the flow of genes between
    them must be negligible. The study of speciation is therefore the
    study of how reproductive isolation evolves, premating or postmating,
    between populations.
    3) Natural selection is the differential survival or reproductive
    success of individuals differing in phenotype within a population.
    Sexual selection, by contrast, is the differential mating success of
    phenotypically different individuals. These two processes are the most
    potent drivers of evolutionary change within populations.
    4) Our understanding of the process of speciation has increased
    greatly since Darwin first proposed a central role for natural
    selection. Much of what we now know has come from research conducted
    over the past two decades. The picture emerging is that speciation is
    a process that results from the same forces responsible for most
    change within species: natural and sexual selection. Nonetheless,
    there are still many areas that require investigation. The 'top down'
    or phenotypic approach to studying speciation has found evidence for
    selection on ordinary phenotypic characters shown also to underlie
    premating and postmating isolation. This approach has yielded little,
    however, about the genetic basis of reproductive isolation. For
    example, we do not know yet if species differences are based on many
    genes of small phenotypic effect, or if few genes of large effect are
    most important in causing divergence and reproductive isolation. This
    has made it difficult to pinpoint exactly how natural selection has
    led to divergence in most cases. Recent studies of speciation in
    monkeyflowers and other taxa are helping to overcome this gap.
    References (abridged):
    1. Coyne, J.A. and Orr, H.A. (2004). Speciation. Sinauer, Sunderland,
    2. Orr, H.A., Masly, J.P., and Presgraves, D.C. (2004). Speciation
    genes. Curr. Opin. Genet. Dev. 14, 675-679
    3. Panhuis, T.M., Butlin, R., Zuk, M., and Tregenza, T. (2001). Sexual
    selection and speciation. Trends Ecol. Evol. 16, 364-371
    4. Rieseberg, L.H. (1997). Hybrid origins of plant species. Annu. Rev.
    Ecol. Syst. 28, 359-389
    5. Schluter, D. (2000). The Ecology of Adaptive Radiation. Oxford
    University Press, Oxford
    Current Biology http://www.current-biology.com
    Related Material:
    The following points are made by S.L. Pimm and J.H. Brown (Science
    2004 304:831):
    1) Pointed disagreements persist about why the tropics have more
    species than other latitudes(1). The many hypotheses (2-4) reflect
    three deeply different approaches. Two date back to the time of Alfred
    Russel Wallace (1823-1913) (2): One stresses ecological processes,
    such as a location's temperature and rainfall, the other historical
    factors, such as whether a region was covered in ice during recent
    glaciations. The third approach is a newcomer that explains species
    richness as a simple, statistical consequence of the observation that
    some species have large geographical ranges, whereas others have small
    ranges. This explanation echoes a classic debate about the patterns of
    communities (5). By "pattern", ecologists mean such features as how
    many species a community houses, or how similar those species are
    2) Most species live in the tropics and, in particular, within moist
    forests. Why do warm, wet places generate diversity? "There are more
    niches," goes one argument, "as demonstrated by their being more
    species to fill them," goes its circular conclusion. Warm, wet places
    are proposed to be more productive and to support more individuals,
    which in turn permit more species to coexist. Unfortunately, tropical
    richness increases much faster than expected with the increase in
    3) Perhaps species are preserved longer or are born more frequently in
    the tropics. Wallace (2) suggested that the tropics avoided the
    devastation of periodic ice ages. Higher birthrates would fill the
    tropics with "young species" -- those with many "siblings" in the same
    genus. The tropics, however, simply have more of every taxonomic
    level. More than half of all bird families are tropical.
    4) Based on the excellent fossil record for marine bivalves, the
    tropics are both the primary source of diversity and the accumulator
    of species. Species appear in the tropics, then expand outwards. High
    latitudes are a "sink" -- that is, places from which species disappear
    forever. Tropical oceans house enormous diversity today because it is
    here that the geographic ranges of old and mostly widespread species
    overlap with those of young and spatially restricted ones.
    5) Proponents of the third approach ask us to imagine a child's pencil
    box wide enough for many pencils. Barely used pencils --analogous to
    species with large ranges -- will span almost the entire length of the
    box. Other pencils will be worn to mere stubs -- analogous to species
    with small ranges. The length of the box is any well-defined gradient.
    Now shake the box end-to-end to randomize the pencils. Pencils larger
    than half the box's length must encompass its middle. With enough long
    pencils, more pencils overlap at the middle than at the ends --
    producing the "mid-domain effect". Such unavoidable constraints
    generate the expected statistical distribution of numbers of species
    along the gradient against which to compare empirical patterns.
    References (abridged):
    1) R. D. Keynes, Ed., The Beagle Record: Selections from the Original
    Pictorial Records and Written Accounts of the Voyage of the H.M.S.
    Beagle (Cambridge Univ. Press, Cambridge, 1979)
    2. A. R. Wallace, Tropical Nature and Other Essays (Macmillan, London
    3. E. R. Pianka, Am. Nat. 100, 33 (1966)
    4. M. R. Willig, D. M. Kaufman, R. D. Stevens, Annu. Rev. Ecol. Evol.
    Syst. 34, 273 (2003)
    5. R. Lewin, Science 221, 636 (1983)
    Science http://www.sciencemag.org
    Related Material:
    The following points are made by L. Margolis and D. Sagan (citation
    1) Charles Darwin's landmark book The Origin of Species, which
    presented to scientists and the lay public alike overwhelming evidence
    for the theory of natural selection, ironically never explains where
    new species come from.
    2) Species are names given to extremely similar organisms, whether
    animals, plants, fungi, or microorganisms. Because we need to identify
    poisons, predators, shelter materials, fuel, food, and other
    necessities, we have long bestowed names on living and once-living
    objects... Until the Renaissance, however, names of live beings varied
    from place to place and were seldom precisely defined. The confusion
    of local names and inconsistent descriptions led the Swedish
    naturalist Carolus von Linne (1709 1789) to bring rigor and
    international comprehensibility to the descriptions. Since Linnaeus
    (his Latin name) imposed order on some 10,000 species of live beings,
    scientists use a first name (the genus -- the larger, more inclusive
    group) and a second name (the species -- the smaller, less inclusive
    group) to refer to either live or fossil organisms.
    3) Most Linnaean names are Latin or Greek. By today's rules the
    species and genus names are introduced into the scientific literature
    with a "diagnosis", which is a brief description of salient properties
    of the organism: its size, shape, and other aspects of its body (its
    morphology); its habitat and way of life; and what it has in common
    with other members of its genus. The diagnosis appears in a published
    scientific paper that describes the organism to science for the first
    time. The paper also includes details beyond the diagnosis, called the
    "description". To be a valid name not only must the names, diagnosis,
    and description be published but a sample of the body of the organism
    itself must be deposited in a natural history museum, culture
    collection, herbarium, or other acknowledged repository of biological
    4) Fossils are dead remains, evidence of former life. The word comes
    from "fosse" in French, something dug up from the ground. Fossil
    species, like the enigmatic trilobite Pamdoxides paradoxis-simus, also
    are given names and grouped on the basis of morphological similarities
    and differences.
    5) The word "species" comes from the Latin word "speculare", to see --
    like spectacles or special. Everyone, knowingly or not, uses the
    morphological concept of species -- dogs look like dogs, they are
    dogs, they are all classified as Canus familiaris. The problems come
    when we try to name coyotes (Canus latrans), wolves (Canus lupus, gray
    wolf, or Canus rufus, red wolf), and other closely related animals.
    Zoologists, those who professionally study animals, have imposed a
    distinct concept of species, which they call the "biological species
    concept". Coyotes and dogs in nature do not mate to produce fully
    fertile offspring. They are "reproductively isolated". The zoological
    definition of species refers to organisms that can hybridize -- that
    can mate and produce fertile offspring. Thus organisms that interbreed
    (like people, or like bulls and cows) belong to the same species.
    Botanists, who study plants, also find this definition useful.
    L. Margolis and D. Sagan: Acquiring Genomes: A Theory of the Origins
    of Species. Basic Books 2002, p.3.

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