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 belong to the same species unless some members become REPRODUCTIVELY ISOLATED from the original population. Speciation is the separation of two previously interbreeding populations into two populations that can no longer mate to produce fertile, viable offpring.

MODES OF SPECIATION

  • allopatric speciation - a single population is divided into two by a physical/geographic barrier.
  • peripatric - a new species arises at the edge of the range of the orignal population.
  • parapatric speciation - a "gradient" of genetic (and possibly phenotypic) difference develops across a species' range.
  • sympatric speciation - speciation occurs without physical separation, within the range of the ancestral population.

    These can be graphically represented as:

    Let's have a LOOK. (Thanks, Berkeley!)

    Yes. You're responsible for reading and understanding those web pages. Don't worry. They're small.


    THE PACE OF EVOLUTION.

    How fast does evolution proceed? It depends.

    PHYLETIC GRADUALISM

    This is the classical, traditional view stating that large changes (reproductive isolation and morphological differentiation) occur due to the gradual accumulation of many genetic changes. The classic example put forth in many natural history museums in the form of a nice display is that of the evolution of the modern horse.

    (NOTE: The modern systematist would not suggest that each of these species evolved into the next, more recent species. Rather, all these ancestral "steps" to the moder horse share a common ancestor that may have looked a lot like the "Dawn Horse," Hyracotherium (formerly known as Eohippus.

    PUNCTUATED EQUILIBRIUM

    This hypothesis was published in 1972 by Niles Eldredge and Stephen J. Gould.
    They suggested that major changes can occur relatively suddenly, and that they "punctuate" long periods of relatively little change. Let's have a LOOK.

    Remember: "suddenly" is a relative term, geologically speaking, and can mean over thousands of generations (quick!) instead of over millions (not so quick!)

    Eldredge and Gould suggested that this could explain how "awkward" intermediate forms such as the reptile-->flying bird and the terrestrial tetrapod-->swimming cetacean might have been "skipped". A major genetic event could have produced a phenotype that was drastically different from the original, and that this trait could become modified and fixed in the population over relatively few generations.

    Known examples:


    SPECIATION is a temporal process. Populations exist in various stages of this process at any given time, and present day populations are even now undergoing microevolutionary processes which may eventually result in macroevolution. Species that are on the verge of becoming separated are known as incipient species.

    HOW DO SPECIES CHANGE INTO DIFFERENT SPECIES?

    Competing hypotheses:

    WHAT PHENOTYPIC CHARACTERS TELL US ABOUT EVOLUTION

    Phenotype (at many levels, including the level of the DNA) provides the the biologist with the most basic information s/he needs in trying to accurately reconstruct evolutionary relationships.

    The goal of the modern biosystematist (i.e., a biologist who studies the evolutionary relationships between organisms) is to construct taxa (classification groups) that are MONOPHYLETIC - derived from a single common ancestor.

    In so doing, the biosystematist considers homologies, analogies, primitive and derived characters in the taxa under study at the level of morphology, ontogeny, and the biological macromolecules (DNA, RNA, proteins) themselves.


    REPRODUCTIVE ISOLATING MECHANISMS

    When one species gives rise to two new species (cladogenesis), what is it that determines whether or not the two can reproduce, if allowed to regain physical contact? (i.e., if they become sympatric once again?). We can help define separate species by considering the mechanisms that restrict successful mating/gene flow between them.

    Two related species may be separated by one or more of these types of reproductive isolating mechanisms.

    PREZYGOTIC ISOLATING MECHANISMS prevent the formation of viable zygotes.

    POSTZYGOTIC ISOLATING MECHANISMS prevent hybrids from passing on their genes.