AEC 360  Ecology

Lecture 27: Speciation

I.  The history of life on earth
    A.  The geologic eras, which are subdivided into epochs
        1.  Precambrian – Fossils extremely rare, primitive aquatic plants
        2.  Paleozoic – 250  – 600 million years ago (Mya).
        3.  Mesozoic – 65 – 250 Mya.  First dinosaurs, birds, mammals, flowering plants
        4.  Cenozoic – since 65 Mya.  Rise in dominance of mammals

    B.  The history of life on earth is characterized by a trend toward more species over time, but with periods of mass extinctions (see Fig. 23.3)

II.  Speciation, or the formation of new species

    A.  What is a species?

       1. Cases of species definitions, butterfly, red wolf, and dusky seaside sparrow

    B.  How do new species form?
        1.  Populations isolated
        2.  Divergence in traits
        3.  Reproductive isolation

    C.  Physical isolation as a barrier to gene flow
        1.  Allopatric model – species to arise when a small number of individuals becomes isolated at the periphery of a species range
        2.  Note that sympatric speciation from non-isolated individuals is possible, but rarely observed
        3.  Founder effects on islands or other areas, where individuals newly colonize places that have not been occupied by the species
         4.  Vicariance events – when a species range is split in two

        5.  Geologic change isolates populations (Fig. 22.25)
            a.  Continental Drift -- The movement of landmasses on the earth’s surface
            b. This process has two consequences for ecological systems
                1) Continental drift creates and breaks down barriers to dispersal
                2)  Positions of continents & major ocean basins profoundly influence weather patterns

        6.  Global climate patterns can also isolate species (Fig. 22.27)
            a.  Pleistocene ice age has seen violent changes in climate
            b.  Repeated advance and retreat of glaciers

    D.  Sympatric speciation, and the case of the soapberry bug

    E.  Mechanisms of divergence
        1.  Once isolated, how do new species form?

    F.  Why do isolated species look so similar?
        1.  Species with different evolutionary origins but similar environments & environmental adaptations may result in similar appearances

III.  Phylogeny
    A.  Phylogeny is the evolutionary history of a group
        1.  Phylogenetic tree summarizes the history
        2.  Tree describes timing and pattern of branching events
        3.  The most closely related species should have the most traits in common
        4.  Choose parsimonious tree that minimizes the amount of evolutionary change 

    B.  Example: the phylogeny of whales 
        1.  In ungulates, Artiodactyls  (hippos, cows, deer, pigs) are separated from others (horses, rhinos) by shape of Astragalus
        2.  Whales have no Astragalus

        3.  If whales are related to hippos, then later evolution would have led to loss of this characteristic

    C.   Phylogenetic history can help us explain current patterns of ecological diversity