Evolution / Chordates / Vertebrates


      Some terminology:

        Species - A population or group of populations that have the potential to interbreed in nature.

        Population - A localized group of individuals belonging to the same species.

      Natural Selection

        Natural selection is the process whereby the organisms best adapted to their environment survive and reproduce at higher rates than those less well adapted.

        Natural selection is possible because of:

        1. variation among individuals in a population and
        2. the tendency for a population to produce more offspring than the environment can support.

        Individuals that are well adapted (fit) survive and reproduce and leave relatively more offspring than individuals that are not well adapted (differential reproduction). In this way, the proportion of "desirable" genes in the gene pool of a population increases.

    Comparative Anatomy

      Evidence of the descent of separate species from a common ancestor can be found by comparing anatomical structures (as well as by other means such as examining DNA and the fossil record). Homologous structures are ones that are anatomically similar but may not serve similar functions. For instance, the forelimbs of all mammals are homologous structures since they are constructed from the same skeletal elements even though they may serve vastly different functions in different species.

      Chordate Features

      Humans are members of the chordate phylum, as are all vertebrates and some invertebrates. Chordates are characterized by possessing the following four features at some point in their development:




        Longitudinal, flexible rod between gut and nerve cord.

        Dorsal, hollow nerve cord

        This forms the central nervous system. It is derived from a plate of ectoderm that rolls into a tube.

        Pharyngeal slits

        These are like gills in fish, although gas exchange may or may not occur.

        Postanal tail

        A tail extending beyond the anus.

      In humans, these features are present in the early embryo.

      Vertebrate Features

      Vertebrates are a subphylum of chordates. They include fish, amphibians, reptiles, birds, and mammals. In addition to possessing the above four chordate features at some point in their development, most vertebrates also have the following features.



        Highly cephalized

        Well-developed sense organs and distinct brains


        Living and growing


        These enclose the nerve cord


        Encloses the brain

        Axial skeleton

        Skull, vertebrae, and chest cage

        Appendicular skeleton

        Pectoral and pelvic girdles and two pairs of limbs

        Closed circulatory system

        With blood pumped by a chambered heart. Oxygen is transported by hemoglobin in red blood cells.


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