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Empirical Evidence for Evolution by means of Natural Selection

  • Evolution of resistant strains of bacteria. Once again, Darwin's theory is not refuted.

    Another Example:

  • Evolution of flower size in flowers, such as Polemonium viscosum (Alpine Skypilot) Is Darwin's idea about Natural Selection testable? What do you think? Can you think of other ways to put the idea to rigorous testing?

    Evidence for Evolution: Homology

    What observable evidence exists to support the idea that life on earth has evolved its diversity through a long process of descent with modification? One of the most powerful forms of evidence is

    Homology

    In biology, a homology is a characteristic shared by two species (or other taxa) that is similar because of common ancestry.

    Morphological Homology

    Structures derived from a common ancestral structure (that may or may not be used for the same function in the species in which it occurs) are called homologous structures.

    A classic example of homology is seen in the skeletal components of vertebrates...

    In contrast, a structure that serves the same function in two taxa, but is NOT derived from a common ancestral structure is said to be analogous. Examples of analogous structures:

    Evolution can be considered a process of "remodeling" a population over the course of many generations, with one of the main the driving forces being the natural selection factors that favor one form over another in specific environments.

  • vestigial structures are those have marginal, if any use to the organims in which they occur. These are some of the most interesting examples of homology.

    Primitive and Derived Characters

  • A primitive character is one that is relatively unchanged from its original, ancestral form. (Also called a plesiomorphy)

  • A derived character is one that is relatively modified from its original, ancestral form. (Also called an apomorphy)

    Note that these are comparative terms. You can't call something "primitive" or "derived" without comparing it to something else. In other words, when you invoke the "primitive" or "derived" state of a particular character, you are examining homology between two or more species derived from a common ancestor.

    Systematists often use the existence of homologous, shared characters among taxa to help reveal their common ancestry.

    As we will see, the more recently two species diverged from a common ancestor, the more synapomorphies they will share.

    In determining increasing recency of common descent, the systematist finds that symplesiomorphies are generally not informative, though synapomorphies are. Why? Let's do an example with our own species and some of our closest vertebrate relatives....

    (See what you missed if you didn't come to class?)

    Stephen Jay Gould first coined the term exaptation: The pre-existence of a character-- initially with no known adaptive significance--that under changed selective pressures "suddenly" conferred a selective advantage to those individuals exhibiting it.

    Ontogenetic Homology

    How are anatomical structures modified from their original form? Clues can sometimes be found by studying ontogeny (from the Greek onto, meaning "being" or "existence" and genesis meaning "origin"): Embryonic Development

    Differences in ontogeny among taxa are not trivial. They reflect the evolutionary relationships of those taxa, and the modified development that resulted in one species becoming two. Some have suggested that developmental innovations alone may have provided the evolutionary mechanisms by which entire new lineages originated.

    Consider members of the Phylum Chordata. All members share these primitive traits (which are derived with respect to all other Phyla) at some point in their development:

    • a tail posterior to the anus
    • dorsal, hollow nerve cord
    • muscles arranged in bundles (sarcomeres)
    • cartilaginous notochord
    In the most primitive chordates (Phylum Cephalochordata - the lancelets), the notochord persists throughout the life of the organism:

    In the somewhat more derived chordates (Phylum Urochordata), the notochord is seen in the larval form, but lost in the adult:

    And in the most derived chordates (Phylum Vertebrata), the notochord is found only in early embryos, and is completely lost in the adult.

    Only by studying the embryological development of vertebrates can one can find this shared chordate character, where it is present in all three taxa. This is one of the most accurate ways to determine whether a structure appearing similar in adult organisms is homologous or analogous. If it is present in the embryo of all taxa being considered, it is most likely a homology.

    Ontogenetic changes in a developing organism, whether due to mutations or to other heritable changes in the DNA, ultimately are reflected in the adult organism.

    What about other characters that appear superficially similar in adult organisms? What does ontogeny tell us about those?

    Example:

    • vertebrates have a hard, round, protective covering around the brain (the cranium)
    • many insects also have a hard, round, protective covering around the brain.
    Are these structures homologous or analogous? Was the model for this "brain case" present in the common ancestor of both vertebrates and insects?

    The answer--no surprise--is NO.

    If one monitors the embryonic development of a vertebrate and an insect, one sees that divergence in ontogeny occurs VERY early--even before the gastrula stage. (First, a very brief overview of early embryo anatomy...

  • In insects (which are protostomes), the blastopore becomes the mouth, and a secondary opening becomes the anus.

  • In vertebrates (which are deuterostomes), the blastopore becomes the anus, and a secondary opening becomes the mouth.

    This is an enormous difference, and the fate of the blastopore is one of the most powerful characters one can use to divide animals into the two major lineages above (Protostomia and Deuterostomia).

  • The very early (and very complete!) divergence of ontogenetic pathways suggests that the most recent ancestor of insects and vertebrates was nothing more than a gastrula-like creature (sometimes called a "gastrea", to distinguish it as a hypothetical adult organism, as opposed to an embryonic stage known today as a gastrula).

  • It didn't even have a brain, let alone a brain case.

  • Study of the embryological development of these two taxa shows clearly that the "brain cases" of insect and vertebrate are analogous--not homologous. They are a result of convergent evolution, the genesis of similar form from disparate ancestral characters in response to similar selective pressure.

    (Though let's consider an interesting side note on homeotic genes)

    Closely related organisms pass through similar embryonic stages. The earlier the divergence of embryonic fates occurs between two taxa, the more distant their most recent common ancestor.

    As development proceeds, the ontogeny of each taxon diverges from the "ancestral model," developing specializations that make it different from related taxa.

    History of a Controversy: "Ontogeny Recapitulates Phylogeny"

    In 1866, Ernst Haeckle (German physician turned zoologist) proposed the Law of Recapitulation (a.k.a. the "Biogenetic Law"). He stated that "ontogeny recapitulates phylogeny." He meant that an organism's embryonic development mirrored the sequence of adult forms from which it was evolutionarily derived. (For example, this would mean that because Homo sapiens passes through embryonic stages resembling a free-swimming larva known in some fish and amphibians, that humans had an ancestor that was like that swimming larval form as an adult.)

    Thus, Haeckle was suggesting that animals undergo embryogenesis because of their evolutionary history.

    In 1922, Walter Garstang published a paper in which he stated that evolution was not so much a succession of increasingly complex adult forms as much as it was a succession of progressivly more complex ontogenies:

    • A change in the adult means that there has been a change in embryogeny.
    • Traces of an ancestral organism's embryogeny are still reflected in extant organisms' embryogeny.
    In 1972, Gosta Jagersten (in Evolution of the Metazoan Life Cycle) suggested:

    • similar, distinctive larval forms in two taxa indicate some degree of shared ancestry

      (For example, all crustaceans pass through a larval nauplius stage, and all molluscs pass through a larval veliger stage.)

    • Even highly divergent groups' shared ancestry can be seen in the similarity of their larval forms.

      (For example, all annelids (segmented worms, such as earthworms) and molluscs (I hope you know what these are!) pass through a larval trochophore stage; the mollusc later develops into the more complex veliger.)

    Numerous papers were written on this controversial subject, many of them little more than arguments over the semantics of what "recapitulation" really meant.

    In 1940, Libby Hyman (one of the most eminent zoologists specializing in invertebrates) wrote :

      "Ancestral resemblance during ontogeny is a general biological principle. There is no need to quibble over the word "recapitulation;" either the usage of the word should be altered to include any type of ancestral reminiscence during ontogeny, or some new term should be invented."
    No new term was invented. But it is important to understand what is really going on when one sees similarity in ontogenies.

    Heterochrony

    Now that we've established that divergent ontogenies result in diversity of adult forms, let's look at some examples of how it works.

    Heterochrony (from the Greek hetero meaning "other" and chronos meaning "time") describes a change in the timing of ontogenetic events between two taxa. These, of course, can be the result of relatively small genetic changes that may not even be alterations in DNA sequence, but in the timing of particular genes being turned "on" and "off" during development via methylation of bases and other factors.

    Are we really just baby chimpanzees?
    A tale of Heterochrony and Allometric Growth

    Many animals undergo isometric growth as they mature from new hatchling to adult. This means that all the body parts grow at approximately the same rate, and the adult proportions are not significantly different from those of the juvenile. For example, see our pal Batrachoseps, one of the few salamanders that has a terrestrial (not a gilled, aquatic) larva:

    A heterochronic change can result from a mutation that causes the rate of one cell line of the body to develop at a rate different from that of other cell lines in the body. This can result in allometric growth.

    In a species that exhibits allometric growth (from the Greek allo meaning "different" and metr meaning "measure", different cell lines/body parts grow at different rates (relative to an ancestral, isometrically growing form) during development from juvenile to adult.

    Humans are a good example of a species that undergoes allometric growth. The head, limbs, and body grow at different rates, resulting in a human adult with proportions completely different from those of the newborn baby:

    .

    Hold that thought.

    Paedomorphy is a result of Heterochrony

  • A somatic cell is defined as one that makes up part of the body, but is not a cell that will become a gamete.

  • A germline cell is one that has the potential to undergo meiosis to become a haploid gamete (in most--but not all--organisms, this means egg or sperm)

    In animals, the body becomes reproductively mature at a very specific stage of somatic (body) development. In some species, a heterochronic mutation can cause the organism to become reproductive relatively sooner than an ancestral species. This can happen in one of two ways:

    progenesis: Somatic development proceeds at the same rate as in an ancestral species, but germ line cell development is accelerated as compared to the rate in that same ancestor. neoteny: Germ line cell development proceeds at the same rate as in an ancestral species, but somatic cell development is retarded as compared to the rate in that same ancestor.

    Here's an analogy that might help you visualize this. The end result is the same in either case.

    In either case, the resulting condition is known as paedomorphy: a reproductive adult that has the juvenile form of the ancestral species. Just looking at an animal that exhibits paedomorphy won't tell you whether it's due to progenesis or neoteny. You must compare the species' development to that of the ancestral (or closely related) species, if possible. Remember that "paedomorphic" is a relative term, too. You must be comparing the species to something else in order to consider it paedomorphic with respect to that something else.

    Examples of paedomorphic organisms we know and love:
    1. The Common Mudpuppy (Necturus maculosus) is a salamander which retains its juvenile gills as an adult
    Most salamander species have aquatic larvae that lose their external gills when they reach adulthood: Juvenile Ambystoma mabeei:

    Adult Ambystoma mabeei:

    The mudpuppy is paedomorphic with respect to other salamander species: It retains its external gills as a reproductive adult due to either neoteny or progenesis:

    2. Many domestic dog breeds (Canis lupus familiaris), which are derived from wolves (Canis lupus)

    3. Homo sapiens, whose prolonged brain development period and relatively flat face may be reflections of a prolonged juvenile period, relative to that of our closest relative, the chimpanzees (Pan paniscus and P. troglodytes)

    Remember:

  • paedomorphy/osis: the condition of an adult organism retaining juvenile features as an adult

  • progenesis and neoteny are two processes by which

    But paedomorphy is not the only possible result of heterochrony. Other phenotypic differences between closely related species also can be a result of differences in developmental timing.

    Timing is Everything

    . The time of onset of certain developmental characters can determine differences in phenotype. In any given species...
    • A characteristic/feature may appear relatively early in embryo development compared to an ancestor's (or related species) embryo development or

    • A characteristic/feature may appear relatively late in embryo development compared to an ancestor's (or related species) embryo development.
    For example, color pattern differences in closely related animal species can be a result of heterochronic changes that affect pigment deposition during ontogeny.

    Molecular Homology

  • As you already know, the genotype of an organism is its genetic code: the sequence of its DNA nucleotides .

    New technologies allow us to see that related taxa share similar DNA and RNA base sequences, often due to shared ancestry.

  • Molecular homology may be of greatest use when we attempt to determine how long ago very distantly related species (as different as plants and animals) diverged from a common ancestor.

    Molecular homologies also can be seen at the level of the finished protein product encoded by the DNA, as shown in this comparison of amino acid sequence in hemoglobin (the protein in your red blood cells that carries the oxygen) of different species...

    The study of molecular homology is the Evolutionary Biology of the Future.
    And the future is now.
    Stay tuned.