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What observable evidence supports the idea that life on earth has evolved its diversity through a long process of DESCENT WITH MODIFICATION?

In biology, a HOMOLOGY is a characteristic shared by two species (or other taxa -- a taxon is the generic term for a classification group such as a Kingdom, a Phylum, etc.) that is similar because of common ancestry.


A structure found in two (or more) different species, but derived from a common ancestral structure is said to be HOMOLOGOUS in those species. The structure may or may not be used for the same function in the species in which it occurs.

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

In contrast, a structure that serves the same function in two species, 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 the driving force being natural selection factors that favor one form over another in specific environments.

  • vestigial structures 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.

    Biosystematists often use the existence of shared characters in related taxa to help reveal their common ancestry.

    The more recently two species diverged from a common ancestor, the more shared, derived characters they will share.

    In determining recency of common descent, symplesiomorphies are generally not informative, but 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?)


    First, let's consider how homologous characters can originate and change.

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

    Flight may have been a "happy accident"--a side effect of these original functions.

    But beware the danger of slipping into the "Just So Story".

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

    Similarities and differences in ontogeny among animals are not trivial.
    They reflect the evolutionary relationships of the taxa in which they occur. Some have suggested that embryonic innovation may have been the evolutionary mechanism by which entire new lineages originate. A "mistake" in embryo development can be the seed of a new, adaptive form!

    Consider a group we'll study in more detail a bit later: Vertebrates. All vertebrates share the following traits at some point in their development:

    ...but not all vertebrates have those characteristics evident as adults.

  • How do we know those characters are common to all vertebrates?
  • We look at their embryonic development!

    These similarities were first noted by eminent evolutionary biologist Ernst Mayr (the above is his illustration) in his book What Evolution Is. In the text, he notes: "embryonic similarities, recapitulation, and vestigial structures . . . raise insurmountable difficulties for a creationist explanation, but are fully compatible with an evolutionary explanation based on common descent, variation, and selection." As Mayr also notes, if evolution is not true, "why should the embryos of birds and mammals develop gill slits, like fish embryos?"

    By studying the ontogeny of various vertebrates we can find shared characteristics that might be lost when the adult is fully developed. (Example: where is YOUR notochord?)

    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 homologous.

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


    Are these structures homologous or analogous? Was an original 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. But to see it, we have to look at a few drawings of early animal embryos.


  • In one large group of animals, the blastopore goes on to become the MOUTH. (These are PROTOSTOME animals)
  • In another lineage, the blastopore goes on to become the ANUS. (These are the DEUTEROSTOME animals)

    Insects, as it so happens, are protostomes. Vertebrates are deuterostomes. So the very last embryonic stage we share in common with insects is that gastrula that hasn't yet made its mouth vs. anus decision.

    In fact, the most recent common ancestor we share with insects proably looked a bit like that confused gastrula--a free-swimming little blob in the ancient oceans. It didn't even have a brain, let alone a case for it! This hypothetical ancestor of all animals is sometimes called a gastrea

    Further comparison of insect and vertebrate ontogeny reveals almost NO similarity between the two groups.

    So are vertebrate cranium and the insect head case, so similar in function and even in appearance, HOMOLOGOUS or ANALOGOUS?

  • The similarity is superficial, and a result of CONVERGENT EVOLUTION, the genesis of similar form from disparate ancestral characters in response to similar natural selection pressure. (Think: What does it mean to "converge"?)

    Bottom Line:

    Closely related organisms pass through similar embryonic stages. The earlier the divergence of embryos occurs between two species, the more distant their most recent common ancestor, as with the vertebrate/insect gastrea example above.

    As development proceeds, the ontogeny of each species diverges from the "ancestral model," and each species develops unique features that make it different from related species.


    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 species. These, of course, can be the result of relatively small genetic changes between an ancestor and its descendant species.

    One type of heterochrony: PAEDOGENESIS In many species, development of the body (a.k.a. somatic development--soma means "body" in Greek) largely ceases when an animal reaches sexual maturity.

    If, by some accident of genetics, an animal becomes sexually mature while the body is still growing, and in its juvenile form, then somatic development will cease. The resulting condition is known as PAEDOMORPHY: the retention of juvenile features in a sexually mature (adult) organism.

    Let's explore...

    A tale of Heterochrony and Allometric Growth

    Many animals undergo ISOMETRIC GROWTH as they mature from new hatchling (fresh from the egg) 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 (i.e., the cells descended from one particular cell of the blastula) of the body to develop at a rate different from that of other cell lines in the body. This can result in ALLOMETRIC GROWTH--different parts of the body grow at different rates.

    In a species that exhibits ALLOMETRIC GROWTH (from the Greek allo meaning "different" and metr meaning "measure" (and also, interestingly "womb")), 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.


    In animals, the body becomes reproductively mature at a very specific stage of somatic (body cell) development. In some species, a heterochronic mutation can cause the organism to become reproductive relatively sooner than its ancestral species.

    The resulting condition is known as PAEDOMORPHY: a reproductive adult that has the juvenile form of the ancestral species.

    Examples of paedomorphic organisms we know and love:
    1. The Common Mudpuppy (Necturus maculosus) is a salamander that 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:

    2. Many domestic dog breeds (Canis lupus familiaris), 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)

    Paedomorphy isn't 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. It is the time of onset of certain developmental characters that determines differences in adult form. In any given species... Even color pattern differences in animals can be a result of heterochronic changes that affect pigment deposition during ontogeny.


  • The GENOTYPE of an organism is its genetic code: the sequence of the DNA "letters" in its genes.

    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 can also 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...

    Now that we have seen the observable evidence for evolution itself...


    Is Darwin's idea about Natural Selection testable?

    It's easy to see the diversity of animals all around us, and doesn't take a great stretch of imagination to guess that the reason they are different is because their genetic codes (as stored in that magical molecule, DNA) differ.

    More subtle are genetic differences WITHIN A SPECIES. Yet this variation is the raw material of evolution. Without it, there would be no genetic variation upon which natural selection (or other evolutionary processes) could act. Consider for a moment...

    These are examples of diversity of gene expression (i.e., the physical result of the genetic code) in a population. In these cases the traits do not appear to be actively under the pressures of natural selection--or anything else that would reduce the genetic diversity now present in the population. But they may have been at one time. And they might be again, in the future. It all depends on whether the environment changes.

    POPULATION: All the members of a single species living in a defined geographic area.

    Though Darwin's idea (natural selection) was probably the most important and influential one in the history of Biological Science, he didn't consider some of the other mechanisms by which evolution also can take place, most of which have to do with RANDOM PROCESSES.

    In order for a population to NOT be evolving (i.e., for its inherent genetic diversity NOT to change from generation to generation), five conditions must be met:

    If any of these conditions is not met, then it is very likely that the genetic composition of the population will change from generation to generation.

  • Microevolution: Genetic change within a population without speciation.
  • Macroevolution: Genetic change within a population with speciation.

    Let's look at a modern-day example of MICROEVOLUTION due to Natural Selection.

  • A TALE OF RESISTANT BACTERIA Darwin's Theory is supported, once again!

    If two once-contiguuous populations are isolated from each other for many generations, their mutations, natural selective pressures and random genetic events will be different. Unless there is GENE FLOW (i.e., exchange of genes via mating) between the populations, each population will change in isolation from the other.

    This eventually can lead to speciation, the reproductive isolation of the two populations from each other This is also known as MACROEVOLUTION.