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.
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.
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.
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.
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.
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.
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?
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.
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.
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:
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.
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:
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)
New technologies allow us to see that related taxa share similar DNA and RNA base sequences, often due to SHARED ANCESTRY.
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...
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...
POPULATION: All the members of a single species living in a defined geographic area.
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:
Let's look at a modern-day example of MICROEVOLUTION due to Natural Selection.
a. Did phenotypic variation exist in the bacterial population? YES
(both sensitive and resistant strains were present in the patient)
b. Was the variation heritable? YES
(Rifampin works by inactivating an enzyme the bacteria use to make protein. Like all the bacteria's proteins, that enzyme was coded by its DNA. In the Rifampin-resistant bacteria, a single mutation of ONE of the DNA "letters" coding for that enzyme made the enzyme UNRECOGNIZABLE to the antibiotic! The Rifampin could not inactivate the mutant enzyme.) c. Did natural selection occur? YES.
(M. tuberculosis with the mutant enzyme (not affected by Rifampin) left more offspring than those that lacked the mutation.)
d. Did a non-random subset of the original population remain after
The populations before and after rifampin administration were significantly genetically different, with resistant bacteria replacing rifampin-sensitive individuals.
This eventually can lead to speciation, the reproductive isolation of the two populations from each other This is also known as MACROEVOLUTION.