This might not have been the case in Darwin's time.
But since the publication of On the Origin of Species, a great deal more has come to light.
There are at least four major lines of physical evidence telling us that evolution has occurred, and is still occurring.
Speciation is the generation of two (or possibly more) reproductively isolated new species from an ancestral species.
Speciation implies that the two new species are no longer able to produce fertile, viable offspring with each other: they are reproductively isolated.
Lines of Evidence: What Rocks and Fossils Tell Us
Despite what you might have heard, fossils are NOT the best line of evidence for evolution.
However, they offer a "snapshot" into various eras of the past and the organisms that lived in those times.
Fossils show us how life has changed over time, with species appearing, giving rise to new species, and often going extinct.
Paleontology and Geology were the first scientific disciplines to yield information indicating that earth was more than a few thousand years old.
Countless mountain ranges filled with marine fossils tell us that the earth we see today was very different long ago.
Note that fossils are rare. Most organisms are consumed and destroyed relatively soon after they die. Only rarely do properties of the organism and environment promote fossilization. If you're going to become a fossil, it helps to have...
Fossil bodies or body parts can be unaltered (composed of actual matter from the organism):
Determining the Age of Fossils: Relative vs. Absolute Dating
Before radiometric dating techniques were invented, paleontologists were able to compare the age of fossils and rocks with relative dating.
Stratigraphy is the study of layers (strata) of sedimentary and volcanic (=igneous) layered rock.
Some common forms of sedimentary rock include
Geologists represent such formations as a stratigraphic column, which describes the vertical placement of rock layers/units in a particular location.
Absolute (Radiometric) Dating
To determine the absolute age of a fossil, radiometric dating techniques must be employed.
Molecules usually exist in a stable form that does not release any of its subatomic particles.
However, sometimes a molecule may exist as an isotope.
Isotopes are atoms of the same chemical element that differ only in their number of neutrons.
A radioactive isotope is one with an unstable nucleus. To become stable, the atom emits ionizing radiation (alpha and beta particles or gamma rays). In the process, the nature of the particles in the nucleus changes, and the radioactive substance becomes a stable isotope of a different element.
...yields the age of the sample.
The half life of a specific isotope is the time needed for half the nuclei in a sample of it to decay to its stable form. After one half life, one half of the original sample will remain radioactive. After two half-lives, one fourth of the original sample will still be radioactive; after three half-lives, one eighth of the original sample will still be radioactive, etc.
Specific isotopes have known rates of decay and known half lives. For example:
The relative proportions of a radioactive isotope and its decay products in a sample tells us how much time has elapsed since the sample was formed.
The greater the proportion of decay product to radioactive isotope, the older the sample.
Different elements and isotopes must be used to date different types of fossils.
Lines of Evidence: Homologies
First, let's establish how evolution does NOT happen.
A question sometimes posed by people who do not understand evolutionary mechanisms is:
2. Humans and monkeys DO share a common ancestor that has since become extinct.
3. About 35 million years ago, that common ancestral lineage split into
4. There are still monkeys because, like humans, they are descended from successful ancestors.
5. The ancestor of humans and monkeys had characteristics common to both types of animals.
6. Modern lineages of monkeys and great apes (including humans) have specializations that make them unique and different from each other and from that common ancestor.
...but they do all share a common ancestor, if you go back far enough.
One of the most powerful forms of evidence for the common descent of all living things is homology. In biology, a homology is a characteristic shared by two species (or other taxa) that is similar because of common ancestry.
A classic example of homology is seen in the skeletal components of vertebrates...
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.
Scientists have several criteria for determining whether structures are homologous.
Let's have a look at some of our own homologies in The Zoo that is YOU.
Shared homologies allow us to devise hypothetical "family trees" of living organisms called phylogenies.
These can be represented as a branching diagram called a phylogenetic tree or evolutionary tree.
Just as humans change across generations, so, too, do natural populations of other species.
By comparing the traits of species that are both closely and distantly related to each other, we can begin to understand their evolutionary history.
Primitive and Derived Characters
We can learn about the process of evolution from studies of both extant and extinct species, and comparing their physical and chemical characteristics.
The long tail of the baboon is a more primitive condition than very reduced (derived) tail of the gorilla or the Very Handsome Man
Latent Genes Can Awaken
Occasionally, we get an
atavistic reminder of our ancestry.
And then there are those pesky supernumerary nipples.
Symplesiomorphies and Synapomorphies
Note that "primitive" and "derived" are relative terms.
You can't call something "primitive" or "derived" without comparing it to something else.
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.
Homology Reveals Relatedness
The synapomorphic astragalus ankle bones of ancient whales (Cetaceans) and cloven-hooved mammals (Artiodactyls) helped biologists determine that these two groups share a relatively recent common ancestor.
When you're trying to determine common ancestry, synapomorphies are far more informative than symplesiomorphies.
To illustrate this, let's consider our own species and some of our closest vertebrate relatives.
(See what you missed if you didn't come to class?)
Analogous Structures: Convergent Evolution
Structures can appear similar for reasons other than common ancestry.
An analogous structure found in two different species
Example: The wing of a butterfly and the wing of a bat.
How do we know the common ancestor of the butterfly and the bat didn't pass on the wings to both species?
To fully understand how we know the wings evolved independently, we must take a trip deep into the past, to meet the last common ancestor of the bat and the bird.
During ontogeny (embryo development), all animals go through this progression:
During the ontogeny of a butterfly and a bat, the gastrula stage is pretty much where developmental physical similarity between them ends.
Protostomes vs. Deuterostomes
More than 520 million years ago, a genetic change in one lineage of animals caused the blastopore to develop into an anus instead of a mouth.
From this ancestral change, the animal lineage we know today as the DEUTEROSTOMES arose.
In most animals, the blastopore becomes the mouth: The bilaterally symmetrical among them are protostomes:
In most animals, the blastopore becomes the mouth:
The bilaterally symmetrical among them are protostomes:
But in one lineage, a genetic change caused the blastopore to develop into the anus. This lineage comprises the deuterostomes:
Back to Butterfly and Bat Wings...
Or possibly the slightly fancier Urbilateria, a simple flatworm that was barely more than a crawling gastrula, at least in some depictions.
The ancient last common ancestor of the butterfly and the bat had nothing like wings.
This tells us that the wings of the butterfly and the wings of the bat evolved independently, long after their ancestral lineages diverged from the gastrula-like ancestor.
The same developmental toolkit may be used to generate analogous structures in distantly related animals.
Reptile scales and bird feathers are homologous. But how did scales turn into feathers without becoming a liability? Some have suggested that elongate reptile scales may have initially been retained in the population because they conferred a selective advantage as...
Flight (certainly an adaptive function) may have been a "happy accident"--an eventual outcome these original functions.
But beware the danger of slipping into the "Just So Story".
Vestigial Characters: Highly Derived and Mostly Non-functional
A vestigial structure is one that has marginal, if any use to the
organism in which it occurs.
These are some of the most interesting examples of homology.
Ever heard the phrase, "Ontogeny Recapitulates Phylogeny"?
The phrase above was coined by German physician Ernst von Haeckel, who quit his medical practice to become an apologist for evolution after reading Darwin's On the Origin of Species by Means of Natural Selection.
Haeckel was the first to notice that the embryonic forms of different species had remarkable physical similarities.
He suggested that every living thing passes through its evolutionary history during its embryo development.
While he didn't quite get his wording right (embryos don't actually pass through the adult forms of all the species that came before them), it is true that ontogenetic similarities across species reflect those species' common ancestry.
Differences in ontogeny among taxa are not trivial.
Such changes reflect the
evolutionary relationships of those taxa, and the modified development that resulted in one species becoming two.
Developmental innovations may have provided the evolutionary "raw material" for the origin of entirely new lineages.
The claws of the baby hoatzin also fall into this category.
In embryo development, Timing is Everything (<--required link!).
More complex adult forms are the result of changes in embryo development. These can be
Consider members of the Phylum Chordata.
At some point in their development, all members of Chordata show these traits, which are unique to Chordates:
In somewhat more derived chordates (Urochordata), the notochord is seen in the larval form, but lost in the adult:
3. Molecular Homology
The molecules that carry the instructions for constructing and running the living organism--DNA and RNA--show homology across species.
The more recently two species descended from a common ancestor, the more similar their DNA sequences.
DNA encodes instructions for every living thing on earth. This in itself is powerful evidence of common ancestry.
Hox Genes: Duplications and Conscription
Whether it's a fruit fly or a mouse, homeo domain (Hox) genes are expressed every animal species known, and in a specific order that corresponds to the anterior --> posterior body axis.
(The homeodomain is NOT amenable to mutation.)
Addition of complexity over time may be partly due to duplications of Hox complexes.
In mice and other vertebrates, there are four Hox complexes on four different chromsomes, and each complex includes 9-11 genes, for a total of 39 Hox genes in vertebrates.
Hox gene mutations cause changes the identity of the segment affected by the gene's action. The Hox genes are homologous among all animal species.
Chromosomal Homology: Synteny
We can see common ancestry in the chromosomes:
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 oxygen) of different species:
Most modern evolutionary biology is the study of molecular homology.