Understanding relationships is a critical part of being a human
being, and it has been throughout our history. Who are your
close relatives, that you can depend upon for support but probably
not for spouses? Is it safe to eat this plant if it closely
resembles one you know is safe? Animals with high-contrast
markings are probably good to avoid. Also, having language, we
love to label things, give them names. Label and categorize,
the science of biological taxonomy, a way of understanding
relationships among living things, was inevitable for humans to
Taxonomy as the U.S. government defines it.
humans first began categorizing Nature and writing down the groups,
they often based their groupings on analogies, resemblances
in abilities or simple external form. In such a system, a
snake, an eel, an earthworm, and a roundworm would all be included
in a "serpent" category, or anything with wings, or fins, would be
assumed to be related. This also led to the first definition
of Kingdoms: one large group for plants, one for
animals, one for rocks. Analogy can sometimes be helpful in
establishing relationships, but it is very limited. Evolution
often produces structures that look the same during the
adaptation-to-similar-environments process. This production of
superficial resemblance is called convergence or
Genesis story in the Bible shows this old way of categorizing - read
days 5 and 6.
A bit more on convergence.
closely at animals, especially at their inner structure, suggests
that analogy can be misleading. A bird and a bat support their
wings with front arms and bones, but the bones at the tip are very
different; insect wings have no bones at all, and in fact an
insect's skeletal structure is outside rather than inside. The
bones of a bat's wings show closer resemblance to our own arms and
hands than they do to a bird's wings; this, and other
similarities in basic architecture, places the bats in a group with
humans, cows, and dogs (the mammals) rather than the birds.
Similar comparisons make it obvious that whales are not fish,
pterosaurs were not dinosaurs, and birds probably are close
relatives of dinosaurs. This architectural similarity is
called homology. It is much more useful than analogy in
linking biological relatives together. Evolution that drives
similar architecture to do different tasks - legs to flippers in
whale evolution, for instance - is called divergence or
divergent evolution. Divergence to one extent or another
is how different types of organisms evolve.
Some homologies in forelimbs (you need much more to truly categorize
Image of vertebrate embryos at comparable stages.
Homologies, as mentioned above, are not confined to just bones
or other obvious adult structures. Structures that exist early
in life, in the first stages of embryos, can hold on to
resemblances that have long disappeared in the adults. There
are two probable reasons for this. First, the earlier a major
change happens, the more dramatically changed the eventual outcome
is likely to be, and dramatic changes are even more rarely useful
than subtle ones. Second, embryos often have much more similar
environments, encased in eggs or seeds, and have needs that haven't
changed in eons (flies and humans, for instance, use very homologous
genes to determine where their eyes will grow).
Comparisons of embryos was the basis for one of the
great cautionary tales of biology history. In the late
1800s, Ernst Haeckel decided that as a mammal embryo developed, it
actually replayed its evolutionary history: it was a fish for
a while, then a reptile, then a primitive mammal, and so on.
This was known as "ontogeny recapitulates phylogeny," and
Haeckel stared for hours through a microscope at embryos, making
many drawings that were labeled to show off his rule. But it
turns out that he was seeing what worked and ignoring what didn't (a
natural human trait that scientists have trouble avoiding);
embryos do show their relatedness to fish and reptiles, but don't
actually become them. Other scientists quickly discovered that
Haeckel's ideas were not supported by the evidence, and he is now
known for a famous mistake.
Stages in a human's development.
Embryonic resemblances in research.
More on Haeckel's embryos.
Some of Haeckel's drawings.
It has been found that control of
very basic layout, such as positions of heads, or limbs, or
structures like eyes, is connected to genes that have been around
for a very long time and can be found in a broad range of even
distantly-related groups. These genes are called HOX genes.
These genes even hang onto things like the order for how parts
of the body develop, as seen in the video link.
More on HOX genes.
Hox genes in a linear sequence.
Since many molecules have complex
structures, including long sequences of components, comparison of
molecules is often used in modern biology to detect relationships.
Differences in sequence especially are useful, especially in DNA,
which has long bits that can apparently change components with no
effects - these point mutations happen, get passed on, and
persist down a family line. How often such changes happen is
related to chance, and can be estimated over a timeline - as a
family tree separates into different branches, each branch over time
will accumulate its own unique point mutations. The more
different mutations, the longer it has been since the tree branched.
These comparisons are called molecular clocks, and are
commonly used in evolutionary biology. They are not perfect;
no one can say for certain that mutation chances stay the same over
long periods, or whether different parts of DNA have different
mutation rates. However, they can be useful in establishing
how closely related different groups are. Such a comparison
can support the idea that fungi are actually more closely related to
animals than plants.
More on molecular clocks.
A critique of believing too much in the regularity of molecular
A molecular clock leads to an estimate of when humans began to wear
the study of Nature became more and more common, with many
people speaking many languages adding to the knowledge, it became
obvious that some agreed-upon approach to classifying living
things was needed. The one that was eventually worked out,
which we still use today, was partly worked out by Carl von Linne
(known in publication, since most was in Latin, as Carolus
Linnaeus). He developed the binomial nomenclature
method of naming species, using an approach similar to how people
get their names.
Here are the rules for species names:
Species names are always two
The first word is always
capitalized, the second one never is.
The first word is the genus
(group the species belongs to), and the second word means
nothing by itself.
The name in English is treated
as a foreign phrase - italicized or underlined.
Species names are commonly
abbreviated with the genus' initial and the second word (E.
coli is an abbreviated species name).
So what is a species, anyway? It's a
particular type of organism, but what makes it different from
closely-related species? It once was based purely on
description. Later, it was based on reproductive compatibility
- different species couldn't make offspring together. But
sometimes they can, but the offspring are often sterile, so the
definition expanded to include that. But...biology is a
science of exceptions, which makes rules tough to make. The
latest definition (probably soon to be replaced with one based on
genetics) uses behavior: a group that keeps to itself
reproductively under natural conditions.
Basically, we let the species define themselves.
And all of this really only applies to sexual
species with a more-than-one-parent system. The others are
Linnaeus brief biography.
odd species names.
internet: Carl's letters.
The Linnaeus legacy.
A discussion of defining species.
With species collected
into a genus, what larger group would a genus fit into? As
more and more relationships were determined, a bigger
all-encompassing system of groups-within-groups was needed.
Here's how it works:
Species are collected into a Genus.
Genera (plural of genus) are collected
into a Family.
Families are in an Order.
Orders are in a Class.
Classes are in a Phylum.
Phyla are in a Kingdom.
To give the system some flexibility, groups can be
additionally linked and split through the use of Supergroups
(a Superfamily contains Families and would be inside an Order or
Suborder) or Subgroups (a Subclass would be contained inside
a Class). This means that if new information is discovered,
connections can be added without shifting the entire "ladder" up or
The position and relationships of living things is
often the subject of debate: someone thinks that a group
belongs somewhere, but others don't. People have disagreed as
to whether giant pandas (Ailuropoda melanoleuca)
belonged to the bear family (Ursidae) or the family
containing raccoons (Procyonidae). There are rules
here, too: you can easily propose that a group be moved, or be
classified as a smaller or larger type of group, but you can't
change the group's name without a long and particular
process. That way, folks may not recognize a group's level,
but they will still recognize the name and know what sort or
organism is being discussed.
A class getting the group organization down, with rhythm.
A whole bunch of ways to remember the order of groups.
Some taxonomic lists showing sub- and supergroups.
In the earliest attempt to categorize
Nature, there was the Plant Kingdom, the Animal Kingdom, and the
Mineral Kingdom. It didn't take long to realize that only the
first two included living things (although with spontaneous
generation, folks thought that members of one kingdom could be
generated by members of another, like snails from stones). It
was eventually realized that fungi, which had been included in with
the plants, were so different that they deserved their own kingdom.
Then along came the microscope and
the discovery that tiny single-celled living things existed.
These often showed combinations of animal, plant, and fungus
features and were very difficult to place in the existing kingdoms.
They were eventually split off in their own kingdom, and as more was
discovered about them, those kingdoms split into two, then three,
kingdoms of single-celled organisms. Today, basic biology
courses generally teach the six-kingdom system, although many people
recognize even more, as well as a bigger category, Divisions.
MODERN SIX-KINGDOM SYSTEM:
(cells have no nucleus); always unicellular
(single-celled). Bacteria. May have plant,
fungus, or animal characteristics.
always unicellular. Often adapted to unusual and/or extreme
conditions, such as very hot, very salty, or no-oxygen
environments. Have several different cellular chemistries from Monera.
(nucleus in cell); mostly unicellular, or collections
of very similar cells. May have plant, fungus, or
multicellular; capable of photosynthesis, production
of complex molecules from simple molecules using light.
multicellular; must obtain complex food molecules from
external source, broken down and absorbed internally.
Usually capable of movement.
almost all multicellular; must obtain complex food
molecules from external source, absorbed through external
surface. Almost never capable of movement.