What is Systematics?
The science of systematics gives us the tools to communicate clearly about the natural world, and imposes order by describing the relationships between living organisms. Scientists name and put organisms into groups based on their evolutionary history (how closely related they are). These groups start very general and large, for example plants or animals, and become increasingly specific. Each species has a scientific name that anyone in the world can recognize.
The field of systematics is an active one, full of debates about the true evolutionary relationship between species and groups of species as well as the details of how we should represent and name those relationships. Systematists (people who study systematics) construct genealogies that represent their best understanding of the evolutionary relationships between species. To do this, they use specific traits called taxonomic characters. These characters are attributes that differ between groups. Historically, taxonomic characters were most often morphological traits such as the form, shape, or structure of an organism’s different parts. For example, taxonomic characters for a group of birds might include beak shape and size and wing shape. In Lepidoptera, important morphological traits include the shape of the genitalia and patterns of wing venation.
In the past few decades, systematists have acquired powerful new tools to study the relationships between species. For example, electron microscopes, which can magnify objects more than 100,000 times their normal size, allow scientists to look at tiny details of small structures such as eggs, scales, or pollen grains (see the scanning electron microscope pictures in Butterfly Sensory Systems earlier in this section). Using biochemistry and molecular biology techniques, systematists can also study the differences between organisms’ DNA and proteins. Molecular similarities and differences between organisms can then be used to judge how closely related the organisms are.
How are Organisms Classified?
People have classified and named organisms for millennia. Some scientists today, for example, study Native American names for plants as they work to classify new species. In the eighteenth century, a Swedish physician and botanist named Carolus Linneaus developed the naming system we use today. Using Latin and Greek words, Linneaus gave thousands of species two-part (binomial) names based on characters such as their appearance or origin. After Linneaus, binomial names replaced common names, such as garden pea or monarch butterfly, in scientific discussions and writing. Since the common name for a species often varies in different places, or may be used for more than one species, this system allows biologists to communicate more clearly.
The first part of a scientific name is called the genus name. A genus is a taxonomic classification of organisms that are closely related. The second part of a scientific name is called the specific epithet and differentiates between members of a genus. Together, the genus and specific epithet provide a unique name for every species, or kind of organism. Some scientific names of familiar species are:
Megaptera novaeangliae—humpback whale
Ursus horribilis—grizzly bear
Canis familiaris—domesticated dog
Danaus plexippus—monarch butterfly
Notice that the scientific names of these species are printed in italics. If you are handwriting a scientific name, you underline it. Scientific names are almost always descriptive. Using Latin or Greek, scientists name an organism for its appearance, its locality, its behavior, or to honor its discoverer. For example, Homo means human in Latin and sapiens means wise or knowing, so Homo sapiens refers to wise humans. Octopus means eight feet and vulgaris means common.
Let us look more closely at Canis familiaris to understand the other groups into which organisms are classified. The species name for dog originates in Latin: Canis means dog and familiaris means common, or familiar. Canis lupus is the scientific name for wolves. Because wolves and domesticated dogs are very closely related, they share the same genus, Canis. But dogs are also put into groups with other animals. This list includes all these groups, starting with a group that contains all animals, and ending with the group that contains only domestic dogs.
Start from the bottom of this list to see how scientists have grouped dogs with other animals. Remember that the scientific name of an organism is just its genus and specific epithet put together.
The specific epithet for domesticated dogs is familiarius. This group contains all domesticated dogs, even though different breeds may look quite unlike each other. They can all interbreed, which is an important criterion used by scientists to determine what constitutes a species. Domesticated dogs are grouped with their close relatives into a single genus, Canis.
Genera (the plural of genus) are grouped into broader categories called families. The “dog” family, called Canidae, also includes coyotes, foxes, and jackals. Common traits of these related genera include an elongated jaw with 42 or 44 teeth, five toes on the front feet, and four toes on the back feet.
Families with similar characteristics are put into the same order. The dog family is in the order Carnivora (Latin, to eat flesh), which also includes bears, cats, and other meat-eating animals. The presence of canine teeth (fangs) distinguishes this order.
Similar orders are grouped into classes. Carnivores are in the same class as us, Mammalia. Mammals are distinguished from other classes of animals by hair and by the birth of live young who feed from specialized mammary glands as infants.
Similar classes are put into the same phylum. Mammals are in the phylum Chordata, along with related classes such as amphibians, reptiles, and fish. All members of this phylum have a nerve cord running down their back. Most members, including humans, are called vertebrates. In vertebrates, the nerve cord is enclosed in a backbone made up of small bone units called vertebrae.
Finally, all phyla (plural of phylum) are grouped into the next largest category, called a kingdom. The animal kingdom includes vertebrates, insects, segmented worms, shellfish, and all other multicellular organisms that cannot make their food from the energy of the sun. Scientists recognize five kingdoms of living organisms: animals, plants, fungi, bacteria, and protists (one-celled organisms that have a different cell structure than bacteria).
The table on the next two pages compares the classification of monarch butterflies and humans, starting with their kingdom, and describes the characteristics of each group to which they belong. (Note: the numbers of living species are accurate as of January 2021. Also remember that these numbers can fluctuate with scientific research and debate. Sources may state different numbers.)
Comparing Monarch and Human Classification
Animalia. Both monarchs and humans are in the animal kingdom. There are more than a million animal species in the world; over three-quarters of these are insects, while only about 3% are vertebrates. We do not know exactly how many animal species there are because many have not been discovered yet. New species are discovered every year. All animals get the food and energy they need by consuming other organisms instead of using the energy of the sun. They are made up of many cells, and most reproduce through a process that involves the joining of egg and sperm cells. There are about 35 animal phyla. Examples of animal phyla include sponges; jellyfishes and corals; flatworms, flukes, and tapeworms; molluscs; segmented worms; starfish and sea urchins; arthropods; and chordates.
Arthropoda. (jointed foot, Greek) This is the largest phylum of animals, containing more than one million species. All arthropods have an external skeleton, called an exoskeleton, and jointed limbs in pairs.
There are eight classes in the phylum, including insects, crustaceans, spiders, millipedes, and centipedes.
Chordata. There are about 65,000 chordate species known today. Members of this phylum all have a nerve cord in the back of their body. The phylum Chordata is divided into three subphyla, which are groups whose relatedness and similarity are in-between a phylum and a class.
Two of the Chordata subphyla have only marine members. The subphylum that includes humans is divided into 7 classes: 3 classes of fishes, amphibians, reptiles, birds, and mammals.
Insecta. There are over a million known species of insects, and several thousand new species are discovered every year. All insects have three divisions in their bodies: a head, thorax, and abdomen.
They have three pairs of legs on their thorax and obtain oxygen through internal air tubes called trachea. There are 29 orders of insects, a few of which include diptera (flies), coleoptera (beetles), hymenoptera (bees, wasps, and ants), odonata (dragonflies), isoptera (termites), and lepidoptera (butterflies and moths).
Mammalia. There are 5500 species of mammals in the world. All mammals are homeotherms, which means that they can regulate their body temperature. They also have a four-chambered heart, hair, and nourish their young with milk produced by mammary glands in the female.
There are 26 orders of mammals, some of which include monotremata (egg-laying mammals), marsupials (pouch-bearing mammals), chiroptera (bats), lagomorpha (hares, rabbits, and pikas), rodents, cetacea (whales), and primates.
Lepidoptera. (scale wing, Greek) The Lepidoptera include all butterflies, moths, and skippers. There are about 180,000 known species of Lepidoptera, most of which are moths. In fact, butterflies constitute only 8%, or about 14,500 species of the Lepidoptera. All have complete metamorphosis, with egg, larva, pupa, and adult stages. As adults, they have four wings with scales on them and a proboscis through which they ingest liquid food.
There are over 70 families of Lepidoptera, six of which are butterflies.
Primates. There are between 190 and 448 species of primates, which include lemurs, tarsiers, monkeys, apes, and humans. Primates have a relatively large brain, an opposable thumb, and eyes that face forward.
There are nine families of primates with living members.
Nymphalidae. This is the largest family of butterflies, with about 6,000 species. A characteristic of this group, which is easy to see on monarchs, is their undeveloped front legs. These legs are much shorter than the other legs and are tucked up under the head. In males, the front legs are often covered with dense tufts of scales, giving this group the name “brush-footed butterflies.”
Hominidae. There is only one member of this family that still lives today, and that is humans. An extinct genus, Australopithecus, is also put into this family, and some anthropologists include Ramaphithecus as well. This family is characterized by an erect posture (walking upright), bipedalism (walking on two legs instead of four), less hair than other primates, and an even bigger brain than other primates.
Danaus. This genus, often called the milkweed butterflies, includes 12 species of both New and Old World butterflies.
Homo. (human, Latin) We are the only living member of this genus. Another species, Homo erectus, is extinct.
plexippus. Only monarchs are given the specific epithet plexippus, making their full name Danaus plexippus.
sapiens. (wise, Latin) We have given ourselves the specific epithet sapiens, and our complete scientific name is Homo sapiens.
Butterflies vs. Moths
Lepidoptera are divided into dozens of families. Generally, we think of these families in two groups: butterflies and moths. However, if we drew a “family tree” that accurately showed the evolutionary relationships between butterflies and other Lepidoptera, it would show that some moths are closer relatives of butterflies than of other moths. Butterflies are best described as a group of moths that has “escaped” being nocturnal and is most active during the day.
Butterflies and moths usually can be distinguished by the following traits. There are, however, exceptions; for ex- ample, a few moths are active during the day and some butterflies have “fuzzy” bodies.
|clubbed antennae||slender or comb-like antennae|
|wings upright when resting||wings open when resting|
|sleek (tightly scaled) body||"fuzzy" body (longer, looser scales)|
|active during the day (diurnal)||active during the night (nocturnal)|
|eggs upright||eggs flat or oblong in most, but upright in cutworms, tiger moths, and gypsy moths|
|pupal stage has no silk covering, called a chrysalis||pupal stage in most species has silk covering, called a cocoon (or just "pupa" if there is no silk covering)|
This content was originally published in the Monarchs & More curriculum (4th Edition).