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| Tree of life https://evogeneao.s3.amazonaws.com/ images/content/es/tree-of-life_2000.png |
In 1735, the Swedish naturalist Carl von Linné (Linnaeus) created the binomial system of biological nomenclature still in use today, and a classification system for living things that is becoming obsolete. This system used at first seven successive categories: kingdom, phylum, class, order, family, genus, and species. The last two categories are used to name the species. Thus, according to Linnaeus's classification, the human species belongs to the animal kingdom, the phylum Chordata, the class Mammalia, the order Primates, the family Hominidae, the genus Homo, and the species sapiens. Its scientific name, according to Linnaeus's binomial system, is therefore Homo sapiens.
As new species of living things were discovered and
described, Linnaeus's classification system proved inadequate. There were too
many genera in each family, too many families in each order, and so on.
Therefore, intermediate categories were introduced: subkingdom; subphylum;
superclass; subclass; superorder; suborder; superfamily; subfamily; tribe;
subgenus; and subspecies.
Based on Darwin's theory of evolution, two distinct
trees coexist in the biological world: a) The tree of life or tree of evolution, which shows the way in which species or groups of
living beings have changed over time. b) The taxonomic tree, the tree of classification, where all living
beings that live today or existed in the past are found in parallel at the
lowest level, the level of species. One advantage of this tree is that it
facilitates the segmentation of biology into simpler branches (zoology, botany,
entomology, ornithology, etc.), each of which can be studied as an independent
field. The degree of relatedness between two species can be measured by the
level of the lowest taxonomic category that includes them.
According to the traditional classification,
vertebrate animals were a subphylum of the phylum Chordata and were divided
into five classes: fish, amphibians, reptiles, birds, and mammals. All extinct
vertebrates, known from their fossils, fell into one of these five classes.
Dinosaurs, for example, were classified among the reptiles.
In 1950, the German biologist Willi Henning
proposed replacing the traditional classification with a new system based on
the tree of evolution, thus creating a phylogenetic or cladistic classification. However, it took quite some time to gain
acceptance, not until the 1980s, and since then it has become dominant. No
other classification is used anymore by biologists and paleontologists.
To achieve this, living species are divided into clades. What is a clade? It is the result of separating a part of the tree of life by cutting
one of its branches. What is separated from the tree is a clade. Therefore, there are as many clades as there are cuts we can make. But only some are
truly important. It depends on which branch of the tree one cuts.
Of Linnaeus's five classes of vertebrates, only two
are clades. Fish are not, because if we cut the tree at the
point corresponding to the first fish in the history of life, amphibians,
reptiles, birds, and mammals split off at the same time, for all of them are
descendants of fish. The same happens if we cut the tree at the point
corresponding to the first amphibian. Birds and mammals are clades, because both groups can be separated from the
tree by cutting it at a single point.
Reptiles, on the other hand, present a problem. If
we cut the tree at the first reptile, birds and mammals follow. The problem is
mitigated because almost at the beginning of reptile evolution (the first amniote animals), they divided into two large groups: synapsids and diapsids.
We therefore have two clades, one of which includes mammals, the other
dinosaurs and birds. Today, the word "reptile" is often used
synonymously with the clade of diapsids, so dinosaurs are still considered
reptiles, but birds are both reptiles and dinosaurs (see figure).
But where does the group of birds begin? Where does
the group of mammals begin? To decide this question, a cladistic rule is
sometimes applied, though not all biologists agree: a clade of
animals with current members (not completely extinct) begins with the most recent common ancestor of all current members
of that clade.
According to this rule, in the accompanying figure, which shows the
phylogenetic tree of synapsids (and therefore mammals), the mammal clade should
be cut off at point 1, since that is the most recent common ancestor of the
three groups of extant mammals: monotremes, marsupials, and placentals.
Other biologists disagree. For them, even though
some groups have become extinct, the mammal clade should split off when the
corresponding groups already exhibit all the features considered typical of
mammals. In the figure, that would be at point 2. Note that this requires the
inclusion of two additional extinct groups in the mammal clade.
Regarding Linnaeus's classification categories,
attempts are still being made to use them, although it is sometimes very
complicated. For example, in the traditional classification, birds and reptiles
were a class; diapsids a subclass; and dinosaurs a superorder. With cladistic
classification, diapsids or reptiles include dinosaurs, which in turn include
birds; therefore the classic hierarchical categories are no longer useful. We
will have to get used to it, although I don't think the new classifications will
soon become part of general knowledge, as primary and secondary education are
very slow to adopt new ideas. Which, in this case, is perhaps a blessing.
The two images showing the clades of dinosaurs and
synapsids were taken from the books The Rise and Fall of the Dinosaurs: A New
History of a Lost World
and The Rise and Reign of the Mammals: A New History, from the Shadow of
the Dinosaurs to Us
by Steve Brusatte. Note: I tried to get an LLM to generate these two images for
me by describing what I wanted, but what I got was useless.
Thematic Thread on Evolution: Previous Next
Manuel Alfonseca
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