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Classification and phylogeny for beginners

In this blog, we usually use therms related with the classification of living beings and their phylogeny. Due to the difficulty of these therms, in this post we will explain them for those who are introducing to the topic. 

INTRODUCTION

Before introducing in the topic, it is necessary to explain two concepts, which are usually confused: systematics and taxonomy.

Systematics is the science of the classification and reconstruction of phylogeny, it means that is responsible for reconstructing the origin and diversification of a taxon (unit that we want to classify, such as a species, a family or an order).

On the other hand, taxonomy is the study of the principles of scientific classification, the order and the name of organisms.

In other words, while systematics is responsible for creating systems of classification, which are represented by trees, taxonomy establishes the rules and methods to identify, name and classify each species in the different taxonomic categories based on systematics.

ABOUT SPECIES AND BEYOND

We cannot begin to talk about how to classify species without knowing what is a species and other classification levels of organisms.

WHAT IS A SPECIES?

Along history, it has been given several definitions to the concept species with different approaches.

  • Morphological concept of species: a species is a group of organisms with fix and essential features that represent a pattern or archetype. This concept is totally discarded nowadays, despite morphological features are used in guides to identify species.
INFO-BALLENA
Despite all guides use morphological features to identify species, morphological concept of species is not used (Picture: Revista Viva).
  • Biological concept of species: a species is a group of natural populations which reproduce among them and reproductively isolated and have their own niche in nature. So, a species has common ancestry and share traits of gradual variation.  This definition has some problems: it is only applicable in species with sexual reproduction and it is not applicable in extinct species.
  • Evolutionary concept of species: a species is a single lineage of ancestor-descendent populations that maintains its identity in front of other lineages and has its evolutionary tendencies and historical destination. This approach and the biological one are, in fact, complementary because they are talking about different phenomenons.
  • Phylogenetic concept of species: according to this point of view, a species is an irreducible group of organisms, diagnostically distinguishable from other similar groups and inside which there is a parental pattern of ancestry and descendants.  This point of view covers sexual and asexual reproduction.
ensatina_phylogeny
According to the phylogenetic definition of species, A, B and C are different species. In the C group, all of them are the same species with different types (Picture: Sesbe).

BEYOND SPECIES

Species are classified into a hierarchical system based on more taxonomical categories. From the highest to the lowest category, organisms can be classified in: Domain> Kingdom> Phylum> Class> Order> Family> Genus> Species> Subspecies> Variety> Form. 

We are giving an example: imagine dogs.  Dogs, like wolf, are included in the same species: Canis lupus, but dog is the subspecies Canis lupus familiaris. The naming of a species is its genus (Canis) followed by the specific epithet (lupus). The other taxonomical categories of dogs are: Eukarya Domain, Animal Kingdom, Chordata Phylum, Vertebrata Subphylum, Mammalia Class, Carnivora Order and Canidae Family.

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Dogs and wolfs are included in the same species, but they are different subspecies (Picture: Marc Arenas Camps).

HOW IS TREE OF LIFE RECONSTRUCTED?

To reconstruct tree of life, it is the relationships between living and extinct species (phylogeny), we use traits. Traits are features of organisms that are used to study the variation inside a species and among them.

To reconstruct the phylogeny, it is used the shared traits among different taxa. We have to distinguish two types of similarity: when similarity of traits is a result of a common lineage is called homology, while when it is not the result of common ancestry is known as homoplasy.

Probably, it will be easier to understand it with an example. The wings of owls and quails are similar because they have the same origin (homology), but the wings of insects, birds and bats, despite they have the same function, they do not have the same origin (homoplasy).

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The wings of insects, birds and bats are an homoplasy (Picture: Natureduca).

There are three types of homoplasy:

  • Parallelism: the ancestral condition of a variable trait (plesiomorphic) is present in the common ancestor, but the derived state (apomorphic) has evolved independently. An example is the development of a four-cavity heart in birds and mammals.
  • Convergence: in this case, the homoplastic trait is not present in the common ancestor. The structures originated by convergence are called analogy. An example is the wings of insects and birds.
  • Secondary loss or reversion: consist on the reversion of a trait to a state that looks ancestral. So, it looks and old state but, in fact, is derived.
paralelismo, convergencia, reversion
Biological parallelism, convergence and reversion (Picture: Marc Arenas Camps).

There are different types of traits that are used to order living beings: morphological, structural, embryological, palaeontological, ethological, ecological, biochemical and molecular.

Species that share derived states of a trait constitute clades and the trait is known as synapomorphy. Synapomorphies are traits that were originated in a common ancestor and are present in that ancestor and all its descendants. So, mammary glands are a synapomorphy of mammals.

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Mammary glands are a synapomorphy of mammals (Picture: Tiempo de éxito).

After the selection of traits, the several classification schools use them in different ways to get the best relationship between living beings.

REFERENCES

  • Notes of the subject Advanced Biology Basics, Degree in Biology (University of Barcelona).
  • Hickman, Roberts, Larson, l’Anson & Eisenhour (2006). Principios integrales de zoología. Ed. McGraw Hill (13 ed).
  • Izco (2004). Botánica. Ed. McGraw Hill (2 ed).
  • Shnek & Massarini (2008). Biología. Ed. Médica Panamericana (7 ed).
  • Vargas (2009). Glosario de Cladística: Introducción a la sistemática filogenética.
  • Cover picture: Tree of life mural, Kerry Darlington

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