What is a Clade?
The term “clade” comes from the Greek “klados,” for “branch.” It’s useful to think of a clade as being one “branch” on the tree of life, where the common ancestor is the place that the branch split from the main trunk.
Because clades are a way of thinking about “branches of the tree of life,” a clade can only contain organisms that do share a common ancestor. A clade also contains all descendants of that branch, excluding none.
Clades are useful in the study of biology because biologists study how life works, and how it changes over time. Seeing how different species have descended from a common ancestor, and how they are similar or different, can help biologists to understand how different characteristics of life evolve.
This graphic below illustrates how clades are identified and defined. The green box does not represent a true clade, because all of the ancestors of the oldest common ancestor are not included. The blue and orange boxes, by contrast, are true clades because they contain a common ancestor and all of the descendants of that ancestor.
The study of cladistics is the study of classifying organisms based on their relationships with each other.
Historically, biologists attempted to determine how closely organisms were related by studying their physical characteristics, such as fur, feathers, and bone structure for similarities. For example, several major branches of mammals were first classified based on shared bone structures inherited from a common ancestor.
Today, it is possible for scientists to sequence an organism’s DNA directly – literally reading the “blueprint” for the organism and seeing where mutations have occurred. This has led to far more accurate understandings of what organisms are related, and how closely.
Function of Clade
Clades are used to help scientists understand similarities and differences between life forms, and how life changes and develops over time.
The idea of classifying organisms based on their relatedness originated with Darwin’s theory of evolution. When Darwin discovered that populations of animals could change their physical characteristics over time, he realized that populations could split into different species over time.
He then hypothesized that species with similar characteristics, such as humans and apes, might have at some point descended from the same common ancestor.
Up until Darwin’s theory of evolution, biologists had attempted to understand and classify species according to superficial traits. For example, mammals were defined as organisms that had hair, had four limbs, and nursed their young – but pre-Darwinian biologists did not believe that mammals were related, only similar.
Under this system, the question of why there are so many different species – and why some are very similar to each other, while others are quite different – had not been answered.
Darwin’s theory of evolution brought answers to those questions. There were so many species of organisms because there were many different environments, and many different “niches” within those environments to survive. Over time, organisms developed many different traits to allow them to successfully survive and reproduce.
Likewise, this explained why some species of animals were so different, while others were very similar. Lions and tigers, for example, had probably diverged from a common ancestor recently; lions and monkeys probably had a common ancestor which had fur and nursed its young, but clearly, that was a very long time ago and much change had happened to both species since.
Today, the study of cladistics continues to help us understand where we came from. Recent developments in genetics have allowed us to find microscopic similarities and differences between life forms, which have led to some surprises.
Some interesting discoveries that have come out of the molecular biology approach to cladistics include:
- Fungi are more closely related to animals than to plants. Upon studying our genome, scientists now think that the ancestors of fungi and animals had diverged after the ancestor of plants was already a separate lineage. This answers questions about the intriguing differences between fungi and plants and helps us to understand how our own cells’ characteristics evolved.
- Archaebacteria – previously classified as just “a weird type of bacteria” – is actually a completely separate branch of life from modern bacteria. They are genetically and molecularly very different from modern bacteria and multicellular organisms.
- Genome analysis suggests that multicellular organisms like ourselves may actually have evolved from ancient archaebacteria. Although their cells look very different from ours now, some genes found in our own cells have been found in archaebacteria that are not found in modern bacteria.
Examples of Clades
Archaebacteria is a “branch” of the tree of life that includes all members of an ancient lineage of bacteria. Archaebacteria are very different from other cells, using different molecular components in their membranes, and having very different genomes.
Once thought to be just “weird bacteria,” archaea have now been discovered to be a totally different branch of life, whose members are uniquely adapted to live in extreme conditions, and who can perform some life functions that members of other lineages can’t.
It has been discovered, for example, that only archaebacteria make methane as a byproduct of their metabolism. Archaebacteria are also commonly found in very hot, very salty, and very acidic environments. Their unique biochemistry – invisible until the advent of molecular biology – makes this possible.
The origin of animals is of special interest to biologists for obvious reasons: we are animals! And so the discovery of Apoikozoa, which happened in 2015, was important.
Animals had long been defined by their obvious characteristics: we are multicellular, we move around, we eat, drink, and breathe, etc.. But how our first ancestors split off from other branches of life such as fungi, plants, etc., had not been clear.
Apoikozoa is the “branch” of life that includes both ourselves – and a group of single-celled organisms called Choanoflagellatea.
Early biologists noticed as far back as the 1840s that these single-celled organisms looked similar to cells seen in sponges – a primitive animal. But it was not discovered until the advent of modern molecular biology that we are almost certainly related.
In addition to the cellular characteristics that could be seen under the microscope, it was found that the single-celled organisms of Choanoflagellatea have some genes which would have been vital for the development of multicellularity in animals.
Very likely, Choanoflagellatea and Animalia diverged after the first ancestor of animals developed multicellularity. From there, animals evolved to be steadily more complex, while the lineage of Choanoglatellatea remained single-celled.
Just as the branch of a tree can have many smaller branches shooting off of it, one clade can contain many other, smaller branches. Such is the case with Apoikozoa and its “daughter” clade, Metazoa – also known as Animalia.
Animalia / Metazoa
For millennia, “animalis” has been the name in the Greek language for creatures that move and breathe.
In the 1870s, biologist Ernst Haeckel coined a new term for multicellular animals to distinguish us from single-celled eukaryotic organisms that had been discovered with the recent advent of the microscope. He called multicellular animals “Metazoa.”
Molecular biology studies have confirmed that all organisms Haeckel classified as Metazoa are related – likely all descendants of the same ancestor that developed multicellularity.
Haeckel’s single-celled “animals” have been found to be cellularly different from us in important ways and moved to a different clade – so “Metazoa” and “Animalia” now refer to the same “branch” of the tree of life.
The study of Metazoa/Animalia has typified the challenges and rewards of biology, and of science in general. Philosophers began trying to understand the nature of “animalis” – living, breathing things – millennia ago, and today, molecular biology has allowed us to begin to learn exactly what mutations allowed us to develop different traits from those of plants and other life forms.