Cladistics

Cladistics is a way scientists group living things by how they are related through their most recent common ancestors. It helps us see how different species, like animals and plants, are connected over time.

In cladistics, scientists look at special traits that groups share. This helps them decide how these groups are related.

When scientists use cladistics, they make groups called "clades." These clades include all the descendants of a common ancestor.

Cladistics is now the most common way scientists classify living things. It is also used in other areas besides biology. Sometimes, it can cause problems for traditional ways of grouping living things, because new discoveries can change how groups are named.

History

The study of grouping living things based on shared traits started with a German scientist named Willi Hennig. He called this method phylogenetic systematics in his book from 1966. Later, other scientists began using the words "cladistics" and "clade" to talk about these grouping methods.

Ideas about grouping living things this way go back to 1901. Over time, this method became very popular. This happened especially when scientists could use computer programs to study lots of data from DNA and other molecules. Today, most scientists use cladistics to learn how different species are related.

Methodology

Main articles: Phylogenetics and Cladogram

See also: Phylogenetic tree

Cladistics is a way scientists group living things based on how they are related through common ancestors. Scientists look at traits that are shared by certain groups to decide how they might be connected. The result of this analysis is a diagram called a cladogram, which shows how these groups might be related.

Scientists use different data, like genetic information or physical traits, to build these diagrams. Different data or methods can lead to different diagrams, so scientists keep studying which diagrams are most accurate. For example, scientists have different ideas about how turtles, lizards, crocodiles, and birds are related.

Cladistics helps scientists understand the history of groups like primates, which include monkeys, apes, and humans. It shows that all primates share a common ancestor, even if they look different today. This method helps scientists describe their relationships fairly and honestly.

Terminology for character states

Here are some important words scientists use to describe traits in living things:

• A plesiomorphy or "close form" is a trait that a group of animals has kept from their ancestors. For example, being cold-blooded is a trait shared by many reptiles but not birds, who are warm-blooded.

• An apomorphy or "separate form" is a new trait that appears in a group. These traits help scientists identify groups of related animals. For example, having digits similar to humans is a trait that helps identify a group of animals called tetrapods.

• Scientists also use the word homoplasy for when different animals share a trait that their common ancestor did not have. For example, both mammals and birds can keep a constant warm body temperature, but this trait evolved separately in each group.

Terminology for taxa

Living things can be grouped in special ways based on how they are related. These groups are called mono-, para-, and polyphyletic taxa. We can learn about them by looking at their family tree and the traits they share.

Criticism

Cladistics has had some questions since it started. Some people think choosing which traits are related can be guesswork.

In the late 1970s, a new idea called transformed cladistics tried to solve some of these problems by not assuming things about family trees before studying them. But this method did not become very popular.

Issues

The cladistic method does not name fossil species as the direct ancestors of a group. Instead, fossils are seen as part of separate extinct branches. While a fossil might be an ancestor, we cannot know for sure. So, scientists assume fossils are related to other fossils and living species based on shared features.

Hybridization can make it hard to clearly define the ancestors of a species. Many species can interbreed for millions of years, creating complex branching patterns. This complexity can make it difficult to create simple family trees.

Horizontal gene transfer is when genetic information moves between different organisms. This can happen through natural processes and does not usually affect the ancestry of the organisms. However, it can make it harder to determine ancestry. Cladistics can help map these gene transfers by studying the family trees of individual genes.

Naming groups can be challenging in cladistics. If relationships are unclear, there may be many possible family trees. Assigning names to each possible group might not be wise. Established names may no longer fit as new groups are discovered. Naming changes happen when our understanding of relationships changes, especially for extinct species. Keeping old names might not accurately show true relationships. For example, groups like Archaea, protists, and worms include humans in a broader sense. Scientists sometimes use a more limited meaning for these names to better match actual relationships.

In disciplines other than biology

Cladograms can be used in many areas besides biology to show how different things might be related. Any group of items that might share a common origin can be compared if we can identify and measure their features.

Cladistic methods help us understand how cultures, stories, and even languages change over time. For example, they have been used to study old myths, folktales, and manuscripts like the Canterbury Tales. In language studies, these methods help us see how languages are related by looking at their features. They also help us understand the history of galaxies in space.