This week, I want to continue on with the discussion on evolution and classification of animals. Today’s post will be centered around cladistics. which is an important term to know in the unit of evolution. I will discuss mainly about clades, traits, and cladograms.
Clades
I have made reference to the idea of evolution several times throughout this month: species evolve over time and split to form new species. A clade refers to a group of organisms that have evolved from a common ancestor. Clades include all the species that exist together with the common ancestral species along with the ones that became extinct. Their size ranges fro as small as a few species to as large as thousands of species.
It is often hard to determine which species came from a common ancestor and thus be included in a clade. The most reliable evidence is from analyzing base sequences of DNA, since species that share an ancestor have similar base or amino acid sequences. Therefore species that have many differences in the amino acid sequence are likely to have diverged from a common ancestor millions of years ago.
These differences in the DNA sequence are the results of mutations. Mutations occur at a constant rate, so when we observe how mutations accumulate gradually over long periods of time, the appearance of mutations can be utilized as a molecular clock. An example is the differences in base sequence of mitochondrial DNA and how a hypothetical ancestry was constructed through these differences. It has been established that the European-Japenese split happened 70,000 years ago, African-European/Japanese split at 140,000 years ago, and the human-chimpanzee split at 5,000,000 years ago.
Analogous / Homologous traits
Traits can be classified into two types. Homologous structures are similar due to similar ancestry, while analogous structures are similar due to convergent evolution. There has been issues with differentiating these two types of traits, which led to mistakes in classification. Thus, scientists now rarely use morphology of organisms to determine members of a clade and instead rely more on molecular biology.
Cladogram
A cladogram is a tree diagram that is based on differences and similarities between species in a clade. They are based on base sequences analyzed by computer programs. These programs measure how species in a clade could have evolved with a few number of changes in the DNA sequence. This method is called the principle of parsimony, which reveals the most probable sequence of divergence in clades. The branching points on cladograms where ancestral species split to form two or more species are called nodes.
There are various types of cladograms, and primate cladograms are one of them. Primates are mammals that adapted to climb trees. Humans, monkeys, baboons, lemurs, and gibbons are included int the primates order. The cladogram was designed as the entire genome of humans, chimpanzees, and bonobos were analyzed.
It is important to design cladograms, but it is much more significant to know how to analyze them. When two clades are linked at a node, they are relatively closely related, but if they are connected through several nodes, they are distantly related. Some cladograms that display numbers of differences in amino acid sequence can be used to estimate how long ago clades split. However, it is important to remember that cladograms do provide hard evidence but do not provide proof. Therefore we must be cautious when analyzing cladograms.
Towards the end of the 20th century, more accurate cladograms could be constructed as information about the base and amino acid sequences became available. Cladistics – the act of constructing cladograms and identifying clades – allowed for some changes in classification of organisms. It supported the argument that classification based on morphology contains some errors. Thus, some groups have been reclassified while other groups merged or divided.
It is true that reclassifying organisms is takes a tremendous amount of time and effort, but it is worthwhile, especially considering that cladistics can reveal new information about groups of organisms. This act of revising a theory is actually an important process in science.
Works Cited
Allott, Andrew. Biology: Course Companion. Oxford University Press, 2014.
“Analogous and Homologous Traits.” CLADISTICS, group4cladistics.weebly.com/analogous-and-homologous-traits.html.
“File:Primate Cladogram.svg.” File:Primate Cladogram.svg – Wikimedia Commons, commons.wikimedia.org/wiki/File:Primate_cladogram.svg.
The Science Herald 
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