Bio 100/103 Lecture 5 September 6th.
Chapter 2 Understanding and organizing the diversity of life.
There are millions of different species of organism on earth today. How do we organize them so we can make sense of them? How do we classify them?
All organisms evolved and are the product of millions of years of evolution. The species that exist today evolved from species that existed in the past and ultimately all organisms are related to each other being derived from a common ancestor. The process of speciation will be covered later in the semester, but it is basically a branching process whereby different groups of individuals from a single species to adapt to different environments and over time their descendants become increasingly different from each other, so much so that eventually they form different species (cannot breed with the other species).
There are many ways in which objects can be classified: color, shape, texture, size, etc. But these are not informative ways to classify objects. For example, if two red objects are grouped together this doesn’t tell you anything else about the two objects other than their color. One might be a ball and the other a fire truck.
We want our classification scheme to be informative and, hence, useful.
Human family trees are branching diagrams that show relationships on the basis of parentage. If you know who the parents are, you can probably infer various things about the offspring because they will inherit traits of their parents. If the parents are red-haired the children likely will be too. So this classification is informative.
Organisms are classified into family trees too on the basis of how closely related they are to each other. However, these evolutionary trees look different from human family trees. In human family trees branches connect parents to offspring
On evolutionary trees in contrast organisms occur on the branch tips and the node at which branches connect represents the most recent common ancestor of all species above it. The common ancestor is thus the ancestral organism from which the two (or more) organisms above it in the evolutionary tree evolved.
How are evolutionary trees put together?
The aim is to reconstruct the evolutionary history of the organisms being classified.
.
Two approaches have been used extensively.
Evolutionary Taxonomy approach.
In this approach systematists (those who classify organisms) attempted to assess relatedness on overall general similarity of organisms using a subjective approach. Sometimes some characteristics of would be considered more important than others in deciding relatedness. The subjectivity of this approach has led to its being largely superseded by the cladistic approach.
The cladistic approach focuses on certain features of organisms that evolved ofr the first time in an ancestral species and that were then passed down to all descendant species and so are found in all descendant species. These features or traits are referred to as shared (because all descendant species have them) derived (because they are derived from a common ancestor) features or characteristics. Systematists using a cladistic approach will use many characteristics to construct an evolutionary tree. This requires the use of computer programs to evaluate all of the data and construct the most parsimonious (simplest) evolutionary tree.
Certain features are not useful in constructing a classification.
Shared ancestral characteristics: A feature present in all of a group of organisms that one is trying to classify provides no useful information. The fact that all mammals have a backbone does not help in deciding whether cats or elephants are more closely related to humans.
Convergent features also are not useful in constructing an evolutionary tree. These are features that just look similar, but are not derived from a common ancestor. For example, the fact that birds, bats, and butterflies all possess wings does not mean they are closely related (their wings are all independently evolved). However, the fact that bird and bat wings are constructed from bones suggests that bats and birds are more closely related to each other than either is to butterflies.
The result of classification is that organisms are grouped into a hierarchical classification. The hierarchy was developed by Carolus Linnaeus in the 1700’s.
Each organism has a unique two word name. E.g. for humans it is Homo sapiens (“wise man”). This name is shared by no other species. Homo is the genus name and sapiens the species name. The closest relatives of human are extinct, but are included in the same genus as modern man. These include Homo erectus (“upright man”), Homo habilis (“handy man”), and Homo neanderthalensis (Neanderthal man).
Scientific names are always italicized. In addition, the genus name is always capitalized Homo and the species (or specific name) sapiens is always lower case. If you can’t use italic type, underline the words to indicate italics.
The hierarchical classification incorporates more and more species as it moves up from genus and species towards the kingdom and domain level.
Classification of humans
Domain Eucarya (includes plants, animals, fungi and single-celled Eukaryotes
Kingdom Animalia (includes all animals)
Phylum Chordata (includes those with a central nerve cord)
Class Vertebrata (includes all animals with a vertebral column))
Order Mammalia (includes all with hair and mammary glands)
Family Primates (includes all with binocular vision, grasping hands, and arboreal habit)
Genus Homo (includes all large brained, opposable thumb, upright walking)
Species sapiens