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TheDiversity of Life

I. AN OVERVIEW OF THE DIVERSITY OF LIFE

We are now going to consider the diversity of living systems that share this planet with us. While all organisms share the basic characteristics of life, HOW they satisfy these requirements (particularly energy acquisition) varies dramatically. We will focus on how organisms differ, and the evolutionary innovations introduced throughout the history of life.

A. Classifying Organisms

Linnaeus based his taxonomic classification system on a "Platonic" view of unchanging, completely separate species. And he based his taxonomy on morphology, alone, classifying species into genera. Other pre-evolutionary scientists built on this idea and grouped genera into families, orders, classes, phyla, and kingdoms. But, while morphology often correlates with biological relatedness, sometimes it doesn't (analogous structures, for instance). So, there might be instances where this taxonomic system does NOT represent the genetic pattern of biological relatedness. For example, humans are very different morphologically from chimps and gorillas, so we classfied ourselves in a separate family (Hominidae) from the others (Pongidae). However, genetica analysis reveals that humans are more similar to chimps than gorillas are - so we really need to change the taxonomy. We should not place chimps and gorillas in one family, if chimps are actually more closely related to humans. (This would be like placing your sibling and first cousin in one 'family', and yourself in another. This misrepresents the gentic reality that you and your sibling are more similar than either of you are to your first cousin.) So, throughout this unit, you should look and see how your book classifies organisms - by showing their evolutionary relationships (phylogenetics) and not just their taxonomic "affiliation".

Nonetheless, using the historical approach, we classified organisms into five kingdoms, with a standard set of subcategories:

So, humans classify as:

        Kingdom - Animalia
                Phylum - Chordata
                  (Subphylum - Vertebrata)
                        Class - Mammalia
                            (subclass: Eutheria...the placental mammals)
                                Order - Primates
                                        Family - Hominidae
                                                Genus - Homo (italicized or underlined)
                                                        Species - Homo sapiens - always both names or abbreviated genus (H. sapiens)

As we discussed, this is a "Platonic" system, and it tends to misrepresent nature as a set of categorically separate groups, rather than as the genetically continuous product that it is. Also, it is important to rememeber that this classification is a human construct, not a biological reality. In fact, the only biologically "real" level are species, because species define themselves by reproductive isolation (which is equivalent to genetic identity).
However, as we noted before, species are potentially reproducing organisms. It may seem like organisms either mate or don't, but that's not the case at the population level. Rather, some groups may "mate a little", or their offspring may be of poor quality. In these cases, two groups (species) hybridize but still maintain their separate identities because the hybrids are either rare or selected against. Since genetic divergence is a continuous process, the process of becoming two different species is a continuous process. As such, the process of becoming reproductively isolated is a continuous process, too, and we should expect to see groups that aren't completely separate yet; they are in the process of separating. This makes identification of species difficult and ambiguous, at times. And, it makes this "Platonic" view of categorically different "boxes" fundamentally the wrong way to look at things.

Modern systematics (phylogenetic analyses and classification) tries to classify organisms according to their biological relatedness. Closely related species should be put in the same genus, closely related genera should be placed in the same family, etc. However, since the Platonic view predates Darwin, we have some historical inconsistencies that arise. For instance, there are some classification categories that DO NOT group closest relatie together, but which we continue to use because of their historical importance. For example, Linnaeus classified humans in our own family, the Hominidae. He classified Chimps and Gorillas in a separate family, the Pongidae. Modern genetic analyses has revealed, however, that chimps and humans are more closely related than chimps and gorillas. So, grouping chimps and gorillas together into the Pongidae, while grouping humans in a separate family, is like grouping yourself on one "family" and placing your sibling in a different "family" with your first cousins. That wouldn't represent your actual degree of biological relatedness. Another example is the "Reptiles". This group of organisms is actually very diverse and not really all that closely related (see fig. 669). For example, crocodiles are more closely related to birds than they are to turtles, lizards, or snakes. Yet we group croc's with turtles, lizards and snakes into the class "Reptilia"..... Systematists are slowly overcoming the historical power of these previous categories by creating new names that more truly represent the patterns of biological relatedness.

B. Kingdoms

Through the 1970's there were five types of life typically recognized by biology as different Kingdoms. They are separated as such:

1. Do the cells have a nucleus? NO ---> Then the organism is in the kingdom MONERA (Bacteria)
YES---> Then the organism is a eukaryote - go to question 2.

2. Is the organism multicellular? NO ---> (unicellular or colonial eukaryote) - PROTISTA
YES ---> multicellular eukaryote - go to question 3.

3. Is the organism a heterotroph? NO ---> (autotrophic multicellular eukaryote) - PLANTAE
YES---> go to question 4

        4. Does the organism have a cell wall and external digestion?
                                            NO ---> ANIMALIA
                                             YES ---> FUNGI

C. Domains

With the advent of large scale gene sequencing, we are able to look directly at the patterns of biological relatedness. This allows us to see where our morphology-based taxonomy worked (keeping animals separate from plants), but it also identified some areas where morphology, alone, is not the best descriptor of relatedness. (Humans, chimps, and gorillas, for instance).

In the 70's and '80's, Carl Woese compared r-RNA sequences from eukaryotes and two types of "monerans" - the eubacteria and the archeabacteria (as they were then known). He found that these three groups were VERY different, and that actually, the archeabacteria were more similar to eukaryotes than to the other bacteria. So, based on genetic similarity, he constructed a new phylogeny and divided life forms into three groups, or "Domains"

        Eubacteria - the "common" bacteria
        Archaea - exploit extreme environments
        Eucarya - eukaryotes.

- As a result of Woese's genetic work, the relationship among life looks like this:

                     |-------Eubacteria
                ---|       |--Archeaea
                     |----|
                            |--Eukarya

- So, it seems that the Archeaea and Eubacteria are very different groups. They are both very diverse, gneetically, as we should expect of these very ancient lineages. They both date back beyond 3.5 bya, while eukaryotic life only extends back about 1.8 by. So, for most of the history of life on this planet, life was bacterial. With such a long evolutionary history, there has been more time for genetic variation to accumulate. thus, these groups are much more diverse, genetically, than all the eukaryotes, combined.

- More recent and complete genetic analyses show that, against the backdrop of their large differences, there are a few genes they have in common. Now, bacteria can "trade" genes..called lateral transfer...that makes reconstructing phylogenic trees very difficult. A recent model of early evolution suggests that eukaryotes may have evolved when Aerchaeal and Eubacterial cells merged. This is called the "ring of life" hypothesis, and it was proposed by Maria Rivera and James Lake of UCLA in Sept. 2004. Go to the library and see the Sept. 9, 2004 issue of Nature to read about it.

D. Timeline

4.5 billion years ago - Earth formed (Evidence: meteorites and moon rocks date to this age)

4.0 bya - oldest rocks (The oldest rocks are not as old as the Earth because the Earth is tectonically active, recycling its crust)

3.8-3.5 bya - origin of life (oldest fossils date to 3.5 bya, from western Australia)

2.0 bya - oxygen accumulating ("red bed" sediments form, demonstrating that oxygen in the atmosphere was oxidizing minerals)

1.9 bya - first eukaryotes (fossils)

0.9 bya - first animals (fossils)

So, for half of life's history, life was exclusively bacterial.

Study Questions

1. classify yourself.

2. What are the three domains of Life?

3. How does a populational approach to taxonomy make classification difficult?

4. Describe two inconsistencies between our historical classification system and patterns of actual biological relatedness reveal by genetic analysis.