Biology - Lesson 4

Linnaeus was classifying organisms based on what they looked like, what characteristics and other similarities they appeared to have in common. This made it difficult to classify organisms that seemed to share characteristics with both kingdoms that Linnaeus proposed, Plants and Animals. For example, fungi including mold and mushrooms do not move (or do they?) so they seem to be plants but, unlike plants, they do not make their own food. Fungi live off of the nutrients they absorb from dead or living plants and animals.

Instead of trying to organize organisms based on the characteristics they appeared to have in common, as the evidence for evolution mounted, the goal of taxonomy became showing how
organisms are actually related, ancestrally. This is called Phylogeny (meaning "tribe-source").

On a wall in Martin Hall, Swarthmore College Dept of Biology

Furthermore, when scientists started to study cells in more detail, they found a new and crucial difference between groups of organisms. Some organisms have cells with a nucleus and others have cells with no nucleus. The former are called eukaryotes and the latter are called prokaryotes. Most eukaryotes are multicellular ("many-celled") beings while most prokaryotes are single-celled or unicellular.

Researchers discovered that they could also organize species based on their rRNA or Ribosomal RNA, an important part of what cells use to grow and reproduce.



rRNA is similar among closely related species and differs more significantly the farther apart species are from each other ancestrally speaking. By studyingn the rRNA of organisms, biologists were able to recatagorize them into three major groups or domains: the Bacteria, another group of prokaryotic, single-celled organisms called the Archaea, and the Eukaryotes (the domain Eukarya). Domain was added then as another taxonomic level. One of the most interesting facts I read about in this chapter of my text is that, based on rRNA, fungi is much more closely related to animals than to plants.





Sidenote: Does this mean that, as a vegetarian, I shouldn't eat mushrooms? Hmmmm.

Biology - Lesson 3

Linnaeus didn't stop at giving organisms just these two classifications either. He came up with a hierarchy of taxonomic groups:


At the top of this is the least specific of grouping, Kingdom, and at the bottom is the most specific grouping, Species.

Humans, for example, are within the Kingdom Animalia, the Phylum Chordata (animals having a notochord, or dorsal stiffening rod, as the chief internal skeletal support at some stage of their development), the Class Mammalia, the Order Primates, the Family Hominidae, the Genus Homo, and the Species Sapiens.

Biology - Lesson 2

That tendency over time for populations of organisms to diverge and become completely different species over time is called speciation.

Ultimately, given the assumption that life arose on Earth just once, scientists would like to be able to map out a family tree of organisms, or tree of life, that shows the origin of all species in one mother ancestor at the very top.



The labeling and classification of organisms is called taxonomy. Carolus Linnaeus was one of the first scientists who attempted to organize all the many organisms being discovered at the time into groups of organisms that had clear commonalities. He gave each type of organism a two-part name consisting of its genus and its species. A species was defined as a group of organisms that regularly breeds together or has characteristics distinct from those of other groups.



The genus was defined as a closely related group of species such as humans or Homo sapiens and other (now extinct) species of beings that walked upright and frequently used tools. So, the scientific name that Carolus Linnaeus gave organisms went from a more general classification to the more specific classification. Linnaeus also said that all types of organisms should be given a unique genus and species name. So, for example, while there are other members of the Homo genus and while there may be sapiens in other genera (plural of genus), humans are the only Homo sapiens. This system of naming organisms in two-parts is called binomial taxonomy.



Phylogeny

Biology - Lesson 1

A theory is an explanation for a very general class of phenomena or observations.

Modern Biology is founded on two key theories: The cell theory and the theory of evolution.

A cell is defined as a highly organized compartment bounded by a thin, flexible structure called a plasma membrane and that contains concentrated chemicals in an aqueous (watery) solution.

The Cell Theory says that all organisms are composed of cells and all cells come from other cells.

The Theory of Evolution by Natural Selection says that new species develop from preexisting species and that all species change through time as a whole through mutations that affect the fitness of the composite organisms.

Fitness is defined as the ability of those organisms to reproduce and to survive.

A trait that improves the fitness of an organism within a particular environment is called an adaptation.


Cell theory contradicted the prevalent previous theory (or rather hypothesis) that life develops spontaneously. In the late 1850s, scientist Louis Pasteur decided to test the Spontaneous Generation Hypothesis by setting up two beakers of 'pasteurized' broth, one open to the air and thus vulnerable to being settled in by bacteria and the other closed to the air. He left both for months during which the first beaker filled quickly with bacteria and fungi and the second beaker did not. His conclusion was that cells arise only from preexisting cells, not spontaneously from nonliving material. This convinced scientists that the All Cells-from-Cells Hypothesis was correct and not the Spontaneous Generation Hypothesis.

Recall that Cell Theory states that all cells come from pre-existing cells (this is called the process component of Cell Theory). If this is so, then in organisms that each have only one cell (Single-Celled organisms), each organism must come from another organism, who must have come from a previous organism, and so on tracing back to a single organism or mother of them all. ----note: I'm not following that last bit of logic. This does not demonstrate that all the single-celled organisms have a common ancestor. If they could all have one common ancestor that didn't come from a cell, then why could there have been two or more lines or two ancestors that developed from something other than a cell and spawned two separate lines of single celled organisms? In other words, clearly not all cells came from other cells unless you believe that there have always been cells going back infinitely into the past, but if there were ancestor cells that came about some other way, why not believe that there were more than one of these original/first ancestor cells?

It was the scientists Charles Darwin and Alfred Russel Wallace that, separately, published essays hypothesizing that all species are connected by a common ancestry. They defined species as all distinct, identifiable types of organisms.

Darwin stated that natural selection changes the characteristics of a wild population over time, just as the deliberate manipulation of "artificial selection" changes the characteristics of a domesticated population over time.