Common Names
Why isn’t it practical for scientists to use common names for organisms ( ie. cat,
dog, daisy) when they want to talk to other scientists about organisms?
- when you say “cat”, not everyone calls what you think of as a “cat” that
particular name ( ie. French -chat )
- also, there are many different species of “cats” (ie. lions, tigers, house cat) which one are you referring to?
- also, within the same language people may use different names for the same
organism (ie. puma, cougar or mountain lion are all the same animal)
- also, common names can be misleading - look at Figure 4.7 on page 112
- this confusion would make it very difficult for scientists to communicate with
one another
Dichotomous Keys
(Also called Identification Keys or Taxonomic Keys)
- people can use these keys as a guide or blueprint to name organisms already
identified by taxonomists
- such keys move from general to specific descriptions
- the keys usually consists of a series of paired statements that describe
alternative (opposite) characteristics of an organism
- these paired statements usually deal with the presence or absence of some
characteristic or structure that is easily seen
- as each pair of statements gets more specific, a smaller grouping of organisms
is produced until the species is finally identified
- you can also create a dichotomous key of your own to identify organisms (or
anything)
- Complete the worksheets - “Fun With Fictitious Animals” and “Classifying”
- LAB - Creating a Dichotomous Key page 110-111
Viruses - Where do they fit?
- both bacteria and viruses cause many diseases for all kingdoms
- however, bacteria are classified as living while viruses are not
- viruses have no cellular structure - no cytoplasm, no organelles, no cell
membrane and they do not carry out respiration or other life processes therefore, they are not classified in any of the kingdoms
- they consist of strands of DNA surrounded by a protective protein coat called
a capsid
- they infect other cells
- there are 160 major groups which differ in size and shape ( Fig. 4.20 page 122
)
- list the 4 shapes and give an example of each:
- viruses do multiply but not on their own
- they depend on the metabolism of prokaryotic and eukaryotic cells to multiply
- refer to the Life Cycle of the “T4" virus
( Fig.4.21 page 123 )
Difficulties with Categorizing
- we must recognize the problems that can arise when we try to put organisms into
groups
- scientists must look at all the characteristics of organisms and decide which
group they should be placed in
- no classification system is carved in stone
- why not?
- newly discovered organisms
- new things discovered about organisms that are already known
- classification systems need to be adaptable
- as mentioned earlier, there were 5 Kingdoms but recently that was changed to 6
- this happened when new information was discovered about bacteria that
determined they should be put into two separate groups ( more later )
Modern Classification Techniques
- the science of taxonomy also involves other biological sciences such as evolution
- taxonomy also attempts to determine the evolutionary history of groups of
organisms
- scientists compare characteristics of different species living today with each
other and with extinct species
- there are several different types of evidence that scientists can use to classify
organisms and study evolutionary relationships
- they include:
(i) radioactive dating
(ii) comparative anatomy ( structural information )
(iii) comparative embryology
(iv) biochemical information ( DNA / Proteins )
(v) cellular structure
(vi) behavior
(i) Radioactive Dating (Video on Carbon Dating)
- recall “Archaeopteryx” from the video “Classifying Living Things” - how do
scientists know the approximate age of this organism from its fossil?
- when fossils are dated, either a relative age or an absolute age is determined.
- relative age - because of the way sedimentary rock forms ( ie. in layers ), the age
of the layers can be determined in relation to each other - the oldest layers are
laid down first and are found at the bottom. Thus, the younger layers are on top
because they were recently added. This dating principle is referred to as the
“Law of Superposition”
- likewise, the relative age of the fossils found in these rocks can be determined.
- NOTE: relative age does not determine the absolute (actual) age of the fossil,
only the age of the fossil relative to the layers of sediment it is in.
- absolute age - Pin-points the exact age and is found when the fossil or the rock in
which the fossil is found is radioactively dated.
- radioactive isotopes break down into new elements at a known rate called a halflife. (Page 113)
- a half-life is the time it takes for ½ of a radioactive sample to break down.
Carbon Dating
Half - life
C14 ----------> C12
5730 yrs
Sample Problem:
Useful range
60 000 yrs
If you had a fossil with 2 units of C14 left in it and you determined that in the
living organism (or one that is similar) has 16 units of C14, you could use one of the
following
methods find the absolute age of the fossil:
Method 1:
1. Determine amount of C14 left in fossil.
2. Determine amount of C14 in a living organism of the same size and type living
today.
3. Calculate the number of half-lives needed to reduce the C14 in the living
organism to the amount that is left in the fossil.
4. Multiply by the half - life ( in this case, 5730 years ) to determine the age of
the fossil.
16 units of C14
C12
8 units C12
C14
C 12
Living Organism
Fossil
3 x 5730 = 17, 190 year old
note: for fossils too old for carbon dating, an isotope with a longer half - life must
be used:
Isotope
U235
K40
U238
half - life
700 million years
1.25 billion years
4.5 billion years
ii) Comparative Anatomy
- Comparing the anatomy of organisms indicates a common ancestry because of:
- homologous structures - structures having a common ancestry but with
different uses in various species.
Eg. Similar bone structure of the forelimb of a bat, whale, horse and human
suggests these different species have a similar evolutionary origin. Page 113,114
& 664
- analogous structures - body parts of organisms that do not have a common
evolutionary origin but perform similar functions. Eg. insect wings and bird
wings are similar in function but not in structure. Page 665
- vestigial organs - small or incomplete organs ( or bones ) that have no apparent
function in one organism but do have a function in another species. This
indicates evolutionary origin from a common ancestor. Page 665
Eg. Human ear muscles
Human appendix
Hip bones in whales
Human tail bone
Leg bones in snakes
Forelimbs in the flightless ostrich
iii) Comparative Embryology
- Comparing the embryos of organisms indicates common ancestry with other
types of living organisms because the embryos have similar stages of
development and early in their development are very similar looking. (eg. gill
slits and tail in human embryos indicates humans share common ancestry with
birds, reptiles and fish) Page 665
iv) Comparative Biochemistry
- Comparing the biology of one species with another at the molecular level (the
molecules from which they are made) can indicate a common ancestry. An
organisms genes (DNA) determine the proteins that form the body of the
organism. Page 115
- human proteins ( amino acid sequences )have more in common with chimpanzee
proteins than frog proteins &
pig or beef insulin can be used to treat human diabetes
v) Cellular Structure (p. 106 -107)
- Studying the structure of cells can give clues to their evolutionary history
- you have studied the two basic types of cells - prokaryotic and eukaryotic
(review Fig.4.4 on p. 106)
- fossil evidence has shown that the first life forms were prokaryotic ( similar
in appearance to bacteria ) and existed approximately 3.5 billion years
ago
- eukaryotes appeared later ( approximately 1.5 billion years ago )
- multicellular organisms appeared only 700 million years ago
- recently prokaryotes which were classified into one kingdom ( Monera ) have
been discovered to be very diverse
- two groups in particular, Bacteria and Archaea, have been seen to be as
different from each other as they are from the eukaryotes and
therefore have been put into different Kingdoms ( thus, the 6 Kingdoms
instead of 5 )
- Also, biologists have created a new level of classification, above Kingdoms,
called domains
- there are three domains - Domain Bacteria, Domain Archaea, and Domain
Eukarya
( Fig. 4.5, page 107 - Important )
vi) Behavior (p. 706)
- how organisms are adapted in how they respond to their environment is called
behavioral adaptations
- eg. include migration, courtship displays, foraging behavior
- it is believed that these adaptations have evolved in response to changes in
environmental conditions as continents formed and moved millions of
years ago
- the favorable adaptations were passed on to the offspring
- note: Biofact p.706
How have classification systems improved as a result of these modern techniques?
- through the use of these techniques, organisms once thought to be closely
related, have been found not to be related and vise versa.