Classification and Macroevolution
Last week you learned about Natural Selection, DNA, and heredity. Here are some important take-a-ways:
Your goals for Chapter 5:
Organisms can be classified according to any number of criteria, including overall similarities, colors, ecological functions, etc. However, it is generally agreed that the most useful way for scientists to organize biological diversity is to group organisms according to shared evolutionary history. This way the grouping not only results in an organized classification, it also contains and conveys information about our understanding of the evolutionary history of these groups.
Principles of classification
Constructing Classifications and Interpreting Evolutionary Relationships
Definition of a Species-
Recognition of a fossil species:
Understand how fossils are formed.
Consider how some environments are conducive to creating fossils while others are not.
Know the following:
Please note that further information on how to construct a cladogram follows, in the event you don’t remember it from your last biology course. The actual lab is at the end.
Diagraming Evolution, or How to read a Cladogram
(the first 3 pages of this handout was adapted from the source listed above)
Remember the Linnaean system of classification you probably learned in grade school or high school (heck, maybe even college)? It went something like this…
Kingdom -> Phylum -> Class -> Order -> Family -> Genus-> Species
Linnaeus started using this classification system in the 1700s and he had a good run; we still use his system in certain situations. However, we’ve moved away from this ranking system mostly because of the discovery of transitional fossils that screwed with Linnaeus’s idea of neat little boxes for all of life to be categorized into. This system falls apart, for example when you have two equivalent ranks, let’s say class Osteichtyes (boney fish) and class Amphibia (amphibians), and find a transition between the two. In the case of our example, let’s say we find a transitional fossil like Tiktaalikwith both fish and amphibian characteristics. Now what? Does this thing belong to class Osteichtyes or class Amphibia? Which box do we put it into? For reasons like this, most modern evolutionary biologists prefer to organize species on a diagram called a cladogram. These take a little getting used to but once you understand them, they give a person a much better sense of how evolution is operating.
Cladograms are branching diagrams where each branch represents an evolutionary lineage. We put the names of specific taxa at the end of each branch and we put the names of groups to which those taxa belong at the intersection of branches (nodes). Check out the example below that uses mammals.
The diagram (above) doesn’t really have axes, but implied in the x direction is some sort of evolutionary distance from each other and implied in the y direction is relative time (the first branch at the bottom happened long ago the latter branches higher up happened more recently). This diagram tells you that all the animals listed are mammals (see the group name Mammalia at the bottommost node). The koala and kangaroo are more closely related to one another than to anything else on the diagram. Their branches intersect each other before they intersect any other lineage. This indicates that they diverged from each other more recently than they diverged from any of the other animals on the diagram. Furthermore, they both belong to the group Marsupialia. Similarly, the bat and the lion are more closely related to each other than they are to either of the marsupials. They are both placental mammals (eutherians).
Now let’s try a slightly more complicated example…
Here we can see some interesting relationships. Notice you (assuming you are a human) are more closely related to a turtle than you are to a frog (you and the turtle are both amniotes, part of the group Amniota). Now, as you work your way down the diagram things start to really get interesting. The tuna is more closely related to humans than it is to sharks. Surprising right? I know what your thinking “Is that really true?” Sure it is. See the distance between the ‘Vertebrata’ node and the ‘Osteichtyes’ node? That distance is shared evolution between humans and tunas that sharks don’t share (they’ve already branched off). Put another way, this means that tunas and humans have a few extra million years of evolution in common that tunas and sharks don’t. The fact that sharks and tunas both live in the ocean and kinda look a lot alike doesn’t count for all that much.
As long as we are examining this diagram notice that the Linnaean ranks we used before as equivalent, Osteichthyes and Amphibia, are certainly not equivalent on this diagram. Amphibia is contained within Osteichthyes. In fact, all land dwelling vertebrates are within Osteichthyes (this is why some evolutionary biologists are fond of saying we’re all ‘fish’).
Now finally, I’ll show you where Tiktaalik fits on the cladogram. It’s closer to being an amphibian than to being a tuna, but then again it’s just as close to being human as it is to being an amphibian.
Cladograms are diagrams which depict the relationships between different groups of taxa called “clades”. By depicting these relationships, cladograms reconstruct the evolutionary history (phylogeny) of the taxa.
Cladograms are constructed by grouping organisms together based on their shared derived characteristics.
On the next pages, you will see alternative examples of cladograms (drawn a little differently). You will also see how the characteristics were used to create the cladogram.
Now, lets out these taxa within nested boxes called a Venn Diagram. This diagram is a pictorial representation of logical sets, pictorially represented as circles or closed curves within an enclosing rectangle.
Based on this cladogram, we know that humans are more closely related to kangaroos then to a bull frog. They are the vertebrates within the class Mammalia (because both organisms have mammary glands). It is important to note that on the cladogram, every organism identified past the evolutionary trait of mammary glands must have that trait. Similarly, every organism added after placenta would need to be a placental mammal. So koalas, which have mammary glands, but are not placenta mammals, would be added next to kangaroos.
Let’s look at a cladogram of humans and other living primates.
Does this cladogram depict the relationships between different groups of taxa? By depicting these relationships, does it reconstruct the evolutionary history (phylogeny) of the taxa? Yes, the cladogram of primates is based on homologous physical traits and genetic similarities that are derived from having a shared common ancestor at some point in the past. Does this mean humans evolve from modern monkeys? No, it does not, but if you trace the branches of our family tree far enough, you'll realize that we share a common ancestor. Later in this course, you will learn about the physical traits and genetic code that we share with other primates.
Here is one last example that may “ring” more clearly to you. It doesn’t represent a living organism but clearly applies physical traits to show shared technological relationships.
Notice how the oldest phone is the first (on the left). Followed by the next oldest with two types branching at the top, suggesting they shared an ancestor at some point and share traits in common with each other (rotary dial). They have more in common with each other than the later phones that poses “advanced” or “derived” traits. The further away a specimen is away from the root of the tree, the more changes have occurred. So, the I-phone “I” at the far right is the most unlike to the old wall phone on the left. In other words, they are the least “alike” in this cladogram.
(the lab is on the next page)
LAB for Week 6: Evolution of Chocolate Candies
Mint Three Musketeers
Reese's Peanut Butter
In this activity, you will construct a cladogram, like Mr. Anderson did in his video (so watch it now if you have not https://www.youtube.com/watch?v=ouZ9zEkxGWg). You will need to apply your critical thinking and analysis skills for this activity, because sometimes there is more than meets the eye when trying to use characteristics to create an evolutionary tree for a group of like organisms (or candies in this case). You don’t need the actual candies in front of you to do this lab, just knowledge of the candies you select.
TO DO THIS, YOU WILL WANT TO HAVE:
EXAMPLE Candy Cladogram:
*note, I would replace “almonds” with the word “nuts” after milk chocolate.