Presentation on theme: "The history of life How to organize our observations regarding changes in the biosphere over time."— Presentation transcript:
The history of life How to organize our observations regarding changes in the biosphere over time
Some definitions Hierarchy = a rank or order of features. “biota” = the largest group of living organisms. “vertebrates” are a subset of biota Characters = traits possessed by organisms. Not a single feature of a single organism but features common to all members of a group. Ex, all mammals have 3 bones comprising their ear. This is a character of mammals
In living organisms, we can describe a wide range of characters: Structural: muscles, bones, teeth, organs, skin, feathers, fur, Genetic: DNA, chromosome structure Embryology: ontogenetic development Ecology/behavior: swim, fly, bipedal For fossils, this list is much more limited
Characters are distributed hierarchically Ex, mammals have hair/fur therefore this character distinguishes this group. Having fur, however, won’t distinguish cats and dogs. We need another character (in this case, skull structure) to distinguish these. Two types of characters: specific: a character that’s diagnostic of all members of a group general: a character that’s non-diagnostic of that group The same character will be specific for one group but general for a subset within that group. ex: fur
Paleontologists and evolutionary biologists have developed a grouping pattern to display hierarchical characters in the biota: cladogram = a branching diagram depicting hierarchies of shared characters Ex: cars and trucks: characteristics: used to carry people? No- this is a function 4 wheels, chassis, engine, steering wheel, headlights- Yes The branch in the diagram indicates that there are some features That cars have that trucks don’t, ex, a trunk = specific character of cars
Add a motorcycle: Two of these items have more in common than with the other, therefore there’s a hierarchy
Note that we can choose the character we’re using to construct the cladogram. This cladogram implies that motorcycles and cars have more in common than cars and trucks
How do you decide which characters to use? The clade shouldn’t change if you add characters. Ex, you can’t add “chassis” or “steering wheel”to B. Likewise, you can also add other motor vehicles, like SUVs or RVs to A. A B
Traits that appear early in the cladogram are called “primitive.” Traits that appear later are called “derived.” Wheels, engine, headlights are primitive traits of motor vehicles. Chassis, steering wheel, passenger space are derived traits.
What has been the impact of cladograms (and cladistics) to the study of the history of life? A way of analyzing patterns Show groups of organisms that are related to one another as a result of shared traits; they also show how closely related they might be.
A few more terms about characters: Homologues = anatomical structures that can be traced back to a common ancestor. Ex, forelimbs of pterosaur, bird, bat, human
Analogues - anatomical structures that perform the same function but have no shared ancestry. Ex, wings of a bird and a fly or human and grasshopper legs Both human and grasshopper legs can move to produce the jumping motion (= similar function). However, their internal structures are totally different. Therefore, they did not arise from a common ancestor.
Phylogeny = the history of descent of an organism over time. based on how characters have changed over time and on hierarchies of shared characters, revealed through cladograms A branch on a cladogram = a clade = a monophyletic group = groups or organisms are more closely related to each other than they are to others. Characterized by derived traits. A cladogram is a “hypothesis of relationships.” How are they tested?
When is a wristwatch a wristwatch? An example of the use of cladograms
There are 6 possible cladograms, but a & b are redundant in that the groups at a node share the characteristics at that node. In other words, in #1 both wind-up watches and digital watches are above the last node. 1a &2a are appealing because digital watches are the newest and most advanced BUT a cladogram is established on the basis of shared traits
1a and 2a say that a digital watch shares the most traits in common with a quartz or wind-up watch. Examine the watches and you see that this isn’t correct: wind-up watches have moving parts and internal gears. digital watches contain microcircuitry with no moving parts, i.e., It’s a small computer! #3 is a more accurate cladogram of the degree of “watch similarity”
From an evolutionary perspective, the wristband and watch case have evolved twice, once for moving parts and again for a very tiny computer. Is this plausible? Likely? A watch is a small instrument for telling time (=a function), so it is not a great analogy of living organisms, which we don’t classify by functionality. That said, we can still use watch types for a discussion of how cladograms work. If a “watch” can include a tiny computer as well as a tiny machine with moving parts, then the cladogram for watches must also include all computers! =
Distinguishing between multiple options of cladograms “parsimony” = a philosophical principle developed by theologian William of Ockham. Often called “Ockham’s Razor”: simplest is best. Ex, bird, bat, human: 2 cladograms are possible: Cladograms are hypotheses of phylogenetic relationships; they make predictions which are testable. Data includes fossils, DNA B is less likely because it requires the loss of fur/hair or mammary glands; it’s not simple!
When we study the history of life we use both cladograms and the more traditional phylogenetic study. Cladograms suggest “degrees of similarity” based on shared traits, with the implication that shared traits are a result of common ancestry. Phylogenies are the actual representation of ancestor-descendent relationships based on observed and measured morphologic changes over time. Shown here is one of the best documented phylogenies: horse evolution in the Cenozoic.
Interrelationships of Vertebrates The chordates are monophyletic, having originated from a Middle Cambrian organism, Pikaia, that looks like a worm, but had a nerve cord within a sheath (notocord), bilateral symmetry, gill slits (pharyngeal gills) in throat. The modern organism that is very similar to Pikaia is called a “sea squirt,” whose larval stage contains a primitive notocord. “Strange as it sounds, the tubular creatures are essentially vertebrates without backbones. They're much more closely related to fish, birds and people than to worms, starfish or other invertebrates. Like humans, they belong to a group of animals called chordates.”
1= nerve cord within a sheath, bilateral symmetry, pharyngeal gills; 2= segmentation 3=eyes, kidneys, layered bone 4=jaws 5= bone in endrochrondal skeleton 6= ray fins 7= fleshy pectoral and pelvic fins 8= 4 limbs The cladogram for vertebrates: Our survey of the history of life will focus above node 3, with organisms with a developed vertebrae (although we’ll return to Pikaia when we discuss the Cambrian radiation of life).
Evolution of fishes Jawless fish: armored, relatively immobile poor swimmers Jawed fish: the Gnathostomes Within this group are the Chondrichthyes or cartilagenous fish, including sharks, and also the Osteichthyes, or bony fish, which include the Actinopterygii,or ray-finned fish, which are what you catch and eat. These are the largest (most numerous group of vertebrates on Earth today). The oldest fish fossils: Ordovician. Major diversification: Devonian, often called the “Age of Fish” Think you know what a fish is?……..
What makes a fish a fish? Live, breath, move and reproduce in water? These are all behaviors, and are therefore not good criteria for defining a group, or clade. It turns out that the term “fish” is not really meaningful in a cladistic perspective. The characters that unite all fish are also shared by other “non-fish” organisms.