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Copyright © 2006 John Chamberlain Jr
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Adaptation
An adaptation is a character of an organism that performs a specific function. Adaptations can be structural
(e.g.. bird wing), behavior (e.g. migration), or biochemical (e.g. photosynthesis). Adaptations are often imagined as examples of perfect design, so much so that some adaptations are sometimes invoked as proof of divine intervention in the organic world. While the effectiveness of many adaptations is apparent, the notion of perfection in adaptation has been seriously misunderstood by many. Organisms actually possess characters that enable them to cope with their environment – but nothing more. It is adequacy of design that characterizes organisms, not perfection of design.
Adaptation in Snail Shells
In this exercise we are interested in investigating the adaptive significance of shell form among marine snails. In the tray labeled “ADAPTATION” are the shells of two modern snail species that we will use to illustrate the relation between form and function among animals. Our goal in this exercise is to understand structural adaptation in terms of the engineering problem that a lineage (i.e., an evolving group of organisms) must overcome. In analyzing these correlations remember that all snails have a muscular foot
(see Figure 1) which they use for locomotion and attachment to the substrates upon which, or in which, they live. The shell is a coiled, hollow tube into which the animal can withdraw for protection.
FIGURE 1: Generalized anatomy of a snail.
Although virtually all snail shells are coiled, shell shape nevertheless varies widely among snails. This variation appears to be related to the needs of the lifestyles adopted by individual species. We will look at two species of tropical marine snails, and evaluate the shape of their shells in terms of the environments in which they live. Our goal will be to discover the functional significance of some attributes of shell form in these species.
Surface Crawlers: Many species of snails live in shallow tropical lagoons, and other reef locales, protected from strong waves. Many of these species could be called “surface crawlers” because they crawl slowly across the surface of loose, sandy substrates that are common in such environments. These snails live exposed to the attacks of large predators, such as starfish, which try to turn them over to get at the soft
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tissues underneath, and fish, which try to swallow them whole. Thus, to survive, these snails must develop defenses against such attacks.
Graspers: Many other snail species living in reef locales do so by fastening themselves securely to hard rock surfaces on wave swept shorelines and on reefs that are exposed to the surge of waves and tides. They grasp algae off rocks as food. Such snails are referred to as “graspers”. In such harsh environments predators are not much of a problem. Instead, grasping snails must contend with the problem of dislodgement by strong waves from the rocks to which they must cling for survival.
Table 1 identifies the snail species that we will investigate with regard to adaptive significance of shell form. Examples of each of these species are in the tray labeled ‘ADAPTATION”. All of the specimens were collected on the reefs at the eastern end of the island of St. Croix, U.S. Virgin Islands.
LIFE STYLE SPECIES ENVIRONMENT
Grasper Nerita tessellata Exposed Rocks
Surface Crawler Murex beaui Loose Sand
TABLE 1: Grasping and surface crawling snails studied in this exercise.
Analysis of Shell Form
In our examination of shell adaptation we shall focus on two attributes of shell form: 1) aperture size; and
2) spines.
Aperture Size: The aperture of a snail shell is the large opening in the shell which leads to the shell interior. In life, the animal’s foot extends outward through the aperture and rests against the flat, smooth, shiny region on the underside of the shell, referred to as the apertural lip. Not all snails have an apertural lip. The size of the aperture plus apertural lip, where present, is an index of the size of the animal’s foot.
Examine the apertural region in each specimen.
FIGURE 2 Definition of geometric parameters for snail shells
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Determine the size of the aperture and apertural lip, if present, relative to the size of the shell in both Nerita and Murex. To do this, we will define aperture area as given in Figure 2, and we shall express shell size in terms of shell area, which is also defined in Figure 2.
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Measure aperture length (A, in Figure 2), aperture width (B), shell length (C), and shell width (D) for Nerita and Murex , and record your results in the appropriate spaces in Table 2. Note that for
Nerita, aperture length is based on the distance between the outer lips of the aperture rather than between the inner edges.
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Calculate aperture area, shell area, and relative aperture area according to the formulae defined in
Figure 2, and record the results of your calculations in Table 2.
Species
N. tessellata
A B C D Aperture
Area
Shell
Area
Relative
Aperture
Area
M. beaui
TABLE 2: Morphometric data for grasping and crawling snails.
Which of the two species has the largest aperture relative to the size of the shell? What is its life style?
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Considering that aperture size is an index of foot size, what adaptive advantage does a large aperture convey to grasping snails? Of what value might this be to snails living on wave-swept rocks?
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Spines: ` Murex, and other surface crawling snails, live on loose, sandy substrates which provide no firm attachment for the animals. As a result, they cannot rely on strength of attachment to prevent being overturned or grabbed by predators. Structures other than those associated with the foot must act in this defensive role. To examine this question, place the Murex shell on the lab table with the aperture down.
This is the normal life position of these snails.
Where does the shell actually touch the table?
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Would the shells be harder or easier to roll or overturn if they had no spines?
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Would they be harder or easier to swallow without spines?
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Based on these observations, what do you think is the function of the spines in Murex?
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Evolution
Evolution is the process by which a lineage of organisms is modified through time. When modification becomes extensive, new species may arise. Processes and patterns of evolution have been the focus of study by biologists and paleontologists for more than a century. This work has established the fact of evolution, and its general attributes, but many details concerning evolutionary mechanisms are hotly debated in the scientific literature. Analysis of fossils is a powerful tool in the study of evolution. Such work is particularly valuable in establishing the course of evolution, in recognizing the rate of evolutionary change, and in identifying the causes of evolution.
Devonian Brachiopods
In the large specimen tray labeled “EVOLUTION”, you will find specimens of three different species of fossil brachiopods of middle Devonian age from upstate New York. Brachiopods are clam-like invertebrates which were very common in Paleozoic marine ecosystems, but have now become quite rare, and are not often encountered in modern marine environments. Each species is in its own small tray. The three species represent different stages of an evolutionary lineage. Tropidoleptus carinatus is the founder of the lineage. It gave rise to Spinatrypa spinosa , and this species in turn produced Pseudatrypa devonica.
Thus, there are two speciation events represented in this lineage: 1) T. carinatus – S. spinosa; and 2) S. spinosa – P. devonica. Figure 3.9 shows the geologic context in which these species are found. The sketch is a profile view of a roadcut 30 miles southwest of the city of Rochester, in Monroe County, NY.
Each species is confined in its occurrence to a specific zone within the sequence of rock layers
(stratigraphic sequence) identified in Figure 3.9. Specimens of a particular species occur only within its zone, and not elsewhere in the stratigraphic sequence. This is because a species originates at the base of its zone, and becomes extinct at the top of its zone. The fossiliferous zones are observed to overlie one another in a simple vertical sequence. Notice that the radiometric ages of horizons within this sequence are also given in the figure.
FIGURE 3: Stratigraphic occurrence of three atrypid brachiopod species in rocks of middle Devonian age,
western New York. Radiometric ages are for the first occurrence of each species.
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Paleoenvironment
It is clear from Figure 3 that these species are found in different types of rock. T. carinatus occurs in sandstone, and the other two species occur in shale. This distribution has important environmental implications because the physical characteristics of a rock are the products of the environment in which it formed. Thus, rock type is an index of paleoenvironment, i.e
., the environment in which a fossil species lived. Sandstones, for example, are generally indicative of environments with clean, clear water, fastmoving currents or waves, and hard, sandy substrates. Shales indicate environments with turbid water, little current activity, and soft, muddy substrates.
The changeover in sediment type from the lower sandstone units in Figure 3 (Ludlowville and Genesee
Sandstones) to the Wanakah Shale imply that a significant changes in environment occurred during the interval represented by this stratigraphic section.
Where in the section did this environmental change occur?
Within the Shale Between the Sandstone and Shale (Circle One) Within the Sandstone
Brachiopod Shell Geometry
To more clearly understand the evolution of these brachiopods, we will need to analyze their shell shapes using the parameters defined in Figure 4.
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Measure the length and height of each species (one specimen of each will do) using the definitions for these terms given in Figure 4.
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Also calculate shell fineness using the equation given in Figure 4.
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Record your data in the first three columns of Table.
FIGURE 4:
Definition of Geometric Parameters Used In
Measuring Brachiopod Shells
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Geologic Age
We will define the geologic age of these brachiopod species as the time of their first appearance in the stratigraphic record. Geologic age for each species may be obtained from the data in Figure 3.
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Enter in the appropriate spaces in the last column of Table 3, the geologic age ( i.e
. the age of first occurrence) for each of the brachiopod species, as determined from Figure 3.
SPECIES
T. carinatus
S. spinosa
Shell
Length
(cm)
Shell
Height
(cm)
Shell
Fineness
(L/H)
P. devonica
TABLE 3: Geometric and age data for three species of atrypid brachiopods
Scale of Evolutionary Change
Geologic
Age
(mya)
The data in Table 3 can be used to examine the scale of evolutionary change in shell shape in this brachiopod lineage. This may be done by subtracting the shell fineness value of the species produced in each of the two speciation events of interest (daughter species) from the shell fineness of the ancestral form in each speciation (ancestor species).
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Perform this calculation and enter the results in Table 4.
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In the last column of Table 4, note whether the magnitude of the change in shell fineness is large or small by placing an “X” in the appropriate space.
Speciation
Event
#1
#2
Ancestor
Species
T. carinatus
S. spinosa
Daughter
Species
S. spinosa
P. devonica
Evolutionary
Change in
Fineness
Scale of
Evolutionary
Change
Large Small
TABLE 4: Data on scale of evolutionary changes in shell fineness
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Rate of Evolutionary Change
We can now estimate the rate of evolutionary change in shell fineness in these brachiopods from the data contained in Table 4. A graphical approach is simplest. Use the graph axes shown in Figure 5, as a basis for conducting your analysis.
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First plot the position of each species in Figure 5 with respect to shell fineness and geologic age.
In doing this, remember that for each species, age is the X-coordinate, and is located with respect to the X (age) axis; and that fineness is the Y- coordinate, and is located with respect to the Y
(fineness) axis. When finished, you should have three data points drawn in Figure 5, one for each species. Label each data point with the appropriate species name.
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Connect the three data points in Figure 5 by drawing straight lines between them. You will thus obtain two lines, one connecting T. carinatus with S. spinosa , and the other connecting S. spinosa with P. devonica . The first line represents the speciation of S. spinosa from T. carinatus
(Speciation #1). The second line represents the speciation of P. devonica from S. spinosa
(Speciation #2)
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The slopes of the two lines in Figure 5 are an index of evolutionary rate. Lines that are nearly horizontal indicate slow rates of change, while steeply lines represent rapid rates of change. How do the two speciation events listed in Table 5 compare with respect to rate of change in shell fineness? Note your conclusions in Table 5 by placing an “X” in the appropriate spaces of the last column of this table.
FIGURE 5: Rate of evolutionary change in shell fineness for a lineage of atrypid brachiopods.
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Speciation
Event
Ancestor
Species
Daughter
Species
Rate of Evolutionary Change
Fast Slow
T. carinatus S. spinosa #1
#2 S. spinosa P. devonica
TABLE 5: Summary of evolutionary rates for a lineage of atrypid brachiopods.
Evolutionary Causation
It is possible to gain some understanding of the causes driving the evolution of this lineage of brachiopods by combining our observations on the following: scale of evolutionary change (Table 4); evolutionary rates (Table 3.5); geologic setting (Figure 3); and paleoenvironment. Use Table 6, below, as a guide in assembling these data. To do this, follow the instructions below.
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Evolutionary Scale: Transfer data ( i.e., the terms “large” or “small”) from the last column of
Table 5 to the first column of Table 6
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Evolutionary Rate: Transfer data ( i.e., the terms “fast” or “slow”) from the last column of Table 5 to the second column of Table 6.
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Rock Type: This term refers to the type of rock in which a species occurs. Enter the term
“same” in the third column of Table 6, if both ancestor and daughter species occur in the same type of rock, as determined from Figure 3, and “different” if they occur in different rock types.
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Paleoenvironment: In the fourth column of Table 6, enter the term “same” if both ancestor and daughter species lived in the same type of paleoenvironment, as determined earlier, and
“different” if they lived in different paleoenvironments.
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Speciation
Event
#1
#2
Evolutionary
Scale
Evolutionary
Rate
Rock
Type
Paleo
Environment
TABLE 6: Evolutionary data for a lineage of atrypid brachiopods.
New species arise in two different environmental scenarios: 1) adaptation to changing environments (Type
1 speciation; and 2) progressive adaptation within a constant environment (Type 2 speciation). In the first case, a lineage produces new species when confronted by changing environmental conditions. The new species thus produced generally exhibit new adaptations which make it possible for them to survive in
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the new environment. In the second case, a lineage produces new species within the context of an unchanging, constant environment. The new species generally exhibit improved adaptive designs that render them more efficient and effective competitors within this environment.
Using the information in Table 6 and the information above, briefly discuss the probable cause of speciation in these brachiopods. Pay particular attention to whether the appearance of each new species was a Type 1 or Type 2 speciation event as defined above.
Speciation #1 ( T. carinatus/S.spinosa
):
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Speciation #2: (S . spinosa/P. devonica:
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What is the likely explanation for the difference in evolutionary rates between the speciation that produced S. spinosa , and the speciation that produced P. devonica ?
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Adaptive Significance of Evolutionary Change in Shell Shape
Brachiopods are filter-feeders, which means that they extract tiny organisms and organic particles from the water in which they live. They do this by generating a current which draws food-laden water into their shells where it is passed through a filtration net, and the food particles removed. Studies with modern brachiopods show that even a small amount of sediment entering the shell with the incoming current can clog this filtration system and kill the animal. Thus, brachiopods must either live in environments with clear water, or adopt shell shapes which minimize sediment influx into their shells.
Consider the environments in which our Devonian brachiopods were living. T. carinatus inhabited clearwater, sandy environments where clogging probably was not much of a problem. More significant a factor in terms of determining shell form were strong currents, which are common in sandstone paleoenvironments. The flattened, streamlined shell shape of Tripidoleptus probably functioned to minimize the flow-induced forces acting to dislodge it from the substrate. On the other hand, as indicated by their association with shale, both S. spinosa and P. devonica inhabited muddy-water environments with soft, mud bottoms, where currents, if any, moved only very slowly. For these brachiopods, clogging must have been a major problem. The rounded, globular shapes of their shells served to anchor them in the soft
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substrate, and keep the margins of the shell clear of the bottom, so that when the shell was opened, the mud in which they sat would not be drawn into the shell.
Considering these functional aspects of brachiopod shell shape, discuss the adaptive significance of changes in shell form associated with the two speciations of interest.
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