Biology 221 - Invertebrate Zoology
Lecture Notes Copyright © 2003, S.M. Shuster
Lecture #3 9/02/03 and Lecture #4 9/04/03
I. Introduction
A. This lecture will concern some of the outcomes of natural selection and the patterns it
produces; if time permits, will lead into classification schemes.
II. Patterns of Evolution, continued
A. Last time left off discussing how natural selection works and how if allowed to
operate for long periods, how it can lead to evolutionary change.
B. Convergence
1. Species don't always diverge.
a. in some cases distantly related species may be faced with similar environmental
circumstances.
b. If selection favors similar phenotypes character CONVERGENCE is likely to
occur.
c. Examples:
1. Sucking insects
2. Limpets and acorn barnacles - conical morphology.
C. Parallelism
1. Species may follow similar evolutionary paths
a. Separation of populations may prevent gene flow.
b. Genetic drift, unique selection pressures may cause some divergence.
c. Primary selective pressures may stay the same.
d. Result: Species are distinct, but retain similar morphology.
1. Examples
a. Bopyrid isopods, other parasitic species
1. host specialization prevents gene flow.
b. Lepidopteran Wing Patterns
1. ancestor to family probably had some patterning of wings
2. different general inhabit similar environments,
a. individual selective pressures, drift, chance lead to similar, yet slightly
different wing patterns
D. Punctuated Equilibrium vs. Gradualism
1. Major patterns in evolution can arise from these patterns.
a. Stabilizing selection
1. The fitness function remains constant for long periods of time.
this can lead to character stasis.
2. Much of the fossil record exhibits this pattern, especially in marine communities.
c. Directional selection
1. Generates directional change.
2. The intensity of selection, heritability of character related to rate of change
3. Sometimes change is slow and gradual -the type envisioned by Darwin.
4. Sometimes change is rapid - change is so rapid that it is not reflected in fossil
record.
2. There are also other explanations for change
a. Extinction
1. Catastrophic or gradual extinction events may remove portions of species
variation.
2. If populations go extinct,
a. competition may be relaxed
b. new adaptive zones may become available
c. populations could respond adaptively.
3. Or, intermediate forms that gave rise to other forms may be lost.
b. Chance
1. Sometimes extinction events can change selective pressures and constellation of
species such that rapid change will occur
2. Small populations and reduced migration rates can reduce genetic variation.
a. This may limit the evolutionary potential of a population.
b. Or, it may produce genetic interactions that produce new phenotypes.
c. New combinations may have higher fitness than old combinations.
1. If populations are suddenly reduced in size, they are called “population
bottlenecks,”
2. If few individuals are isolated in new locations these evolutionary changes
are called “founder effects.”
c. Two major patterns of evolutionary change are observed in the fossil record.
1. The combination of morphological stasis and rapid evolutionary change
a. called Punctuated Equilibrium by Stephen J. Gould and Nils Eldridge.
2. The Darwinian process of gadual change over long time
a. called Phyletic Gradualism – by palentologists in general.
3. Both patterns exist in fossil record.
a. We will come to this again shortly.
E. Evolution as Progress
1. G. Vermeji 1987. Evolution and Escalation: an Ecological History of Life Princeton
Univ. Press.
a. This book suggested that much of what we see today is the result of biological
interactions of the past.
b. Large scale patterns exist in the fossil record.
1. example: the location of gastropod predator drill holes in bivalve shells changes
over evolutionary time – a predator prey race.
2. Vermeji documented this entirely by touch – he is completely blind.
c. Prediction: Certain evolutionary patterns will persist over time, and will evolution
occur in repeatable ways; with enough time, there will tend to be Evolutionary
Progress.
1. Examples appear to include
a. gastropod predators, bivalve prey
b. siphon formation in bivalves
c. stem and holdfast formation in crinoids
d. zooid integration and determinate growth in bryozoa
e. large, crushing chelae in crabs
d. Alternative explanation:
1. ecological relationships among taxa are irrelevant over long periods of time
because of mass extinction events.
2. periods of cataclysm that indiscriminately wipe out large numbers of taxa without
any ecological pattern.
a. Predictions:
1. there will be no pattern to the extiction probability among taxa along
morphological or ecological lines.
2. also, environmental patterns will not predict morphology over long periods
of time
b. Examples
1. Jablonski & Raup 1993; 1995: end cretaceous bivalve extinctions
2. They show that there is no relationship between ecological position,
habitat type, habitat location and physical size and the probability of
extinction.
c. What is the significance of this difference??
1. Evolution viewed as progress can lead to the conclusion that life on Earth
becomes increasingly highly evolved.
2. Jablonski and Raup’s data suggest that natural selection and evolutionary change
may simply be a process that arises spontaneously in replicating entities.
3. It is certainly the simplest explanation for what we see.
4. It also shows the importance of contingency on evolution
a. this is the idea that evolution on Earth (or anywhere) would not procede the
same way twice if allowed to run again.
b. Like in “It’s a Wonderful Life,” things are very different without George
Bailey.
c. It suggests that humans are a very unlikely evolutionary event indeed.
III. Phylogenetic Systematics
A. Why mention these things?
1. Because biological classification schemes attempt to make their designations in a way
that reflects evolutionary history.
a. As mentioned in lab, this requires:
1. identifying characters that distinguish taxa
2. determining whether these characters are
a. ancestral - representing earlier forms
b. derived - representing more specialized forms
2. Constructing a framework that represents the probable line of descent.
a. The only way to link related taxa is by identifying synapomorphies shared, derived
characters.
a. syn = shared
b. apo = away from the stem
c. morph = form
3. However, the variety of evolutionary processes described above can cause difficulties in
character identification.
a. There may be considerable modification of morphology by selection character
polarity –
b. parallelism, convergence and character loss can make it difficult to determine
which characters arose first.
c. So given that pitfalls exist, what are the goals and guideposts?
4. The Goal
a. Given that only one true phylogeny occurred, the primary goal is:
1. Identification of related taxa (of MONOPHYLETIC groups)
2. Representation of true phylogenies requires avoiding two mistakes
3. The Mistakes are:
a. The exclusion of related taxa
1. This makes both groups PARAPHYLETIC
a. example: butterflies from other insects
b. Avoiding inclusion of nonrelated taxa
1. This makes the genealology POLYPHYLETIC
a. example: insects and crustaceans
5. Guideposts to Phylogenetic classification
a. Unless contrary evidence exists, assume that similarities represent homologies
1. Homology: tissues with common embryonic origin
2. This is necessary FOR TWO REASONS:
a. Evolution appears to be a conservative process
1. Characters are unlikely to evolve multiple times independently
2. Heritable characters often persist within lineages.
3. Shared, derived characters identify the in-group
a. example: Donidae
b. Donidae + outgroups
c. ancestral taxa are considered the out-group
4. What constitutes contrary evidence?
a. repeated patterns under same ecological circumstances
1. example: aposomatic coloration and grouping.
2. example: Drosophila ejaculatory ducts
B. Evidence of common ancestry is provided only by the presence of shared, derived
characters (synapomorphies).
1. Convergent, parallel characters occur independently within lineages after divergence
a. They therefore provide no information about relationships among lineages.
b. Example:
1. Cestode scolex characteristics apparently evolved within different hosts thus
scoleces are distinct.
2. tapeworm morphology, however, does appear homologous (gut habitat) useful in
separation of cestodes from other flatworms.
a. flukes (no segments, distinct life cycle)
b. turbellarians (eyes, gut, mouth)
2. How do you do this?
a. example of character matrix
b. note that ancestral and derived characters are recognized
1. this is not always so easy
2. sometimes easier to recognize presence or absence
a. group with most ancestral characters is often used as the outgroup
b. i.e., a group that lacks synapomorphies necessary for comparison
c. group with shared derived characters is the ingroup
c. Example: construction of tree with 7 characters a. need to reconcile all of the data
simultaneously, arrive at simplest tree
C. When information from two character series generate different cladograms, the most
parsimonious tree (the one with the fewest character reversals) is the one accepted as
true.
1. the principle of parsimony is important
a. Occam's Razor (14th century): "that which can be done with fewer assumptions is
done in vain with more"
b. simplest explanation is best
1. this is because it is most conservative
2. evolution is a conservative process
a. mutation, distinct selection patterns rarely occur many places at once
b. favored characters persist within lineages
D. Identification of outgroups and character polarity
1. Homologous characters found among members of an ingroup as well as among
members of an outgroup are considered ancestral.
2. Homologous characters found only among members of an ingroup are considered
derived.
a. see diagram
b. note that sometimes it is necessary to look back quite a long way before some
concensus is reached
c. occasionally, character is so labile that it is difficult or impossible to use.
II. Examples of how this is done
A. There are many schemes for classification of animal taxa
1. Previous approaches have concentrated on several sources of
information:
a. morphology
a. anatomy (structures, esp. body cavity, bauplans)
b. behavior
c. chemistry
d physiological information
e. how systems appear to work
f. development
2. Often this last one is a source of debate
a. often thought to be best because development is assumed to be conservative
1. like a city, new plans are usually extensions of existing networks
2. difficult to start from scratch and still survive.