Chapter 12 - Adaptive Radiation

Notes for Understanding Evolution - Chapter 12

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General guide on these review questions here

Notes for Chapter 12: Adaptive Radiation

Introduction

I. Galápagos Islands

Key Terms: adaptive radiation, endemism

RQUE12.1: What are a few ways in which animals endemic to the Galápagos are adapted to life on these arid tropical islands?

Featured Scientist: Charles Robert Darwin

source of image

Links: 1 - 2 - 3 - 4

Excellent notes are available from Dennis O'Neil's (Palomar College) website:
Evidence of Evolution and Darwin and Natural Selection

Browse Darwin's Origin of Species (1st Edition) online

The Evidence for Darwin’s Evolutionary Theory:

A. Living and fossil organisms are constantly changing
B. Sedimentary rock layers indicate an ancient Earth(Notion of “Deep” Time - Geologist Lyell)
C. Many evolutionary trends are apparent (e.g., horses)
D. Patterns in nature are hierarchically nested as expected from Darwin’s “common descent” (click here for an example: birds are nested within a clade of feathered therapod dinosaurs)
E. Homology between structures is evidence for evolution because the explanation that similarity is due to shared ancestry is more parsimonious than alternative explanations.
F. Animals tend to be more similar earlier in ontogeny
G. Multitude of species on Earth suggests branchinghistory
H. Gradual changes can add up, given enough time (Examples from lecture: Darwin's worm stone and coral reef formation, also used as an examples of Darwin'sapplication of the notion of uniformitarianism to ecological and geological studies, respectively).

source of image

II. Darwin's Finches

Featured Organisms: Darwin's Finches

Links: 1 - 2 - 3 - 4 - 5 - 6 - 7 - 8 - 9 - 10

Links to other interesting endemic fauna/flora: 1 - 2 - 3 - 4

RQUE12.2: Darwin's finches have become adapted to a number of ecological opportunities in the Galápagos. What evidence is there that they are not "ideally" adapted, instead having adaptations that reflect their ancestral heritage?

III. Factors in Diversification

RQUE12.3: What is a likely explanation for why an adaptive radiation of this particular group of finches has not occurred on the continental mainland, to nearly the same extent it has occurred in the Galápagos?

IV. Competitive Exclusion

RQUE12.4: How are interactions between the Galápagos finches, in some ways, like those between two strained cultures of Paramecium that Gause studied?

V. Coexistence

Key Terms: Gause's principal, niche, avoidance of competition, niche diversification,

RQUE12.5: What is meant by the concept known as Gause's principle (or the "niche requirement")? If this concept is accurate, then how do closely related species manage to live in close proximity?

VI. Classification

RQUE12.6: The hypothesized phylogenetic relationships postulated by the tree diagram in Fig. 12.7 is based on the inference that the "tree finch" morphology arose only once, and subsequently geographic isolation led to speciation, producing six separate tree finch species. Likewise, the tree implies that the common ancestor of the six "ground finches" was also a ground finch. What is at least one alternative explanation that might have led to the observed pattern of separate tree and ground finches on the separate islands? Can you suggest how you might test between these alternative hypotheses?

Note: The following notes on phylogenetic analysis and classification are much more extensive than the two paragraphs devoted to classification at the end of Chapter 12, but this is as good a place as any to introduce the terminology and concepts presented below, which we will emphasize throughout the remainder of the course.

Key Terms: phylogeny, characters, homology, homoplasy

A. Using Character Variation to Reconstruct Phylogeny

Key Terms: ancestral vs. derived character states, polarity (the ancestral state is generally the one that is also present in the outgroup, by outgroup comparison), clade (ancestor plus all its descendents), synapomorphy (derived novelty that helps us recognize a clade, e.g., feathers in birds), plesiomorphy (ancestral or "primitive" state, not necessarily the most "simple" state), nested hierarchy, symplesiomorphic (shared "primitive" similarities - these do NOT help us recognize clades, e.g., lack of backbone in a fly and a snail), cladogram vs. phylogenetic tree (similar, but the "y-axis" of a cladogram means nothing, whereas it might in a phylogenetic tree, e.g., geological time) - Note: Don't worry, these terms are difficult at first and we will be reinforcing them over the entire semester -- See Cladogram Exercise 1.

1. Estimating a phylogeny depends on characters (traits)

a. only characters that vary are interesting
b. the different forms of the charater are termed states

2. One observes similarities that could be homologous

a. a homology is a similarity due to common ancestry
b. this means the common ancestor had the same state
c. a homologous similarity only has to evolve once

3. Alternatively, similarities might have evolved separately (convergently)
4. Any similarity not due to homology is termed a homoplasy (includes convergence)
5. The parsimony criterion is used to choose some trees as better than others

a. The most parsimonious tree explains as much as possible by homology
b. A tree is more parsimonious than another when it requires fewer changes
c. The most parsimonious tree is the one with the least homoplasy
d. This is because homoplasies require extra changes; homologies do not

B. Study of Character Variation Can Reveal Ancestral Conditions

1. Given a phylogeny, one can determine which character state is ancestral
2. The ancestral state of a character is the state found in the ancestor
3. Character states arising later are termed derived states
4. In practice, we cannot normally observe the common ancestor

a. Instead we use a closely related taxon as an outgroup to estimate ancestral state
b. We can also use multiple outgroups
c. Example: we observe no teeth in birds and teeth in lizards; which is primitive?

1) We note that outgroups (e.g., mammals, salamanders and fish) all have teeth
2) Thus, the common ancestor of birds and lizards probably had teeth
3) Thus, the presence of teeth in lizards is a primitive state
4) Thus, the lack of teeth in birds is a derived state

5. A clade is a natural taxon of organisms bound in space and time

a. A clade is defined as a common ancestor and all of its descendants
b. In practice, we recognize a clade by its derived similarities
c. Example: clade - birds includes ancestor of birds and all its descendants
d. Feathers is a derived similarity found only in birds (no living outgroup has feathers)
e. It is most parsimonious to suppose that the common ancestor of birds was feathered
f. Because lizards lack feathers, feathers probably arose after lizards and birds diverged
g. In other words, the last common ancestor of lizards and birds lacked feathers

6. Technically, a derived character state is termed an apomorphy
7. A shared derived character state is termed a synapomorphy
8. Synapomorphies are typically nested hierarchically

a. Example: All placental mammals have a placenta; placenta is a synapomorphy
b. All marsupial mammals have a marsupial pouch and lack a placenta
c. Both placentals and marsupials have hair and mammory glands (as do all mammals)
d. mammals and lizards both have an amnion around their eggs (as do all amniotes)
e. amniotes and salamanders both have four limbs (as do all tetrapods)
f. tetrapods and sharks both have jaws (as do all gnathostomes)

9. An ancestral (not derived) state is termed a plesiomorphy
10. A shared ancestral state is termed symplesiomorphy.

C. Sources of Phylogenetic Information

Key Terms: comparative morphology, biochemistry, and cytology

1. Morphology: includes shape, size, and development

a. Examples: Skull or limb bones, scales, hairs, feathers
b. Can be observed in fossils as well as living specimens

2. Biochemical comparison (now the most common evidence used)

a. Examples: Protein or DNA sequence comparison
b. Occasionally fossils have remnants of DNA preserved, but not easy to recover

3. Cytological comparison

a. Examples: examines variation in number, shape and size of chromosomes
b. Only used for living organisms

4. Dating a fossil is possible (with radioactive dating methods)
5. Estimating when lineages diverged is also possible with sequence comparisons

D. Theories of Taxonomy

Key Terms: monophyly, paraphyly, polyphyly

1. Three types of groupings are recognized

a. Monophyletic: includes common ancestor and all its descendants
b. Paraphyletic: includes common ancestor and only some of its descendants
c. Polyphyletic: does not include the most recent common ancestor of its members

2. Evolutionary and cladistic systematists only disagree about the case of paraphyletic groups

a. Both agree monophyletic groupings should be recognized
b. Both agree polyphyletic groupings should be rejected
c. Only evolutionary systematists allow paraphyletic groups
d. In contrast, cladists only formally name groups thought to be monophyletic
e. The cladistic principle is known as the "rule of monophyly"
f. Many taxa in widespread use are paraphyletic

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or back to Chapter 11 or ahead to Chapter 13