Notes for Understanding Evolution - Chapter 13

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General guide on these review questions here

Under Construction

Notes for Chapter 13: Major Adaptive Radiations

Introduction (Temporary links from Biology 404)

Introduction

The Origin of Land Plants

The Earliest Land Floras

Comparing Plant and Animal Evolution

The First Land Animals

Rhipidistians (and Other Lobefins)

The First Amphibian


Editorial Comments about Understanding Evolution Ch. 13

I have had a great deal of trouble coming up with review questions for this chapter

because...

• This is an extremely old-fashioned presentation of the evolution of land vertebrates

• There are numerous factual errors

• There are great "bad examples" of misconceptions about evolution throughout

• These can seem familiar because such misconceptions are common in biology texts

• The different conceptual approach in modern systematics makes a huge difference

• The idea is to abandon grades as non-phylogenetic, adopt the "Rule of Monophyly"

• This notion was introduced on the web page for Ch. 12

 

Where to start?

• Recognize that this chapter uses grades not clades

     – a grade is a trend or progression which might or might not reflect the actual phylogenetic history
     – examples include fish -> amphibian –> reptile –> mammal; prokaryote –> protist –> invertebrates –> vertebrates
     – we will try to avoid speaking about "primitive" groups giving rise to more "advanced" groups
     – we will especially avoid ladder-like grade hypotheses -- refer to my Discussion Board comments on horse evolution
     – try to reserve the term "primitive" for describing the older of two or more character states compared across taxa
     – the organism or taxon itself is not "primitive" because all organisms/taxa have both "primitive" and "derived" traits
     – for example, a duck-billed platypus has primitive features (e.g., egg laying) and derived features (e.g., toothless bill)
     – do not refer to monotremes as primitive mammals, but you can say they are reproductively primitive
     – you can call them a "basal" mammal lineage, which means they split off near the base of the mammal tree
     – remember, a platypus has been evolving for exactly as many years as we have since we last shared a common ancestor
     – when you recall how we said speciation occurs, you should realize that "only species speciate, genera do not generate"
     – in other words, higher taxa (e.g., reptiles) do not give rise to other higher taxa (e.g., birds)

 

• Recognize that this chapter uses paraphyletic taxa

     – a paraphyletic taxon is a taxon that includes a common ancestor and only some of its descendants
     – examples include amphibians, reptiles (in conventional sense), prokaryotes, protists, invertebrates
     – the alternative is to use only monophyletic taxa, which are hypothesized to be clades
     – a monophyletic taxon includes a common ancestor and all of its descendants
     – examples include tetrapods, reptiles (in cladistic sense), the clade of all life, eukaryotes, metazoans (animals)
     – tetrapods include various early "amphibians" but also the amniotes (reptiles + synapsids -- including mammals)
     – early tetrapods were actually more like lizards (except in their reproduction) than they were like salamanders
     – reptiles in the conventional sense includes all early amniotes, excluding birds and mammals
     – in conventional classifications, Reptilia, Aves, Mammalia, are all ranked at the same (class) level
     – this leads one to think of them as separate groups, each one distinct
     – texts like ours typically assert that "birds evolved from reptiles" or "mammals evolved from reptiles"
     – again, recognize that such statements imply that reptiles must be paraphyletic (see above)
     – for decades now, systematists have rejected such notions -- it is time for textbook authors to catch up!
     – monophyletic taxa are supported by synapomorphies -- shared-derived features hypothesized to be homologous
     – paraphyletic taxa are arbitrarily defined taxa -- for example, "invertebrates" are all animals except those with backbones
     – in philosophy, these sorts of definitions are termed "not A" because a group is arbitrarily separated into "A" and "not A"
     – for example, animals can be divided into vertebrates (A) and invertebrates (not A)
     – more examples: life –> eukaryotes (A) and prokaryotes (not A); eukaryotes –> plants, fungi, animals (A groups), protists (not A)
     – in each case, each "A" group is more closely related to some members of the "not A" grouping than it is to other "not A" members
     – paraphyletic taxa are the corresponding monophyletic group with some of its descendants excluded

• Learn to contrast this approach with the cladistic alternative


     – cladists strictly follow the "rule of monophyly" and reject paraphyletic taxa as unnatural
     – classifications based on this rule are simpler to learn -- learn the best-supported cladogram, the classification follows
     – there can be alternative cladogram hypotheses, each implying a different classification
     – in practice, systematists might publish a "conservative" classification only formally naming the most stable cladogram nodes
     – to compare a cladistic classification, get used to thinking of taxa as hierarchically nested clades
     – birds are nested within reptiles, reptiles are nested within amniotes, amniotes are nested within tetrapods
     – this may seem odd at first, but you don't have any problem extending this deeper with more familar taxon names
     – for example, tetrapods are nested within gnathostomes (vertebrates with jaws), which are nested within vertebrates, etc.
     – as another example, humans are nested within primates, within placental mammals, within all mammals, within vertebrates
     – by analogy, CSUF is within Fullerton, which is within Orange Co., which is within California, which is within U.S.A.
     – returning to birds, there is now firm evidence that birds are a specific lineage of feathered dinosaurs
     – birds are a subclade of maniraptors, maniraptors are a subclade of therapods, therapods are a subclade of dinosaurs
     – dinosaurs and crocodiles are subclades of archosaurs, archosaurs and lizards are subclades of reptiles
     – reptiles, as redefined cladistically, include birds, crocodiles, squamates ("lizards" and snakes), and turtles
     – reptiles, as redefined cladistically, do not include the "mammal-like reptiles" (basal or stem synapsids, e.g., "pelycosaurs")
     – instead, think of the first tetrapod vertebrate with an amnionic egg as the common ancestor for the clade Amniota
     – get used to referring to "basal amniotes" not "reptiles giving rise to mammals and birds"
     – the text uses the paraphyletic taxon "cotylosaur" which was rejected decades ago (because it is paraphyletic)
     – this would basically correspond to the first amniote and all its descendants, except "modern" reptiles (including birds) and mammals

• Reevaluating the history of tetrapod vertebrates with "water-tight" eggs (the amniotes)


     – there is good evidence that amniotes split early into two distinctive lineages: reptiles (or sauropsids) and synapsids
     – reptiles (in this class) now refers to the sister taxon of synapsids; still living are: turtles, lizards, crocodiles, and birds
     – usually, reptiles are subdivided into anapsids (including turtles) and diapsids (lizards, crocs, birds, etc.)
     – some biologists not support grouping turtles as close relatives of crocodiles and birds (i.e., the archosaurs)
     – mammals are a subclade of synapsids -- more on that below
     – synapsids dominated in the Paleozoic, for example, Dimetrodon (Fig. 13.7, p. 143) lived in the Permian era
     – the text uses the paraphyletic taxon "pelycosaurs" to refer to all synapsids (including their common ancestor) except therapsids
     – therapsids are the only group of synapsids to survive the giant extinction event at the end of the Permian
     – therapsids rebounded during the early Triassic, and were extremely common, especially in Gondwanaland
     – they included large lumbering herbivores (especially dicynodonts, e.g., Lystrosaurus) and smaller carnivores
     – later in the Triassic, the herbivorous therapsids went extinct, perhaps outcompeted by a new group of reptiles, the dinosaurs
     – the carnivorous therapsids include the first mammals, about as old as the first dinosaurs
     – most of the Mesozoic Era (specifically, the Late Triassic, Jurassic, and Cretaceous) was dominated by dinosaurs
     – other reptilian groups also were important ecologically, including pterosaurs in the air and lots of aquatic reptiles
     – aquatic reptiles included crocodiles and turtles plus extinct groups such as icthyosaurs, sauropterygians, and mosasaurs
     – Mesozoic mammals were mostly small and nocturnal
     – dinosaurs suffered many extinction events in their long history
     – for example, the long-necked sauropods were dominant in the warm Jurassic but were extinct by the Cretaceous
     – the most famous dinosaur extinction occurred at the end of the Cretaceous (65 Mya), probably caused by a meteorite impact
     – big dinosaurs such as T. rex went extinct then, but birds survived (remember, birds are dinosaurs)
     – there are more species of dinosaurs alive today than there are mammals

 

I. Invasion of Land

RQUE13.1: Describe Romer's old hypothesis for lobefin fishes ("crossopterygians") and their incentive to become increasingly adapted to dry land. Next, realize that modern lungfish will rarely if ever leave a drying temporary pool to set out searching for another pool, and it is highly doubtful that our ancient semi-aquatic ancestors did that either. So what alternative hypotheses might account for increasing adaptations to come out onto dry land? A possible test might be to compare when or why modern semi-aquatic animals come out of the water.

RQUE13.2: Review the concepts above and then compare the old-fashioned figure of the evolution of land vertebrates in the geological past (Fig. 13.1, p. 137). How does this figure obscure or even contradict the nested pattern of relationships I describe above? According to the "rule of monophyly" advocated, which taxon names would need to be rejected? Can you draw a cladogram equivalent?

II. Conquest of Land

RQUE13.3: How did the amniotic egg permit vertebrates to become more fully terrestrial? Which parts of the amniotic egg (yolk sac, embryo, chorion, amnion, allantois, shell) are primitive and which are novelties, first arising in the common ancestory of amniotes? How do these derived egg components function?

III. Adaptive Radiation of Amniotes (Synapsids and Reptiles)

RQUE13.4: Do the same for Figure 13.6 (p. 142) that you did for Figure 13.1 in RQUE13.2.

RQUE13.5: Suggest two alternative hypotheses for the sail of the early synapsid, Dimetrodon.

IV. Extinction and Replacement

RQUE13.6: What evidence suggests that forelimbs are homologous across the tetrapod vertebrates in Figure 13.8 (p. 145)? Make sure that you define what you mean by homology. How are some of these forelimbs analogous (not homologous) to structures in other animals, likely due to convergent evolution? What evidence is there that the bird and bat forelimbs are convergent as wings, even if they are homologous as forelimbs?

RQUE13.7: Compare the "primitive pattern" hypothesized in Figure 13.8 with the forelimbs of early tetrapods such as Acanthostega (see "The First Amphibian" links above). What is different?

PBS link: Adaptive Radiation: Mammalian Forelimbs (pdf)



Resources

American Museum of Natural History - Teacher guide to introducing cladistics (K-6) and OLogy Website or Here
Arizona Tree of Life Project: What is Phylogeny? or Main Page
UC Berkeley Paleobotany Lab's Phylogenetics Lab
UC Berkeley Museum of Paleontology's Cladistics in Brief or Phylogeny Wing
Australian Systematic Biologists' Introduction to Phylogenetics
GenBank's Taxonomy Browser organizes all organisms with sequences available according to a cladistic hierarchy
Lecture Notes on Systematics: 1 - 2 - 3 - 4 - 5 - 6
BBC's Walking with Dinosaurs and Walking with Beasts (Cenozoic Mammals)

 

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This page created 6/8/02 © D.J. Eernisse, Last Modified 7/19/02, Links Last Completely Checked 7/13/02