Chapter 6 - The Accidental Toolkit

Notes for Evolution - The Triumph of an Idea - Chapter 6

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General guide on these review questions here

Chapter 6 - The Accidental Toolkit - Chance and Constraints in Animal Evolution

PBS Website Links: 1 - 2

Objectives:

a) Get introduction to Hox genes and other "homeotic" genes that are used to regulate other genes in the development of animals, especially those that have bilateral symmetry (bilaterians).

b) Understand how these genes make up part of a common "genetic toolkit" that was almost certainly already present in the last common ancestor of bilaterians.

c) Explore how the evolution of this genetic toolkit might be related to the fantastic early diversification of bilaterian animals that is characterized as the Cambrian explosion.

d) Consider some ideas about why this diversification did not continue once all the major groups of animals still present today had appeared in the fossil record, over 500 million years ago.

e) Focus on the early evolution of vertebrates, including emphasis on the duplication of Hox gene clusters and the evolution of tetrapod vertebrate limbs.

f) Use the example of the fossil record of whales to address whether transitional fossils do, in fact, exist in the fossil record.

Introduction

Featured Scientist: Ed Lewis

source of image

Links: 1 - 2 - 3 - 4 - 5 (PBS article in pdf format)

I. Introduction

RQ Ev-6.1: Why is it claimed that a "standard tool kit of body-building genes" must have evolved before the Cambrian explosion?

II. Evolution's Monsters

Key Terms: Hox genes (also see links above for Ed Lewis and these web articles: 1 - 2)

RQ Ev-6.2: Contrast what is now known about the mutant fly discovered with legs growing where its antennae should be with Bateson's notion of freakish "homeosis" variations.

III. The Master-Control Genes

RQ Ev-6.3: What is meant by a "master control" gene compared with other sorts of genes, such as genes that code for enzymes or structural proteins?

IV. The Genes Behind the Cambrian Explosion

RQ Ev-6.4: Why is it thought that the "genetic toolkit" present in all animals with bilateral symmetry is possibly involved in the Cambrian explosion?

RQ Ev-6.5: What evidence links flies and vertebrates with respect to their dorso-ventral axis of symmetry, and what change in orientation is necessary in order to support this link?

V. Gene Duplication and the Dawn of Vertebrates

RQ Ev-6.6: How was gene duplication of at least the Hox gene cluster possibly involved in early vertebrate evolution, in comparison to non-vertebrate chordates such as the lancelet?

VI. Lighting the Cambrian fuse

RQ Ev-6.7: What physical factors on Earth shortly before the time of the Cambrian might have set the stage for the Cambrian explosion?

VII. The Party's Over

RQ Ev-6.8: The Cambrian Period is famous for its Burgess Shale-like animals but in what sense was the "party over" already with respect to animal evolution by the time of the Burgess Shale, even though this is some 505 million years back?

RQ Ev-6.9: Eyes of humans, octopus, and flies have conventionally been treated as stunning examples of evolutionary convergence, but what recent discoveries have suggested that there are actually some common elements to eye evolution in all of these, despite their functional and embryological differences?

VIII. Fish Fingers and Life on Land

RQ Ev-6.10: The evolution of limbs in tetrapod vertebrates used to be thought of as an adaptation to life on land. How did discoveries by Jenny Clack for the early tetrapod, Acanthostega, lead to a revision of that view?

IX. Forward Into the Past: The Origin of Whales (Links: 1 - 2 - 3 - 4 - 5 - 6 - 7 - 8 - 9)

RQ Ev-6.9: How do recent discoveries of ancient whales and their near relatives provide evidence that transitional fossils do occur in the fossil record?

Click link to return to Biology 409 Schedule
or back to Chapter 5 or ahead to Chapter 7