The History of Life on Earth

Page
9

Hypothetical vertebrate

ancestor (invertebrate)

with a single Hox cluster

Second Hox

duplication

First Hox

duplication

Slide 57

Changes in developmental genes can result in new morphological forms

Hox gene 6

Hox gene 7

Hox gene 8

About 400 mya

Drosophila

Artemia

Ubx

Slide 58

Concept 25.6: Evolution is not goal oriented

Evolution is like tinkering—it is a process in which new forms arise by the slight modification of existing forms.

Most novel biological structures evolve in many stages from previously existing structures.

Complex eyes have evolved from simple photosensitive cells independently many times.

Exaptations are structures that evolve in one context but become co-opted for a different function.

Natural selection can only improve a structure in the context of its current utility.

Slide 59

Evolution: new forms arise by the slight modification of existing forms

(a) Patch of pigmented cells

Optic

nerve

Pigmented

layer (retina)

Pigmented cells

(photoreceptors)

Fluid-filled cavity

Epithelium

Epithelium

(c) Pinhole camera-type eye

Optic nerve

Cornea

Retina

Lens

(e) Complex camera-type eye

(d) Eye with primitive lens

Optic nerve

Cornea

Cellular

mass

(lens)

(b) Eyecup

Pigmented

cells

Nerve fibers

Nerve fibers

Slide 60

Horse Evolution

Recent

(11,500 ya)

Neohipparion

Pliocene

(5.3 mya)

Pleistocene

(1.8 mya)

Hipparion

Nannippus

Equus

Pliohippus

Hippidion and other genera

Callippus

Merychippus

Archaeohippus

Megahippus

Hypohippus

Parahippus

Anchitherium

Sinohippus

Miocene

(23 mya)

Oligocene

(33.9 mya)

Eocene

(55.8 mya)

Miohippus

Paleotherium

Propalaeotherium

Pachynolophus

Hyracotherium

Orohippus

Mesohippus

Epihippus

Browsers

Grazers

Key

Slide 61

The appearance of an evolutionary trend does not imply that there is some intrinsic drive toward a particular phenotype

Millions of years ago (mya)

1.2 bya:

First multicellular eukaryotes

2.1 bya:

First eukaryotes (single-celled)

3.5 billion years ago (bya):

First prokaryotes (single-celled)

535–525 mya:

Cambrian explosion

(great increase

in diversity of

animal forms)

500 mya:

Colonization

of land by

fungi, plants

and animals

Present