1

Bio1B Evolution 11Last lecture:Species & speciation• Hybridization - hybrid zones, reinforcement & hybrid-speciation

TodayMACROEVOLUTION• The fossil record & background extinction• Mass extinctions - the Cretaceous/Paleogene (“K/T”)

boundary - cause & consequenceTransitional forms• Terrestrial vertebrates (tetrapods)• Birds & evolution of feathersEvolution of development programs [Text Pp. 525-530]• The eye, vertebrate limbs

2

The fossil record:billions of years ofsedimentation =>

recoverable history

• E.g. Grand Canyon &environs - layeredsediments from recent topre-Cambrian

Fig. 22.3Fig. 25.5

3

Major transitionsin earth history

• Earliest prokaryotes - fossilstomatolites 3.5 Bya

• Increase O2 - 2.7 Mya• Fossil eukaryotes 2.1Bya;

multicell algae 1.2 Bya• Complex metazoa 0.55 Bya• Marine -> land 0.5 Bya• [Hominids only 0.005 Bya]

Fig 25.7

4

The Cambrian “explosion”- Burgess shale &

appearance of modernphyla

Fig. 25.10

Burgessshale

“Hallucigenia”

Weirdstuff…

5

The big 5mass

extinctions

• Evidence from analyses of extinction (red) and blues(diversity) or families of marine invertebrates

• Permian-Triassic - 96% species extinction, 8/27 orders ofinsects; Volcanism in Siberia?

• Cretaceous-Paleogene (“K/T”), 65 Myr - demise ofdinosaurs & large terrestrial animals => mammalianradiation

6

The asteroid impacthypothesis - Luiz & Walter

Alvarez, UC Berkeley(see Science, 5th March, p1214)

Faunal turnover Carbon cyclechange

Iridium spike

7

Are we the cause of the 6th massextinction? (Barnosky et al. Nature 2011)

8

Understanding the transition of tetrapodvertebrates from water to land

Fig 34.20

Tiktaalik

Acanthostega

9

Modification of existing structures for newpurposes: ears and feathers

Fig. 34.31. Bones of inner ear ofmodern mammals are derived fromjaw joint of ancestors (see also Fig.25.6

LateJurassicfeathereddinosaur

Feathers:for displayor warmthbeforeflight?

Recent discovery:dinosaur featherswere colored -display?

10

Evolution ofdevelopmental genes=> phenotypic novelty

• Molecular homology: geneswith common ancestrycontrolling development (topright)

• Changes in timing andspatial pattern of expression=> change in phenotype

• E.g Ubx suppresses legdevelopment in flies, but notshrimp

Fig. 25.22

11

Origin of novelties:The vertebrate limb

• Are the fish “fin” andvertebrate “limb”homologous?

• Very different anatomy, yet…• Similar patterns of Hox gene

expression• Anatomic differences could

be due to modification oftiming/duration ofexpression?

Shubin etal. 2009Nature457:818

12

Origin of novelty:The eye

Shubin et al.2009 Nature457:818

Convergent evolution ordescent withmodification?

Molecular homology ofkey genes - Pax-6 &opsin pathway

Subsequentmodification ofpathways andstructures

Both inannelids