The Cambrian Explosion and BeyondFig. 2.18 The geological time scaleLimitations of the fossil recordEarly animals – Ediacaran fauna (565 – 544 mya)Kimberella http://www.ucmp.berkeley.edu/vendian/kimberella2.htmlDickinsonia http://www.ucmp.berkeley.edu/vendian/dickinsonia.jpgThe Cambrian explosionThe first animals: phylogeny and fossils (Fig. 17.12)Macroevolutionary PatternsAdaptive RadiationDarwin’s Finches (Fig. 3.4)Adaptive radiation (Fig. 17.13)Punctuated EquilibriumThe controversyPatterns of morphological change: punctuated equilibrium and gradualism (Fig. 17.15)Explaining the fossil recordTesting punctuated equilibriumPunctuated change in cheilostome Bryozoa (Jackson and Cheetham 1994) (Fig. 17.16)Who wins?Genetic and morphological change in two arthropod clades (Fig. 17.18) a. Horseshoe crabs today are almost identical to those that lived 150 million years ago b. Hermit crabs and allies Genetic distances based on 16s rRNA gene sequencesExtinctionDistribution of species extinction intensities (Raup 1994) (Fig. 17.20)Patterns of extinctions of families through time (Benton 1995) (Fig. 17.21)Survivorship curves for taxaSurvivorship curves for genera and families (Van Valen 1973) (Fig. 17.22)How long does a species of marine bivalve exist? (Jablonski 1986) (Fig. 17.23) Species with planktonic larvae have longer durations than do species with direct developmentGeographic range affects the survivorship of bivalve and gastropod species (Jablonski 1986) (Fig. 17.24)The Cretaceous – Tertiary (K-T) extinctionIridium anomaly at the K-T boundary (Fig. 17.25 b)Location and shape of the Chicxulub crater (Schultz and D’Hondt 1996) (Fig. 17.26)Effects of a 10 km asteroid impactProbability of extinction at the K-T boundary and geographic range of marine bivalves (Jablonski and Raup 1995) (Fig. 17.28)Anthropogenic extinctions - 1Anthropogenic extinctions - 2Exinction of forest birds on the island of ‘Eua (Tonga) (Jablonski and Steadman 1995) (Fig. 17.29)Deforestation in the Brazilian Amazon (Skole and Tucker 1993) (Fig. 17.30) a. 1978 b. 1988 During this period of time the annual loss of forested area was about 15,000 km2, an area approximately equivalent to the state of Connecticut per year1The Cambrian Explosion and BeyondChapter 172Fig. 2.18 The geological time scale3Limitations of the fossil record•Hard parts – shells, bones, teeth – most likely to be fossilized because they decay slowly and are more durable•In order to be fossilized, a specimen generally needs to be covered quickly by water-borne or wind-borne sediments (sand, mud, ash)•Lack of oxygen is favorable for fossilization•The fossil record consists primarily of hard parts left in depositional environments such as river deltas, beaches, flood plains, marshes, lakeshores, and the sea floor — the fossil record is biased4Early animals – Ediacaran fauna(565 – 544 mya)•First multicellular animals appear about 565 million years ago (mya)•Simple, small, asymmetric or radially symmetric – sponges, jellyfish•Few bilaterally symmetric forms, such as Kimberella, which has uncertain affinities, but appears to be mollusc-like•Dickinsonia considered by some to be an annelid worm, and by others to be cnidarian (jellyfish) polyp5Kimberellahttp://www.ucmp.berkeley.edu/vendian/kimberella2.html6Dickinsoniahttp://www.ucmp.berkeley.edu/vendian/dickinsonia.jpg7The Cambrian explosion•Cambrian period 543 – 495 mya•“Explosive” appearance of large, complex, bilaterally symmetric animals, segmented animals with limbs, antennae, shells, external skeletons, and notochords – including arthropods, molluscs, annelids, and chordates within the period 543 – 506 mya•Almost all living animal phyla are present•Burgess shale fauna (Simon Conway Morris; Stephen Jay Gould, Wonderful Life, 1989)8The first animals: phylogeny and fossils (Fig. 17.12)9Macroevolutionary Patterns•Adaptive radiation•Punctuated equilibrium versus gradualism•Extinction•Taxon survivorship curves•Mass extinctions•The Cretaceous – Tertiary (K-T) impact extinction•Anthropogenic extinction10Adaptive Radiation•An adaptive radiation occurs when a single or a small group of ancestral species rapidly diversifies into a large number of descendant species that occupy a wide variety of ecological niches•Adaptive radiations can occur when:–A species colonizes a new region where there are no or few competitors (i.e., lots of empty niches) – Galápagos finches, Hawaiian Drosophila and silver swords–A taxon acquires an important adaptation – evolution of flight in birds–A taxon is released from competition after extinction of a dominant group – radiation of mammals after extinction of the dinosaurs (?)11Darwin’s Finches (Fig. 3.4)12Adaptive radiation (Fig. 17.13)13Punctuated Equilibrium•Niles Eldredge and Stephen Jay Gould, 1972•The fossil record for some groups reveals that morphological evolution consists of long periods of stasis with little or no change and very short periods during which morphological change occurs in association with speciation•Presented as a challenge to the “conventional” picture of morphological evolution described by the modern synthesis14The controversy•Phyletic gradualism–New species arise by transformation of large ancestral groups (often without splitting – anagenesis)–Transformation occurs over all or a large part of the ancestral species geographic range–Transformation is even and slow–Evolution occurs more or less at the same rate during and between speciation events•Punctuated equilibrium–A small subgroup of the ancestral form gives rise to a new group by splitting – cladogenesis–New species originates in a small part of the ancestral species geographic range – peripheral isolates model–New species develop rapidly, then may replace ancestral species–Between speciation events there is stasis15Patterns of morphological change: punctuated equilibrium and gradualism (Fig. 17.15)16Explaining the fossil record•Darwin was aware of this “problem” and he explained the apparent discontinuities and sudden transitions in the fossil record as being due to the incompleteness of the fossil record•Eldredge and Gould argued that sudden transformations are not artifacts – speciation occurs rapidly and in small populations and is, therefore, unlikely to leave a fossil record17Testing punctuated equilibrium•Strong tests of punctuated equilibrium vs.