The evolution of biological diversity. The evolution of biological diversity. The evolution of biological diversity. Introduction

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1 MYA EON ERA PERIOD EPOCH MAJOR EVENTS Development of agriculture and human Recent 0.01 civilization Quarternary Appearance and world-wide spread of the genus C Pleistocene Homo. Repeated glaciations. Extinctions of large E 1.8 mammals and birds. N O 5.2 Pliocene Z 24 O Miocene Continued diversification of modern birds, I 34 Tertiary Oligocene placental mammals, snakes, teleost fish, C pollinating insects, grasses and angiosperms. 56 Eocene 65 Paleocene 1 Introduction 2 M Diversification of flowering plants, birds and 144 E Cretaceous mammals. P S H O 206 A Jurassic 1 st birds and angiosperms. Dinosaurs abundant. Z N O E I 1 Triassic st dinosaurs and mammals. Gymnosperms 251 R C become abundant. Continents moving apart. O Z Diversification of reptiles, including mammal-like O Permian species. Land masses form single continent, 290 I Pangea. C 1 st reptiles and winged insects. Warm humid Carboniferous conditions result in huge forests of primitive 354 P plants, which formed extensive coal deposits. A L 1 st amphibians and true insects. Atmospheric E Devonian oxygen at present levels or higher. Continents 409 O moving toward one another. Z O Silurian 1 st land plants. Atmospheric oxygen about I percent. C 500 Ordovician 1 st fish. Mass extinctions All living organisms are descended from an ancestor that arose between 3 and 4 billion years ago. Currently, the diversity of life on earth includes some 5 million to 50 million species! 543 2,500 Cambrian PROTEROZOIC 1 st shelled organisms. Trilobites abundant. Probably all metazoan phyla present, including arthropods and early chordates. Atmospheric oxygen reaches about 2%. Abundant prokaryotic life. Eukaryotes may have appeared by 2,000 million years ago. Atmospheric oxygen about 0.2%. 3,600 ARCHEAN Oldest known rocks and prokaryotes. 4,600 HADEAN Earth forms. No geological record. Introduction Before discussing the observed patterns, it is worth thinking about the possible ways in which the number of species present at any point in time may have changed over the history of the earth. Millions Number of species One 3.5 BYA Time Now 3 The patterns and rates of diversification reflect the rates of speciation (S) and extinction (E) of taxa. At the simplest conceptual level, the number of species (N) present on earth will change over time according to the formula:!n = (S - E) N!t where!n is the change in number of species over an amount of time!t and where S and E are rates of speciation and extinction per species. 4

2 !N = (S - E) N!t If S and E remain approximately constant over time, exponential growth will result. Millions Number of species One 3.5 BYA Time Now 5 Both speciation and extinction rates may vary over time and from species to species depending on: resource and habitat availability ("niche space") interactions among species key adaptations (changing "adaptive zones") climactic changes catastrophes If (S - E) goes down as N increases, then logistic growth in the number of species is expected, potentially resulting in a global equilibrium. Diversity-dependent growth: An alternative possibility is that either the rate of speciation or extinction (or both) depends on diversity levels (N). Why might this be true? Millions Number of species One 3.5 BYA Time Now

The number of species is difficult to determine from the fossil record. Are allopatric populations the same or different species?

Consequently, paleontologists tend to study diversity by counting the number of higher taxa (e.g. families).

throughout the 543 MY of the Phanerozoic.

3 The number of species is difficult to determine from the fossil record. Are allopatric populations the same or different species? Are populations at different points in time the same or different species? Are there sufficient fossil remains of most species? Consequently, paleontologists tend to study diversity by counting the number of higher taxa (e.g. families). 9 Two major databases have been compiled: John Sepkoski (1984) compiled data on the temporal ranges of more than 4000 marine skeletonized families throughout the 543 MY of the Phanerozoic. Michael Benton (1993) compiled Fossil Record 2, containing 7186 families of all groups of microbes, algae, fungi, protists, plants, and animals. The pattern of diversification depends on the taxa examined The entire Fossil Record 2 database Tetrapod families For several groups, an exponential rise in the number of families best fits the data.

4 13 14 For example, the growth of marine families appears to show a number of steps: For other groups, the patterns are more complicated. Sepkoski interpreted this as the result of the radiation of three distinct "evolutionary faunas" 15 Cambrian fauna with broad habitat and trophic requirements (e.g. trilobites, inarticulate brachiopods...) 16

5 Palaeozoic fauna with more specialized requirements (e.g. crinoids, cephalopods, soft corals...) 17 Modern fauna with predatory and defensive strategies (e.g. bivalves, hard corals, malacostracans, fish...) 18 Why might marine fauna have increased rapidly and then reached a plateau between 400 and 250 MYA, whereas tetrapods and insects have not plateaued? 19 Similarly, the number of families of vascular land plants appears to have increased in steps. Knoll et al. (1984, 1985) interpreted this as the appearance and radiation of major baupläne (body forms) 20

6 Devonian flora (early vascular plants) Carboniferous flora (club mosses, ferns, conifers) Triassic flora (gymnosperms) Cretaceous flora (angiosperms) The subsequent rise of angiosperms has proceeded exponentially. 21 Surprisingly, these data suggest that the number of species on earth has not reached a global equilibrium. If anything, the most prevalent pattern is of exponential growth. Can exponential growth of species numbers be sustained? How? 22 Exponential growth of species may occur by changing patterns of "species packing", with generalists being replaced by more specialized forms and with an increase in the complexity of the ecological community. 23 Patterns of speciation & extinction Taxa with high rates of increase (S-E) tend also to have high extinction rates What might explain this odd result? 24

7 Patterns of speciation & extinction Specialists may be more likely to speciate because of their patchy distribution but may also be at higher risk of extinction. 2. Species with small population sizes may be more likely to speciate (if drift is important) but are at higher risk of extinction. 3. Species with low dispersal rates may be more likely to speciate (lower gene flow) but may be more likely to go extinct following local environmental changes. Speciation rates are higher in some lineages than others and at certain times over others. Here we explore several possible explanations. Ecological Opportunity There are numerous examples of spectacular radiations following the colonization of isolated habitats, e.g.: honeycreepers on Hawaii drosophilids on Hawaii cichlids in African rift lakes amphipod crustaceans in Lake Baikal in Siberia 27 Ecological Opportunity In these cases, the fauna was locally depauperate before the arrival of the original colonist. "Vacant niches" existed into which the newly arrived organisms diversified. 28

8 Ecological Opportunity Similarly, there are several examples in the fossil record where the decreased representation of one group is followed or accompanied by a proliferation of another group. The new group may cause the extinction of the former group (displacement) The new group may be released from competition by the extinction of the former group (incumbent replacement) Ecological Opportunity Cox (1998) For example, rodent-like, non-placental mammals (multituberculates) decreased in diversity and went extinct following the appearance of placental rodents. 31 Ecological Opportunity 32 Ecological Opportunity Cox (1998) Does the correlated pattern of increase and decrease suggest displacement or incumbent replacement? Lineages often are seen to radiate following the mass extinction of another group. For example, mammals radiated in the Tertiary period following the mass extinction of dinosaurs near the K-T boundary (the Cretaceous-Tertiary boundary ~65MYA). Does this suggest displacement or incumbent replacement?

9 33 Key adaptations Speciation rates within a group may rise after the evolution of a new adaptive trait. How can we test the hypothesis that a trait increases speciation rates? We need: 1. A method that counts the number of species with and without the trait 2. A method that accounts for the amount of time that each lineage has had to speciate 3. Multiple independent comparisons. 34 Key adaptations Ehrlich and Raven (1964) suggested that defenses against herbivores (e.g. latex and resin canals) promoted diversification in plants. Mitter, Farrel and colleagues (1988,1991) tested this hypothesis using independent sister-group comparisons. 35 Key adaptations In 13/16 sister groups of plants, the canal-bearing groups contained more species than their sister clades. 36 Key adaptations Similarly, the fossil record suggests that key adaptations in marine organisms (specialization, predation, swimming, hard shells) promoted their diversification. Key adaptations allow organisms to evolve into a greater variety of niches, creating a more complex and tiered community structure.

10 37 Provinciality Speciation rates will depend on the extent to which organisms are distributed through space. There has been a general trend over the last 250MY from wide-spread distributions to more localized distributions. 38 Provinciality As Pangaea began to break apart during the Triassic, land and ocean masses became more spatially separated. Ocean currents also changed, leading to a more pronounced temperature gradient. These changes have increased the number of biological "provinces" (a self-contained region wherein speciation rather than colonization dominates the appearance of new taxa). 39 Provinciality 40 Provinciality An example (in reverse) is the extinction caused when the separate land provinces of North and South America became connected by the Isthmus of Panama (~2MYA). 23 families of mammals were endemic to South America (e.g. sloths, armadillos, opossums, raccoons), 25 to North America (e.g. mammoths, mastodons, sabertoothed cats, and camels), and 2 occurred in both.

41 Provinciality The Great American Interchange describes the migration of animals that resulted from the formation of the Isthmus of Panama.

11 41 Provinciality The Great American Interchange describes the migration of animals that resulted from the formation of the Isthmus of Panama. Diversity, supported by provinciality, was lost following the interchange: only 38/50 families remained, 24% went extinct. 42 Extinction The tree of life has been severely and often pruned. The vast majority of species that have ever lived have gone extinct. Extinction rates vary over time, but are dominated by the "big five" mass extinction events. 43 Extinction Extinction Event MYA Putative Cause End-Cretaceous 65 Meteoroid End-Triassic 208 End-Permian 245 Increased rainfall? Sea-level fluctuations? Late Devonian 367 Global cooling? End-Ordovician 439 Glaciation 44 Extinction Climate change has been cited as a major factor involved in each mass extinction event. The end-permian extinction is associated with a number of catastrophic climate changes including: major sea level regression ocean anoxia (decreased oxygen) Siberian flood basalts (magma flows) over 1.5 million km 2 increased CO 2 global warming

45 Extinction Major climate changes also surround the end- Cretaceous extinction (K-T), possibly resulting from a massive meteoroid hitting the earth.

12 45 Extinction Major climate changes also surround the end- Cretaceous extinction (K-T), possibly resulting from a massive meteoroid hitting the earth. A buried crater has been detected in the Yucatan peninsula of Mexico with a diameter of 180 km... Correlation or causation?? 46 Extinction These mass extinctions have played a major role in shaping the biota we see today. Regardless of how adapted a species may be in "normal" times, if it succumbs to extinction during a massive climate change, it will play no further role in evolution. For instance, at the K-T boundary, insects, amphibians, crocodilians, mammals, and turtles suffered few extinctions, whereas several bird species, ammonites, dinosaurs, and other large reptiles went extinct. 47 Extinction What determines which groups survive a mass extinction event, and which don t? Although we generally do not know why extinction events are so selective, some patterns have emerged. 48 Extinction Jablonski (1986) found that bivalves and gastropods with wider geographic distributions were less likely to go extinct at the end of the Cretaceous.

13 49 Extinction 50 Chance & repeatability Without these mass extinctions, we might expect the world to be a very different place. For one thing, the rise and diversification of mammals may never have happened. The "big five" mass extinction events decimated a large fraction of the organisms alive at the time. Yet many of these extinctions occurred randomly. Besides being wide-spread, there is no clear attribute that protected a group of organisms from extinction. 51 Chance & repeatability 52 Chance & repeatability Stephen J. Gould (1989) in his book, Wonderful Life: The Burgess Shale and the Nature of History, argues that chance has played a defining role in evolution. He asks us to re-run the tape of life, and imagine possible alternative worlds What if eukaryotes had not evolved? "Run the tape again, and the first step from prokaryotic to eukaryotic cell might take twelve billion instead of two billion years [beyond the expected ten billion year life span of the sun] - and stromatolites, never awarded the time needed to move on, might be the highest mute witnesses to Armageddon."

53 Chance & repeatability 54 Chance & repeatability What if a different collection of metazoans had survived the Cambrian?

Run the tape again, and primitive chordates bite the dust and some other oddball creature sneaks through.

55 Chance & repeatability Cox (1998) "Mammals spent their first hundred million years - two-thirds of their total history -- as small creatures

56 Chance & repeatability "The divine tape player holds a million scenarios, each perfectly sensible.

14 53 Chance & repeatability 54 Chance & repeatability What if a different collection of metazoans had survived the Cambrian? A large number of unusual, high-level taxa evolved during the Cambrian, most of which went extinct. Run the tape again, and primitive chordates bite the dust and some other oddball creature sneaks through. What if an asteroid had not hit the earth at the end of the Cretaceous? Would dinosaurs have gone extinct? 55 Chance & repeatability Cox (1998) What if an asteroid had not hit the earth at the end of the Cretaceous? Would dinosaurs have gone extinct? "Mammals spent their first hundred million years - two-thirds of their total history -- as small creatures living in the nooks and crannies of a dinosaur's world." If the dinousaurs had not gone extinct, would mammals have proliferated? 56 Chance & repeatability "The divine tape player holds a million scenarios, each perfectly sensible. Little quirks at the outset, occurring for no particular reason, unleash cascades of consequences that make a particular future seem inevitable in retrospect... The end results are so different, the initial perturbations so apparently trivial." Gould, Wonderful Life

15 57 Chance & repeatability 58 Chance & repeatability: examples So, has all of evolution been a quirky twist of fate? I don't know. If we played the tape of life again, we certainly would not arrive at the same place today. But perhaps some striking similarities would appear We see tanatalizing examples of repeatability in evolution by comparing marsupials & placental mammals 59 Chance & repeatability: examples and Anolis lizards in the West Indies. Lizards that share the same niche on different islands are more similar in appearance and behavior than they are to more closely related lizards on the same island. Trunk-ground lizards on Jamaica & Hispaniola 60 Chance & repeatability: examples and Anolis lizards in the West Indies. Lizards that share the same niche on different islands are more similar in appearance and behavior than they are to more closely related lizards on the same island. Twig lizards on Jamaica & Hispaniola

16 61 Chance & repeatability: examples Jamaica Hispaniola 62 Chance & repeatability: examples Trunk-ground Twig Overall, it s unclear how relatively important chance processes have been in the patterns of diversity and adaptation on Earth. These examples show that non-chance processes (here in the form of convergent evolution) can lead at some scales to very similar outcomes from very different beginnings.

The Evolution of Biological Diversity. All living organisms are descended from an ancestor that arose between 3 and 4 billion years ago.

The Evolution of Biological Diversity All living organisms are descended from an ancestor that arose between 3 and 4 billion years ago. The diversity of life on earth currently includes some 5 to 50 million