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 species! 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: Number of species Millions One 3.5 BYA Present Time
Number of species Millions One 3.5 BYA Present Time Number of species Millions One 3.5 BYA Present Time
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. If S and E remain approximately constant over time, exponential growth will result.
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 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? If (S - E) goes down as N increases, then logistic growth in the number of species is expected, potentially resulting in a global equilibrium.
The number of species is difficult to determine from the fossil record. Are allopatric populations the same or different species? Are populations from 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). 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 the 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. For several groups, an exponential rise in the number of families best fits the data. (1) All of the families in the Fossil Record 2 database: Time (MYA) Geological Time
Number of Families Number of Families (2) Tetrapod families: Time (MYA) Mammals Amphibians Reptiles Birds (3) Insect families: Geological Time Geological Time (REMEMBER: Extinctions are happening throughout.)
Number of Families For other groups, the patterns are more complicated. For example, the growth of marine families appears to show a number of steps: 800 400 543 251 65 Geological Time (MYA)
Sepkoski interpreted this as the result of the radiation of three distinct "evolutionary faunas": Cambrian fauna with broad habitat and trophic requirements (e.g. trilobites, inarticulate brachiopods...) Palaeozoic fauna with more specialized requirements (e.g. crinoids, cephalopods, soft corals...) Modern fauna with predatory and defensive strategies (e.g. bivalves, hard corals, malacostracan crustaceans, fish...) Why might marine fauna have increased rapidly and then reached a plateau between 400 and 250 MYA whereas tetrapods and insects have not plateaued?
Number of Species Similarly, the number of species of vascular land plants appears to have increased in steps: Geological Time (MY) Knoll and colleagues (1984, 1985) interpreted this as the appearance and radiation of new major baupläne (= body forms): Devonian flora of early vascular plants Carboniferous flora of club mosses, ferns, conifers Triassic flora of gymnosperms Cretaceous flora of angiosperms The subsequent rise of angiosperms has proceeded exponentially.
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 be sustained? How? Earth Earth That is, 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.
(S-E) per million years Are the rates of speciation and extinction the same for different organisms? S >> E Trilobites Ammonites Graptoloids Birds Mammal families Insects Freshwater Fishes S ~ E Low E High E Average species duration (MY) No!
Interestingly, those taxa with high rates of increase (S-E) also tend to have high rates of extinction. What might explain this odd result? Specialists may be more likely to speciate because of their patchy distribution but may also be at higher risk of extinction. Species with small population sizes may be more likely to speciate (if drift is important) but are at higher risk of extinction. 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.
Factors Affecting the Origin of Biological Diversity Speciation rates are higher in some lineages than others and at certain times over others. Here we explore several possible explanations. (1) Ecological Opportunity: Living organisms present numerous examples of spectacular radiations following the colonization of isolated islands or bodies of water, e.g.: honeycreepers on Hawaii drosophilids on Hawaii cichlids in African Great Lakes amphipod crustaceans in Lake Baikal in Siberia
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. 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)
For example, rodent-like, non-placental mammals (multituberculates) decreased in diversity in North America following the appearance of placental rodents. MYA 34 56 65 Multituberculates Rodent Genera The correlated pattern of increase and decrease suggests 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 (Cretaceous-Tertiary boundary ~65MYA). Does this suggest displacement or incumbent replacement?
(2) Key Adaptations: Speciation rates within a group may rise after the evolution of a new adaptive trait. How can we tell whether a trait increases speciation rates? Replicated Sister-Group Comparisons
For example, 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 by identifying 16 sister groups of plants with and without these canals. In 13/16 cases, the canal-bearing groups contained more species than their sister clades. Conifers Gingko (+ canals) (- canals) 559 1 Fig family Elm family (+ canals) (- canals) 1703 150 Number of living species 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.
(3) 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. 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).
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 (incl. sloths, armadillos, opossums, raccoons), 25 to North America (incl. mammoths, mastodons, saber-toothed cats, and camels), and 2 occurred in both. The "Great American Interchange" describes the resulting migration of animals across the Isthmus of Panama. Migrants from the South Migrants from the North Diversity, supported by provinciality, was lost following the Interchange: only 38/50 families remained, 24% went extinct.
Percent Extinct Extinctions and The Decline of Biological Diversity The tree of life has been severely and often pruned. The vast majority of species that have ever lived have gone extinct. Geological Age Extinction rates vary over time, but are dominated by the "big five" mass extinction events. 3 1 2 4 5 [Note: Peak in Cambrian probably reflects low diversity at that time.]
Extinction Event MYA Family Loss Species Loss End-Cretaceous 65 ~14% ~76% End-Triassic 208 ~30% ~80% End-Permian ** 245 ~60% ~95% Late Devonian 367 ~30% ~83% End-Ordovician 439 ~23% ~85% [Family extinctions reflect all organisms in the Fossil Record 2 database. Species extinctions reflect loss of marine species estimated by Jablonski (1991,1995).]
Climate change has been cited as a major factor involved in each mass extinction event. The largest of the extinction events, at the end of the Permian, is associated with a number of catastrophic climate changes (the "world-went-to-hell" hypothesis) including: major sea level regression ocean anoxia (= decreased oxygen) Siberian flood basalts (=magma flows) over 1.5 million km2 increased CO 2 global warming Major climate changes also surround the end-cretaceous extinction (K-T), possibly resulting from a massive asteroid hitting the earth. (A buried crater has been detected in the Yucatan peninsula of Mexico with a diameter of 180 km!)
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.
% of Genera Extinct Although we generally do not know why extinction events are so selective, some patterns have emerged. Jablonski (1986) found that bivalves and gastropods with wider geographic distributions were less likely to go extinct at the end of the Cretaceous. Number of Provinces Inhabited Without these mass extinctions, the world would be a very different place. For one thing, the rise and diversification of mammals may never have happened.