General Ecology: EEOB 404

Evolutionary ecology: speciation and extinction Topics for this class: Species richness of communities depends on both speciation and extinction processes Nature of speciation concepts, mechanisms Example: beetle herbivores & speciation on angiosperm plants Nature and impact of extinction on species richness Impact of humans on contemporary rates of extinction

Species richness depends on both speciation and extinction Species richness is another form of biological diversity than genetic diversity within a species Species richness is a characteristic of evolutionary clades, and of ecological communities High rates of speciation can boost species richness over time, just as high rates of extinction can decimate it

Species concepts Evolutionary biologists still debate which of a number of species concepts apply most widely in nature Ernst Mayr’s Biological Species Concept probably applies most widely (at least in animals): Accordingly species are reproductively isolated units No one species concept applies to all organisms

Mechanisms of speciation Similarly, mechanisms of speciation are still poorly understood by evolutionary biologists Several are well documented: Allopatric speciation (probably most vertebrates) Sympatric speciation (especially insects) Polyploidy and chromosome rearrangements in small peripheral populations (especially plants) We need not concern ourselves in this class with particular species concepts or mechanisms to get the main points about the importance of evolutionary phenomena to understand ecological questions

Effect of speciation on species richness: an example Outstanding question: “Why are there so many beetle species?” Background: Insects are overwhelmingly rich in species: 751,000 species, ca. 50% of all described species Of insects, beetles are overwhelmingly dominant: >50% of all insects are beetles Of beetles, leaf-eating families are dominant groups (e.g., Chrysomeloidea = leaf beetles, Curculionoidea = weevils)

Famous anecdote J.B.S. Haldane is a well known evolutionary biologist, one of the architects of “The Modern Synthesis” A theologian reputedly asked Haldane, late in life, “What can we conclude about the nature of the creator from looking at the creation?” Haldane replied: The Creator has an inordinate fondness for beetles”

Question of beetle diversity was recently addressed by phylogenetic study Study by Farrell,B.D. 1998. Science 281:555. (copy on class reserve) Study first devised phylogeny of 115 phytophagous beetles (representative families) Sequenced entire 18s ribosomal subunit, 2117 nucleotide positions-->355 informative characters (i.e., those that varied among the species) 2nd data set,scored 212 morphological characters Parsimony analysis-->most likely “tree” of evolutionary relationships (= cladistic analysis)

Adaptive radiations of beetles began in late Jurassic, continued to diversify up until today; Herbivores diversified disproportionately 20000 100 200 300 400 Herbivores Number of beetle genera 10000 Carnivores Saprophages Permian Triassic Jurassic Cretaceous Tertiary Recent

Next step in analysis... Superimpose onto phylogeny feeding habitats of living species, timing at which groups show up in fossil record, number of extant species Results (next slide): Five independent cases in which phytophagous beetles became associated with angiosperm plants Each of these cases associated with increased species richness This pattern general, because we see it replicated 5 times, independently in beetles’ evolutionary history These beetle radiations took place in Cretaceous & Tertiary, corresponding with adaptive radiation of angiosperms (flowering plants)

1 2 3 4 5 Tr Ju Cr Ter Clytrinae(947) Nemonychidae(85) Attelabinae(800) Rhynchitinae(1200) Antliarhininae(24) Apioninae(1500) 1 Rhynchophoridae(1100) Curculionidae(40502) Oxycoryninae(10) Allocoryninae(20) 2 Belinae(150) Prioninae(770) Aseminae(75) Spondylinae(3) 3 Cerambycinae(10000) Lamiinae(14000) Lepturinae(1000) Palophaginae(3) 4 Megalopodinae(350) Zeugophorinae(50) Orsodacninae(8) Aulacoscelidinae(18) Synetinae(11) Eumolpinae(3200) Megascelidinae(60) Lamprosomatinae(190) Cryptocephalinae(2290) Clytrinae(947) Chlamisinae(360) 5 Chrysomelinae(2000) Galerucinae(5300) Alticinae(8000) Cassidinae(3000) Hispinae(3000) Donaciinae(165) Criocerinae(1400) Pachymerinae(77) Amblycerinae(400) Bruchinae(3000) Tr Ju Cr Ter early Oligocene

Interpretation of Farrell’s study? Angiosperms diverged greatly in plant chemistry. Each plant group provided new “island archipelago” with opportunities for colonization, diversification in how the insects could use these new plants (e.g., chew leaves, mine leaves, eat seeds, mine stems, chew flowers) Basically flowering plant speciation appears to have triggered massive adaptive radiation of plant-feeding insects, i.e., co-evolutionary arms-race Thus, we see evolutionary tools applied to addressing ecological question (distribution & abundance of species)

Extinction is the other evolutionary process that influences species richness Rates of extinction have varied enormously in Earth’s history from negligible (background) rates to a number of mass extinctions (5--class overhead) E.g., extinction of dinosaurs (and many other birds, mammals, marine reptiles, etc.) 65 MYA, response to asteroid impact Extinctions opened opportunities for subsequent radiations, such as mammals and birds following dinosaur mass extinctions Extinction is the rule, rather than the exception

Humans causing extinctions today, higher rate than mass-extinctions! Increasing rate of extinction (e.g., birds, mammals) correlated with increasing human population Disproportionate vulnerability of island populations Entire clades wiped out by human over-hunting (e.g., Moas of New Zealand; native flightless birds of Hawaii) Almost all islands world-wide are missing species due to human-caused extinctions (on order of 50% of spp.) However mainland species also vulnerable--e.g., passenger pigeon, Carolina parakeet, ivory-billed woodpecker, Bachman’s warbler = U.S. birds extinct since 19th century

What are some lessons about extinction & its causes? Causes of known animal extinctions (484 spp.): Introduced animals (17%)--e.g., brown tree snake, Guam Habitat destruction (16%) Over-hunting (10%) Other (1%) or unknown (56%) causes Different causes of extinction in different taxa, different locations E.g., Hawaiian organisms particularly affected by introduced organisms Hawaiian birds particularly affected by disease (avian malaria) Many island populations overexploited (e.g., hunted)

Conclusions Understanding evolutionary processes (such as speciation, extinction) provides critical information on present-day patterns of organism distribution and abundance Speciation and extinction processes have both had a profound impact on ecological patterns and processes observed today Extinction and speciation processes affect taxa differently, and geographic locations differently Humans today, and deep into our evolutionary past, have profound impacts on ecological systems...why?