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Origins of Biodiversity Species formation and the hierarchy of life Speciation results in the splitting of an ancestral species into two (or more) descendent species. Results in a sequence of speciation events extending over great expanses of time, creating a branching tree of historical relatedness. How do new species originate? How do “phylogenetics” relate to conservation? Why is the speciation process so variable in intensity around the world? Biological Species Concept Two individuals belong to the same species if their gametes can unite with each other under natural conditions to produce fertile offspring. This concept emphasizes that a species is an evolutionary unit. 1 Problems with the BSC Can you test it on museum specimens or fossil data? What if all members of a species do not live in the same place (e.g., individuals occupying different parts of a range)? What about hybrids that occur [naturally] under some circumstances? •“Jefferson complex” salamanders •Hybrid lineages 3.9 millions years old •Very ancient, female-only lineages One of the greatest wonders of zoology… Ambystoma platineum = all-female, triploid species two sets of chromosomes from A. jeffersonianum one from A. laterale Ambystoma tremblayi (Tremblay’s) is an all-female triploid species 1x A. jeffersonianum 2x A. laterale Female A. platineum mate with males of A. jeffersonianum, but the sperm make no genetic contribution to the offspring Female A. tremblayi mate with males of A. laterale (but no male genetic contribution) Phylogenetic species concept Diagnosable geographic forms of the same basic "kind" of organism should be treated as distinct species. Each form has a unique evolutionary history All a PS needs to be “diagnosed” is a single shared and derived character Problems with the PSC Much less restrictive than the the BSC Any population with a distinct feature is considered a “species” There would be many more species classified under the PSC than under the BSC – too many to cope with? Does a single, fixed nucleotide difference between populations warrant separate protection status for each population? 2 The operational definition: Does a “species” have the potential to interbreed and does it represent a separate evolutionary lineage? Bad taxonomies can kill… Case study Methods Examined the mtDNA sequence of red wolves (zoo animals and from DNA obtained from museum pelts from 1905 to 1930) as well as gray wolves and coyotes. The red wolf occurred only in regions where gray wolves and coyotes were. One of the last wild red wolves to be captured in Louisiana. 3 gray wolves, coyotes, and red wolves wolf-like canids Red wolf on right percentage sequence divergence Legal issues Conclusions Along with the geographic information, concluded that the "species" red wolf is (was) actually a hybrid of the gray wolf and the coyote. Other conclusions Grey wolves and coyotes have overlapping ranges in the northern US But red wolf phenotype is not present in hybrids in the north. Therefore, the red wolf phenotype is not only a product of the hybridization, but of environment as well. The red wolf has been formerly classified as an endangered species but US Fish and Wildlife does not extend endangered species classification to hybrids. U.S. Fish & Wildlife Service in 1996 hybrid policy: The Service now believes the responsibility to conserve endangered and threatened species under the Act extends to intercross progeny if (1) the progeny share the traits that characterize the taxon of the listed parent, and (2) the progeny more closely resemble the listed parent’s taxon than an entity intermediate between it and the other known or suspected non-listed parental stock. 4 Species formation Lloyd’s hedgehog cactus How do we get cladogenesis -- the splitting of one lineage into two? This question is critical, because it is what produces many species from few, results in evolutionary trees of relatedness, and generates what we now call biodiversity. Allopatric (geographic) speciation Different geographic regions are likely to have different selective pressures: Temperature Rainfall Predators Competitors Populations may also differentiate because New mutations New genetic combinations occur by chance in each owing to genetic drift Thus, selection will have different raw material to act upon in each population. Often species coming back into contact not yet fully reproductively isolated. Natural selection may reinforce reproductive barriers and isolation. Species isolated by post-zygotic barriers will subsequently evolve pre-zygotic barriers. Why? 5 “Ring Species” A ring of populations encircles an area of unsuitable habitat. At one location in the ring of populations, two distinct forms coexist without interbreeding, and hence are different species. Around the rest of the ring, the traits of one of these species change gradually, through intermediate populations, into the traits of the second species. Salamanders in the Ensatina eschscholtzii group Greenish warblers Means of sympatric speciation 'sym' = with; 'patris' = country Sympatric Speciation • One large, randomly mating population • Differences among individuals cause them to choose different habitats and mates. • Habitat specialization and non-random mating cause individuals to become genetically different. • Examples? 6 Sympatric Speciation New, reproductively isolated species may arise instantaneously in the same location due to multiplication of the entire complement of chromosomes. The process is known as polyploidy and is very important in plants About half have arisen in this fashion! Global patterns of diversity Where has the speciation process been most prolific? The tropics 23o north to 23o south latitude At least 75% of all species (plants, animals, microorganisms) exist in the tropics Species–latitude relationship: birds in grid cells (611,000 km2) across the New World. 7 Africa Much of the region between 23o north and south latitude is the most inhospitable desert on earth. The Sahara covers 25% of the continent. Rain forest is restricted to the west central part of Africa and covers less than 9% of the continent. South America Asia The Special Case of Central America and Indonesia/New Guinea The tropics of South America are an extremely complex mosaic of lowland wet and dry forest. Also ecosystems that change with elevation such as high-elevation shrublands (paramo) and grasslands (puna). Rain forest covers about 32% and savanna about 38% of the South American continent. Mostly lowland rain forest A significant feature of southeast Asia is the preponderance of islands. Islands serve to isolate populations of organisms and facilitate allopatric speciation Plate tectonics have only recently brought the areas together Thus, modern biotas are fusions of long isolated biotas associated with distinct land masses Some notable exceptions Sea birds and marine mammals - feeding on marine crustaceans and fish Productivity accounts for this disparity 8 Some notable exceptions Lichens - highest diversity and abundance occurs at high altitude or northern climates, particularly where higher plants cannot grow they are out competed by plants yet nutrients still available for those species that can cope with cold and lack of light, e.g., lichens Higher rate of productivity More solar energy = greater resource base to build communities Warm and humid conditions are simply more benign to physiological processes Longer active seasons Why so many species in the tropics? Many theories... Species–energy relationships. a, Mean monthly summer temperature (°C) and richness of breeding birds in Britain b, Mean annual sea surface temperature and richness of eastern Pacific marine gastropods. c, Evapotranspiration (mm yr-1) and richness of Epicauta beetles (Meloidae) in North America More stability and time Tropical communities more stable and have not been “scraped” regularly by glaciation. Hence more opportunity for specialization and co-evolution 9 Habitat subdivision during the glacial periods During inter-glacials, the tropics became drier and cooler, with wet forest forming large “islands” surrounded by savanna. Islands large enough to avoid local extinctions. Enhanced the process of allopatric speciation. 18,000 y bp Disease Intense disease and parasitism in tropics creates many rare species, none of which are dominant Cold controls disease at higher latitudes, permits single species to dominate. 10 End species formation 11
