Lecture 2: Biodiversity What is biological diversity? Natural selection Adaptive radiations and convergent evolution Biogeography Biodiversity and Distributions Types of biological diversity: Genetic diversity Species diversity Ecosystem diversity Fundamental question: How is biodiversity distributed around the globe, and why? Biodiversity and Distributions What determines the distributions of individual species? 1. Historical factors - Evolutionary history/geography - Speciation events 2. Ecological factors - Environment varies dramatically across globe - Each species is specialized to deal with specific ecological conditions - Adaptations that result from natural selection The process of natural selection Charles Darwin, Origin of Species (1859) What is natural selection? process by which organisms become uniquely adapted to their environment Acts on individuals, but affects make-up of populations 1
Evolution by Natural Selection Variation in traits 50% green/brown Differential Reproduction = Fitness Natural Selection Heredity Speciation & Adaptations Evolution! The process of natural selection Requires the following 3 conditions in a population: 1. variation among individuals in some trait e.g. height, speed, coloration 2. This variation is heritable trait is coded by specific genes 3. There is a relationship between the trait and the fitness of individuals their survival and/or reproduction End result: 100% brown Grants studies on the Galapagos finches Example: medium ground finch (G. fortis) Entire population found on Daphne island Finches use their beaks to open seed cases and eat seeds inside They measured the beak sizes of individuals and monitored reproduction and survival over many years 1. Tons of individual variation in beak size Number of individuals (frequency) 7 mm 12 mm /size 2
2. Beak size is heritable of offspring (when full size) of parent Each point is one offspring plotted against their own parent Steepness of line indicates heritability of trait (i.e. no slope = not heritable) Can only be measured if offspring do not disperse 3. Beak variation has ecological consequences Seed size consumed Birds with larger beaks tended to eat larger and harder seeds Birds with smaller beaks ate smaller and softer seeds 4. Beak variation had survival consequences - Big drought in 1977 - Many plants died, much less food available - Shift to seeds that were harder and larger Massive die off of finches from 1500 to 200 - Smaller beaked birds suffered most mortality Next generation of birds had larger beaks - Evolutionary shift in beak size in one generation! 4. Beak variation had survival consequences of offspring of parent Mean of survivors (post-drought) Mean of all birds (pre-drought) Evolutionary shift towards birds with larger beaks in one generation 3
Case studies of natural selection Summary: The study of natural selection in progress requires these critical factors: Variation in a trait that is related to fitness Heritability of that trait Populations with limited dispersal A selection event to occur Natural selection: Important points 1. NS operates on individuals! - Does not work to ensure the survival of the species 2. There is a lot of potential for NS because most traits are heritable - Think of artificial selection in domesticated animals - Mainly acts on existing genetic variation (not necessarily on new mutations) Natural selection: Important points 3. NS is an evolutionary process (genetic change) but is also profoundly ecological - Due to ecological interactions (biotic and abiotic) 4. NS can be very strong and rapid Adaptive radiations = Speciation + ecological specialization Definition of adaptive radiation: rapid diversification of a lineage into several species that exploit a variety of different resources 4
Adaptive radiations Classic example: Darwin s finches (again!) Speciation: 14 species total 13 in archipelago of Galapagos islands 1 on Cocos island in Costa Rica Molecular work confirms common ancestor and that speciation occurred within past 1 million years Ecological specialization Massive variation among species in beak size and shape This morphological variation is associated with different diets among species How do adaptive radiations occur? 1. Empty region is colonized by one species 2. Isolation of populations allowed for subsequent differentiation (no gene flow) 3. Competition leads to further divergence Prediction: adaptive radiations require many isolated populations w/ NS Test Recall: 13 species in archipelago, but only 1 on extremely remote Cocos island One island so no speciation could occur Still is ecological selection for specialization Behavioral feeding specialization Insect-eaters, ground foragers, nectar specialists Adaptive Radiations Two ways to become specialists: morphology (across species) or behavior (within a species) Adaptive radiation (speciation + ecological specialization) only occurred in archipelago!! Cool examples: Hawaiian honeycreepers, African cichlid fishes 5
Despite divergence, share a common ancestor, so there are still many similarities Do such similarities among species always indicate that adaptive radiation has occurred? Hawaiian honeycreepers African Rfit Valley cichlids Flying Squirrel Convergent evolution Different taxonomic lineages converge in ecological roles Evolutionary convergence is presumed to be due to adaptation to similar environments i.e. placental mammals and marsupials Flying Marsupial Convergent evolution 1. Similarities between forms can be very strong - Suggests that certain conditions or niches favor very specific adaptations, regardless of the taxonomic group that fills the role 2. Differences between forms still exist - - Due to the fact that organisms come from different taxonomic lineages that have very different evolutionary histories e.g. carnivores in Australia will always be marsupials, whereas those in America will always be placentals 6
Biogeography The study of the geographic distributions of organisms History of the earth is a major factor of interest Different continents have different histories and different taxonomic groups Wallace s Line Alfred Russel Wallace, co-discoverer of Nat. Sel. Visited 2 islands separated by 26 miles On Bali found birds of Asian descent On Lombok found birds of Australian descent One side of line has Asian taxa: tigers, monkeys, bears, woodpeckers Other side of line has Australian taxa: kangaroos, marsupials, birds of paradise Why? Continental drift and Isolation (= Vicariance) Biogeographic example: S. continents Curious pattern many similar looking taxa between S. America and Africa Hummingbirds (S. America) and Sunbirds (Africa) both nectar feeders S. American and African Cattle egrets Ratites (flightless birds) Emu in Australia Ostrich in Africa Rhea in S. America Biogeographic example: S. continents What are the possible mechanisms? 1.Convergent evolution - These species are not close relatives but NS has caused them to resemble each other 2.Vicariance biogeographic hypothesis - Originally stemmed from common ancestor in Gondwanaland 3.Dispersal - These birds are related and dispersed to continents over water 7
Biogeographic example: S. continents Hummingbirds (S. America) and Sunbirds (Africa) not related species: convergent evolution S. American and African Cattle egrets Dispersal between continents was documented Ratites (flightless birds) Are distantly related species Cannot disperse across continents Most likely due to continental drift Biodiversity Hotspots Threats to Biodiversity Habitat destruction/fragmentation Invasive Species Overexploitation Pollution Climate Change 8