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Speciation Chapter 17 Morphology & Species • Morphological traits may not be useful in distinguishing species – Members of same species may appear different because of environmental conditions – Morphology can vary with age and sex – Different species can appear identical Biological Species Concept “Species are groups of interbreeding natural populations that are reproductively isolated from other such groups.” Ernst Mayr Species • Similar in morphology and behavior • Able to reproduce under natural conditions • Produce fertile off-spring Speciation & Natural Selection • Natural selection can lead to speciation • Speciation can also occur as a result of other microevolutionary processes – Genetic drift – Mutation Barriers to Gene Flow • Whether or not a physical barrier deters gene flow depends upon: – Organism’s mode of dispersal or locomotion – Duration of time organism can move Reproductive Isolation • Cornerstone of the biological species concept • Speciation is the attainment of reproductive isolation • Reproductive isolation arises as a by-product of genetic change Genetic Divergence • Gradual accumulation of differences in the gene pools of populations • Natural selection, genetic drift, and mutation can contribute to divergence • Gene flow counters divergence Genetic Divergence populations of one species (gold) time A time B time C time D populations of a daughter species (green) Reproductive Isolating Mechanisms • Prezygotic isolation – Mating or zygote formation is prevented • Postzygotic isolation – Takes effect after hybrid zygotes form – Zygotes may die early, be weak, or be sterile Prezygotic Isolation Ecological Isolation Temporal Isolation Behavioral Isolation Mechanical Isolation Gametic Mortality Postzygotic Mechanisms • Zygotic mortality • Hybrid inviability • Hybrid sterility Mechanisms of Speciation • Allopatric speciation • Sympatric speciation • Parapatric speciation Allopatric Speciation • Speciation in geographically isolated populations • Probably most common mechanism • Some sort of barrier arises and prevents gene flow • Effectiveness of barrier varies with species Extensive Divergence Prevents Inbreeding • Species separated by geographic barriers will diverge genetically • If divergence is great enough it will prevent inbreeding even if the barrier later disappears Archipelagos • Island chains some distance from continents – Galapagos Islands – Hawaiian Islands • Colonization of islands followed by genetic divergence sets the stage for speciation 1 Speciation on an Archipelago A few individuals of a species on the mainland reach isolated island 1. Speciation follows genetic divergence in a new habitat. Later in time, a few 1 individuals of the new species colonize nearby island 2. In this new habitat, speciation follows genetic divergence. Speciation may also follow colonization of islands 3 and 4. And it may follow invasion of island a by genetically different descendants of the ancestral species. 3 2 4 2 1 3 2 4 Hawaiian Islands • Volcanic origins, variety of habitats • Adaptive radiations: – Honeycreepers - In absence of other bird species, they radiated to fill numerous niches – Fruit flies (Drosophila) - 40% of fruit fly species are found in Hawaii Hawaiian Honeycreepers FOUNDER SPECIES Speciation without a Barrier • Sympatric speciation – Species forms within the home range of the parent species • Parapatric speciation – Neighboring populations become distinct species while maintaining contact along a common border Sympatric Speciation in African Cichlids • Studied fish species in two lakes – Species in each lake are most likely descended from single ancestor • No barriers within either lake • Some ecological separation but species in each lake breed in sympatry Parapatric Speciation Adjacent populations evolve into distinct species while maintaining contact along a common border BULLOCK’S ORIOLE BALTIMORE ORIOLE HYBRID ZONE We’re All Related • All species are related by descent • Share genetic connections that extend back in time to the prototypical cell Evolutionary Trees extinction (branch ended before present) new species branch point (a time of divergence, speciation) a single lineage branch point (a time of divergence, speciation) a new species a single lineage dashed line (only sketchy evidence of presumed evolutionary relationship) Gradual Model • Speciation model in which species emerge through many small morphological changes that accumulate over a long time period • Fits well with evidence from certain lineages in fossil record Punctuation Model • Speciation model in which most changes in morphology are compressed into brief period near onset of divergence • Supported by fossil evidence in some lineages Adaptive Radiation • Burst of divergence • Single lineage gives rise to many new species • New species fill vacant adaptive zone • Adaptive zone is “way of life” Adaptive Radiation Extinction • Irrevocable loss of a species • Mass extinctions have played a major role in evolutionary history • Fossil record shows 20 or more large-scale extinctions • Reduced diversity is followed by adaptive radiation Who Survives? • Species survival is to some extent random • Asteroids have repeatedly struck Earth destroying many lineages • Changes in global temperature favor lineages that are widely distributed From Primates to Humans “Uniquely” human traits evolved through modification of traits that evolved earlier, in ancestral forms Hominoids • Apes, humans, and extinct species of their lineages • In biochemistry and body form, humans are closer to apes than to monkeys • Hominids – Subgroup that includes humans and extinct humanlike species Adaptations to an Arboreal Lifestyle • During the Eocene, certain primates became adapted to life in trees – Better daytime vision – Shorter snout – Larger brain – Forward-directed eyes – Capacity for grasping motions First Hominids • Earliest known is Ardipithecus ramidus – Lived 4.4 million years ago in Africa – More apelike than humanlike • Numerous australopiths evolved during the next 2 million years – Large face, protruding jaw, small skull – Walked upright Humans Arise • First member of the genus Homo is H. habilis • Lived in woodlands during late Miocene Homo erectus • Evolved in Africa • Migrated into Europe and Asia about 1.5 million - 2 million years ago • Had a larger brain than H. habilis • Was a creative toolmaker • Built fires and used furs for clothing Homo sapiens • Modern man evolved by 100,000 years ago • Had smaller teeth and jaws than H. erectus • Facial bones were smaller, skull was larger Earliest Fossils Are African • Africa appears to be the cradle of human evolution • No human fossils older than 1.8 million years exist anywhere but Africa • Homo erectus left Africa in waves from 2 million to 500,000 years ago Multiregional Model • Argues that H. erectus migrated to many locations by about 1 million years ago • Geographically separated populations gave rise to phenotypically different races of H. sapiens in different locations • Gene flow prevented races from becoming species African Emergence Model • Argues that H. sapiens arose in subSaharan Africa • H. sapiens migrated out of Africa and into regions where H. erectus had preceded them • Only after leaving Africa did phenotypic differences between races arise 40,000 years ago Fig. 24.30, p. 402