Two Patterns of Speciation1. Gradualism (Anagenesis)A slow, gradual accumulation ofheritable changes (adaptations) ina population, due to many smallepisodes of natural selection. So,one species changes slowly, step-by-step, until it looks so differentthat we call it a new species.2. Branching (Cladogenesis)A more rapid splitting of one ormore new species from anoriginal species that may or maynot continue to exist. So onespecies branches into two ormore new ones. This process isthe basis for all biologicaldiversity.
How does one species evolve into two or more new species by branching? By a 2-step process…1. Geographic isolation - A single population gets divided into two (or more) populations. The members of the different populations (which are now separated from each other) are therefore no longer able to interact with each other.2. Evolution of reproductive barriers - Over time, as they adapt to their different environments due to natural selection, the two populations evolve one or more reproductive barriers, which prevent interbreeding.
Step 1: Geographic IsolationGene flow between two populations isinterrupted (or reduced) between twopopulations because they are geographicallyseparated. What causes this to happen?a) Geological ChangeNew lava flows, the gradual formation of a rivervalley or mountain range, or the slow movement sof tectonic plates can isolate populations if theorganisms are unable to disperse/travel betweenadjacent locations (eg., Harris’ antelope squirreland white-tailed antelope squirrel)
b) Geographic isolation can also result fromcolonization of a new area (as in the founder effect)The separation of a small “founding” population from the mainpopulation is a crucial event in the formation of new species,especially on islands.The combination of natural selection and genetic drift increasethe likelihood that the population will change over time, making itless and less similar to the main population.
Adaptive Radiation: many diversely-adapted species evolved from a common ancestor• Sometimes founder populations on island groups formdifferent species on each island; or sometimes differentspecies in each valley.
Adaptive Radiation: many diversely-adapted species evolved from a common ancestorHawaiian Honeycreepers • 51 endemic species of honeycreepers have evolved from 1 species of finch that colonized Hawaii millions years ago. • More than 1/3 of the species are now extinct (15 within historical times)
Adaptive Radiation: many diversely-adapted species evolved from a common ancestor 30 species of plants in Hawaiian Silversword alliance evolved from one species that colonized Kauai about 5 million years years ago.Closest non-Hawaiian relative is a tarweed that lives on the west coast of N. America Online Activity 15.1
Important point to remember:Geographic Isolation alone is not sufficient to lead to speciation.Even when changes in thegene pool result in newadaptations of an isolatedpopulation to a localenvironment, speciation maynot occur.Speciation occurs only whenchanges in their gene poolsresult in the formation ofreproductive barriersbetween the two (or more)isolated populations.
Step 2. The formation of reproductive barriers between species• What are reproductive barriers? Any morphological, physiological, or behavioral trait that prevents different organisms from successfully interbreeding (so they cannot produce viable, fertile offspring).• There are several types, which can be classified as either pre-zygotic or post-zygotic barriers
Pre-zygotic Reproductive BarriersPrevent the formation of a zygote by preventingmating or fertilizationThere are 5 types:
Pre-zygotic Reproductive Barriers 1. Habitat IsolationTwo species that occupy two differenthabitats within the same geographicalarea may never interact because theynever (or very rarely) encounter eachother.e.g. two species of garter snakes thatlive in the same area, but one isprimarily terrestrial and the other isaquatic
Pre-zygotic Reproductive Barriers• Prevent the formation of a zygote by preventing mating or fertilization 1. Habitat Isolation 2. Temporal Isolation (a matter of timing) Species that breed during different times of the day, different seasons, or different years cannot mate. e.g. ranges of the eastern and western spotted skunk overlap, but the eastern species (c) mates in late winter and the western one (d) mates in the fall
Pre-zygotic Barriers• Prevent the formation of a zygote by preventing mating or fertilization 1. Habitat Isolation 2. Temporal Isolation 3. Behavioral Isolation Little or no sexual attraction between males and females of different species, perhaps due to unique courtship behaviors e.g. Eastern and western meadowlarks are almost identical in shape, color and habitat, but they remain separate species because their courtship rituals differ
Pre-zygotic Barriers• Prevent the formation of a zygote by preventing mating or fertilization 1. Habitat Isolation 2. Temporal Isolation 3. Behavioral Isolation 4. Mechanical IsolationReproductive structures are physicallyincompatible (lack of “fit”). (e.g., The differently-shaped penises of closely-related insectsprevent cross breeding)In plants, the pollinators may be different. (e.g.,The differently-shaped and colored blossoms offlowers attract different types of pollinators;hence cross-pollination is extremely rare.
Pre-zygotic Barriers• Prevent the formation of a zygote by preventing mating or fertilization 1. Habitat Isolation 2. Temporal Isolation 3. Behavioral Isolation 4. Mechanical Isolation 5. Gametic incompatibility Sperm from one species is unable to fertilize the eggs of another species. e.g. Two closely-related species of sea urchins may breed at the same time on the same coral reef, but their gametes are not compatible.
Post-zygotic Reproductive Barriers• Occur after the formation of a zygote by preventing the formation of healthy or fertile offspring• There are 2 types:
Post-zygotic Reproductive Barriers• Occur after the formation of a zygote by preventing the formation of a viable, fertile adult offspring 1. Reduced hybrid viability A hybrid zygote fails to survive embryonic or juvenile development. e.g. Some salamanders in the genus Ensatina live in the same regions and habitats, where they occasionally interbreed; however, the hybrid offspring rarely complete embryonic development, and those that do are frail and do not live long. Same is true for sympatric frogs in genus Rana.
Post-zygotic Reproductive Barriers• Occur after the formation of a zygote by preventing the formation of a viable, fertile adult offspring 1. Reduced hybrid viability 2. Reduced hybrid fertilityEven if hybrids are healthy, they may be sterile.If the chromosomes of the two parent species differ in structure ornumber, meiosis in the hybrid offspring may fail to produce normalgametes, preventing gene flow between the two species.e.g. the hybrid offspring of a donkey and a horse, a mule, is healthybut sterile
Concept Check1. Why are donkeys and horses considered different species?Their offspring (mules) are sterile (not fertile).2. What is macroevolution?Major evolutionary changes, often evident in the fossil record,due to speciation and the evolution of major new adaptations3. Give an example of a reproductive barrier that may separatetwo similar species.prezygotic: behavioral, habitat, temporal (timing), mechanical,gametic,postzygotic; reduced hybrid viability or fertility
Concept Check4. Describe conditions that could make a new island a likelyplace for adaptive radiation.The island may consist of varied habitats with environmentalconditions that differ from one another and from otherlandmasses.Organisms may adapt to the varied conditions in differentways, resulting in species diversity.5. How does that branching model of evolution relate toDarwins theory of natural selection?Suggests that natural selection causes most change as newspecies begins. Major change is less common once a speciesis established in an area.