Species Concepts & Isolation Mechanisms PDF

What is a Species? What is a Species?  A species includes all members of a population that can interbreed under natural conditions and produce viable offsprings.  Some species are quite distinct based on their morphology but some species are only distinguished by...

What is a Species? What is a Species?  A species includes all members of a population that can interbreed under natural conditions and produce viable offsprings.  Some species are quite distinct based on their morphology but some species are only distinguished by genetic comparisons Mechanisms of Reproductive Isolation  any biological factor that prevents the two populations from interbreeding when living in the same region. Mechanisms of Reproductive Isolation  Prezygotic Mechanisms: -prevent interspecies mating and fertilization ◦Different breeding seasons, physical or behavioral traits, habitat preferences or the incompatibility of gametes Mechanisms of Reproductive Isolation Postzygotic mechanisms: -prevent maturation and reproduction in offspring from interspecies reproduction Mechanisms of Reproductive Isolation Prezygotic Temporal Isolation: Species have different breeding seasons. Example: Western spotted skunks breed in the fall but the Eastern species breeds in late winter Western Eastern  Temporal isolation  Species that breed during different times of day, different seasons, or different years cannot mix gametes Eastern spotted skunk (L) & western spotted skunk (R) overlap in range but eastern mates in late winter & western mates in late summer Mechanisms of Reproductive Isolation Prezygotic Behavioural Isolation: Species may have different courtship or mating behaviors. Example: Eastern meadowlark and western meadowlark are almost identical in everything, except their courtship rituals and songs Eastern Western Behavioral isolation  Unique behavioral patterns & rituals isolate species ◦ identifies members of species ◦ attract mates of same species   courtship rituals, mating calls Blue footed boobies mate only after a courtship display unique Mechanisms of Reproductive Isolation Prezygotic Mechanical Isolation: Species may have reproductive structures that are physically incompatible. Example: Snails of the same species have reproductive structures that align, but snails from two different species part the other species Mechanical isolation  Morphological differences can prevent successful mating Plants Even in closely related species of plants, the flowers often have distinct appearances that attract different pollinators. These 2 species of monkey flower differ greatly in shape & color, therefore cross- pollination does not happen. Mechanisms of Reproductive Isolation Prezygotic Ecological Isolation: very similar species may occupy different habitats within a region Example: The mountain bluebird lives at high elevations, while the eastern bluebird prefers lower elevations. The mountain bluebird and eastern blue bird do not encounter each other Ecological Isolation  Species occur in same region, but occupy different habitats so rarely encounter each other ◦ reproductively isolated 2 species of garter snake, Thamnophis, occur in same area, but one lives in water & other is terrestrial lions & tigers could hybridize, but they live in different habitats:  lions in Mechanisms of Reproductive Isolation Prezygotic Gametic isolation: male gametes may not be able to recognize and fertilize an egg of a different species Example: corals, clams and sea cucumbers release their sperm and eggs into open water. The sperm recognize the egg of their own species through chemical markers on the egg surface.  Gametic isolation  Sperm of one species may not be able to fertilize eggs of another species Sea urchins release sperm & eggs into surrounding waters where they fuse & form zygotes. Gametes of different species— red & purple —are unable to fuse. Mechanisms of Reproductive Isolation Postzygotic Zygotic Mortality: mating and fertilization are possible, but genetic differences result in a zygote that is unable to develop properly Example: If you have sheep and goats on a farm, some species of sheep and goat are able to mate, but the zygote is not viable. Mechanisms of Reproductive Isolation Postzygotic Hybrid Inviability: a hybrid dies early in development, or lives to maturity but is not healthy and suffers an early death. Example: Some salamanders species may mate with each other, but most of the offspring do not live to adulthood, or if they do they are weak.   Genes of different parent species may interact & impair the hybrid’s development Species Species of of salamander salamander genus, genus, Ensatina, Ensatina, may may interbreed, interbreed, but but most most hybrids hybrids do do not not complete complete development development & & those those that that do do are are frail. frail. Mechanisms of Reproductive Isolation Postzygotic Hybrid Infertility: The hybrid offspring mature into adults but are infertile. are infertile.   Even if hybrids are vigorous they may be sterile ◦ chromosomes of parents may differ in number or structure & meiosis in hybrids may fail to produce normal gametes Mules are vigorous, but sterile Horses have 64 Donkeys have chromosomes 62 (32 pairs) chromosomes Mules have 63 (31 pairs)  Hybrid breakdown  Hybrids may be fertile & viable in first generation, but when they mate offspring are feeble or sterile In strains of cultivated rice, hybrids are vigorous but plants in next generation are small & sterile. Speciation Introduction  Species continue to evolve  Changes that occur within species (microevolution) are easy to understand ◦ Faster-running cheetahs ◦ Colourful male songbirds ◦ Larger brains in humans  But what factors lead to speciation- the evolution of an entirely new species? Modes of Speciation  New species can evolve in many ways  Always includes the evolution of distinct features that isolate the new species reproductively, therefore genetically, from other species  So Reproductive Isolation is the key!  Once isolated, populations will evolve independently  Allopatric Speciation  Most new species form when a single species is separated into two geographically isolated populations  Once physically separated, populations cannot share genetic information  Over many generations the populations will gradually become less alike  Mutations and genetic drift may have caused populations to change  Reproductive isolating mechanisms may have developed  Allopatric Speciation White-tailed Harris’s antelope antelope squirrel squirrel Sympatric Speciation  The evolution of populations within the same geographic area into separate species  Individuals within a population become genetically isolated from the larger population Sympatric Speciation  Separation can be due to diet, behavior, or genetics BUT not physical barriers Specifically: 1. Polyploidy 2. Differences in mating preferences 3. Changes in behavior like mating calls, feeding habits Exercise Identify the given isolating mechanisms Exercise 1. Two species of garter snakes live in the same region but one lives in water and the other on land 2. Two species of meadowlarks (birds) with different mating songs Exercise 3. Two species of trout(fish) that breed in different seasons 4. Mule is the sterile offspring of a horse and a donkey Exercise 5. Two species of plants flower at different months