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Speciation – How Species Form Section 9.2 Species Physiology, biochemistry, behaviour, and genetics are used to distinguish one species from another . Species Physiology, biochemistry, behaviour, and genetics are used to distinguish one species from another Species – individual members can interbreed to reproduce viable and fertile offspring Speciation The formation of a new species from existing species . Speciation The formation of a new species from existing species Occurs when some members of a sexually reproducing populations change so much that they are no longer able to produce viable, fertile offspring with members of the original population Micro vs. Macro Microevolution population = evolution within a Macroevolution species = formation of a new Reproductive Isolation . Reproductive Isolation 2 populations may become reproductively isolated (become 2 species) over time if there is little or no gene flow between them . Reproductive Isolation 2 populations may become reproductively isolated (become 2 species) over time if there is little or no gene flow between them Gene flow can be prevented pre-zygotic or post-zygotic Reproductive Isolating Mechanisms Pre-zygotic Post-zygotic Prevention of Mating Prevention of Fertilization Prevention of Hybrids Behavioural isolation Mechanical isolation Zygotic mortality Temporal isolation Gametic isolation Hybrid inviability Ecological/habit at isolation Hybrid infertility Pre-zygotic Isolating Mechanisms (5) Impede mating between species or prevent fertilization of the eggs if individuals from different species attempt to mate 1. Behavioural Isolating Mechanisms Any special signals or behaviours that are species specific prevent interbreeding with closely related species Examples: bird songs, courtship rituals, pheromones, etc. 2. Habitat Isolating Mechanisms Two species may live in same general region but in different habitats and therefore rarely encounter one another Example – 2 species of North American garter snakes – one prefers open areas and avoids water while another is commonly found near water 3. Temporal Isolating Mechanisms Separation by temporal or timing barriers Different mating schedules (different times of day, different seasons, or in different years) Example: orchids that bloom for a single day as a response to stimuli in the weather…1 blooms after 8 days, 1 after 9 and 1 more after 10 days 4. Mechanical Isolating Mechanisms Failed fertilization due to incompatible anatomy Genital anatomy is distinctive in many organisms and can be used to classify species 5. Gametic Isolating Mechanisms If gametes from different species do meet, gametic isolation ensures they won’t fuse and form a zygote Various techniques for various species Example – male sperm will not survive environment of female reproductive tract of another species Post-zygotic Isolating Mechanisms Prevention of hybrid zygote development even if the sperm of one species successfully fertilizes the egg of another species and a zygote is formed 3 methods 1. Hybrid Inviability Genetic incompatibility of the interbred species may stop development of the hybrid during development Normal mitosis is prevented after fusion of the nuclei in the gametes Zygotes of sheep and goats dies early in development…long before birth 2. Hybrid Sterility Two species mate and produce a hybrid offspring, but the offspring will be sterile and unable to reproduce Meiosis fails to produce normal gametes in hybrid b/c chromosomes of 2 parent species differ in number or structure Example – Mule born of a female horse and male donkey 3. Hybrid Breakdown First generation hybrids are fertile Second generation hybrids are sterile or weak (even if mating with an individual from either parent species) Types of Speciation Sympatric Populations living in same habitat diverge and become reproductively isolated Allopatric Populations are separated by a geographical barrier and then diverge genetically Sympatric Speciation – Ex 1 Chromosomal changes (in plants) and non-random mating (in animals) alter gene flow More A common in plants than animals new species can be generated in one generation if a genetic change results in a reproductive barrier between the offspring and parent generations What is non-disjunction yo? Non-disjunction can lead to polyploidy (extra sets of chromosomes) which can lead to speciation A polyploidy organism has 3+ sets of chromosomes (rather than 2) If chromosomes don’t separate in meiosis, gametes have two sets of chromosomes (diploid instead of haploid) If two diploid gametes fuse, the offspring will be tetraploid (4 of each chromosome) If tetraploid survive, they can undergo meiosis and produce diploid gametes Organism can reproduce with other tetraploids, but not parent generation b/c that would form triploids *see page 364 Illustration – page 364 In one generation, a reproductive barrier has been established in a population b/c gene flow was interrupted Sympatric Speciation – Ex 2 Two species interbreed to produce a sterile offspring Offspring is sterile but can reproduce asexually resulting in a separate population Many plants do this: wheat, cotton, oats, potatoes Allopatric Speciation Population is split into 2+ isolated groups by a geographic barrier Geographic barriers glacier or lava flow, and fluctuations in ocean levels (creation of islands) Gene pool of split population becomes so distinct that groups are unable to interbreed even if brought back together Once separated populations begin to diverge b/c of natural selection, mutation, genetic drift, and/or gene flow Isolation doesn’t need to last forever, but must last long enough for populations to become reproductively incompatible before they are re-joined Small populations on the periphery are more likely to become the new species Small populations are more susceptible to genetic drift and mutations and even natural selection through selective pressure if the environment is different. *not all isolated populations will survive long enough to change Darwin’s Finches - Allopatric Birds ended up on the Galapagos islands in the past Only birds on the island and therefore had many ecological niches to undertake (ecological role and physical distribution of a species in its environment) Ancestral species divided into different populations and some evolved into new species due to the various selective pressures they experienced Main differences are genetic an beak length Practice Questions Page 363 #15, 16, and 18 Page 365 #19, 22, 23, and 24
