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Speciation
The Evolution of new Species
Speciation
• The formation of new species from existing species
• Macroevolution• Species = population/group in nature whose
individual members can interbreed to produce viable, fertile offspring
• When some members of a sexually reproducing population change so much that they are no longer able to produce viable, fertile offspring with members of the original population, speciation has occurred.
Speciation
REPRODUCTIVE BARRIERSREPRODUCTIVE BARRIERS
• Any mechanismmechanism that impedesimpedes two species from producing fertile fertile and/or viable hybrid offspringand/or viable hybrid offspring.
• Two barriers:Two barriers:
1.1. Pre-zygotic barriersPre-zygotic barriers
2.2. Post-zygotic barriersPost-zygotic barriers
• Pre-fertilization barriers• Impede mating between species or prevent
fertilization of the eggs• 5 types of isolating mechanisms
– Behavioural– Habitat– Temporal– Mechanical– Gemetic
1.1. PRE-ZYGOTIC PRE-ZYGOTIC BARRIERSBARRIERS
Behavioural Isolating Mechanisms
• Any special signals or behavious that are species specific prevent interbreeding with closely related species
Habitat Isolating Mechanisms
• Two species may live in the same general region but in different habitats, so they will probably never encounter each other
Temporal Isolating Mechanisms
• Species kept separate by timing barriers
• Two species may occupy the same habitat but mate or flower at different times of day, in different seasons, or in different years.
Mechanical Isolating Mechanisms
• Species that are closely related may attempt to mate but fail to achieve fertilization because they are anatomically incompatible
Gametic Isolating Mechanisms
• Gametes = egg and sperm
• Gametes of different species will rarely fuse to form a zygote
POST-ZYGOTIC BARRIERS
• A barrier that prevents hybrid zygotes from developing into viable, fertile individuals; also called post-fertilization barrier– Hybrid inviability– Hybrid sterility– Hybrid breakdown
Hybrid inviability• Development of the hybrid zygote may
stop
• E.g. embryos between sheep and goats die in early development before birth
• Due to genetic incompatibility
Hybrid Sterility• Hybrid fails to produce functional
gametes
• Example:A mule is the offspring of a male donkey and a female horse. Horses and donkeys are different species, with different numbers of chromosomes. All male mules and most female mules are infertile.
Hybrid breakdown
• Offspring of hybrids are weak or infertile
• E.g. different species of cotton plants can produce fertile hybrids, but the offspring of the hybrids die as seeds or early in development
Types of Speciation
• The process of speciation requires populations of organisms to become, and largely remain, genetically isolated from one another.
• Two types of speciation based on how gene flow is disrupted within a population:– Sympatric
- Allopatric http://bcs.whfreeman.com/thelifewire/content/chp24/2402001.html
Sympatric Speciation
• Enables populations that live in the same habitat to diverge genetically
• Result of a radical change in the genome that produces a reproductively isolated sub-population within the parent population (very rare)
SYMPATRIC SPECIATION • the ancestors of apple maggot flies laid their eggs only
on hawthorns—but today, these flies lay eggs on hawthorns (which are native to America) and domestic apples (which were introduced to America by immigrants and bred). Females generally choose to lay their eggs on the type of fruit they grew up in, and males tend to look for mates on the type of fruit they grew up in.
ALLOPATRIC SPECIATIONALLOPATRIC SPECIATION• happens when the ancestralancestral
population becomes separatedseparated by a geographical barrier/isolation.geographical barrier/isolation.
• Isolation might occur because of great distance or a physical barrier, such as a desert or river, as shown below
Speciation Tutorial
http://www.google.com/search?um=1&hl=en&biw=804&bih=871&q=types+of+speciation+tutorial+animation&ie=UTF-8&sa=N&tab=iw#sclient=psy&hl=en&biw=804&bih=871&source=hp&q=speciation+animation&aq=f&aqi=&aql=&oq=&gs_rfai=&pbx=1&fp=6f32b8af52b7e0b8
ADAPTIVE RADIATIONADAPTIVE RADIATION• Emergence of numerous speciesEmergence of numerous species
from a common ancestorcommon ancestor introduced to new and diverse environments.
• Example:Example:
Darwin’s FinchesDarwin’s Finches
http://bcs.whfreeman.com/thelifewire/content/chp24/2402001.html
INTERPRETATIONS OF INTERPRETATIONS OF SPECIATIONSPECIATION
• Two theories:Two theories:
1.1. Gradualism (Neo-Darwinian):Gradualism (Neo-Darwinian):
Slow changes in species overtime.
2.2. Punctuated Equilibrium:Punctuated Equilibrium:
Evolution occurs in spurts of relatively rapid change.
CONVERGENT EVOLUTIONCONVERGENT EVOLUTION• SpeciesSpecies from different evolutionary branchesevolutionary branches may
come to resemble one another if they live in very very similar environments.similar environments.
• Example:Example:1.1. Sidewinder (Mojave Desert) andSidewinder (Mojave Desert) and
Horned Viper (Middle East Desert)Horned Viper (Middle East Desert)
CONVERGENT EVOLUTION - EXAMPLE
• 1.1. Ostrich (Africa) and Emu Ostrich (Africa) and Emu (Australia).(Australia).
COEVOLUTIONCOEVOLUTION
• Evolutionary changeEvolutionary change, in which one species act as a selective forceselective force on a secondsecond species, inducing adaptations that in turn act as selective force on the firstfirst species.
• Example:Example:1.1. Acacia ants and acacia treesAcacia ants and acacia trees
Video
COEVOLUTION - EXAMPLE
2.2. Humming birds and plants with Humming birds and plants with flowers with long tubesflowers with long tubes
http://www.mhhe.com/biosci/esp/2001_gbio/folder_structure/ev/m3/s2/evm3s2_4.htm
Consequences of Human Activities
• Human-made barriers may prevent gene flow between the split populations
• Isolated populations may undergo adaptive radiation
• Severely fragmented populations may eventually die out if there is insufficient genetic diversity
• E.g. giant panda in China
Speciation and Mass Extinctions
• Five major mass extinctions have been identified