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Unformatted text preview: Lecture 14: The Origin of Species Branches in the Tree of Life Speciation and the Tree of Life Fig. 26.1 What is a SPECIES?
s No definition that applies in all cases s Biological Species Concept - BSC
x Potential to interbreed under natural conditions and produce viable, fertile offspring x Largest unit of potential genetic exchange x "Some" hybridization o.k., as long as it doesn't occur naturally enough to overwhelm the boundary x Most widely used species concept And then there were hybrids... More Hybrids What is a species?
s Biological Species Concept - Limitations
x Can be difficult to apply Not always clear who has the "potential" to interbreed x Doesn't apply to asexual organisms
3 Is every bdelloid rotifer a distinct species? x Can't be applied to fossils s Biological Species Concept - Limitations
x Boundaries arbitrary
3 How What is a species? much hybridization is too much? See also 26.11 Ring Species These salamanders are found around the San Joaquin Valley and vary in form from place to place. They have been given a number of taxonomic names. Some of the forms interbreed and some do not. What is a species?
s Biological Species Concept
x Still, generally useful for sexual species x Emphasizes reproductive isolation s Many other species definitions:
x Morphospecies concept is a practical substitute, and usually coincides with BSC. x Phylogenetic- more later What separates species? Prezygotic vs. Postzygotic barriers
s Prezygotic barriers
x Prevent formation of a zygote or fertilized egg
s Postzygotic barriers
x Prevent development of viable or fertile offspring What separates species? Prezygotic barriers s 1. Habitat isolation
x May occupy the same range and be potentially able to hybridize, but prefer different habitats so never (or rarely) mate. x E.g. Rhagoletis (Maggot fly) races (subspecies) What separates species? Prezygotic barriers Temporal Isolation
x May potentially interbreed, but are "ready" at s 2. different times x Many plants, animals breed at different times.
3 E.g. Rhagoletis - Apple maggot flies and Hawthorn maggot flies emerge at different times 3 E.g. Breeding times in frogs
/ http://www.npwrc.usgs.gov What separates species? Prezygotic barriers What separates species? Prezygotic barriers
s 3. Behavioral Isolation
mate because of differences in courtship or other behaviors.
3 Ex. x Species may encounter each other, but don't Bird courtship What separates species? Prezygotic barriers
s 4. Mechanical Isolation - E.g. Lock and Key
x In many insects x Flower anatomy s 5. Gametic Isolation
due to different receptors x E.g. Gametes don't recognize each other What separates species? Postzygotic barriers s 1. Reduced hybrid viability:
x Hybrid offspring don't develop x Hybrid offspring don't survive as well
3 E.g. Sticklebacks: Benthics vs. Limnetics What separates species? Postzygotic barriers
s 2. Reduced hybrid fertility
etc. x E.g. Mules, Tigons What separates species? Postzygotic barriers
s 3. Hybrid breakdown Speciation - How do these differences arise? 1. Allopatric speciation s Physical barrier divides population (vicariance)
x E.g. rise of a mountain range, formation of a river or valley, changes in sea level See Fig. 26.5 Speciation - How do these differences arise? Allopatric speciation s Physical barrier divides population (dispersal)
x E.g. migration to an island or a new habitat. Speciation - How do these differences arise? Allopatric speciation
s Populations become different due to...
x Founder effect at outset (especially if one or both of the new populations are small) through genetic drift. x Natural selection under different conditions. May result in physical or behavioral differences that inhibit breeding even if contact is restored. Speciation - How do these differences arise? Allopatric speciation s Physical barrier divides population s Populations become different Speciation - How do these differences arise? Allopatric speciation differences acquired in isolation
x Reinforcement - Hybrid offspring s Re-contact may reinforce or overwhelm have lower survival, so selection favors assortative mating x E.g. Australian green-eyed tree frog http://animals.nationalgeographic.com/animals/amphibians/green-eyed-tree-frog.html Reinforcement drives rapid allopatric speciation. Hoskin et al. 2005. Nature 27 Oct.: 1353-56
Figure 1 Speciation - How do these differences arise? Allopatric speciation
s Recontact may reinforce or overwhelm differences acquired in isolation
x Fusion - Large areas of hybridization and highly fit hybrids may fuse the incipient species back into one. Speciation - How do these differences arise? Sympatric speciation
s Sympatric speciation
x No physical barrier separating diverging populations x E.g. Rhagoletis. Show fidelity to host plant
3 Fed and laid eggs only on Hawthorn trees 3 < 300 years ago, apples introduced to New World 3 Some Rhagoletis switched to apples 3 Apple maggots and Hawthorn maggots are now distinct in their reproductive patterns, and pretty well isolated genetically Speciation - How do these differences arise?
x E.g. Soapberry bugs. s Sympatric speciation Fig. 26.7 x Autopolyploidy same species (> 2 chr.sets) x Duplication of chromosome number reproductively isolates offspring from parent population
3 Common in plants (50%?) 3 Facilitated by ability to self-fertilize 3 See pages 535-6; Fig. 26.8 Speciation - How do these differences arise?
s Sympatric speciation Autopolyploidy (Page 536) Speciation - How do these differences arise?
s Sympatric speciation: x Allopoloploidy
3 Hybridization and errors in meiosis/mitosis lead to polyploid offspring with chromosomes from 2 different species 3 E.g. Spartina, wheat See figure 26.10 and pages 536-7 Speciation on Islands
s Adapative Radiation- evolution of many diversely adapted species from a common ancestor s Species difficult to define Summary x BSC most widely used for sexual organisms x Emphasis on restriction of gene flow s Species separated by pre/post zygotic barriers s Barriers arise via allopatric speciation
x Physical division of population x Divergence due to founder effect, drift, selection x Possible reinforcement if recontact s ...And sympatric speciation
x Ecological isolation, auto and allopolyploidy s Adaptive radiation ...
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- Spring '10