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BIS 2B Speciation Handout.2010

BIS 2B Speciation Handout.2010 - SPECIATION AND THE...

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SPECIATION AND THE GENERATION OF BIOLOGICAL DIVERSITY _____________________________________________________________________________________ I. D ARWINIAN E VOLUTION AND THE O RIGIN OF S PECIES 1. Darwin’s explanation: a) Natural selection leads to sufficient divergence among “races” so that they eventually become full-fledged species. b) Problems : 1) Lack of explicit mechanism causing reproductive isolation. 2) How does/can reproductive isolation evolve by selection? c) Anagenesis vs . Cladogenesis 1) Anagenesis : changes due to selection and random forces ( e.g ., founder effects, bottlenecks, etc .) that occur within a lineage through time. 2) Cladogenesis : the generation of new clades or lineages through time ( i.e ., lineage splitting). II. D EFINITIONS OF S PECIES 1. The Biological Species Concept : A species consists of groups of actually, or potentially, interbreeding natural populations of organisms that are reproductively isolated from other such groups . a) Breeding units: 1) Populations/Demes/Gene Pools 2) Populations vs . Species b) In other words… species are breeding units that are evolutionarily independent from other breeding units . c) In still other words, SPECIES ARE FUNDAMENTAL EVOLUTIONARY UNITS . 1) Individuals do NOT evolve (their genetic composition – barring mutation – doesn’t change). 2) Populations and species DO evolve. 3) Species represent separate evolutionary units that can no longer exchange genes, hence evolve independently of each other. d) Problems with the Biological Species Concept 1) Actual vs. potential mating opportunities in allopatric (geographically separated) populations. 2) What’s “natural”? Mating in the lab vs . in nature. 3) What about extinct organisms? 4) What about asexual organisms? III. T YPES OF G ENETIC B ARRIERS OR R EPRODUCTIVE I SOLATING M ECHANISMS “The evolution of a new species is equivalent to the evolution of genetic barriers to gene flow between populations.” (Doug Futuyma) WHAT MECHANISMS LIMIT GENETIC EXCHANGE (GENE FLOW) BETWEEN POPULATIONS? HOW CAN THESE GENETIC BARRIERS EVOLVE? 1. Prezygotic Mechanisms : Prevent gametes from uniting to form zygotes .
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BIS 2B, Spring 2010 Speciation Page 2 a) Spatial/Geographical Isolation: immediately limits gene flow if there is little dispersal across a geographic barrier 1) The importance of allopatry . 2) Example : speciation in snapping shrimp across the Isthmus of Panama b) Ecological Isolation: if two species utilize different habitats (& mate in those habitats), they can be ecologically isolated 1) Example : benthic and limnetic forms of 3-spine sticklebacks c) Temporal Isolation: species that breed at different times (daily, seasonally, or yearly) have limited opportunity to mate 1) Example : periodic cicadas that breed in different prime-numbered years d) Behavioral Isolation: species-specific mate recognition systems (courtship behaviors, songs/acoustical signals, chemical signals/pheromones, visual signals) may effectively limit mating between species 1) Example : species-specific flashing and flight patterns in fireflies e) Mechanical Isolation: anatomical mismatches between males and females may make
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