Notesale: Turn your study into money

Document Preview

Extracts from the notes are below, to see the PDF you'll receive please use the links above

---

Chapter  24:  The  Origin  of  Species    
24
...
   
• such  barriers  block  gene  flow  between  species  limit  the  forma>on  of  hybrids,  offspring  that  result  
from  an  interspecific  ma>ng    
• Prezygo'c  barriers  (before  the  zygote)  block  fer>liza>on  from  occurring    
1
...
by  preven>ng  an  aCempted  ma>ng  from  being  completed  successfully    
3
...
2  Specia6on  can  take  place  with  or  without  geographic  sepera6on    
• allopatric  specia'on,  gene  flow  is  interrupted  when  a  popula>on  is  divided  into  geographically  
isolated  subpopula>ons    
• ex-­‐  water  level  in  a  lake  may  subside,  resul>ng  in  two  or  more  smaller  lakes  that  are  now  home  to  
separated  popula>ons    
The  Process  of  Allopatric  Specia6on    
• one  geographic  separa>on  has  occurred,  the  separated  gene  pools  may  diverge    
• different  muta>ons  arise,  the  natural  selec>on  and  gene>c  driQ  may  alter  allele  frequencies  in  
different  ways  in  the  separated  popula>ons    
Polypoloidy    
• a  species  may  originate  from  an  accident  during  cell  division  that  results  in  extra  sets  of  chromosomes,  
a  condi>on  called  polyploidy    
• polyploid  specia>on  occasionally  occurs  in  animals    
• two  dis>nct  forms  of  polyploidy  have  been  observed  in  plant  popula>ons  
• autopolyploid  is  an  individual  that  has  more  than  two  chromosomes  sets  that  are  all  derived  from  a  
single  species    
• ex-­‐  a  failure  of  cell  division  could  double  a  cell’s  chromosome  number  from  the  original  number  (2n)  
to  a  tetraploid  number  (4n)    
• a  tetraploid  offspring  can  produce  fer>le  tetraploid  offspring  by  self-­‐pollina>ng  or  by  ma>ng  with  other  
tetraploids  —  reproduc>vely  isolated  from  2n  plants  of  the  original  popula>ons,  because  the  triploid  
(3n)  offspring  of  such  unions  have  reduced  fer>lity    
• a  second  form  of  polyploidy  can  occur  when  two  different  species  interbreed  and  produce  hybrid  
offspring  
• in  subsequent  genera>ons,  various  mechanisms  can  change  a  sterile  hybrid  into  a  fer>le  polyploid  
called  an  allopolyploid,  fer>le  when  ma>ng  with  each  other  but  cannot  interbreed  with  either  parent  
species;  thus,  they  represent  a  new  biological  species    
Habitat  Differen6a6on    
• sympatric  specia>on    can  occur  when  a  subpopula>on  exploits  a  habitat  or  resource  not  used  by  the  
parent  popula>on  



---