Visit Your Local PBS Station PBS Home PBS Home Programs A-Z TV Schedules Watch Video Donate Shop PBS Search PBS
Click to return to the Evolution Home Page
darwin change extinction survival sex humans religion
sex width=    
thumbnail image of amourous coupleessay: The Advantage of Sex

microscopic photo of sickle-shaped cells
In sickle cell
deforms blood
cells to sickle

Keeping variety in store  
Sexual species can call on a "library" of locks unavailable to asexual species. This library is defined by two terms: heterozygosity, when an organism carries two different forms of a gene, and polymorphism, when a population contains multiple forms of a gene. Both are lost when a lineage becomes inbred. What is the function of heterozygosity? In the case of sickle cell anemia, the sickle gene helps to defeat malaria. So where malaria is common, the heterozygotes (those with one normal gene and one sickle gene) are better off than the homozygotes (those with a pair of normal genes or sickle genes) who will suffer from malaria or anemia.  
One of the main proponents of the Red Queen hypothesis was the late W. D. Hamilton. In the late 1970s, with the help of two colleagues from the University of Michigan, Hamilton built a computer model of sex and disease, a slice of artificial life. It began with an imaginary population of 200 creatures, some sexual and some asexual. Death was random. As expected, the sexual race quickly died out. In a game between sex and "asex," asex always wins -- other things being equal. That's because asexual reproduction is easier, and it's guaranteed to pass genes on to one's offspring. sea anemone
Sea anemones
page 1 | 2 | 3 | 4 | 5 | 6 adding parasites to the mix
  related web activities  
Sex and the Single Guppy
Learn how exhibitionism has an evolutionary payoff.
The Mating Game
Help our contestants find the perfect date.
  related topics  
  Adaptation and Natural Selection  
  Evolution of Diversity  
  Evolution Since Darwin  
Source Credits      
Videos Web Activities Site Guide About the Project FAQ Glossary Site Map Feedback Help Shop