Natural selection on single-gene traits can lead to changes in allele frequencies and thus to evolution. Organisms of one color, for example, may produce fewer offspring than organisms of other colors.
How does natural selection affect polygenic traits? • Natural selection can affect the distributions of phenotypes in any of three ways: • directional selection • stabilizing selection • disruptive selection
Directional Selection • When individuals at one end of the curve have higher fitness than individuals in the middle or at the other end, directional selection takes place. • The range of phenotypes shifts as some individuals survive and reproduce while others do not. How does this influence gene and allele frequency?
Stabilizing Selection • When individuals near the center of the curve have higher fitness than individuals at either end of the curve, stabilizing selection takes place. • This keeps the center of the curve at its current position, but it narrows the overall graph. • How does this influence gene and allele frequency?
Disruptive Selection • When individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle, disruptive selection takes place. • If the pressure of natural selection is strong enough and long enough, the curve will split, creating two distinct phenotypes. • How does this influence gene and allele frequency?
Graphs of Types of Selection On the next slide, there are 3 graphs to depict 3 ways in which a hypothetical deer mouse population with heritable variation in fur coloration from light to dark might evolve. The graphs show how the frequencies of individuals with different fur colors change over time. The large white arrows symbolize selective pressures against certain traits.
Sexual Selection …Not a type of natural selection!
Sexual Selection • Sexual selection: selection towards a physical display/characteristics that leads to differences among males and females of the species
Behaviors Associated with Sexual Selection • Dancing • http://www.milkandcookies.com/link/64174/detail/ • http://www.pbs.org/wnet/nature/episodes/andes-the-dragons-back/video-dance-of-the-flamingos/3104/ • Singing/ Calling • http://www.music4musicians.com/cardinal_mating_call.htm • http://animal.discovery.com/videos/return-of-the-fire-cat-mating-season.html • http://www.hear.org/AlienSpeciesInHawaii/species/frogs/index.html#frogcalls
Behaviors Associated with Sexual Selection-Continued • Physical Display (Weaponry or Lovely) Lovely Weaponry How does this influence gene and allele frequency?
Artificial Selection …Not a type of natural selection
Artificial Selection • Definition: Human intervention in animal or plant reproduction to ensure that certain desirable traits are represented generation to generation How does this influence gene and allele frequency?
Example: Breeding of Greyhounds • Breeding of greyhound dogs: • Early breeders were interested in dog with greatest speed. • carefully selected those who ran the fastest. • From offspring selected those dogs who ran the fastest. • By continuing this selection gradually produced a dog breed that could run up to 40mph. • used to hunt the fastest of game, fox and deer. • End Result= Artificial selection for extreme speed in the greyhound dogs
Artificial Selection Examples Continued • Artificial selection is the selective breeding of organisms for a desired trait, such as • Breeding rose plant to produce larger flowers • Breeding a chicken to lay more eggs. • Breeding a cat for specific characteristics • Etc.