Natural Selection 2. If humans select traits for dogs, pigeons and the five major mammals, who selects the traits of all those Lemurs, Frogs, and other wild animals? WHICHEVER TRAITS WORK BEST FOR THE ENVIRONMENT THEY LIVE IN ARE THE ONES THAT ARE PASSED ON THROUGH THE GENES NOBODY Animals not possessing the beneficial traits don’t do as well, or just die
Example #1: n The neck of the Giraffe n And Lamarck n The slow hypothetical evolution: Response to Acacia Trees n The Red Queen Hypothesis...
Example #2: Versicolor Moth n Early trees had light-colored bark n Only the light-colored moths survived. Selection was for less melanin. n After industrialization, the tree bark was darker. n Only the darker colored moths now survived. Selection was for more melanin.
What serve as pressures? Weather: changes in local weather (e.g. when an Ice Age comes on), migration to a new area (e.g. Alaska down into California) Change in surroundings/cover: change in density of vegetation (e.g. scrub to forest) change in color of vegetation (e.g. versicolor moth) Change in food or predators: food grows defenses (e.g. Acacia Trees and Giraffes) only fastest predators survive (e.g. speed of Cheetahs)
The Main Types of Selection Pressures n Directional Selection –Easiest to identify –Camouflage and mimicry usually fall here n Disruptive Selection –Causes species to diverge n Stabilizing Selection –Drives a population to specialize n Sexual Selection
Directional Selection n Most quantifiable changes in a population are directional, for example... n Neck of Giraffe n Moth color (melanin) n Camouflage/Mimics n Many sexually selected traits
Sexual Selection: Females Females must behave in a way that ensures that their offspring survive and mate, and that they have as many offspring as possible If they behave this way, the genes for this behavior are passed on. If they don’t behave this way, the genes are not passed on. (remember, over millions of years all of your ancestors have successfully mated…) Behavior #1: Pick a mate who will help you raise the young (if young have long infancy period) Behavior #2: Pick a mate who has the “fittest genes” for your environment and mate with them (if young have short infancy period)
Sexual Selection: Males Males also behave in a way that maximizes the propogation of their genes. The ones who possess traits that allow them to mate and have many surviving offspring pass on those “prolific” genes. Behavior #1: Choose one female to raise the young with (if young have long infancy period) Behavior #2: Convince the females that they have the “fittest genes” for their environment (if young have short infancy period)
Behavior #1: male and female identical and best adapted (Gibbons, Albatrosses, many seals)
Behavior #2: Males compete so that females choose them (Elephant Seals, Elk, Lions, Gorillas, Widowbird)
These “Tournament Species” tend to be Sexually Dimorphic Males and females of the same species have different forms and are easily distinguishible The male sexual characteristics identify which males are dominant Often the superficial sexual characteristics imply more essential ones - the probiscus, size and deepness of voice of elephant seals are probably all controlled by the same gene Some of these characteristics allow males to fight each other more effectively. But what about the ones that don’t help them physically compete against each other?
Males and females of the same species have different forms and are easily distinguishable Males and females of the same species have different forms and are easily distinguishable
The male sexual characteristics identify which males are dominant
Often the superficial sexual characteristics imply more essential ones- the probiscus, size and deepness of voice of elephant seals are probably all controlled by the same gene
Some of these characteristics allow males to fight each other more effectively. But what about the ones that don’t help them physically compete against each other?
Do the females really go for it? 1. Catch long-tailed birds and cut off tails. 2. Paste these tails onto short-tailed widowbirds. 3. Catch some long-tailed birds, cut tails and paste back on. Findings: The long-tailed widowbirds were more successful at mating and had more offspring (whether their tails were long naturally or pasted on)
How did these characteristics come about if they advertise to predators? Theory #1: A bottleneck occurred in the population. An outward, superficial gene was linked on the chromosome near a more essential one. The females began choosing the best mate on superficial cues. The superficial cues have been undergoing selection ever since. Theory #2: That dumb characteristic risks the life of the male. If he can survive with all those colors and obvious feathers, he must be some kind of stud.