Biology 203 Lab

Natural Selection

Natural selection - the changing of allele frequencies by differential survival and differential reproduction.

Normal Distribution - A theoretical frequency distribution for a set of variable data, normally represented as a bell-shaped curve (also known as a Gaussian distribution), symmetrical around the mean.

Differential survival refers to some individuals living where others die. If, for instance, some individuals have more cryptic coloration, they will be eaten by predators less often than non-cryptic individuals. If some individuals find food better, they are less likely to starve than other individuals.

Differential reproduction refers to some individuals leaving more offspring than others. This may be due to these individuals surviving better than other individuals, or possibly having larger numbers of offspring, or more of those offspring surviving, etc.

For natural selection to work, there are three requirements:

1. Traits are heritable
2. Variation in these traits is present in the population
3. Some of these variants confer greater survival and reproductive success on their bearers.

An excellent example of Natural Selection are the Galapagos finches.  The Galapagos Islands and Cocos Island are home to 14 species of finch, generally belonging to 4 groups: the ground finches (Geospiza), the tree finches (Camarhynchus), the warbler finch (Certhidea) and the Cocos finch (Pinaroloxias).  

These finches all evolved from a single species similar to the Blue-Back Grassquit Finch Volatina jacarina, commonly found along the Pacific Coast of South America.  Once in the Galapagos Islands the finches adapted to the new habitat and the size and shape of their bills began to reflect their feeding specializations - insects, flowers, leaves, or seeds.  

The Vegetarian and Ground Finches all have crushing bills.  Ground Finches eat ticks which they remove with their crushing beaks from tortoises and iguanas, and kick eggs into rocks to feed upon their contents.  On Isla Wolf, the Sharp Beaked Ground Finch, known as the "Vampire Finch", jumps on the backs of other birds, pecking at their flesh and feeding on their blood.  

Tree Finches have a grasping bill most likely specialized for insect and seed feeding. 

The Cactus Finch, Warbler Finch and Woodpecker Finch have probing bills.   Woodpecker and Mangrove Finches, two species of tree finches, use small twigs and cactus spines as tools to dine on the larva stored in dead tree branches.

Though they have adapted for specialized feeding, most finches are generalized eaters most of the time.  The individual specializations allow the birds to compete and survive during extreme conditions, like times of drought, when food is scarce.

The Vampire and Mangrove Finches are highly endangered in the wild.

Natural selection works on the trait via a selective agent.  This environmental agent can affect the natural selection of populations in 3 different ways - stabilizing, directional and disruptive  

Under stabilizing selection, even though over generations, the frequency distribution may look the same, the extreme data from both ends of the frequency distribution are eliminated.  This appears to be the most common form of natural selection, and is often mistaken for "no selection".  A real-life example is that of birth weight of human babies.

Under directional selection, one extreme trait of the frequency distribution is favored, so the distribution in the subsequent generation is shifted from where it was in the parental generation, towards that of the favored trait.  This is what is usually thought of as natural selection.  The fossil evolution of the horse is such an example.

Under disruptive (diversifying) selection, both extreme traits are favored at the expense of intermediate varieties. This is uncommon, but of theoretical interest because it suggests a mechanism for species formation without geographic isolation.  Some closely related frog species, that once could interbreed, show disruptive selection by evolving to breed at different times in the spring - reproductive isolation.

The EvoTutor website has a nice Natural Selection simulation.

Review Questions

- Who first proposed natural selection?
- Give one example of natural selection?
- Natural selection is a mechanism of what process (What does natural selection lead to)?
- What does natural selection act on? Phenotype, genotype or both?
- Does an individual evolve?
- Draw a normal distribution demonstrating rainfall (in.) vs. time (hr).
- What would you expect to happen to the color distribution of moths after several generations if the same colors of individuals were selected against?
- Define the 3 different types of selection and draw their curves - directional, stabilizing and disruptive.
- State whether each of the following is most likely to illustrate directional, stabilizing or disruptive selection:
        1.  Selection that favors medium body size.
        2.  Selection that favors small or large body size.
        3.  Selection that favors only large body size.
- What is a selective agent?