PDS AP Environmental Science 5th Period 2010-11

According to our text, biological evolution is the genetic change in populations of organisms across generations. But to begin our discussion on evolution, we had to first consider how biodiversity is calculated. Speciation – Extinction = Biodiversity. Speciation is the only method known to add species and increase biodiversity, and extinction–the disappearance of a species–decreases biodiversity. In class today, we focused on speciation. So let’s imagine a certain population of a species and call them Population A. Within the population, natural selection occurs, which drives change in the population. The change is seen in the changing allele frequencies of the gene pool. This is termed microevolution. Microevolution usually occurs over just several generations.

Now, there are 4 Factors of Natural Selection:

1. Genetic Variation (Sources of variation include mutation and sexual reproduction/meiosis.)
2. Overproduction of Offsprings
3. Struggle for Existence
4. Differential Rate of Survival and Reproduction

Genetic variation allows different combinations of genes to produce offsprings with different traits through sexual reproduction. Furthermore, overproduction of offsprings via sexual reproduction allows many offsprings with all sorts of different combinations of genes. Due to the limited amount of resources, the large number offsprings will struggle to survive, and only the offsprings with the best traits for survival and reproduction will pass on it’s genes. These four factors are all interconnected and essential in driving change within populations. These populations change in order to survive, thus certain traits are favored. There are 3 Types of Natural Selection that illustrate the different ways that a population can change.

1. Directional Selection – One extreme trait is favored over the other extreme trait.
2. Stabilizing Selection – An intermediate trait is favored over the two extremes.
3. Disruptive (Diversifying) Selection – The extreme traits are favored over the intermediate trait.

This site has great graphs that can help visualize the three types: http://www.tutorvista.com/biology/types-of-natural-selection.

As populations change, they can become so different that Population A becomes Population A1 and Population A2. Once these two populations are so dramatically different that they cannot reproduce together, they are classified as two completely different species. Reproductive Isolation essentially determines if a species has diverged or not. This is termed speciation or macroevolution. Unlike microevolution, macroevolution occurs after millions of generations. Furthermore, speciation is often triggered by geographic isolation.

There are 2 Types of Speciation:

1. Allopatric – Speciation is due to geographic seperation.
2. Sympatric – Speciation occurs in the same location, but for some reason, a species separates.

This site illustrates the difference between the two modes of speciation: http://scienceblogs.com/evolvingthoughts/2007/03/basic_concepts_allopatry_and_s.php.

*From Mr. W, here is the brief video example on allopatric speciation shown in class: