Evolutionary Change and Natural Selection – Introduction

Evolution is descent with modification and includes small-scale evolutionary change (microevolution) as measured by changes in gene frequency in a population from one generation to the next and large-scale evolutionary (macroevolution) change as evidenced by speciation events. Several mechanisms contribute to evolutionary change, such as natural selection, a process in which individuals with certain beneficial traits are more likely to survive and reproduce. Natural selection is also responsible for the loss of lethal or detrimental traits from a population. It is important to keep in mind that evolution does not act on individuals; it acts on populations. Natural selection, however, does act on individuals within a population and can result in evolutionary change of that population over time.

Populations do not always change due to Natural Selection, since there are several conditions that must be met in order for Natural selection to occur:

• Individuals within a population must vary; they do all exhibit identical traits.
• Some traits are “better” than others or “worse” than others.
• The traits that vary are heritable and not simply acquired.
• The “better” individuals have more success reproducing and produce more offspring.

IF these conditions are met, then in successive generations, more offspring will exhibit the beneficial trait (or conversely, a detrimental trait will be lost).

Recall that DNA contains sequences that code for particular proteins or traits, these sequences are called genes. The alternate forms of genes are called alleles and these alleles exist in pairs because chromosomes exist in pairs. A dominant allele is one that masks another (recessive) allele; dominant does NOT mean a given allele is more frequent or necessarily better. A recessive allele is one that requires two copies to be expressed. Evolutionary biologists are interested in the frequency of alleles within a population and how they change over time (= microevolution).

Be sure that you read through our Unit 7 online lecture this unit on Evolution as well as your text book readings. Open the Unit 7 Experiment Answer Sheet and work through the first two exercises.

Mechanisms of Evolutionary Change – Introduction

There are other mechanisms besides Natural Selection that can lead to evolutionary change. These include:

• Genetic drift (random changes in the gene pool)
• Mutations
• Gene flow (e.g., immigration and emigration)
• Nonrandom mating (e.g., inbreeding, sexual selection, assortative mating)

Be sure to review our online lecture on Evolution and pp 260-264 in your book before starting this exercise. You will be using the following website for this exercise. Be sure you are able to open it and use it:

BioMan Biology. No date. Biology Games and Virtual Labs: Evolution
http://biomanbio.com/GamesandLabs/EvoClassGames/aaevo.html  (Links to an external site.)

When you are ready, open your Unit 7 Experiment Answer Sheet and complete Exercise 3: Mechanisms of Evolutionary Change.