LESSON 1: THE PATTERNS OF EVOLUTION

There is perhaps no greater unifying concept in biology today than evolution. Understanding how evolution works not only allows us to accurately investigate and describe the history of life on earth, but also greatly aids in our understanding of how life and organisms work today.

In fact, it would be nearly impossible to find a discipline within biology that does not in some way utilize the concept of evolution. In the medical sciences, for example, researchers are constantly searching for new antibiotics to battle populations of bacteria that have evolved resistance to previous antibiotics. In agriculture, a similar struggle is occurring between scientists and herbicide- or pesticide-resistant populations of weeds and pest insects. Scientists charged with developing recovery plans for endangered species must incorporate evolutionary theory to insure the best chance for survival for their species of interest. Even those researchers involved in understanding and combating global warming must incorporate evolution into their predictions and recommendations.

In the following lessons, you will be introduced to the basic tenets of evolution - for example, what causes evolution, how evolution works, and how evolution relates to the diversity of life on our planet. In this final segment of Cells and the Evolution of Life, it is our hope that this information will both strengthen your understanding of the previous topics we have covered, and increase your understanding of the amazing life that we are a part of on this planet.

Learning Objectives

• Know who Charles Darwin was, and how two of his simple observations led to the development of the theory of evolution by natural selection.
• Be able to clearly define evolution.
• Understand how the link between environment and evolution.
• Understand how we can determine whether or not a population is evolving for a specific character.
• Be familiar with the different agents of evolution.
• Understand how one species may diverge into two species.
• Comprehend why genetic isolation is an essential part of speciation.

Topics covered in this Lesson

• Charles Darwin
• Evolution of Populations I
• Evolution of Populations II
• Speciation
• Intrinsic Mating Barriers

Charles Darwin

Charles Darwin is perhaps the most famous figure in biology. Darwin, of course, is best known for developing the theory of evolution by natural selection. The concept of natural selection, while first presented nearly 150 years ago, has stood the test of time and is today widely accepted as the most important mechanism of evolution.

Darwin based his theory of natural selection on two relatively simple observations:

• organisms could reproduce exponentially, but they do not

• offspring of all organisms resemble their parents, but they are not identical to them or to their siblings

How do these observations lead to natural selection, and hence to evolution? Listen in to find out…

Learning Objectives

• Be able to clearly define evolution.
• Understand how Darwin’s observations led him to develop the theory of evolution by natural selection.
• Understand stabilizing, directional and disruptive selection.
• Know that natural selection is a dynamic process, because most environments are dynamic by nature.

Evolution of Populations I

Evolution is defined as a change in the genetic constitution of a population. Most simply, then, evolution may occur in two ways: 1) by changing the frequency of the alleles of a gene or genes in a population, or 2) by changing the alleles themselves, for example by introducing new alleles into a population.

Scientists often investigate the process of evolution by comparing trends of actual populations to hypothetical, non-evolving populations. Populations (real or hypothetical) that are not evolving for a given character are said to be in Hardy-Weinberg equilibrium. Such populations exhibit no change in allele and genotype frequencies from generation to generation.

In this lesson you will learn how to calculate whether a population is in Hardy-Weinberg equilibrium, and what factors can disturb this equilibrium.

Learning Objectives

• Be able to calculate allele and genotype frequencies of a population.

• Understand the characteristics of Hardy-Weinberg populations. Are the evolving? Why or why not? Do allele frequencies change over time? Do genotype frequencies change over time? Why or why not?

• Understand the assumptions behind Hardy-Weinberg populations, and how breaking these assumptions leads to evolution of a population.

Evolution of Populations II

There are many ways by which populations may evolve, or by which the genetic makeup of populations may change. In fact, breaking any of the assumptions of Hardy-Weinberg equilibrium leads to evolution. For example, populations may evolve due to natural selection, or by migration of individuals between populations, or by mutation of the DNA of individuals within the population.

Learning Objectives

• Understand the definition of evolution.

• Be able to describe/define the following agents of evolutionary change and how they affect the genetic makeup of populations
  

  • gene flow
  • genetic drift
  • founder effects / bottlenecks
  • non-random mating
  • natural selection
  • gene flow
  • mutation
    

• Understand that most populations evolve due to a combination of factors.

Events Leading to Speciation

Speciation is not a black-and-white event. As we have seen already, the genetic make-up of populations is changing most of the time. When the genetic composition of a particular population gets to the point where it differs from that of another population so much that gene flow between the two populations ceases, we say that speciation has occurred. In this lesson we will look at several processes by which speciation can occur, along with several contemporary examples of speciation.

Learning Objectives

• Understand the general course of events involved in speciation.

• Know what allopatric, sympatric and parapatric speciation are and how they occur.

• Know what a ‘hybrid zone’ is.

• Understand how different factors may affect speciation rates.

Intrinsic Mating Barriers

In order for two populations to fully diverge into different species, gene flow between the populations must cease. In other words, there must be a barrier of some kind that prevents successful interbreeding between the two populations. There are two general types of such mating barriers – those that occur before a zygote is formed by the mating of members of different populations (prezygotic mechanisms) and those that occur after a zygote is formed (postzygotic mechanisms).

Learning Objectives

• Understand the significance of blocking gene flow between populations.
• Understand the difference between pre- and postzygotic mating barriers, and be able to give several examples of each.