Introduction to evolution

Biology 391 (Organic Evolution) Lecture: Introduction to evolution and natural selection

NOTE: These are lecture notes for Biology 391, Organic Evolution, at The University of Tennesee at Martin. Anyone outside of UT Martin wishing to use these notes or to contact me for additional information should first read the information obtained by clicking here.

Goals: The goals of this lecture are to give a general overview of evolution and introduce natural selection, one of the main ways evolution occurs.

Related Textbook Material: Freeman and Herron (2001) Chapter 1

Lab Manual Questions over this material are in Lab Manual Chapters I and II

The Lecture:

Organic evolution is defined as change in genetics of a population over time (generations)

A population refers to all individuals of the same species living in a defined area at the same time.

Organic evolution can be studied at two different levels:

1. microevolution, which refers to small-scale genetic changes within populations

2. macroevolution, which refers to the large-scale results of genetic changes in populations, including the formation of new species and the evolution of large scale trends seen across species in what traits they have.

INTRODUCTION TO MICROEVOLUTION:

Microevolution occurs through several mechanisms. The first of these that was discovered, and the form which is best known, is natural selection. It was discovered independently by Charles Darwin and Alfred Russel Wallace.

Natural selection is evolution that occurs because individuals with some traits survive and reproduce better than do individuals with other traits. As a result, those traits that result in high survival and reproduction are passed from generation to generation, through reproduction, at a higher rate than are other traits. These traits become increasingly more and more common in populations.

Fitness refers to the degree to which individuals with certain traits are expected, on average, to survive and reproduce. Natural selection can also be said to occur because of difference in fitness within a population.

Adaptation refers either to the process of natural selection, or to a trait that has evolved through natural selection.

There are four properties of a population (your textbook calls these "Darwin's four postulates") that, together, result in natural selection. These are:

More young are produced each generation than can survive to reproduce. This is generally observed in species; many of the offspring born to any generation die before reproduction.
Individuals in a population vary in their characteristics. This is also generally observed in species; individuals are not identical to one another.
The differences among individuals are based on genetic differences. We know the genetic basis for many traits in natural populations and often have observed that the differences among individuals are present because of genetic differences.
Individuals with some characteristics survive and reproduce better (have higher fitness) than do individuals with other characteristics. This has now been observed in hundreds of populations.

If these four postulates are all true of a population, they result in natural selection; those individuals with higher fitness end up being the ones who survive each generation, and as a result reproduce more (you don't reproduce when you're dead); since the traits are genetic, they get passed on to the next generation, and therefore become more common than they were in the previous generation.

The peppered moth: One of the earliest examples of natural selection to be described occurred in the peppered moth. Click here to study this example; it shows how the four postulates work, and how natural selection occurs.

HIV evolution: Chapter 1 of your textbook describes evolution in HIV. The example of AZT resistance in HIV is an example of natural selection. It also illustrates how the four postulates work, and how natural selection occurs. Read this section in your textbook for another example of natural selection.

Make sure you understand these examples (the peppered moth, and AZT resistance in HIV) before you proceed further; you need to understand how natural selection works in real situations to understand the next material. At this point, you should also be able to answer the questions in Chapter II of your lab manual; try some of these questions (especially question 3) before proceeding further.

INTRODUCTION TO MACROEVOLUTION:

Evolution within populations (microevolution) can occur through several other mechanisms which we will consider later in the course. Now we will look briefly at some of the main forms of macroevolution and the patterns it produces.

Speciation refers to the formation of new species; it occurs when one ancestral species evolves into more than one (typically two) descendant species.

Species are typically defined as groups of organisms that are so similar to each other that they can reproduce and produce healthy fertile offspring (later in the term we will consider other ways of defining species, but this definition is good for now.) The idea is that if organisms belong to the same species then they can reproduce and their offspring can also survive and reproduce. If organisms belong to different species, they can't reproduce with each other or, if they do, their offspring die or are sterile.

Since speciation occurs when one species evolves into more than one new species, it increases the number of species that exist. Note that evolution does not necessarily cause speciation -- for example, peppered moths evolved from gray to black but no new species were formed.

Phylogeny refers to the evolutionary history of species, or history of speciation; it refers to the family tree of all life, indicating how all living things are related. It is typically diagrammed as a tree.

The process of speciation, and how it leads to phylogenetic history, which can be diagrammed as a tree, is illustrated here:

Phylogenies can be drawn in several different ways. Some of these are illustrated here:

We will use phylogenies, drawn as trees, extensively in this course. At this point, you should study the phylogeny of HIV and related immunodeficiency viruses in Chapter 1 in your textbook and make sure you understand what such a diagram indicates about the history of life.

Study Tips:

practice applying Darwin's four postulates to real situations (as in qu. 3 in ch. II of the lab manual)
start being very careful in how you use the words "individual", "population", and "species." They have different meanings, and if you use the wrong one by mistake you say something very different from what you mean (and will lose points on homework, exams, and papers as a result.)
take some phylogenies (from this page, lecture notes, or your textbook) and practice drawing the same phylogeny in different ways so that it becomes obvious to you when two drawings represent the same phylogeny.

Return to index of lectures