The Four Forces of Evolution

Chapter 2: Biology and Evolution

Learning Objectives

By the end of this chapter, having fully read it and reviewed the central themes, students should be able to:

1. Describe Darwin’s theory of natural selection.

2. List and discuss the four main evolutionary forces that are responsible for the vast biological variation present in human and non-human populations.

3. Explain the differences between Creation stories and evolution.

4. Trace the history of species classification, from the Great Chain of Being to the modern taxonomic system.

5. Compare the similarities and differences between Linnaeus’ classification system and contemporary taxonomic systems.

6. Differentiate analogies and homologies.

7. Explain how European exploration and Europe’s industrial revolution were key factors in the emergence of evolutionary theory.

8. Trace out the history of Charles Darwin and how he formulated his theory of natural selection.

9. Understand the importance behind Gregor Mendel’s experiments with pea plants.

10. List and discuss the main processes and components that explain heredity.

a. Genes

b. Law of Segregation

c. Independent Assortment

d. Chromosomes

e. DNA

f. Alleles

g. Mitosis

h. Meiosis

i. Homozygous and Heterozygous

j. Dominant and Recessive

k. Genotypes and Phenotypes

11. Explain how population genetics work and their role in evolution.

a. Mutation

b. Genetic drift

c. Founder’s effect

d. Gene flow

e. Natural selection

f. Adaptation

12. Describe the importance of sickle-cell anemia in understanding how evolution works and how traits are context dependent.

13. Describe the role of clines in understanding how and why traits are spread across geographic space.

14. Explain the processes behind macroevolution and speciation, including how speciation may occur and at what rate.

Detailed Chapter Outline

I. CHALLENGE ISSUE AND STUDENT LEARNING OBJECTIVES

II. EVOLUTION AND CREATION STORIES

A. A common part of all cultures is a mythology that explains how humans came about on earth

a. Mythologies and creation stories differ from evolution, which relies upon scientific language

b. Creation stories depict the relationship between humans and the rest of the natural world

B. Evolution is the scientific way to explain how species appear and change over time

a. Evolution is based on testable ideas, which separates it from creation stories

b. Evolution is not ‘just’ a theory

c. Evidence for evolution is so overwhelming that most scientists accept it as fact

d. At the same time historical and cultural processes contribute to scientific thought

III. THE CLASSIFICATION OF LIVING THINGS

A. How humans have classified the world and life forms have changed over time and continue to exhibit variation

a. Early classification systems, such as the Great Chain of Being, were challenged based on early European exploration

B. Avoiding the biases of the Great Chain of Being, Carolus Linnaeus developed the Systema Naturae classification system

a. Linnaeus’ classification system was based on:

i. Body structure

ii. Body function

iii. Sequence of bodily growth

C. Modern taxonomic systems retain Linnaeus’ classification structure; however, they are also based on protein structure and genetic material

D. Modern taxonomic systems are also based on cross-species comparisons, which enable the examination of analogies and homologies

IV. THE DISCOVERY OF EVOLUTION

A. European exploration and European industrialization challenged previously held ideas on species “fixity” or that species did not change over time

B. Ideas such as “catastrophism” were invoked to help explain the disappearance of certain species, while still being faithful to religious doctrine

C. Charles Lyell proposed a non-religious theory to account for variations in the earth’s surface known as uniformitarianism

D. Charles Darwin, however, formulated the theory of natural selection “that has withstood the test of time”

a. Darwin’s theory was influenced by the works of Erasmus Darwin and Thomas Malthus

b. Darwin’s theory of natural selection culminated in his seminal text On the Origin of Species (1859)

E. While Darwin provided the idea behind how species change over time or evolve, parts of his theory were incomplete

a. Two questions haunted Darwin’s theory:

i. “How did the variation arise in the first place?”

ii. “What was the mechanism of heredity by which variable traits could be passed from one generation to the next?”

F. Contemporary and historical advancements in heredity, molecular genetics, cellular biology, and population genetics provide support for Darwin’s evolutionary theory

V. HEREDITY

A. The work of Gregor Mendel would eventually advance the field of cellular biology and molecular genetics

B. Gregor Mendel was instrumental in explaining:

a. Inheritance as particulate rather than blending

b. Law of Segregation

c. Independent Assortment

C. Other components of contemporary/historical genetic research that support Darwin’s theory of natural selection are:

a. Chromosomes

b. Deoxyribonucleic acid (DNA)

c. Genes and alleles

d. Cell division

i. mitosis

ii. meiosis

e. Heterozygous and homozygous

f. Genotype and phenotype

g. Dominance and recessiveness

h. Polygenetic inheritance

VI. EVOLUTION, INDIVIDUALS, AND POPULATIONS

A. At individual levels, the study of genetics illustrates how parents pass traits on to their offspring

B. At the level of the group or population, the study of genetics reveals evolutionary processes and how they affect the levels and patterns of biodiversity on earth

C. Four evolutionary forces active at the population level are:

a. Mutation

i. They are the ultimate source of new genetic material due to the constant induction of new genetic material

b. Genetic drift

i. A particular type of genetic drift is the “Founder’s Effect”

ii. This was likely to have been significant in human evolution given small population sizes

c. Gene flow

d. Natural selection

i. Adaptation in natural selection is the collection of beneficial adjustments that a population makes to the environment

ii. It can only act on pre-existing genetic variation

iii. It is different from the concept of design seen in religious theories because it works with existing store of genetic variation; it cannot create something entirely new

D. A well-studied case of human adaptation to particular environment types is sickle-cell anemia

a. The presence of high rates of sickle-cell anemia correlate with world regions that have high rates of malaria

b. In these malarial environments sickle-cell was an adaptive and beneficial genetic trait

c. In world regions without high rates of malaria, sickle-cell anemia became a disadvantageous trait

VII. ADAPTATION AND PHYSICAL VARIATION

A. Biological diversity is often studied and mapped out in clines

B. This approach of analyzing the spatial distribution of certain human traits enables anthropologists to identify the correlation between the trait’s adaptive function and its relationship to the environment

a. For example, people in cold climates tend to exhibit greater body bulk and people in hot climates tend to be tall and slender

VIII. MACROEVOLUTION AND THE PROCESS OF SPECIATION

A. Microevolution

a. Is the change in a population’s allele frequency

B. Macroevolution

a. Is larger in scale and focuses on the process of speciation, or the formation of new species

C. The micro-evolutionary forces of gene flow, genetic drift, mutation, and natural selection ultimately may lead to speciation, especially when combined with isolating mechanisms

D. Speciation may occur in two branching modes:

a. Cladogenesis

b. Anagensis

E. Speciation may also occur at various speeds:

a. Darwinian gradualism

b. Punctuated equilibria

IX. CHAPTER CHECKLIST AND QUESTIONS FOR REFLECTION

Key Terms

Adaptation

Alleles

Anagenesis

Analogies

Chromatid

Chromosomes

Cladogenesis

Clines

DNA (deoxyribonucleic acid)

Dominant

Evolution

Founder Effects

Gene Flow

Gene Pool

Genus, Genera

Genes

Genetic Drift

Genome

Genotype

Hemoglobin

Heterozygous

Homologies

Homozygous

Law of Independent Assortment

Law of Segregation

Macroevolution

Mammals

Meiosis

Mitosis

Mutation

Natural Selection

Phenotype

Polygenetic Inheritance

Population

Primates

Punctuated Equilibria

Recessive

Sickle-cell Anemia

Speciation

Species

Taxonomy