Chapter 12: The Forces of Evolutionary Change – Study Notes

Chapter 12: The Forces of Evolutionary Change

What is Evolution?

Evolution is the process that explains the features of all organisms, from microbes to humans. It is a central concept in biology that accounts for the diversity of life on Earth.

• Definition: Evolution is genetic change in a population over time.

• It occurs when there are changes in heritable traits from generation to generation.

• Some changes can lead to the development of new species.

• Evolution explains both the unity and diversity of life.

Evolution Occurs in Populations

Evolution does not occur in individuals, since an individual's alleles do not change. Instead, evolution is observed as changes in allele frequencies within populations over generations.

• Allele Frequency: The proportion of a specific allele among all alleles for a gene in a population.

• Allele frequencies can increase or decrease due to evolutionary forces.

Calculating Allele Frequencies

Allele frequencies can vary between populations. For example, the frequency of alleles that produce black hair is high in Asian populations and low in Swedish populations.

• Equation for Allele Frequency:

$\text{Allele frequency} = \frac{\text{Number of copies of an allele}}{\text{Total number of alleles for the same gene in the population}}$

Gene Pools and Evolution

The gene pool is the collection of all alleles for all genes in a population. Evolution is detectable by examining changes in the gene pool over time.

• If populations interbreed, allele frequencies in the gene pool can change, indicating evolution has occurred.

Key Concept Check: Who Can Evolve?

• Only populations (e.g., a group of fir trees in Oregon) can evolve, not individuals, fossils, or non-living things.

How Allele Frequencies Determine Population Characteristics

• The combination and frequency of alleles for each gene determine the traits observed in a population.

• Changes in allele frequencies can alter the population's characteristics over time.

Historical Development of Evolutionary Theory

Contributions of Scientists

• Uniformitarianism: Changes in nature are gradual (Hutton, Lyell).

• Catastrophism: Brief, violent events produce changes in nature (Cuvier).

• Lamarck proposed testable ideas about how species change.

• Lyell argued that Earth must be very old, since natural processes occur slowly.

Fossil Evidence

• Fossils in old rock layers suggest that living organisms evolved from common ancestors.

• Cuvier's principle of superposition: Older layers contain more ancient species.

Darwin and Wallace

• Darwin's voyage to South America and the Galápagos Islands provided evidence for evolution.

• He observed that finch beak types matched their food sources, leading to the idea of "descent with modification." All finches descended from a common ancestor.

Natural Selection

• Darwin proposed that the environment influences which individuals survive and reproduce.

• Individuals with traits best suited to their environment are more likely to survive and reproduce ("survival of the fittest").

Table: The Logic of Natural Selection

Observation	Inference
Genetic variation	Individuals compete for resources (struggle for existence)
Limited resources	Some individuals are more likely to survive and reproduce (unequal reproductive success)
Overproduction of offspring	Over generations, populations change (descent with modification)

Artificial Selection

• Humans select for desired traits in plants and animals, increasing the frequency of those alleles.

• Artificial selection can also increase the frequency of undesired alleles (e.g., hereditary diseases in purebred dogs).

Support for Evolutionary Theory

• Extensive research and data have corroborated and expanded Darwin's findings.

Mechanisms and Outcomes of Evolution

Natural Selection Molds Evolution

• Natural selection does not create alleles; it selects for alleles that arise by chance.

• Genetic variation is necessary for natural selection to act.

• Adaptations are heritable traits that enhance survival and reproduction.

Examples of Natural Selection

• Bacteria resistant to antibiotics survive and reproduce, increasing the frequency of resistance alleles.

• Porcupinefish inflate and have spines—adaptations that increase survival.

Evolution is Continuous and Has No Goal

• As environments change, the traits favored by natural selection also change.

• Evolution does not have foresight or a predetermined goal.

• Fitness is defined as an organism's genetic contribution to the next generation.

Common Misconceptions About Evolution

Misconception	Biologist's Explanation
Biological evolution explains the origin of life	Evolution did not begin until life existed
Evolution is a random process	Some mechanisms are random, but natural selection is not
All individuals develop beneficial adaptations simultaneously	Adaptations become fixed over generations as individuals with beneficial traits survive and reproduce

Fitness and Reproductive Success

• Fitness includes surviving, reproducing, and having offspring that survive to reproduce.

• Even if an organism does not survive, its alleles may persist if passed on to offspring.