Chapter 22 Notes

Chapter 22: Evolution and the Evidence for Natural Selection

I. Charles Darwin and His Many Influences

Charles Darwin was a 19th-century naturalist whose observations and ideas revolutionized biology. His theory of evolution by natural selection provided a scientific explanation for the diversity of life on Earth.

• Influences: Darwin was influenced by earlier scientists, naturalists, and his own observations during the voyage of the HMS Beagle.

• Key Concept: Darwin proposed that species change over time through a process called natural selection, where individuals with advantageous traits are more likely to survive and reproduce.

• Historical Context: Darwin’s work built upon ideas from geology (e.g., Charles Lyell), economics (e.g., Thomas Malthus), and earlier evolutionary thinkers (e.g., Jean-Baptiste Lamarck).

• Publication: His most famous work, On the Origin of Species (1859), laid the foundation for modern evolutionary biology.

II. Adaptations and Evolutionary Fitness

Adaptations are inherited characteristics that enhance an organism’s ability to survive and reproduce in a specific environment. Evolutionary fitness refers to an organism’s success in passing its genes to the next generation.

• Adaptation: A trait that increases an organism’s fitness in its environment.

• Evolutionary Fitness: Measured by the number of viable offspring an individual contributes to the next generation.

• Example – Ear Size: The arctic hare has small ears to minimize heat loss in cold environments, while the black-tailed jackrabbit has large ears to dissipate heat in hot climates.

• Example – Body Size: The fennec fox (small, large ears) is adapted to desert heat, while the arctic fox (compact, small ears) is adapted to cold environments.

Additional info: These examples illustrate Allen’s Rule, which states that endothermic animals in colder climates tend to have shorter extremities to reduce heat loss.

III. Natural Selection

A. Key Features of Natural Selection

Natural selection is the process by which individuals with certain heritable traits survive and reproduce more successfully than others, leading to evolutionary change in populations over generations.

• Observations:

  1. Populations have the potential to increase exponentially if all offspring survive.

  2. Populations tend to remain stable in size over time.

  3. Resources are limited.

  4. Individuals in a population vary in their traits.

  5. Much of this variation is heritable.

• Inferences:

  1. There is a struggle for existence among individuals.

  2. Individuals with advantageous traits are more likely to survive and reproduce (differential survival and reproduction).

  3. Favorable traits accumulate in the population over generations.

• Central Points:

  • Natural selection acts on individuals, but only populations evolve.

  • Evolution is not goal-oriented; it is a response to environmental pressures acting on random variation.

• Example: Seahorses with seaweed-like appendages blend into their environment, avoiding predation and increasing their chances of survival and reproduction. Over generations, this trait becomes more common.

Additional info: It is important to note that traits do not arise because of a need; rather, random mutations that confer an advantage are naturally selected.

B. Types of Natural Selection

Natural selection can take different forms, depending on environmental conditions and the distribution of traits in a population.

• Stabilizing Selection: Selects against extreme phenotypes, favoring the average. Example: Human birth weight.

• Directional Selection: Favors one extreme phenotype over others, often in response to a changing environment. Example: Insect resistance to pesticides.

• Disruptive (Diversifying) Selection: Favors individuals at both extremes of the phenotypic range, potentially leading to polymorphism. Example: Beak size in birds where both large and small seeds are available, but not medium-sized seeds.

IV. Evidence Supporting Evolution

Multiple independent lines of evidence support the theory of evolution, demonstrating the relatedness of all life forms.

• Homologous Structures: Anatomical features in different species that are similar due to shared ancestry (e.g., forelimbs of mammals).

• Vestigial Structures: Remnants of features that served important functions in ancestors (e.g., human appendix).

• Fossil Record: Provides chronological evidence of past life and evolutionary transitions.

• Embryology: Similarities in embryonic development among different species suggest common ancestry.

• Biochemical Data: Comparison of amino acid or nucleotide sequences (e.g., cytochrome c) reveals evolutionary relationships.

Biochemical Evidence: Cytochrome c Comparison

Cytochrome c is a protein found in all aerobic eukaryotes, playing a key role in cellular respiration. Comparing its amino acid sequence across species can reveal evolutionary relationships.

Speci	Cytochrome c Sequence (positions 1-16)
Human	Gly Asp Val Glu Lys Gly Lys Lys Ile Phe Ile Met Lys Cys Ser
Pig	Gly Asp Val Glu Lys Gly Lys Lys Ile Phe Ile Met Lys Cys Ser
Chicken	Gly Asp Val Glu Lys Gly Lys Lys Ile Phe Ile Met Lys ys Ser
Rhesus Monkey

Key Points:

• Humans and rhesus monkeys differ by only 1 amino acid in cytochrome c, while humans and chickens differ by more.

• Greater similarity in amino acid sequence indicates a more recent common ancestor.

• These data support the inference that humans are more closely related to monkeys than to chickens.

Additional info: The entire cytochrome c protein is 104 amino acids long; the more similar the sequence, the closer the evolutionary relationship.

Other Molecular and Biochemical Evidence

• DNA Hybridization: Measures the similarity between DNA sequences of different species by forming hybrid DNA molecules and determining the temperature required to separate them.

• Immunological Studies: Use antibodies to detect similarities in blood proteins between species; greater reactivity indicates closer evolutionary relationships.

• Mitochondrial DNA (mtDNA): Inherited maternally and mutates at a predictable rate, useful for tracing recent evolutionary events and relationships within species.

• Fibrinopeptide Analysis: Compares rapidly evolving protein segments to resolve relationships among closely related species.

V. Evolution of Complex Structures

Complex structures, such as the vertebrate eye, can evolve through a series of small, incremental changes, each conferring a selective advantage. This process is supported by evidence from comparative anatomy, embryology, and genetics.

• Key Point: Even highly complex organs can arise through natural selection acting on simple, functional intermediates.

• Example: The evolution of the eye can be traced through a series of increasingly complex light-sensitive structures in different organisms.