Evolution Notes
Evolution (Natural Selection) Unit
Evolution is a central, unifying theory in biology.
Supported by substantial evidence, evolution is no longer a hypothesis.
The History of Evolution
Plato & Aristotle
Believed species were fixed and complexity-based arrangement possible.
Carolus Linnaeus
Developed Binomial Nomenclature in the 18th century for classifying organisms.
Each organism has a two-part name (e.g., Homo sapiens).
Typically in Latin.
Genus (capitalized) + species (lowercase).
Italicized (typed) or underlined (handwritten).
George Cuvier
Explained fossil record changes (18th century) via catastrophes causing extinctions.
Repopulation from other regions led to perceived change over time.
Pre-Darwin, the world was considered young, and species were thought not to change.
Jean-Baptiste Lamarck
Proposed descent with modification; organisms adapt to environments.
Inheritance of Acquired Characteristics: Organisms adapt during their lifetime and pass traits to offspring.
Law of Use and Disuse: Giraffe necks lengthened as generations reached for higher leaves.
Charles Darwin
Too sensitive for medical career; studied biology and geology with Reverend John Henslow.
In 1831, at 22, accepted naturalist position on HMS Beagle for a 5-year voyage.
Darwin read Charles Lyell’s Principles of Geology.
Stated geological changes result from slow, continuous processes.
Observed earthquakes raising the Argentinian coast and marine shells inland in the Andes.
Voyage observations led to the idea of slow species change over time.
Compared organisms in South America and Galapagos Islands.
Adaptations to the environment cause diversification and new species.
Galapagos Islands: Volcanic islands off South America.
Island species varied from mainland and island to island.
Tortoises: Neck length varied by island vegetation; shell shapes varied due to diet.
Finches: Resembled mainland finch but had more types/varieties.
Bill shapes adapted to different food gathering means.
Varied by nesting sites, beak sizes, and eating habits.
Natural Selection - Darwin’s Theory of Evolution
Natural Selection: Individuals with variations suited to their environment survive and reproduce more.
Evidence from biogeography, genetics, and anatomy supports Darwin’s ideas.
Also known as “survival of the fittest”.
Populations evolve, not individuals.
Proposed by Alfred Russell Wallace and Charles Darwin.
Both read Thomas Malthus’ essay stating human populations outgrow resources, causing a struggle.
Adaptation: A trait that suits an organism to its environment (e.g., camouflage).
Darwin believed adaptations develop over time.
Fitness: An organism’s reproductive success.
Environment selects for fittest organisms, leading to population adaptation.
Fitness is determined by the environment.
Extinction: Occurs when adaptations are no longer suitable due to environmental change.
Variations: Exist among population members, mainly due to inheritance.
Adaptation-enabling variations are passed down.
Overproduction: More individuals are produced than can survive/reproduce.
Adaptations: Better-adapted individuals survive/reproduce.
Causes gradual changes in populations with favorable characteristics accumulating.
Struggle for existence: Members compete for food, shelter, mates, etc.
Conditions for Natural Selection
Darwin’s Book – Origin of Species
Darwin waited over 20 years to publish after returning to England in 1836.
Published Origin of Species in 1859 after reading Alfred Russell Wallace’s similar hypothesis after observations in Southeast Asia.
Both concluded life forms descend from a common ancestor.
Agreed natural selection is the mechanism for species change and new species.
Natural Selection versus Artificial Selection
Humans can create substantial change quickly; nature does so over a long time.
Natural Selection: Nature decides; individuals with best-suited traits survive and reproduce; example: Finch beaks.
Artificial Selection: "Man" decides; selective breeding; can lead to inbreeding; example: Dalmatians.
Five Types of Evidence for Evolution
Biogeography
Physical factors like continent location determine population spread.
Study of geographic distribution of life forms.
Example: Marsupials diversified in Australia after it separated, while placental mammals arose later in other areas.
Fossils
Relics or impressions of ancient organisms (most over 10,000 years old).
Include skeletons, shells, insects in amber, imprints, frozen organisms (wooly mammoth), or organisms trapped in tar (saber-toothed tiger).
Found in layers (strata) of sedimentary rock.
Radioactive dating estimates Earth is about years old.
Transitional forms link groups of organisms (e.g., therapsids - mammal-like reptiles; pterosaurs - bird-like reptiles).
Comparative Anatomy
Closely related organisms have anatomical similarities due to common descent.
Homologous Structures: Similar anatomical structures with different purposes; indicate evolutionary origin (e.g., whale flipper, human arm, bird wing).
Analogous Structures: Structures with common function but not structure; different evolutionary origins (e.g., wings of birds, bees, and bats).
Vestigial Structures: Inherited anatomical structures that lost original function (e.g., dolphin/python hip/leg bones).
Comparative Embryology
Vertebrates share post-anal tail and pharyngeal pouches during development.
Similar embryonic development indicates common ancestry.
Genetics and Molecular Biology
Living organisms use basic biochemical molecules (DNA, RNA, ATP, enzymes).
Similarities in amino acid sequences and DNA codes indicate common descent.
Genetic code is nearly identical across organisms, suggesting evolution from common ancestors sharing this code.
Example: Genetic difference between humans and chimpanzees is minimal (1.2%).
Types of Evolution
Microevolution: Evolution within a species due to genetic variation and struggle to survive/reproduce. Example: Camouflaged mice survive/reproduce more, increasing camouflage over time.
Macroevolution: Evolution between different species, focusing on how diverse groups change. Example: Dolphins (mammals) and sharks (fish) have streamlined bodies.
Types of Selection
Natural Selection
Stabilizing Selection: Favors average phenotypes (e.g., human birth weight between 6-8 lbs).
Directional Selection: Favors extreme forms of a trait (e.g., dark peppered moths in polluted areas).
Disruptive Selection: Favors both extreme phenotypes; limited medium phenotypes (e.g., bird beaks for small/large seeds).
Sexual Selection: Females select mates based on traits/courtship (e.g., lion mane, peacock feathers).
Artificial Selection
Selective Breeding: Humans breed species to increase desirable traits (e.g., hypoallergenic dogs).
Patterns of Evolution
Divergent Evolution
One species gradually develops into two, becoming increasingly different and adapted to unique environments (e.g., woolly mammoth vs. modern elephant).
Convergent Evolution
Two different species share analogous traits due to shared environmental pressures (e.g., whales and fish having similar body shapes).
Coevolution
Two separate species evolve independently but in step, often with coexisting or symbiotic species (e.g., sugar gliders in Australia and flying squirrels in North America).
Genetic Drift
Natural selection isn't the sole source of evolutionary change.
In small populations, traits may become more common by chance.
Genetic Drift: Change in gene frequency due to random chance.
Types of Genetic Drift
Bottleneck Effect: Genetic drift due to drastic population reduction, often by disaster/disease; smaller gene pool differs from original, reducing diversity (e.g., Northern Elephant Seals).
Founder Effect: Few individuals colonize a new habitat, carrying different alleles; new gene pool starts with different frequencies, reducing diversity (e.g., Amish with polydactyly).
Speciation
Process of forming new species from an existing one.
Requires population isolation from the original group.
Reproductive isolation occurs when two populations no longer breed.
Reproductive Isolation
Occurs when two populations no longer interbreed, leading to separate species.
Over time, differences prevent mating and fertile offspring, even if isolation ends.
Types of Reproductive Isolation
Geographic Isolation: Populations separated by physical barriers like rivers/mountains (e.g., Grand Canyon squirrels).
Behavioral Isolation: Populations isolated by differences in courtship/behavior (e.g., different mating rituals in birds).
Temporal Isolation: Populations isolated by reproduction at different times (e.g., mayflies emerging at different times).