Natural and Artificial Selection in Populations
Biological Variation and Adaptations
Diversity of Species: Earth is home to thousands of bird species, each with unique adaptations for survival. * Steller’s Jay: Possesses beaks enabling them to crack hard seeds and nuts. * Belted Kingfisher: Adapted to catch and consume fish. * Dietary Variations: Birds have adapted to eat insects, rodents, small animals, seeds, and nectar.
Interplay of Organisms and Environment: There is a fundamental relationship between an organism’s adaptations (e.g., a bird’s beak) and its specific environment.
Key Biological Concepts: * DNA mutations generate genetic diversity in populations. * Natural selection favors traits that improve an organism's fit with its environment. * Natural selection is a driver in the formation of new species. * Mutations can be caused by environmental factors. * Artificial selection involves humans choosing traits to pass to subsequent generations.
Foundation of Genetic Variation and Mutations
Variation within Species: Even within the same species, individuals show variety (e.g., human classmates), which is driven by genes.
Mutation Definition: A mutation is a permanent change in the genetic material of an organism and serves as the primary source of new genetic variation.
Origin of Mutations: * Spontaneous: Occurs during DNA replication before cell division. * Environmental Agents: Triggers like ultraviolet (UV) radiation.
Outcomes of Mutations: * Harmful: Significant DNA alterations can cause cell death, malfunction, or uncontrolled multiplication. * Neutral/Beneficial: Many mutations have no effect, while some provide advantages.
Inheritance Patterns: * Somatic (Body) Cells: Mutations disappear when the organism dies and are not passed on. * Gametes (Egg/Sperm): Mutations in gametes can be inherited by offspring as new alleles, forming the starting point for genetic variation in populations. * Example: Kittens in a single litter (Figure 1.21) display different fur patterns and colors based on the combination of alleles inherited from their parents.
Natural Selection and Selective Advantage
Selective Advantage: A genetic advantage that improves an organism's chance of survival, typically within a changing environment. It allows an organism to survive and reproduce better than its competitors.
Natural Selection Definition: The process where a population's characteristics change over generations as individuals with heritable traits suited to local conditions survive and reproduce, passing on alleles.
Adaptation: A structural, behavioral, or physiological feature/process that assists an organism's survival and reproduction in a specific environment.
The Situational Nature of Selection: Natural selection has no internal "will" or "direction." A trait that is irrelevant today may become vital if the environment changes.
Case Study: Staphylococcus aureus: * Individual bacteria reproduce asexually and very rapidly. * Antibiotic Resistance: Random mutations may grant resistance to an antibiotic. When treated, only resistant individuals survive. * Survivors pass resistance to daughter cells, changing the whole population's resistance over time. Note: Individuals do not change; the population does.
Selective Pressure: The environment exerts pressure by "selecting for" certain traits and "against" others. * Abiotic Factors: Non-living conditions like sunlight levels. * Biotic Factors: Living factors like predators, parasites, and competition. * Forest Example: In dense forests, trees that can grow in the shade of taller trees (shade-tolerant) reproduce more successfully. If light levels suddenly increase, this once-advantageous trait may no longer be beneficial.
Speciation and Adaptive Radiation
Species Definition: A population/group in nature whose members can interbreed to produce fertile offspring.
Speciation: The formation of new species from existing ones. This occurs when members of a population change so much they can no longer produce fertile offspring with the original population.
Geographical Barriers: Speciation is often triggered by physical isolation, such as glaciers, lava flows, or changes in ocean levels (e.g., a peninsula becoming an island).
Adaptive Radiation: The diversification of a common ancestral species into a variety of differently adapted species.
Galapagos Islands Finches: * Finches reached the volcanic islands (likely blown off course) and found unoccupied ecosystems. * Subjected to different selective pressures, the ancestral species divided into populations that evolved distinct beak shapes and sizes based on food sources (e.g., seeds on lava beds vs. tree-dwelling insects).
Galapagos Tortoises: Diverse tortoise species arose as individuals found their way to different islands with varying environmental conditions.
Red Crossbill (Loxia curvirostra): * Features a twisted beak specialized for prying open conifer cones. * Scientists Anna Lindholm and Craig Benkman (1991) experimented by trimming beaks. Birds with clipped bills could not open closed cones but could still eat from open ones. * The specialized bill developed gradually through selective pressure across generations.
Environmental Factors and Mutagens
Mutagen: A substance or event that increases the rate of mutation.
Physical Mutagens: Cause physical changes to DNA (e.g., X-rays, UV radiation).
Chemical Mutagens: Molecules that react chemically with DNA (e.g., nitrites used in food preservation, gasoline fumes).
Carcinogen: A mutagen that causes cancer (e.g., UV radiation causing skin cancer, cigarette smoke/second-hand smoke causing lung cancer).
Impact on Proteins: Mutations change gene instructions, potentially disrupting the production of specific proteins.
Artificial Selection in Agriculture and Domestic Animals
Artificial Selection Definition: Selective pressure exerted by humans on populations to improve or modify desirable traits (also called selective breeding).
Historical Impact: * Wild Mustard Plant (Brassica oleracea): Over , humans began selecting for different traits, creating kale (leaves), broccoli (flower buds and stem), cabbage (terminal leaf bud), cauliflower (flower buds), Brussels sprouts (lateral leaf buds), and kohlrabi (stem). * Corn: Developed from a weed-like grass called teosinte.
Consequences of Artificial Selection: * Positive: Increased milk production in cows, faster-growing meat chickens, drought-resistant crops. * Negative: * English Bulldogs: Flat faces cause severe respiratory issues. * German Shepherds: Selective breeding for large size leads to hip dysplasia. * Genetic Diversity: Domestication reduces diversity.
Monoculture: The repeated planting of the same varieties of a species over large expanses of land. * Risk: Lack of genetic diversity means a single disease or environmental change can wipe out an entire crop.
Environmental Extinction and Selective Pressure
Extinction: Occurs when a species completely disappears from Earth.
Mass Extinction Events: Sharp declines in the number of species. Five major mass extinctions have occurred in the last . * $250 \text{ million years ago}$: Approximately of all species went extinct. * $65 \text{ million years ago}$ (Dinosaurs): Likely triggered by a large asteroid impact, causing forest fires and soot that blocked the sun for months, killing over half of marine species and many terrestrial families.
Diversity Trends: Despite mass extinctions, biological diversity has generally increased since .
Sustainability: Polycultures and Reverse Speciation
Reverse Speciation: A process where two species that previously diverged from a common ancestor become extinct and are replaced by a single hybrid species. * Enos Lake Sticklebacks (Vancouver Island): Two distinct species (one lakeshore insect-eater, one mid-lake zooplankton-eater) existed for thousands of years. * In the mid-1990s, the introduction of invasive crayfish (likely via human activity) changed nesting/breeding behaviors or forced territories to overlap. * The two species interbred, and by , the original species were extinct, replaced by a hybrid.
Sustainable Agriculture: Methods meeting present needs while enhancing land health for future generations.
Polyculture: Contains a diversity of crops grown on the same plot, mimicking natural ecosystems. * China Rice Study: Mixing rice varieties (some fungus-resistant) increased yields by and reduced fungal infection by . * Companion Planting ("Three Sisters"): Corn (structure), beans (nitrogen for soil), and squash (ground cover to retain moisture and prevent weeds).
Human-Driven "Sixth Extinction"
IPBES Report (March 2018): States that combating land degradation and restoring land is an urgent priority.
Modern Drivers: Habitat destruction and over-exploitation are driving many species toward a new mass extinction event caused by humans.
Quantitative Data and Measurements
Beak Depth in Geospiza fortis: In the Galapagos, beak depth (strength) fluctuated over 8 years. Drought years (e.g., years 1, 4, 6) favored birds with deeper beaks to crush larger, harder seeds, while wet years (e.g., year 8) provided abundant small seeds.
Height Distribution: Human height follows a bell-shaped curve in typical populations, ranging from roughly to , with most individuals clustered near the average height (e.g., around to ).
Species Identification: Leopard frogs, once thought to be a single species (Rana pipiens), are now recognized as at least eight distinct species identified via call analysis (e.g., Northern, Southern, Rio Grande, Plains, Relict, Florida, Ramsey Canyon, Lowland).
Questions & Discussion
Identifying Preconceptions: How are organisms, species, and populations related? * Discussion Point: Groups are composed of species, and populations are localized groups of those species.
Inferring Change (Peppered Moth): * Question: Why did moth color change in 19th-century England? * Answer: Soot from industrial pollution darkened trees, making light-colored moths vulnerable to predators. Dark-colored moths became predominant. When air cleaned up in the , light-colored moths became common again.
First Peoples Perspectives: * The original North American horse went extinct . * After the Spanish reintroduced horses in the , Indigenous groups utilized selective breeding to develop unique modern breeds.
Medical Connectivity: * Question: Why avoid antibacterial soaps? * Answer: Frequent use creates selective pressure, allowing antibiotic-resistant bacteria to survive and populate, rendering treatments less effective.