UNIT 2 SG CH 3

Natural Selection

Process where individuals with traits that improve survival/reproduction leave more offspring.

Directional selection

is a mode of natural selection where environmental conditions favor individuals at one extreme end of a trait's spectrum. Over time, this causes the entire population’s average phenotype (observable physical trait) to shift toward that extreme, while the opposite extreme and intermediate traits are selected against.

• A population already possesses genetic variation for a trait (e.g., varying levels of drug resistance or varying body sizes).

• Selective Pressure: A change in the environment introduces a specific challenge (e.g., a drug, a drought, or a predator).

• Differential Survival & Reproduction: Individuals with the trait at one extreme survive better and pass those advantageous alleles to their offspring.

• Population Shift: Over generations, the favored extreme becomes the new norm for the population.

In a normal bacterial population, most individuals are sensitive to antibiotics, but a few rare individuals carry a random mutation granting resistance. When an antibiotic is introduced, it kills the sensitive majority. The resistant bacteria survive, reproduce, and pass on their resistance genes. Over generations, the population shifts toward the extreme of total resistance.

One extreme trait

Slides the bell curve toward one side

Stabilizing selection

Stabilizing selection is a mode of natural selection where environmental conditions favor the intermediate (average) traits and select against both extreme ends of the spectrum.

Visually, if you picture a bell curve, stabilizing selection squeezes the curve from both sides toward the middle, making the peak taller and narrower while reducing genetic variation.

1. Differential Survival & Reproduction: Individuals with average, intermediate traits survive best and pass their genes on to the next generation.

2. Population Shift: Over generations, the population remains centered around the optimal average, while extreme phenotypes become increasingly rare.

• Human Birth Weight: Infants born with very low birth weights struggle to regulate body temperature and face higher risks of infection or organ complications. Conversely, infants born with very high birth weights can cause severe birth complications for the mother and themselves during delivery. As a result, babies born in the average weight range (roughly 5.5 to 8.5 lbs) have the highest survival rates.

Disruptive selection

Disruptive selection (also called diversifying selection) is a mode of natural selection where environmental conditions favor individuals at both extreme ends of a trait's spectrum while selecting against the intermediate (average) traits.

Visually, if you picture a single bell curve, disruptive selection dips the center of the curve downward and pushes the two ends upward, creating a double-peaked (bimodal) curve. Over time, this strong division can split a single species into two distinct populations, which can lead to speciation.

Mutations

Random changes in DNA; source of genetic variation.

Speciation

formation of new species.

Allopatric

geographic isolation.

Allopatric speciation (from the Greek allo meaning "other" and patris meaning "fatherland") occurs when a physical, geographical barrier physically splits a single population into two or more isolated groups.

Because the groups are separated, gene flow stops completely. Over time, each isolated population adapts independently to its unique environment through natural selection, genetic drift, and mutation until they can no longer interbreed—even if brought back together.

Sympatric

occurs when a new species evolves from a single ancestral species without any physical or geographic barrier.

Because the populations live in the exact same location, reproductive isolation is driven instead by genetic mutations, behavioral shifts, or ecological preferences that stop interbreeding and halt gene flow.

Divergent evolution

Occurs when two or more species that share a common ancestor accumulate genetic differences over time and evolve different traits, often adapting to different environments, niches, or survival challenges.

Key Features

• Common Ancestry: Starts with a single original species or ancestral population.

• Homologous Structures: Produces physical body parts that share a similar underlying anatomical structure or origin, even if they now perform entirely different functions.

• Driver: Driven by different selective pressures, such as new food sources, distinct habitats, or new environmental challenges.

Convergent evolution

Occurs when unrelated or distantly related species independently evolve remarkably similar traits or adaptations because they face similar environmental pressures, challenges, or ecological niches.

Instead of sharing a recent common ancestor, these species start from entirely different evolutionary backgrounds and "converge" on the same functional solution.

• No Recent Common Ancestor: The species do not inherit the similar trait from a shared ancestor.

• Analogous Structures: Produces body parts that serve the same function and look similar outwardly, but have different internal anatomical structures and developmental origins.

• Driver: Driven by similar environmental demands (e.g., needing to swim efficiently in water or fly through the air).

Mutation

• Mechanism: A random change in an organism's DNA sequence caused by copying errors during cell division or environmental factors like radiation/chemicals.

• Role in Evolution: Mutations are the ultimate source of all new genetic variation. They occur purely at random regarding whether they will be helpful, harmful, or neutral to an organism's survival.

Genetic Drift

• Mechanism: Fluctuations in gene (allele) frequencies from one generation to the next caused entirely by chance, rather than natural selection.

• Key Characteristic: Unlike natural selection, genetic drift affects allele frequencies regardless of whether the traits are advantageous or harmful. It has a much stronger impact on small populations, where chance events can cause rare alleles to disappear entirely or become fixed (100% frequency).

Bottleneck Effect

• Mechanism: A sudden, catastrophic event (e.g., natural disaster, disease outbreak, severe habitat destruction, or overhunting) drastically reduces the size of a population at random.

• Evolutionary Impact: The surviving individuals are a small, random sample of the original group. Many alleles are lost forever, resulting in a sharp decrease in genetic diversity in subsequent generations.

• Classic Example: Northern elephant seals were hunted down to fewer than 100 individuals in the late 19th century. Although their numbers have rebounded to over 100,000, they carry extremely low genetic diversity compared to Southern elephant seals.

Founder Effect

• Mechanism: A small group of individuals breaks off from a larger main population to colonize a brand-new, geographically isolated habitat (e.g., an island).

• Evolutionary Impact: The new founding population carries only a fraction of the original population's genetic diversity. Rare alleles present in the founders may become disproportionately common in the new population.

• Classic Example: The Amish population in Pennsylvania was founded by a small group of German immigrants. Because a few founders carried the gene for Ellis-van Creveld syndrome (a rare form of dwarfism and polydactyly), the condition occurs at a significantly higher frequency in this population than in the general public.

Biological Diversity

Biodiversity refers to the overall variety and variability of life on Earth.

Genetic Diversity:

The total variety of genetic information (genes and alleles) contained within a single species.

• Why it matters: High genetic diversity allows a population to adapt to changing environments, diseases, or climate shifts.

Species Diversity:

The number of different species and their relative abundance within a specific habitat or ecosystem.

• Why it matters: Ecosystems with high species diversity are generally more resilient to environmental disturbances.

Ecosystem Diversity:

The variety of habitats, biological communities, and ecological processes across biomes (e.g., rainforests, coral reefs, deserts, wetlands).

• Why it matters: Varied ecosystems provide essential services like water purification, soil fertility, and climate regulation.

Species

A group of organisms whose members can naturally interbreed with one another and produce viable, fertile offspring.

Population ecology

is the branch of ecology that focuses on how and why populations of organisms change in size, structure, and spatial distribution over time.

Size

# of individuals

Density

individuals per area

Distribution

Clumped (Most Common)

Individuals gather in distinct groups or patches.

Uniform (Even)

Individuals are evenly spaced throughout the habitat.

Random

Position of each individual is independent of others; unpredictable pattern.

adaptation

is a genetically inherited trait that increases an organism's likelihood of surviving and reproducing in a specific environment.

Extinction

The complete and permanent disappearance of a species from Earth, occurring when the last surviving individual of that species dies.

Endemic Species

A species that is natively and naturally restricted to one specific geographic location and found nowhere else on Earth.

Habitat

The specific physical environment and ecological setting in which a particular species naturally lives, feeds, shelters, and reproduces.

Specialist Species

Organisms with a narrow ecological niche. They rely on specific food sources, require very particular habitats, or have low tolerance for environmental changes.

• Advantages: Highly efficient at competing for their specific resource within their stable habitat.

• Disadvantages: Extremely vulnerable to extinction if their habitat changes or if their primary food source vanishes.

Classic Example: Giant Pandas

• Diet: Almost exclusively bamboo (makes up over 99% of their diet).

• Impact: If bamboo forests are cleared or undergo natural die-offs, pandas cannot easily adapt to alternative food sources.

Generalist Species

• Organisms with a broad ecological niche. They can eat a wide variety of foods, thrive in diverse habitats, and easily tolerate environmental fluctuations.

• Advantages: High adaptability; can quickly relocate, switch food sources, or thrive in human-altered landscapes.

• Disadvantages: Less competitive than specialists when occupying a highly stable, specialized environment.

• Classic Example: Raccoons

  • Diet: Omnivorous (eats fruits, nuts, insects, small animals, human trash, and pet food).

  • Habitat: Adapts easily to forests, marshes, suburban neighborhoods, and dense cities.

Natality

Birth rate (adds individuals).

Mortality

Death rate (subtracts individuals).

Immigration

Individuals moving into a population from another area (adds individuals).

Emigration

Individuals moving out of a population to another area (subtracts individuals).

Growth Rate Formula

Exponential Growth

J-Shaped Curve

• Conditions: Occurs when resources (food, space, water) are unlimited and environmental resistance is absent.

• Characteristics: The growth rate accelerates over time because the individual birth rate remains constant as the total population expands.

• Real-World Reality: Unsustainable long-term; typically seen in invading species entering new habitats or populations recovering after a catastrophic crash.

Logistic Growth

(S-Shaped Curve)

• Conditions: Occurs when resources are limited, causing growth to slow and eventually stabilize.

• Characteristics: Starts exponentially, slows down as population density increases, and levels off at the habitat's carrying capacity ($K$)—the maximum population size the environment can sustainably support.

Limiting Factors

Limiting factors are environmental conditions that restrict population growth, preventing populations from growing indefinitely.

Density-Dependent

Impact increases as population density grows. Acts as negative feedback to regulate population size around carrying capacity.

• Competition for food, water, or shelter

• Disease/Parasitism (spreads faster in dense crowds)

• Predation (predators target abundant prey)

• Toxic waste accumulation

Density-Independent

Impacts a population equally regardless of its density or size.

• Natural disasters (fires, floods, hurricanes)

• Severe weather & climate (droughts, extreme freezes)

• Human activities (deforestation, pesticide application)

Survivorship Curves

A survivorship curve plots the number of individuals surviving at each age for a given species, typically on a logarithmic scale.

Type I

(Convex Curve):

• Pattern: High survival rates throughout early and middle life, followed by a steep drop in survival in old age.

• Characteristics: Species produce few offspring but invest heavy parental care to ensure high juvenile survival.

• Examples: Humans, whales, elephants, large mammals.

Type II

(Linear / Diagonal Curve):

• Pattern: Constant mortality rate throughout the organism's entire life. The probability of dying is equal at any age.

• Characteristics: Vulnerable to predation, disease, or accidents at all life stages at roughly equal rates.

• Examples: Songbirds, rodents, lizards, hydra.

Type III

(Concave Curve):

• Pattern: Extremely high mortality rate early in life, but the few individuals that survive past the juvenile stage enjoy high survival rates for the remainder of their lives.

• Characteristics: Species produce massive numbers of offspring with little to no parental care.

• Examples: Frogs, marine fish, oysters, oak trees, insects.

r-Selection

(Rate-driven): Thrives in disturbed or unpredictable environments where rapid population expansion is advantageous. Producing large numbers of offspring ensures at least a small percentage survive chance environmental hazards.

K-Selection

(Capacity-driven): Thrives in stable habitats crowded near carrying capacity (K), where resources are fierce. Investing heavy energy and parental care into a few well-equipped offspring ensures they can successfully compete for limited resources.