Evolution and Ecology Study Guide

Darwin and the Foundations of Evolutionary Theory

• Charles Darwin’s Observations: While traveling on the HMS Beagle, Darwin recorded observations that formed the basis of his theory of evolution. His research included:

  • Environmental Variability: He noted that different species inhabited different environments and exhibited traits suited to those specific locales.

  • Fossils: He studied fossil remains that showed similarities to, yet differences from, modern organisms.

  • Adaptations: He observed that organisms possessed specific physical or behavioral characteristics that enhanced their survival.

  • Galápagos Finches: A primary example was the variation in beak shapes among finches on the Galápagos Islands, which were adapted to specific food sources.

• The Main Idea of Evolution: Darwin proposed that species are not static; they change over time through the mechanism of Natural Selection.

• Alfred Russel Wallace:

  • Wallace was a contemporary of Darwin who independently developed the same theory of evolution by natural selection.

  • His correspondence with Darwin, sharing these identical ideas, provided the necessary impetus for Darwin to finally publish his work.

Natural Selection and Genetic Diversity

• Definition of Natural Selection: This process occurs when organisms with specific inherited traits have a higher probability of surviving and reproducing. Consequently, these advantageous traits are passed on to the next generation at higher frequencies.

• Requirements for Natural Selection: For natural selection to occur within a population, three conditions must be met:

  1. Variation: Individuals within the population must possess different traits.

  2. Inheritance: These traits must be capable of being passed from parents to offspring.

  3. Differential Survival and Reproduction: Some individuals must be more successful at surviving and producing offspring than others due to their traits.

• Effects on Genetic Diversity: Natural selection does not have a single effect on diversity; it can either increase or decrease the genetic variation within a population depending on the specific environmental pressures and the type of selection occurring.

Specific Types of Natural Selection

• Directional Selection:

  • Mechanism: One extreme phenotype (trait) is favored over the average or the opposite extreme.

  • Example: Antibiotic-resistant bacteria. When exposed to antibiotics, the "extreme" trait of resistance allows survival, while non-resistant bacteria die.

  • Result: The entire population distribution shifts toward one extreme over time.

• Stabilizing Selection:

  • Mechanism: The intermediate or average trait is favored, while both extremes are selected against.

  • Example: Human birth weight. Infants who are very small or very large have historically faced higher mortality rates, favoring a middle-range weight.

  • Result: The average remains the most common phenotype, and the data curve becomes narrower as extremes decrease.

• Disruptive Selection:

  • Mechanism: Both extreme phenotypes are favored over the intermediate/average trait.

  • Example: Bird beak sizes. If an environment has only very large seeds and very small seeds, birds with medium-sized beaks might struggle to eat either, while those with large or small beaks thrive.

  • Result: The population can split into two distinct groups.

Genetic Mechanisms: Polymorphism and Genetic Drift

• Polymorphism: This refers to the occurrence of two or more distinct forms (morphs) of a trait within a single population. An example is the different color patterns seen in certain species of butterflies.

• Genetic Drift:

  • Definition: These are random, stochastic changes in allele frequencies within a population, as opposed to changes driven by fitness or selection.

  • Population Size: Genetic drift is most impactful and common in small or isolated populations.

  • Effects: Drift generally decreases genetic diversity and can lead to allele fixation, where a specific allele becomes the only version of a gene in the population.

• Founder Effect: This is a specific type of genetic drift that occurs when a small group of individuals breaks away from a larger population to establish a new colony. The new population's gene pool is limited to the alleles present in the founders.

  • Example: High frequencies of specific genetic disorders within Amish populations, resulting from the small original group of settlers.

Hardy-Weinberg Equilibrium and Allele Frequencies

• Hardy-Weinberg Equilibrium Definition: A theoretical state where a population is not evolving, meaning allele frequencies remain constant from one generation to the next.

• Required Conditions for Equilibrium: To maintain this state, five strict conditions must be met:

  1. No mutations must occur.

  2. Mating must be completely random.

  3. There must be no natural selection.

  4. The population size must be extremely large.

  5. There must be no migration or gene flow (no individuals entering or leaving).

• Calculating Genotype Frequencies:

  • Formula: $\text{Frequency} = \frac{\text{Number with specific genotype}}{\text{Total population}}$

  • Example Calculation: Given a population where $AA = 40$, $Aa = 40$, and $aa = 20$ (Total = 100):

    • Frequency of $AA = \frac{40}{100} = 0.40$

    • Frequency of $Aa = \frac{40}{100} = 0.40$

    • Frequency of $aa = \frac{20}{100} = 0.20$

• Calculating Allele Frequencies ($p$ and $q$):

  • Let $p$ represent the frequency of the dominant allele ($A$).

  • Let $q$ represent the frequency of the recessive allele ($a$).

  • Equations:

    • $p = f(AA) + \frac{1}{2}f(Aa)$

    • $q = f(aa) + \frac{1}{2}f(Aa)$

  • Example Calculation (Using previous frequencies):

    • $p = 0.40 + \frac{1}{2}(0.40) = 0.40 + 0.20 = 0.60$

    • $q = 0.20 + \frac{1}{2}(0.40) = 0.20 + 0.20 = 0.40$

• The Hardy-Weinberg Equation:

  • $p^2 + 2pq + q^2 = 1$

  • Where $p^2$ is the frequency of the $AA$ genotype, $2pq$ is the frequency of the $Aa$ genotype, and $q^2$ is the frequency of the $aa$ genotype.

  • Determination of Evolution: If the expected frequencies calculated using this equation do not match the observed frequencies in a real population, that population is not in equilibrium and is currently evolving.

• Allele Fixation: This occurs when a single allele's frequency reaches $100\%$. Generally, alleles that start with a higher frequency are more likely to become fixed, while rare alleles are more likely to be lost entirely.

Real-World Applications of Evolutionary Principles

• Antibiotic Resistance: In medicine, bacteria evolve resistance when exposure to antibiotics kills off susceptible strains, leaving only the resistant individuals to multiply.

• Cancer Treatment: Oncologists observe that cancer cell populations can evolve over time, developing resistance to chemotherapy drugs, which complicates treatment.

• Pest Control: Similar to antibiotic resistance, populations of insects can evolve resistance to pesticides, necessitating the development of new chemical controls.

Species Relationships and Interactions

• Predation: An interaction where the predator kills and consumes the prey. Example: A lion eating a zebra.

• Parasitism: A relationship where one organism (the parasite) benefits at the expense of the other (the host), which is harmed. Example: A tick feeding on a dog.

• Mutualism: A symbiotic relationship where both involved species benefit. Example: Bees collecting nectar while pollinating flowers.

• Competition: Occurs when two or more species compete for the same limited resources, such as food, water, or territory.

• Commensalism: A relationship where one organism benefits while the other is neither helped nor harmed. Example: Barnacles attaching themselves to whales for transportation and food access.

Ecosystem Dynamics and Special Species

• Keystone Species: A species that exerts a disproportionately large effect on the stability and structure of its ecosystem relative to its abundance. Example: Sea otters, which control sea urchin populations to prevent the destruction of kelp forests.

• Trophic Cascade: A phenomenon where changes (such as the removal or addition of a predator) at one trophic level trigger a chain reaction of changes through other levels of the food web.

• Ecosystem Engineer: An organism that physically alters or creates habitats, significantly changing the environment for other species. Example: Beavers, which create wetland ecosystems by building dams.

Speciation and Reproductive Isolation

• Biological Species Concept: This concept defines a species as a group of organisms that can naturally interbreed and produce fertile offspring. Different species are maintained through reproductive isolation.

• Types of Speciation:

  • Allopatric Speciation: Evolution of a new species occurring when a population is separated by a physical, geographic barrier.

  • Sympatric Speciation: Speciation occurring within the same geographic area, often through behavioral or genetic changes.

  • Peripatric Speciation: A specific form of speciation that happens when a small, isolated population at the edge of a larger population's range evolves into a new species.

• Reproductive Isolation Barriers:

  • Pre-Zygotic Barriers (Prevent fertilization):

    • Temporal Isolation: Species breed at different times of the day, season, or year.

    • Behavioral Isolation: Differences in courtship rituals or mating calls.

    • Habitat Isolation: Species occupy different habitats within the same general area.

    • Structural Isolation: Physical incompatibility of reproductive organs.

  • Post-Zygotic Barriers (Occur after fertilization): The offspring are produced but are either non-viable (weak) or sterile. Example: A mule is the sterile offspring of a horse and a donkey.

Ecological Niches and Survival Limits

• Ecological Niche: The specific "role" or "job" an organism has within its environment, including its resource use and interactions.

• Fundamental Niche: The full range of environmental conditions and resources an organism could theoretically occupy or use in the absence of competition.

• Realized Niche: The actual conditions and resources an organism occupies or uses, typically narrower than the fundamental niche due to competition and predation.

• Competitive Exclusion Principle: The rule that two species competing for the exact same resources cannot coexist indefinitely in the same niche; one will eventually outcompete the other.

• Niche Partitioning: A process where species divide resources or space to reduce direct competition, allowing them to coexist.

Population Growth and Regulation

• Exponential Growth:

  • Occurs under ideal conditions with unlimited resources.

  • Formula: $\frac{dN}{dt} = rN$

  • Characterized by: A J-shaped growth curve.

• Logistic Growth:

  • Occurs as resources become limited and the population nears its carrying capacity.

  • Formula: \frac{dN}{dt} = rN\t\left(1 - \frac{N}{K}\t\right)

  • Variable $K$: Represents the Carrying Capacity, or the maximum population size the environment can sustain.

  • Characterized by: An S-shaped (sigmoidal) growth curve.

• Factors Affecting Population Density:

  • Density-Dependent Factors: These have an increasing effect as the population size grows. Examples include disease transmission, competition for food, and predation.

  • Density-Independent Factors: These affect populations regardless of their size or density. Examples include natural disasters such as fires, floods, and hurricanes.

Trophic Levels and Energy Transfer

• The Trophic Hierarchy:

  1. Producer: Organisms (like plants) that create their own food through photosynthesis.

  2. Primary Consumer: Herbivores that eat producers (e.g., a rabbit).

  3. Secondary Consumer: Carnivores that eat primary consumers (e.g., a snake).

  4. Tertiary Consumer: Top-level predators that eat secondary consumers (e.g., a hawk).

• Energy Transfer (The 10% Rule): During the transfer of energy from one trophic level to the next, only approximately $10\%$ of the energy is conserved and stored as biomass. This results in significantly less total biomass at the top of the energy pyramid compared to the base.

• Biomagnification:

  • This refers to the increasing concentration of toxic substances (like heavy metals or pesticides) in the tissues of organisms at higher trophic levels.

  • Impact: Top predators suffer the highest concentrations of toxins.

  • Example: The accumulation of mercury in large fish like tuna.