Unit 3_Evolutionary Mechanism and Natural Selection
Charles Darwin and the Development of Evolutionary Theory
The Galápagos Islands Observations: Charles Darwin observed that different islands within the Galápagos archipelago possessed their own unique varieties of animals. He noted that somewhat similar species existed across the islands but were specifically suited to their particular environments.
The Principles of Natural Selection
Core Definition: Natural selection is the process where organisms possessing traits best adapted to their current environment are more likely to survive, breed, and pass their genetic information to the next generation. This process leads to changes in the overall population over time.
Source of Pressure: Nature acts as the source of "selection pressure."
Survival of the Fittest: This is a synonymous term for natural selection.
Biological Fitness: Defined as the ability of an individual organism to both survive AND successfully reproduce to pass on its genes.
Process of Elimination: Organisms with less beneficial traits are eliminated from the gene pool because they either die prematurely or fail to produce as many offspring as better-adapted individuals.
Factors Affecting Natural Selection:
Environmental conditions.
Genetic variation.
Fitness.
Survival of the fittest.
Documented Examples of Natural Selection:
MRSA (Antibiotic-Resistant Bacteria): Bacteria that have evolved to survive treatments meant to kill them.
Pesticide-Resistant Insects: Insects that survive chemical applications.
Galápagos Ground Finches: Variations in beak shape and size based on food sources.
Peppered Moths: Shift in population color due to industrial soot.
Rock Pocket Mice: Camouflage variations based on the color of the ground/rock.
Galápagos Tortoises: Variations in shell shape and neck length.
The Four Main Points of Natural Selection
For natural selection to occur in an ecosystem, all four of the following points must be present. If they are not (as is often the case with modern humans), natural selection will not occur.
1. Genetic Variation:
Variation must be present within the population and must be heritable (passed from parent to offspring).
Sources of Variation:
Mutations: Random changes in the DNA sequence.
Meiosis: Specifically the process of "crossing over" which reshuffles genetic material.
Random Mate Selection & Fertilization: The unpredictable nature of which individuals mate and which sperm fertilizes which egg.
2. Overproduction of Offspring:
Living things tend to produce more offspring than can actually survive to maturity.
High numbers of offspring increase the statistical chance that at least some will survive.
Result: Better fitness leads to higher reproductive success. Over time, this causes a shift in observable beneficial traits within the population.
3. Competition:
There is a finite amount of limited resources, creating a struggle for existence.
Resources contested: Water, food, space, and mates.
Logic: Without competition for resources, there is no "fittest" because every individual would have enough to survive regardless of traits.
4. Survival of the Fittest (Selection Pressures):
Individuals with the best adaptations survive and reproduce, while others do not or do so to a significantly lesser extent.
This causes a steady shift in the common traits seen in a population.
Scientific Framework: The Theory of Evolution by Natural Selection
Overproduction: Every species tends to produce more individuals than can survive to maturity.
Variation: The individuals of a population have many characteristics that differ from one another.
Selection: Some individuals survive longer and reproduce more than others do.
Adaptation: The traits of those individuals that survive and reproduce will become increasingly more common in a population over successive generations.
Types of Natural Selection Patterns
Diversifying (Disruptive) Selection:
The intermediate (average) variation or characteristic is selected against.
Extremes are selected FOR.
Example: In a fish population, small fish can sneak in to fertilize eggs, and large fish can defend territory to fertilize eggs. Medium-sized fish are unsuccessful because they are too large to sneak in but too small to defend territory; thus, they are selected against.
Stabilizing Selection:
The intermediate or average variation is selected FOR.
The average trait becomes the most common, while extreme variations are selected against.
Directional Selection:
One extreme variation or characteristic is selected FOR.
The population shifts entirely toward that one extreme.
Example: Giraffes with long necks are more successful at reaching food than those with short or medium necks. Nature pushes the population toward the long-neck characteristic.
Adaptations: Morphological, Structural, and Behavioral
An adaptation is a change or process by which an organism becomes better suited to its environment, thereby increasing its fitness. Adaptations are random; they can be helpful, harmful, or neutral.
Morphological Adaptations:
Camouflage: Allows an organism to blend into its physical environment to avoid detection.
Mimicry: A species evolves to physically resemble another, often more dangerous or unpalatable, species.
Antibiotic Resistance: A physiological adaptation in bacteria allowing survival against medical treatments.
Structural Adaptations: Physical body parts that enhance survival.
Examples: Opposable thumbs in monkeys for manipulation/opening objects; gills in fish for underwater respiration.
Behavioral Adaptations: These are often learned behaviors resulting from external stimuli that are passed to future generations.
Using Tools: Monkeys using twigs to extract and eat insects.
Migration: Birds flying south for winter to access better food sources.
Hibernation: Bears receding into caves during winter to conserve energy when resources are scarce.
Long-term Effects: Organisms with favorable traits survive and reproduce, increasing the frequency of those traits in the ecosystem's gene pool over time. Individuals with less ideal adaptations die out or have fewer offspring, lowering the frequency of their genes.
Speciation and Reproductive Isolation
Speciation is the evolutionary process where one species gives rise to a new, distinctly different species. This typically occurs as advantageous traits become more common and populations diverge.
Reproductive Isolation: The inability of a species to breed successfully with related species due to various barriers.
Geographic Barriers: Physical separation that prevents breeding.
Examples: Finches isolated on islands vs. mainland; volcanic eruptions creating new physical barriers; tiger populations inhabiting different, separated regions.
Temporal Barriers: Mating is prevented because groups reproduce at different times (e.g., different seasons or times of day).
Behavioral Barriers: Differences in courtship or communication.
Examples: Different mating calls of the Eastern and Western Meadowlark; specific courtship rituals of Bower Birds.
Physiological Barriers (Mechanical): The physical functioning or structure of the body prevents reproduction.
Examples: Reproductive structure incompatibility (organs do not fit); extreme size differences in different dog breeds.
Genetic Barriers:
Incompatibility at the cellular level where egg and sperm cannot fuse.
Post-zygotic issues: The egg is fertilized, but the offspring does not develop (miscarriage) or the resulting offspring is sterile (e.g., a mule).
The Five Mechanisms of Evolution
For a population to evolve, these mechanisms must be active. It is important to note that individuals do not evolve; only populations do.
The Gene Pool: Represents all the different genes and versions of genes (alleles) within a population. Evolution is essentially changes to this gene pool.
Recombination (Crossing Over): Occurs during meiosis; leads to genetic variation and explains why siblings from the same parents look different.
Mutations: Create variation by changing DNA sequences. This changes the amino acid sequence, which alters protein structure and function, eventually changing the phenotype and fitness.
Natural Selection: Environmentally driven selection pressure.
Gene Flow: The movement of individuals and alleles in and out of populations.
Examples: Seed and pollen distribution by wind or insects; migration (moving in) and emigration (moving out) of animals.
Effect: Causes genetic mixing across regions and increases variation within individual populations. Note: Organisms of different species cannot reproduce despite gene flow.
Genetic Drift: Change in the gene pool due to random chance events.
Founder Effect: A small group splinters off from a main population to start a new colony, skews the gene pool of the new population.
Bottleneck Effect: A disaster or factor reduces a population to a very small number. As the population recovers, the gene pool is narrowed because many alleles were lost during the reduction.
Questions & Discussion
Q: What are the three types of selection and an example of each?
A: 1. Diversifying (e.g., small and large fish vs. medium fish). 2. Stabilizing (e.g., average weight human babies). 3. Directional (e.g., Giraffe neck length).
Q: Practice Scenario: Oysters range from dark greyish brown to light greyish brown. Researchers found most were either dark or light, but very few were medium. What does this indicate?
A: This indicates Diversifying/Disruptive Selection. The environment likely has very dark and very light areas (like dark rocks and light sand), providing camouflage for the extremes but leaving the medium-colored oysters visible to predators.
Q: Practice Scenario: Before the industrial revolution, peppered moths were white with spots; after, they were dark with spots. What type of selection is this?
A: Directional Selection. The environment changed (soot on trees), pushing the population toward the darker extreme.
Q: Practice Scenario: Lionfish are an invasive species with no predators in the Gulf of Mexico. How will they affect prey and what selection might occur?
A: Local prey fish populations will likely decrease. Selection might favor prey fish that are either too small to be seen easily, too large to be swallowed, or those with behavioral adaptations to hide better or swim faster.
Q: True or False: Natural Selection causes changes in populations, NOT individuals.
A: True. Individuals are born with their traits; they do not adapt during their lifetime. The population changes as certain traits are favored over generations.
Q: What is the benefit to organisms that overproduce offspring?
A: It increases the probability that at least some offspring will survive environmental pressures and reach reproductive age.
Q: Scenario: Running over an ant with a scooter. Which mechanism is this?
A: Genetic Drift. It is a random, chance event that was not based on the ant's fitness.
Q: Scenario: A goat named Eddie moves to a neighboring farm to mate with a goat of a different color. Which mechanism is this?
A: Gene Flow. It involves the movement of an individual and its alleles into a new population.