Natural Selection and Adaptation

Natural Selection and Adaptation

High School

​

Learning Objectives

• Define natural selection and explain its four core principles.

• Describe different types of physical and behavioral adaptations with examples.

• Differentiate between directional, stabilizing, and disruptive selection.

• Explain how reproductive isolation can lead to speciation.

• Understand the concepts of extinction and its significance in biodiversity.

Key Vocabulary

​

Natural Selection

Organisms better adapted to their environment are more likely to survive and reproduce successfully.

​

Adaptation

A physical feature or behavior that helps an organism survive and reproduce in its environment.

​

Speciation

The evolutionary process by which new biological species arise from existing ones through different forms of isolation.

​

Reproductive Isolation

The inability of a species to breed successfully with related species due to various barriers.

​

Variation

A different characteristic between organisms in a species, like differences in body size or fur color.

​

​

Darwin and Natural Selection

Charles Darwin

• ​Charles Darwin was an English naturalist who developed the theory of evolution by natural selection.

• ​​His revolutionary ideas were shaped by his observations of species on the Galapagos Islands.

• ​He used fossil evidence to support his conclusion that all species change over time.

Natural Selection

• ​This is the process where organisms with beneficial inherited traits are better suited to their environment.

• ​​These organisms are more likely to survive, reproduce, and pass these traits to their offspring.

• ​This mechanism of evolution is also known as ‘survival of the fittest’.

​

Solved Example 1
A population of 100 moths lives in a dark-colored forest. 75 are dark-colored and 25 are light-colored. After one generation, only 15 light moths remain, while the dark moth population increases by 10%. What is the new population distribution?

​

Step 1: Analyze and Sketch the Problem

• Goal: Calculate the new number of dark and light moths and the total population size.

• Knowns: Initial population = 100 moths; Initial dark moths = 75; Initial light moths = 25; Final light moths = 15; Dark moth increase = 10%.

• Unknown: Final number of dark moths; New total population.

​

Solved Example 1
A population of 100 moths lives in a dark-colored forest. 75 are dark-colored and 25 are light-colored. After one generation, only 15 light moths remain, while the dark moth population increases by 10%. What is the new population distribution?

​

Step 2: Solve for the Unknown

​

Solved Example 1
A population of 100 moths lives in a dark-colored forest. 75 are dark-colored and 25 are light-colored. After one generation, only 15 light moths remain, while the dark moth population increases by 10%. What is the new population distribution?

​

Step 3: Evaluate the Answer

• The number of light moths decreased significantly (from 25 to 15), while the number of dark moths increased (from 75 to 83).

• This change reflects natural selection, where the environment favors the dark-colored moths, leading to their increased survival and reproduction.

• The total population size changed from 100 to 98.

Multiple Choice

According to the slide, what is the definition of natural selection?

• A theory developed by Charles Darwin based solely on studying fossils.

• The process where individuals with inherited beneficial adaptations are better suited to their environment, leading to survival and reproduction.

• A process where species can change their traits within their lifetime to suit their needs.

• The idea that all organisms are in a constant state of competition, regardless of their traits.

​

​

Core Principles of Natural Selection

• Organisms produce more offspring than can be supported, leading to competition.

• ​Individuals in a population have inherited variations in their physical traits.

• Individuals with traits best suited to their environment are more likely to survive.

• These favorable traits are passed on and become more common over time.

Multiple Choice

Which core principle states that individuals with the most suitable variations for their environment are more likely to survive and reproduce?

• Overproduction

• Variation

• Survival of the Fittest

• Competition

​

​

Key Aspects of Natural Selection

• The environment determines which genetic traits are favorable for an organism's survival.

• Natural selection acts on populations over generations, not on single individuals.

• It works on existing genetic variations and does not create traits on demand.

• Selection acts on physical traits, changing the gene frequencies in a population.

​

Multiple Choice

Based on the provided information, on what level does natural selection operate?

• It acts on populations over generations.

• It only affects an organism's genetic code, not its physical traits.

• It grants organisms the specific traits they need to survive.

• It acts on individuals, allowing them to change during their lifetime.

​

​

Adaptations and Natural Selection Examples

Galapagos Tortoises

• Tortoises on the Galapagos Islands showed variations in their neck and leg lengths.

• Tortoises with short necks lived on islands with abundant low-growing plants.

• Long-necked tortoises could reach taller plants where low vegetation was scarce.

Mudskippers

• Competition for resources led some mudskippers to spend more time on land.

• Individuals with stronger fins were better at moving on land to find food.

• This heritable trait became more common in the population over many generations.

Multiple Choice

How did the environment influence the evolution of Galapagos tortoises?

• The type of predators on each island determined the thickness of their shells.

• The height of available vegetation led to the selection of different neck and leg lengths.

• The availability of water caused some tortoises to develop better swimming abilities.

• The color of the soil influenced the camouflage patterns on their shells.

Mechanisms of Adaptation

​

Pesticide Resistance

• ​Pesticides kill most pests, but a few resistant ones survive.

• ​​These survivors reproduce and pass on their resistance to offspring.

• ​Over time, the pest population becomes predominantly resistant to pesticides.

​

Mimicry

• ​A harmless organism evolves to look like a harmful organism.

• ​​This resemblance provides protection from predators who avoid the harmful species.

• ​An example is the harmless Scarlet King Snake mimicking a venomous snake.

​

Camouflage

• ​An organism evolves to blend in with its natural environment.

• ​​This adaptation helps the organism avoid detection by its predators.

• ​It is a common defense in animals like fish, crabs, and frogs.

​

Solved Example 2
In a pest population of 10,000, a pesticide is 95% effective. If 10% of the survivors reproduce, each having 50 offspring, what is the size of the next generation of potentially resistant pests?

​

Step 1: Analyze and Sketch the Problem

• Goal: Calculate the size of the next pest generation.

• Knowns: Initial population = 10,000; Pesticide effectiveness = 95%; Survivors that reproduce = 10%; Offspring per survivor = 50.

• Unknown: Number of survivors, number of reproducing survivors, and the size of the next generation.

• Formula: No single formula, but a sequence of percentage and multiplication calculations.

​

Solved Example 2
In a pest population of 10,000, a pesticide is 95% effective. If 10% of the survivors reproduce, each having 50 offspring, what is the size of the next generation of potentially resistant pests?

​

Step 2: Solve for the Unknown

​

Solved Example 2

In a pest population of 10,000, a pesticide is 95% effective. If 10% of the survivors reproduce, each having 50 offspring, what is the size of the next generation of potentially resistant pests?

​

Step 3: Evaluate the Answer

• The initial population was reduced by the pesticide, but the resistant survivors were able to reproduce, leading to a new generation.

• The calculation shows how a small number of resistant individuals can lead to a significant new population, and the answer of 2,500 pests is a reasonable outcome.

Multiple Choice

What is the key difference between mimicry and camouflage?

• Mimicry is for attracting mates, while camouflage is for hiding from predators.

• Mimicry involves resembling another organism, while camouflage involves blending with the environment.

• Mimicry is a behavioral adaptation, while camouflage is a physical one.

• Mimicry is only found in insects, while camouflage is found in many animals.

Types of Natural Selection

​

Directional Selection

• Favors one extreme phenotype over others due to environmental changes.

• The population's average trait shifts toward that one extreme phenotype.

• Long-necked giraffes reaching higher leaves is a classic example.

​

Stabilizing Selection

• Favors the intermediate phenotype and selects against the two extremes.

• This process decreases the genetic diversity within a given population.

• Medium-height plants get sun but avoid wind damage affecting talls.

​

Disruptive Selection

• Favors both extreme phenotypes, selecting against the intermediate one.

• This type of selection can ultimately lead to new species development.

• Short and tall plants are favored if a specific pollinator disappears.

Multiple Choice

A population of plants has either very short or very tall individuals favored by pollinators, while medium-height plants are not. What type of selection is this?

• Disruptive Selection

• Stabilizing Selection

• Directional Selection

• Artificial Selection

​

​

Speciation and Reproductive Isolation

• A species is a group of individuals that can interbreed successfully.

• Speciation is the formation of a new species from an existing one.

• It occurs when two populations become reproductively isolated from each other.

• For example, a horse and a donkey create a sterile mule.

​

Solved Example 3
A species of finch has a population of 10,000 on an island. After a volcanic eruption, only 50 finches with a specific beak size survive. What percentage of the original population remains?

​

Step 1: Analyze and Sketch the Problem

• Goal: Find the percentage of the finch population that survived.

• Knowns: Original population = 10,000; Surviving population = 50.

• Unknown: Percentage of survivors.

• Formula: Percentage = (Part / Whole) * 100

​

Solved Example 3
A species of finch has a population of 10,000 on an island. After a volcanic eruption, only 50 finches with a specific beak size survive. What percentage of the original population remains?

​

Step 2: Solve for the Unknown

​

Solved Example 3
A species of finch has a population of 10,000 on an island. After a volcanic eruption, only 50 finches with a specific beak size survive. What percentage of the original population remains?

​

Step 3: Evaluate the Answer

• The calculation is arithmetically correct.

• The result of 0.5% represents the small fraction of the population that survived the catastrophic event, which is a realistic scenario in natural selection known as a population bottleneck.

Multiple Choice

What is the primary outcome of reproductive isolation between two populations?

• It results in all offspring being sterile, like mules.

• It can lead to the formation of a new species (speciation).

• It increases the genetic diversity within each population.

• It causes the two populations to merge into one.

Types of Reproductive Isolation

​

Behavioral Isolation

• ​Populations have different courtship rituals or other unique mating behaviors.

• ​​These behavioral differences prevent them from interbreeding with one another.

• ​For example, bird species may have unique songs or dances.

​

Geographical Isolation

• ​Populations are physically separated by barriers like rivers or mountains.

• ​​This physical separation prevents the groups from breeding with each other.

• ​Arctic and Gray Foxes are separated by their geographic ranges.

​

Temporal Isolation

• ​Populations reproduce at different times, such as day or seasons.

• ​​This difference in timing prevents them from mating with each other.

• ​For example, two frog species might breed in different months.

Multiple Choice

If two species of fireflies in the same forest mate at different times of the night, what kind of isolation is this?

• Behavioral Isolation

• Temporal Isolation

• Reproductive Selection

• Geographical Isolation

​

​

What is Extinction?

• Extinction is the complete death of a species and a natural evolutionary process.

• Mass extinctions are rapid, widespread events that cause huge losses of species.

• Earth has experienced five major mass extinctions that have reshaped its biodiversity.

• The current "Sixth Extinction" is an ongoing event driven by human activities.

​

Multiple Choice

What is the defining characteristic of a mass extinction?

• A rapid and widespread loss of a large number of species.

• An extinction event caused exclusively by human activity.

• The slow, gradual disappearance of a single species.

• The complete death of every individual of one specific species.

​

Common Misconceptions

Misconception	Correction
Individuals can adapt during their lifetime.	Adaptation is a process that occurs in populations over many generations.
Natural selection 'gives' organisms what they 'need' to survive.	Natural selection acts on existing random genetic variations within a population.
Natural selection is a process that happens to individuals.	Natural selection is a process that happens to populations, not individuals.

Multiple Choice

In a population of insects, a pesticide is applied, killing most individuals. However, a few insects have a pre-existing genetic trait that makes them resistant and they survive to reproduce. What concept does this scenario best illustrate?

• Natural selection acting on existing variation.

• Individuals developing a need-based resistance.

• Learned behavior being passed to offspring.

• Geographical isolation leading to speciation.

Multiple Choice

What is the primary difference in outcome between stabilizing selection and disruptive selection?

• Stabilizing selection leads to new species, while disruptive selection maintains the current species.

• Stabilizing selection favors one extreme, while disruptive selection favors the opposite extreme.

• Stabilizing selection is caused by environmental change, while disruptive selection is not.

• Stabilizing selection favors the average individual, while disruptive selection favors individuals at both extremes.

Multiple Choice

A single population of salamanders is split into two by a new, fast-flowing river. After thousands of years, the two populations can no longer interbreed. Which of the following best explains this outcome?

• The salamanders on both sides of the river developed identical adaptations through stabilizing selection.

• Directional selection caused one population to outcompete the other, leading to extinction.

• Geographical isolation led to reproductive isolation, resulting in speciation.

• Behavioral isolation occurred because the salamanders developed different mating calls.

Multiple Choice

Analyze the relationship between genetic variation and natural selection. How would a lack of genetic variation affect a population's ability to adapt to a changing environment, such as the introduction of a new disease?

• Without variation, there would be no resistant individuals for natural selection to favor, making the population vulnerable to extinction.

• A lack of variation would cause the population to evolve faster, as all individuals would adapt simultaneously.

• The population would be unaffected, as natural selection can create new traits 'on demand' to fight the disease.

• The population would experience disruptive selection, splitting into two new species immediately.

​

Summary

• Natural selection is the main mechanism for evolution, driven by several key factors.

• Adaptations, like mimicry and camouflage, are inherited traits that boost survival and reproduction.

• Selection can be directional, stabilizing, or disruptive, affecting which traits are favored.

• Reproductive isolation can lead to the formation of new species, or speciation.

Poll

On a scale of 1-4, how confident are you about the concepts covered in today's review?

• 1

• 2

• 3

• 4