Natural Selection and Reproduction

Natural Selection and Reproduction

Middle School

​

Learning Objectives

• Explain the four core principles of natural selection.

• Describe the relationship between genes, proteins, and traits, and identify sources of genetic variation.

• Use Punnett squares to predict the probability of offspring inheriting specific traits.

• Analyze how environmental factors influence whether a trait is adaptive or non-adaptive.

• Explain how the distribution of traits in a population changes over many generations.

Key Vocabulary

​

Natural Selection

The process where organisms with advantageous traits survive and reproduce more successfully.

​

Trait

A specific characteristic of an organism, like eye color or height, passed down through genes.

​

Gene

A segment of DNA that holds instructions for building a specific protein, which determines a trait.

​

Adaptation

An inherited trait that helps an organism survive and reproduce in its specific natural environment.

​

Genetic Variation

The differences in genes between individuals or populations, which leads to a variety of traits.

​

Genotype

The specific combination of genes that an organism has for a particular trait.

Key Vocabulary

​

Phenotype

The observable physical or behavioral traits of an organism, like its appearance.

​

Allele

An allele is one of two or more alternative forms of a gene.

​

Punnett Square

A Punnett Square is a diagram used to predict the genotypes of offspring from a cross.

​

Mutation

A mutation is a change in the DNA sequence of a gene, creating new genetic variation.

Introduction to Natural Selection

• Natural selection is the primary mechanism that drives the process of evolution.

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

• They then reproduce and pass these advantageous traits to the next generation.

Multiple Choice

What is the role of natural selection?

• It is the primary mechanism that drives evolution.

• It is a process that prevents any changes in a species.

• It ensures all individuals in a population are identical.

• It is a process that only applies to plants, not animals.

Multiple Choice

According to the principles of natural selection, what happens to individuals with traits best suited to their environment?

• They are more likely to survive and pass these traits to their offspring.

• They choose to develop new traits during their lifetime.

• They prevent other individuals from reproducing.

• These traits are learned from other individuals in the population.

Multiple Choice

If a population's environment changes, making it harder for some individuals to hide from predators, what is the most likely long-term result of natural selection?

• The trait for better camouflage will become more common in the population over generations.

• The entire population will immediately develop better camouflage.

• The environment will change back to its original state to suit the population.

• The predators will stop hunting the population, regardless of camouflage.

​

​

Inherited vs. Acquired Traits

Inherited Traits

• Traits are passed from parents to offspring through genes.

• Genes are located on chromosomes inside the cells of an organism.

• Examples of inherited traits include fur color and blood type.

Acquired Traits

• Traits are developed during an individual's lifetime through experiences.

• These traits cannot be passed on to the next generation.

• Examples include a scar from an injury or learning to ride a bike.

Multiple Choice

What is the fundamental difference between an inherited trait and an acquired trait?

• Inherited traits are passed to offspring, while acquired traits are not.

• Acquired traits are determined by genes, while inherited traits are not.

• Inherited traits appear only in parents, while acquired traits appear in offspring.

• Acquired traits are more common in nature than inherited traits.

Multiple Choice

How does an organism receive its inherited traits?

• They are developed through life experiences and learning.

• They are passed from parents to offspring through genes.

• They are caused by environmental factors like diet.

• They are chosen by the organism to help it survive.

Multiple Choice

A cat has a specific fur color and a scar from an injury. If this cat has a kitten, what is the most likely outcome for the kitten's traits?

• The kitten will be born with both the same fur color and the scar.

• The kitten will be born with the scar, but not the same fur color.

• The kitten might inherit the fur color, but it will not be born with the scar.

• The kitten will not inherit the fur color or be born with the scar.

​

​

How Genes Determine Our Traits

• Genes are segments of DNA that contain instructions for building proteins.

• These proteins are the workers that are responsible for your traits.

• The relationship follows a simple path: Gene → Protein → Trait.

• A change in the gene can alter the protein and its resulting trait.

​

Multiple Choice

What is the fundamental role of a gene within a cell?

• To provide the instructions for building specific proteins.

• To directly form the physical traits of an organism.

• To break down complex traits into simple molecules.

• To store all of the body's energy in the cell's nucleus.

Multiple Choice

Which statement best describes the relationship between a gene, a protein, and a trait?

• A gene contains the code for a protein, which then produces a trait.

• A protein contains the code for a gene, which then produces a trait.

• A trait contains the code for a protein, which then produces a gene.

• A gene, a protein, and a trait are all the same thing.

Multiple Choice

If a gene's DNA sequence is changed, what is the most likely consequence for the organism?

• The protein may be constructed incorrectly, leading to an altered trait.

• The trait would stay the same, but the protein would be different.

• The gene would create a completely new and unrelated protein.

• The cell would use a different gene to create the correct protein.

​

​

Sources of Genetic Variation

Sexual Reproduction

• ​Offspring inherit a unique combination of genes from two different parents, creating genetic diversity.

• ​​This process reshuffles existing alleles, leading to new combinations of traits in the population.

• ​It is a primary source of variation that allows for adaptation over time.

Mutation

• ​A mutation is a random change that occurs in an organism's DNA sequence.

• ​​These changes can create entirely new alleles, which are different versions of a gene.

• ​The effects of mutations can be beneficial, harmful, or neutral to the organism.

Multiple Choice

What do sexual reproduction and mutation have in common?

• They are both sources of genetic variation in a population.

• They both create offspring that are identical to the parents.

• They both guarantee that only beneficial traits are passed on.

• They both work by combining the DNA of three or more parents.

Multiple Choice

How does the variation from sexual reproduction differ from the variation from a mutation?

• Sexual reproduction creates new alleles, while mutations only reshuffle existing ones.

• Sexual reproduction reshuffles existing alleles, while mutations can create entirely new alleles.

• Sexual reproduction results in harmful traits, while mutations result in beneficial ones.

• Sexual reproduction involves one parent, while mutations involve two parents.

Multiple Choice

If a sudden environmental change occurs, what is the most likely way that genetic variation would help the population survive?

• The presence of varied traits from past mutations and sexual reproduction may allow some individuals to survive and reproduce.

• All organisms in the population will instantly develop new, beneficial mutations to survive the change.

• Only organisms that reproduce sexually will be able to adapt, as mutations are always harmful.

• The population will not be able to survive because genetic variation only occurs over a very long time.

​

​

Genotype vs. Phenotype

• Your genotype is your specific set of genes, like BB, Bb, or bb.

• Your phenotype is the observable physical trait, such as having brown eyes.

• A dominant allele’s trait can be of the genotype is BB or Bb.

• A recessive allele’s trait will be of the genotype is bb.

Multiple Choice

Which statement correctly defines genotype and phenotype?

• A genotype is the combination of genes for a trait, while a phenotype is the observable physical trait.

• A genotype is the observable physical trait, while a phenotype is the combination of genes for a trait.

• A genotype describes dominant traits, while a phenotype describes recessive traits.

• A genotype is represented by a single letter, while a phenotype is represented by two letters.

Multiple Choice

What must be true for a recessive trait, like blue eyes, to be expressed as an organism's phenotype?

• The organism's genotype consists of two recessive alleles (bb).

• The organism's genotype contains at least one dominant allele (BB or Bb).

• The organism's phenotype is determined by its environment.

• The organism has an equal number of dominant and recessive genes.

Multiple Choice

If an organism has a genotype of 'Bb' for eye color, what conclusion can be drawn about its phenotype?

• The organism will have brown eyes because the dominant allele (B) is present.

• The organism will have blue eyes because the recessive allele (b) is present.

• The organism will have a mix of brown and blue eyes because both alleles are present.

• It is impossible to predict the eye color from the genotype.

​

​

Predicting Traits with Punnett Squares

• A Punnett square predicts the genotypes and phenotypes of offspring from a cross.

• Place one parent's alleles on top and the other parent's on the side.

• Filling in the squares shows all possible resulting genotype combinations for offspring.

• You can calculate the probability of an offspring having a particular trait.

Multiple Choice

What is the primary purpose of using a Punnett square?

• To predict the potential genotypes and phenotypes of offspring.

• To determine the exact traits an adult organism will have.

• To show how an organism's traits change over its lifetime.

• To map the complete genetic sequence of a single parent.

Multiple Choice

How does a Punnett square show the possible genotype combinations for offspring?

• By combining the alleles from each parent that are placed on the top and side of the grid.

• By randomly guessing the allele combinations for the offspring.

• By listing only the dominant alleles from the parents in the squares.

• By averaging the traits of the two parents to fill the squares.

Multiple Choice

If a completed Punnett square shows that three out of the four possible squares result in a genotype for a dominant trait, what is the most logical conclusion?

• The offspring will certainly have the dominant trait.

• There is a 75% probability that an offspring will have the dominant trait.

• The recessive trait is more powerful than the dominant trait.

• Three of the four offspring will have the dominant trait.

​

​

Adaptation and the Environment

Adaptive Trait

• A polar bear’s thick, white fur is an adaptive trait in its native arctic environment.

• The thick fur provides excellent insulation, keeping the polar bear warm in freezing temperatures.

• Its white color provides camouflage, helping it to blend in with the snow and ice.

Non-Adaptive Trait

• That same thick fur becomes a non-adaptive trait if the polar bear is in a desert.

• This dense insulation would cause the animal to quickly overheat in the hot desert climate.

• The white fur would make it stand out against the sand, losing its camouflage advantage.

Multiple Choice

What is the most important factor in determining whether a trait is adaptive?

• The environment an animal lives in

• The color of an animal's fur

• The age of an animal

• The food an animal eats

Multiple Choice

What is the function of a polar bear's thick, white fur in its native arctic environment?

• It helps the bear attract a mate.

• It allows the bear to swim faster in icy water.

• It provides both insulation from the cold and camouflage against the snow.

• It makes the bear appear larger to scare away predators.

Multiple Choice

Based on the relationship between traits and environment, what is the most likely outcome for a polar bear in a desert?

• The bear's fur would change color to brown to match the sand.

• The bear would be unable to regulate its body temperature and would be easily spotted against the sand.

• The bear would be better insulated against the desert's cold nights.

• The bear's white fur would reflect sunlight, keeping it cool.

​

Common Misconceptions

Misconception	Correction
Individuals evolve.	Populations evolve over generations.
Organisms try to adapt.	Adaptation is a result of random variation.
'''Survival of the Fittest''' means only the strongest survive.	Fitness refers to reproductive success.
All mutations are harmful.	Mutations can be beneficial, neutral, or harmful.

​

Summary

• Natural selection drives evolution, changing trait distribution in a population over time.

• Genes provide instructions for proteins, which determine an organism's inherited traits.

• Genetic variation is created by sexual reproduction and random mutations.

• The environment determines if a trait is adaptive, and Punnett squares predict inheritance.

Poll

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

• 1 (Not confident)

• 2 (A little confident)

• 3 (Mostly confident)

• 4 (Very confident)