AP Biology - Big Ideas from Unit #7: Natural Selection

Comprehensive Quizlet for AP Biology Unit #7: Natural Selection, aligned to the 2020 (May 2025) and 2025 (May 2026+) CED – Topics 7.1–7.13. 🎯 Covers evolution, H-W equilibrium, speciation, phylogenies, variation, and origin of life. ✅ Includes content recall, EVALUATE prompts, scenario-based application, and original AP-style MCQs with justification and SP alignment (SP 1.A, 1.C, 2.A, 4.B, 5.B, 6.E). Built for clarity and exam-level thinking.

Evolution

Change in gene pool over time from generation to generation, caused by 5 mechanisms of evolution. Change in the genetic makeup of a population over time; supported by various evidence.

5 Mechanisms of Evolution

1) Mutations

2) Natural Selection

3) Genetic Drift

4) Gene Flow

5) Non-random mating & sexual selection

Mutations

A random change in an organism's DNA that can introduce new alleles into a population, providing raw material for evolution.

Natural Selection

Organisms that have beneficial adaptations suited for a particular environment have a greater chance of survival and reproduction. It is the ONLY mechanism of evolution that causes adaptations.

Notes...

1) Natural Selection is a process of editing, not a creative mechanism.

2) Evolution by Natural Selection may occur rapidly (ex. few years or decades)

3) Natural Selection favors adaptations depending on time and environment.

4) Evolution is a blend of chance and "sorting" (Chance, as in the creation of new genetic variations - mutations and Sorting, as in Natural Selection, favors certain alleles over others depending on selection pressures). Due to this sorting process, the outcome of Natural Selection is NOT random. It increases the frequencies of alleles that provide greater evolutionary fitness, leading to adaptative evolution.

Examples of Natural Selection (Must know these, stated in AP Biology CED)

1) Sickle Cell Anemia (Sickle Cell Trait is Selected For)

2) DDT Resistance in Insects

Why can't Natural Selection produce perfect organisms?

1) Selection can act only on existing populations

2) Evoluton is limited by historical restraints

3) Adaptations are often consequences

4) Chance, Natural Selection and the Envirionment interact (The environment changes over time)

Adaptive Evolution

Traits that enhance survival or reproduction tend to increase in frequency over time.

Genetic Drift

A random change in allele frequencies in a population, especially significant in small populations. Examples include the bottleneck effect and the founder effect.

Campbell Biology Definition: "Chance events that can cause allele frequencies to fluctuate unpredictably from one generation to the next, notably in small populations."

Founder Effect

When few individuals become isolated from a larger population, this small group may establish a new population with different geene pool from the intial, source population.

These few individuals that get isolated act as "founding fathers".

It is a form of Genetic Drift.

Bottleneck Effect

A sudden change/catastrophe in environment (ex. fire/flood) may drastically reduce population. A deer might be in the wrong place at the wrong time and due to that, die.

It is a form of Genetic Drift.

Summary of Genetic Drift Effects

1) GD is significant in small populations.

2) GD can causes allele frequencies to change at random.

3) Genetic Drift can lead to a loss of genetic variation in populatons.

4) GD can cause harmful alleles to become fixed.

Gene Flow

The movement of alleles into or out of a population due to migration of individuals or gametes, which can increase genetic diversity and reduce genetic differences between populations.

Think: Migration of Alleles

Non-Random Mating

When individuals select mates based on phenotype or relatedness, which can shift allele frequencies. Includes assortative mating and inbreeding.

Sexual Selection

Process in which individuals with certain inherited characteristics are more likely than other individuals of the same sex to obtain mates

Often result of Sexual Selection

Sexual Dimorphism, refering to the difference in secondary sexual characteristics between men and women of the same species.

Ex. Size Differences, Color Differences, Ornamentary and Behavior Differences

Sexual Dimorphism

The difference in secondary sexual characteristics between men and women of the same species.

Intrasexual Selection

Selection within same sex, individuals compete directly for mates of the opposite sex.

Ex. "Male-on-male normally," Male elephants fighting using tusks.

Intersexual Selection

Mate choice, individuals of one sex (normally, females) are choosy in selection of mates from opposite sex.

Ex. Male lizard dewlaps to attract female attention

Directional Selection

1 extreme phenotype is favored.

Ex. Peppered Moths (unpolluted = lichen/moss --> light colored; coal w/ industrialization polluted soot on trees --> dark colored; clean air act --> light)

Stablizing Selection

Intermediate phenotypes favored.

Ex. Infant baby weight (small babies couldn't survive on own due to being too small; bigger babies got stuck in the birth process)

Disruptive Selection

Both extreme phenotypes are favored.

Ex. Rock Pocket Mouse (Volcanic: Dark Rock --> Dark Colored; Light Sand: Light Rock --> Light Colored; If born with the medium, intermediate phenotype, rock pocket mice with this phenotype would die as they would stick out against their environment, making it an easy target for predators)

Divergent Evolution

When two or more species sharing a common ancestor become more different over time; to become a different species

Adaptations

Inherited characteristics of organisms that enhance their survival and reproduction in specific invironments.

Ex. Webbed feet of duck to aid in swimming

T/F: Individuals do NOT evolve. Populatons evolve.

True! Individuals don't evolve, but populations do evolve!

Evolutionary Fitness

Ability of an organism to survive and produce fertile offspring. We meausre evolutionary fitness by reproductive success (production of offspring).

Heritability

Ability to pass on adaptations to successive generations

Does Fitness depend on Specific Environmental Conditions?

Yes, Fitness depends on specific environmental conditions

Using your understanding of evolutionary fitness, what is the relative fitness of a sterile mule?

0 as the mule doesn't contribute to the gene pool due to sterility, meaning it can't reproduce

An individual's fitness is relative to what?

An individual's fitnessis relative to environmental condition as favorable adaptations can change based off changes in the environment, influencing selection pressures.

How are the rate and direction of evolution determined?

Ecosystem Stability

Genetic Variation

Describes genotype and phenotype differences between individuals in a population.

2 ways genetic variaton is preserved in a population

Balancing Selection!

1) Frequency-Dependent Selection

2) Heterozygote Advantage

Frequency Dependent Selection

Fitness of a phenotype depends on commonality/frequency in a population

Heterzygote Advantage

Occurs when heterozygous individuals at a particular locus have greater evolutionary fitness than both homozygous dominant and homozygous recessive individuals.

Defined in Genotype terms, not phenotype.

Ex. Sickle Cell Trait. In Sickle Cell Anemia, those who are heterozygous have both normal and sickle cell shaped blood cells but not enough sickle shaped blood cells to have Sickle Cell Disease

Selective Pressure(s)

Any biotic/abiotic factors in the environment that influence survivalbility and reproductive success.

Describe the causes of Natural Selection

According to Darwin's theory of natural selection, competition for limited resources results in differential survival. Individuals with more favorable phenotypes are more likely to survive and produce more offspring, thus passing traits to subsequent generations.

Artificial Selection

Process where humans have modified other species over many generations by selecting and breeding individuals that possess desired traits. As a result, crops, livestock animals, and pets bear often little resembelance/similarity to their wild ancestors.

Convergent Evolution

Process by which similar environmental conditions select for similar traits in different populations or different species over time. It can be summarized as "Similar problem, similar solution." Convergent evolution results in analogous structures.

Ex. (1) Wings in Birds and Bats, (2) Streamlined Body Shape in Sharks and Dolphins.

Analogous Structures

Structures that have a similar function but do not reflect a common ancestry between species. Similar solution to a similar problem.

Think: SIMILAR FUNCTON, NO COMMON ANCESTRY! Result of similar selection pressures in similar environments.

When does Convergent Evolution occur?

Convergent Evolution occurs when similar selective pressures result in similar, independent phenotypic adaptations in different populations or species.

Describe the importance of phenotypic variation in a population

a) Natural selection acts on phenotypic variations in populations

b) Environments change and apply selective pressures to populations.

c) Some phenotypic variations significantly increase or decrease fitness of the organism in particular environments

Impact of New Genes on Genetic Variation; CollegeBoard may word this as, "Describe the change in the genetic makeup of a population over time"

Migration (aka GENE FLOW!) can introduce NEW genes into a population.

⬆️ New Genes = ⬆️ Genetic Variation

However, note that continued migration (gene flow) between populations can also REDUCE genetic diversity/differences between populations over time.

CED Answer: Mutation results in genetic variation, which provides phenotypes on which natural selection acts

T/F: Evolution can occur without genetic variation.

FALSE! Evolution CANNOT occur without Genetic Variation.

Explain how random occurrences affect the genetic makeup of a population.

Evolution is also driven by random occurrences—

a) Mutation is a random process that contributes to evolution.

b) Genetic drift is a nonselective process occurring in small populations— i. Bottlenecks. ii. Founder effect.

c) Migration/gene flow can drive evolution

Hardy-Weinberg Equillibrium

States that allele and genotype frequencies in a population will remain constant from generation to generation in the absence of other evolutionary influences. Essentially, no evolution is happening.

HWE is used to describe and predict allele frequencies in a non-evolving population.

5 conditions for Hardy-Weinberg Equillibrium

No mechanisms of evolution.

1) Large population size (no Genetic Drift)

2) Absence of Migration (no Gene Flow)

3) No net mutations (no genes modified, deleted, or duplicated -- no Mutations)

4) Random Mating (No Sexual Selection)

5) No Natural Selection

From CED: "Conditions for a population or an allele to be in Hardy-Weinberg equilibrium are—(1) a large population size, (2) absence of migration, (3) no net mutations, (4) random mating, and (5) absence of selection. These conditions are seldom met, but they provide a valuable null hypothesis"

T/F: Concerning HWE, Allele frequencies in a population can be calculated from genotype frequencies.

TRUE! In HWE, Allele frequencies in a population can be calculated from genotype frequencies by using the formulas.

Formula for Genotype Frequencies - HWE

p² + 2pq + q² = 1

Where...

p² = frequency of homozygous dominant genotype

2pq = frequency of heterozygous genotype

q² = frequency of homozygous recessive genotype

Formula for Allele Frequencies - HWE

p + q = 1

Where...

p = frequency of dominant allele

q = frequency of recessive allele

Explain the impacts on the population if any of the conditions of Hardy-Weinberg are not met

1) Changes in allele frequencies provide evidence for the occurrence of evolution in a population.

2) Small populations are more susceptible to random environmental impact than large populations.

Factors that can disrupt Hardy-Weinberg Equillibrium

The 5 Mechanisms of Evolution

1) Mutations

2) Natural Selection

3) Gene Flow

4) Genetic Drift

5) Non-random Mating (Sexual Selection)

The ability of a population to respond to changes in the environment is influenced by what?

Genetic Diversity (ex. Mutations!). The ability of a population to respond to changes in the environment is influenced by genetic diversity.

Consequences on a population with less genetic diversity

Populations with little genetic diversity...

1) more likely to SHRINK

2) become EXTINCT!

Explain the advantage that populations that are more genetically diverse populations have in regards to survive environmental changes.

(Think: Why are populations that are more genetically diverse more likely to survive environmental changes?)

Genetically diverse populations are more likely to survive as they're more likely to contain individuals who can withstand new environmental pressures.

Sample environmental pressures: climate change, catastrophic gelogical events, habitat loss, human interference, change in food source, predation

Explain the idea behind "genes aren't good or bad"

Genes and traits aren't essentially bad -- the selective pressures of an environment determine whether a trait's an advanage or disadvantage. Traits can be deleterious or adaptive. Whether or not an allele or a phenotype provides an advantage to an organism depends on the environment. If the environment changes, a previously advanatageous allele can become disadvantageous.

Illustrative Examples used in AP Curricula for 7.12 Variations in Populations -- "Population ability to respond to changes in the environment is influenced by genetic diversity. Species and populations with little genetic diversity are at risk of decline or extinction"

- California condors

- Black-footed ferrets

- Prairie chickens

- Potato blight

- Corn rust

- Genetic diversity and selective pressures

- Antibiotic resistance in bacteria. (Not all individuals in a diverse population are susceptible to a disease outbreak.)

These examples are explictly outlined in the AP Biology CED. Know them!

Deletrious Traits

Traits that reduces chances of survival

Adaptive Traits

Traits that increase chance of survival

Types of Evidence that support Evolution

1) biogeography (geographical)

2) molecular level comparisons (DNA and Amino Acid Comparisions) (biochemical)

3) morphological homologies (homologous and vestigial structures) (physical)

4) comparative embryology

5) mathematical modeling

From CED: "Evolution is supported by scientific evidence from many disciplines (geographical, geological, physical, biochemical, and mathematical data)."

Biogeography

Scientific study of past and present geographic distributions of species. Fossils of different groups of an organism may be found on different continents.

Molecular Level Comparisions (DNA and Amino Acid Comparisions)

A comparison of DNA nucleotide sequences and/or protein amino acid sequences provides evidence for evolution and common ancestry.

Comparative Embryology

the study of the similarities and differences in the embryos of different species - embryos across most species are very similar!

Morphological Homologies

Represent features shared by common ancestry - Homologous and Vestigial Structures.

From CED: "Morphological homologies, including vestigial structures, represent features shared by common ancestry."

Homologus Structures

Variations on structure that was present in a common ancestor.

Think: Not necessarily similar function, yes COMMON ANCESTRY

Ex. Whale Fin, Bat Wing

Vestigial Structures

Reduced/obsolte features that serve little/no purpose for an organism. They are the remnants of features that served function in organism's ancestors.

Ex. Human Appendix

Does the Fossil Record provide evidence for Evolution?

Yes.

What methods can fossils by dated?

Fossils can be dated by a variety of methods.

These include:

1) The age of the rocks where a fossil is found

2) The rate of decay of isotopes including carbon-14

3) Geographical data

Researchers dated several fossils using Carbon-14--how can researchers tell what species is the oldest using Carbon-14?

Carbon-14 is incorporated into living organisms based on the percent of carbon−14 in the atmosphere. After the organism dies, carbon−14 breaks down by radioactive decay. Therefore the specimen with the lowest percent of the original carbon−14 remaining would be the oldest.

Think: Lowest amount of Carbon-14 = oldest

3 pieces of biochemical evidence for evolution

Comparision of DNA Nucleotide Sequences and/or protein Amino Acid Sequences provides evidence for evolution and a common ancestry.

1) Same or highly similar genetic code

2) Same or highly similar process of gene expression

3) same genetic language of DNA and RNA

Differentiate between extant and extinct species

extant species = currently living today on earth

extinct species = no longer living on earth

Think:

(a) Extant Species --> Still Existing

(b) Extinct Species --> gone forever

Describe the fundamental molecular and cellular features shared across all domains of life, which provide evidence of common ancestry

a) Many fundamental molecular and cellular features and processes are conserved across organisms.

b) Structural and functional evidence supports the relatedness of organisms in all domains

3 types of structural evidence supporting COMMON ANCESTRY of all EUKARYOTES

1) membrane-bound organelles

2) linear-shaped chromosomes

3) genes that contain introns

Membrane-Bound Organelles

Variations other eukaryotic organelles have have a similar structure, and thus, a similar function, as structure determines function! These theme connects to Unit #2.

Ex. ER, Golgi Apparatus, Vacuoles, Lysosomes

Structural Similarites of Chloroplasts and Mitochondria

1) double membrane present

2) contain circular genomes (DNA)

3) contain ribosomes

4) endosymbiotic theory

Endosymbiotic Theory

Mitochondria and Chloroplast were once prokaryotes but englufed by another prokaryote and were conserved due to symbiosis as Mitochondria and Chloroplast produced energy (ATP!) for the cell.

This connects to topic 2.11 of the AP Biology Curriculum.

Differentiate between Eukaryotic and Prokaryotic Genomes

Eukaryotic genomes contain multiple linear chromosomes. Prokaryotic genomes contain singular circular chromosomes, stored (occurs) in the cytoplasm, and has small genomes.

Eukaryotic genomes have chromosomes made up on tightly coiled DNA w/ histone proteins, chromsomes capped w/ telomeres, found inside nucleus of large genomes.

Eukaryotic genes contain introns but prokaryotic genes do not.

Why does life evolve?

Life evolves because environments continue to change

3 mechanisms of Genetic Change

1) changes in DNA

2) cell division

3) envrionmental disruptions

Changes in DNA

Gene Mutations, Chromosomal Mutations

Cell Division

Meiosis & Sexual Reproduction increases GAMETE Diversity due to Independent Assortment during Metaphase I and Crossing Over in Prophase I

Environmental Disruptions

Sudden changes in environment, changes in allele and/or genotype frequencies within a population

What do transitional fossils show us?

Evolutionary changes as one group evolves into another

Why does resistance to chemicals evolve?

Natural Selection - If having the resistance to chemicals trait is advantageous (selected for) or leads to greater fitness, Natural Selection will select for it. It is an ongoing process.

Pathogens

Infectious agents that can produce a disease

Pathogens _______ withthe host.

Pathogens COEVOLVE with the host.

Pathogenic genomes experience high mutation rates --> generates ncreased diversity

Pathogens are chemically compatible with host --> coevolve with host

Presence of pathogens can change phenotypes selected for or against in host population

Explain how evolution is an ongoing process in all living organisms.

Populations of organisms continue to evolve.

All species have evolved and continue to evolve—

a) Genomic changes over time.

b) Continuous change in the fossil record.

c) Evolution of resistance to antibiotics, pesticides, herbicides, or chemotherapy drugs.

d) Pathogens evolve and cause emergent diseases

Phylogenetic Trees & Cladograms

Phylogenetic trees and cladograms both show relationships between lineages, but phylogenetic trees show the amount of change over time calibrated by fossils or a molecular clock.

Shared, derived characteres indicate what?

Shared, derived characters indicate common ancestry and are informative for the construction of phylogenetic trees and cladograms.

Out-group

The out-group represents the lineage that is least closely related to the remainder of the organisms in the phylogenetic tree or cladogram.

Does having molecular data lead to making a more accurate, reliable phylogenetic tree or cladogram?

YES! Molecular data typically provide more accurate and reliable evidence than morphological traits in the construction of phylogenetic trees or cladograms.

Describe how DNA sequence Divergence can give scientists info about relatedness

DNA Sequence Divergence can give scientists info about relatedness as if there's less divergence, it means the organism is more closely related. Likewise, if there's more divergence, it means less closely related.

Think: ⬇️ Sequence Divergence = ⬆️ Closely Related

Explain how a phylogenetic tree and/or cladogram can be used to infer evolutionary relatedness.

Phylogenetic trees and cladograms can be used to illustrate speciation that has occurred. The nodes on a tree represent the most recent common ancestor of any two groups or lineages.

Phylogenetic trees and cladograms can be constructed from morphological similarities of living or fossil species and from DNA and protein sequence similarities

Phylogenetic trees and cladograms represent hypotheses and are constantly being revised, based on evidence

Speciation

Process by which one species splits into two or more species

Microevolution

changes over time in allele frequencies in a population

Macroevolution

Broad pattern of evolution above the species level

Ex. Origine of new groups of organisms like mammals or flowering plants

Species Definition, using the Biological Species Concept

A group of populations whose members jave potential to interbreed in nature and produce viable fertile offspring but do NOT produce viable, fertile offspring with members of other such groups.

From CED: The biological species concept provides a commonly used definition of species for sexually reproducing organisms. It states that species can be defined as a group capable of interbreeding and exchanging genetic information to produce viable, fertile offspring

What's required for the formation of a NEW species?

Repductive Isolation

Reproductive Isolation

Refers to existence of factors (barriers) that block/impede members of 2 species from interbreeding and producing viable, fertile offspring. Blocks gene flow between species.

Reproductive Isolation Barriers can be divided into Prezygotic Barriers and Postzygotic Barriers

Prezygotic Barriers

Before zygote, block fertilzation from happening

Ex. Habitat isolation, Temporal Isolation, Behavioral Isolation, Mechanical Isolation, Gametic Isolation

Postzygotic Barriers

After zygote

Ex. Reduced Hybrid Viability, Reduced Hybrid Fertility, Hybrid Breakdown