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AP Bio Unit 7 Cheat Sheet (DRAFT) by

AP Biology Unit 7 Natural Selection

This is a draft cheat sheet. It is a work in progress and is not finished yet.

Natural Selection Overview

organisms with heritable traits that favor survival (provide a compet­itive advantage) tend to survive longer and are able to create a greater amount of offspring that are likely to inherit this favorable trait
As organisms with favorable traits are able to create more offspring with this trait, the presence of the trait (often repres­ented by allele frequency) will increase within the population over time

Types of Natural Selection

Artificial Selection
humans select desirable traits and breed organisms to produce these traits, rather than allowing organisms to reproduce (evolve and change gradually) without human interf­erence
Stabil­izing Selection
occurs when selective pressures work against two extremes of a trait in favor of the interm­ediate or "­mid­dle­" trait
Direct­ional Selection
selective pressures work in favor of one extreme of a trait
Disruptive Selection
selective pressures work in favor of two extremes of a trait against the interm­ediate trait

Stabil­izing Selection

Direct­ional Selection

DIsruptive Selection


Describe the types of data that provide evidence for evolution
evolution is supported by scientific evidence from many different discip­lines - geogra­phical, geolog­ical, physical, bioche­mical, and mathem­atical data
Explain how morpho­log­ical, bioche­mical, and geological data provide evidence that organisms have changed over time
molecular, morpho­log­ical, and genetic evidence from present and extinct organisms adds to our understand of evolution fossils can be dated by a variety of methods - the age of the rocks where a fossil is found, the rate of decay of isotopes, and geogra­phical data morpho­logical homologies represent features shared by common ancestry a comparison of DNA nucleotide sequences and/or protein amino acid sequences provides evidence for evolution and common ancestry
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: genomic changes over time, continuous change in fossil records, evolution of resistance to antibi­otics, pestic­ides, herbic­ides, or chemot­herapy drugs, and pathogens evolve and cause emergent diseases evolution ensures that organisms are fully adapted to their surrou­ndings, and gives rise to new species, as well as making others extinct
Describe the types of evidence that can be used to infer an evolut­ionary relati­onship
phylog­enetic trees and cladograms show evolut­ionary relati­onships among lineages phylog­enetic trees show the amount of change over time calibrated by fossils or a molecular clock traits that are either gained or lost during evolution can be used to construct phylog­enetic trees and cladograms molecular data typically provides more accurate and reliable evidence than morpho­logical traits in the constr­uction of phylog­enetic trees or cladograms


Importance of phenotypic variation
Phenotypic variation is important because the enviro­nment may change at any point to favor different traits. If there is not variation in a population when the enviro­nment changes the population may not be able to survive to change with the enviro­nment.
Acquired charac­ter­istics
modifi­cations caused by an indivi­dual’s enviro­nment that can be inherited by its offspring
Population variation
distri­bution of phenotypes in a population
genetic differ­ences among indivi­duals in a population


Effects of enviro­nment on changes in the population
change in an organisms enviro­nment forces the organism to adapt to fit the new enviro­nment, eventually causing it to evolve into a new species convergent evolution occurs when similar selective pressures result in similar phenotypic adapta­tions in different popula­tions or species
heritable trait or behavior in an organism that aids in its survival and reprod­uction in its present enviro­nment
Allopatric speciation
speciation that occurs via geographic separation
Habitat isolation
reprod­uctive isolation resulting when popula­tions of a species move or are moved to a new habitat, taking up residence in a place that no longer overlaps with the other popula­tions of the same species
Bottleneck effect
magnif­ication of genetic drift as a result of natural events or catast­rophes
Geogra­phical variation
differ­ences in the phenotypic variation between popula­tions that are separated geogra­phi­cally
Selective pressure
enviro­nmental factor that causes one phenotype to be better than another

Hardy Weinberg Equili­brium

The Hardy-­Wei­nberg equation operates under the following assump­tions:
The population contains only diploid organisms that reproduce sexually. Genera­tions do not overlap and mating occurs randomly. The population size is infinitely large. Allele freque­ncies are roughly equal between the sexes. There is no mutation, migration, or selection occurring in the popula­tion.
Hardy–­Wei­nberg principle of equili­brium
a stable, non-ev­olving state of a population in which allelic freque­ncies are stable over time
explain the impacts on the population if any of the conditions of Hardy-­Wei­nberg are not met
changes in allele freque­ncies provide evidence for the occurrence of evolution in a population small popula­tions are more suscep­tible to random enviro­nmental impact than large popula­tions leads to variation in a population

Hardy Weinberg Eq.


Random Occurr­ences

explain how random occurr­ences affect the genetic makeup of a population
evolution is also driven by random occurr­ences mutations is a random process that contri­butes to evolution genetic drift is a nonsel­ective process occurring in small popula­tions: bottle­necks and founders effect migrat­ion­/gene flow can drive evolution
Bottleneck effect
magnif­ication of genetic drift as a result of natural events or catast­rophes
Founder effect
event that initiates an allele frequency change in part of the popula­tion, which is not typical of the original population


Gene flow
flow of alleles in and out of a population due to the migration of indivi­duals or gametes
Gene pool
all of the alleles carried by all of the indivi­duals in the population
Genetic drift
effect of chance on a popula­tion’s gene pool
Genetic structure
distri­bution of the different possible genotypes in a population
Genetic variance
diversity of alleles and genotypes in a population
Genotype frequency
the proportion of a specific genotype in a population relative to all other genotypes for those genes that are present in the population


Reprod­uctive isolation
situation that occurs when a species is reprod­uct­ively indepe­ndent from other species; this may be brought about by behavior, location, or reprod­uctive barriers
Assort­ative mating
when indivi­duals tend to mate with those who are phenot­ypi­cally similar to themselves
Evolut­ionary fitness
indivi­dual’s ability to survive and reproduce
measure of successful reprod­uction, the passing on alleles to the next generation
mating of closely related indivi­duals
Nonrandom mating
changes in a popula­tion’s gene pool due to mate choice or other forces that cause indivi­duals to mate with certain phenotypes more than others