Author’s Notes

hello there, and welcome to my unit 7 cram sheet on natural selection !! majority of the info is gathered from my teacher lectures and the AP daily videos :)

• like this cram sheet? I made more ! check it out at my profile ;))

• external links to other resources will also be linked to help you better understand this topic !!!!

╭ Other Resources ::: cararra ﹒﹒ 34 min ap bio review video based from the campbell biology 11th edition textbook!

:: khan academy ﹒﹒the entire unit 7 course from khan academy!!

:: fiveable ﹒﹒ reviews unit 7 with articles and quizzes for you to practice your knowledge on!

﹙✦﹚﹒﹒please note that abbreviations will be used throughout this guide (kept at a minimum!)

﹙7.1-7.3, 7.12 - Natural Selection & Diversity﹚

✦﹒natural selection is the process by which organisms, having adaptations suited for a particular environment, have a greater chance of survival and reproduction, thereby passing the adaptations to subsequent generations

• evolution is the change in genetic makeup of a population over time and is supported by multiple lines of evidence

  • natural selection is a major mechanism of evolution

  • driven by random occurrences, cannot continue without genetic variation

• influenced by the following—

  • environmental stability of biotic and abiotic environments

    • environments can experience major disruptions and change quickly/slowly overtime

    • more unstable → faster rate of evolution (stable → slow rate)

  • genetic variation, which refers to genetic differences among organisms within a population

    • mutations and sexual reproduction increase variation

    • reduction of this can increase differences between populations of the same species

  • adaptations, which are traits that provide an advantage in a particular environment

    • those with adaptions will have a greater chance of survival

    • opposite of this → deleterious traits reduce the chance of survival

    • whether or not a trait is advantageous or deleterious is determined by selective pressures of the environment

  • fitness, which refers to the ability of an organism to survive and produce fertile offspring

    • reproductive success refers to the production of offspring

      • traits with more of this will become more common in the population

    • heritability is the ability to pass on adaptations to successive generations

• can result in an increase OR decrease of specific phenotypes within a population over time

✦﹒phenotypic variation increases the probability a population will continue in unstable environments

• natural selection acts on phenotypic variation by selecting individuals with the most advantageous traits based on certain environmental conditions

  • changes in these conditions can change which individuals in a population will be selected for or selected against

• any phenotypes that increases an organism’s chances of survival and reproduction will significantly increase an organism’s fitness

✦﹒artificial selection is the process in which humans select which traits would reproduce instead of nature

• may result in phenotypes that would not exist in nature

• can lead to more/less genetic diversity

• convergent evolution is the process in which similar environmental conditions select for similar traits in different populations or different species over time (analogous structures fit this; see 7.6 for info)

✦﹒genetic diversity is very important in populations

• little genetic diversity risks extinction

• population with more genetic diversity are more resilient to environmental change

  • more likely to contain individuals who can withstand environmental pressures

✦﹒resistance to chemicals become evident thru natural selection

• can be resulted from mutations

• leads to a species’ evolution

• ex: antibiotic resistance in bacteria

  • antibiotic kills all but the ones that are resistant → resistant bacteria are able to reproduce and take over

    • some bacteria can give their drug resistance to other bacteria (recall from unit 6)

╭ Other Resources :

:: amoeba sisters﹒﹒7 min video on natural selection, with examples!

﹙✦﹚﹒all of these concepts are important.. watch the videos (even the evolution one linked), maybe take notes to digest this info

﹙7.4 - Population Genetics﹚

✦﹒genetic drift is the random change in the frequency of a particular allele within a population (nonselective)

• generally occurs in small populations

• ex: four worms of different colors eventually become all the same color over generations)

• bottleneck events can contribute to this (bottleneck → large population is reduced to a small population)

✦﹒migration/gene flow is the movement of individuals between populations causing an exchange of alleles between populations

• introduces new gene info → increases genetic variation

• continued migrated reduces genetic diversity over time

﹙✦﹚﹒very short, but DONT FORGET THIS! its pretty darn important to know for this unit

﹙7.5 - Hardy-Weinberg Equilibrium﹚

✦﹒Hardy-Weinberg is a model for describing and predicting allele frequencies in a nonevolving population

• these conditions must be met to be in a Hardy-Weinberg Equilibrium—

  • large population → no genetic drift

  • absence of migration → no gene flow

  • no mutations → no genes are modified, deleted, or duplicated

  • random mating → no sexual selection

  • no natural selection

• rarely met, but provides a “ideal” population and null hypothesis regarding the chi-square test

• disrupted by opposite conditions

• the equation: p² + 2pq + q² = 1

  • p² → frequency of homozygous dominant genotype

  • 2pq → frequency of heterozygous genotype

  • q → frequency of homozygous recessive genotype

  • determines genotype and/or phenotype frequencies of individuals in a population

  • ANOTHER equation: p + q = 1

    • p → frequency of dominant allele

    • q → frequency of recessive allele

╭ Other Resources :

:: amoeba sisters﹒﹒9 min video talking about the equilibrium with examples and tips upon its usage!

:: bozeman science﹒﹒11 min video on the equilibrium also if amoeba sisters isn’t enough or you prefer bozeman science

﹙✦﹚﹒another math problem.. one big thing is to PRACTICE! there will likely be a mcq regarding this and wanting to know the frequency of an allele or genotype

﹙7.6-7.7, 7.13 - Evolution & Ancestry﹚

✦﹒evolution is supported by many pieces of scientific evidence such as—

• geographical - based on characteristics of a habitat or land area

• geological - based on environmental features of earth over time (fossils)

  • fossils can be dated by the rate of decay of isotopes (carbon-14)

• physical - based on phenotypes of species

• biochemical - based on chemical composition (ex: biomolecules like DNA and proteins) of organisms

  • more amino acid differences between organisms→ more time that has passed since their divergence

• mathematical - based on calculations and statistics (chi-square)

✦﹒morphological homologies represent modified traits shared among different species

• homologous structures are variations in a structure that was present in a common ancestor (ex: bone structure of human arms, cat and horse legs, dolphin fins)

• vestigial structures are reduced/obsolete features that serve little or no purpose for the organism (ex: human tailbone)

• analogous structures are in which distantly related/unrelated organisms have similar traits (ex: fish and mammals have traits like fins under a similar aquatic environment)

✦﹒membrane-bound organelles, linear chromosomes, and introns provide evidence of common ancestry of all eukaryotes

✦﹒many pieces of scientific evidence suggest the origins of life

• chemicals experiments prove that it is possible to form complex organic molecules from inorganic molecules in the absence of life

  • the RNA World Hypothesis proposed that RNA could have been the earliest genetic molecule

• models for the origin of life indicate that life was extraterrestrial or early Earth atmospheric conditions were ideal for forming monomers for life

  • primitive Earth provided inorganic precursors from which organic molecules could have been synthesized

  • there was a presence of free energy and an absence of atmospheric oxygen (O₂)

╭ Other Resources :

:: amoeba sisters﹒﹒this video is VERY in depth (and less old) about evolution and spans talking topics covered in the natural selection part of this sheet

﹙✦﹚﹒best bet is to know the structures and the evidences regarding the origins of life (RNA hypothesis and absence of O₂)

﹙7.9 - Phylogeny﹚

✦﹒phylogentic trees and cladograms are diagrams that show evolutionary relationships among species

• constructed based off of the morphological similarities from living/fossil species and DNA and protein sequence similarities (more accurate)

• out-group is the species that is least closely related to the remainder of the organisms

• node is where the two lines meet, representing the most recent common ancestor

• root represents the common ancestor of all species

╭ Other Resources :

:: pbs﹒﹒fun game that brings the construction of cladograms in practice! it gets harder the farther you go :)

﹙✦﹚﹒definitely get a good grip of how to create a cladogram/phylogenetic tree based off of DNA via differences and understand how to interpret the model itself

﹙7.10 - Speciation﹚

✦﹒speciation is the creation of new species

• results in diversity of life forms

• allopatric speciation is the evolution of a new species due to geographical isolation of the same species (ex: geographical barriers, like a river)

  • no gene flow, and separation may expose populations to different selective pressures

• sympatric speciation is the evolution of a new species due to reproductive isolation from a surviving ancestral population

  • can result from sexual selective, habitat differentiation, and genetic mutations (polyploidy)

• divergent evolution occurs when adaptations to new habitats result in phenotypic diversification, with the possibility of rapid speciation rates

✦﹒reproductive isolation prevents gene flow between populations

• PREzygotic barriers prevent production of a fertilized egg

  • habitat isolation is where species occupy different habitats and rarely come in contact

  • temporal isolation is where species breed during different times of day, seasons or years

  • behavioral isolation is where species have different courtship behaviors/mate preferences

  • mechanical isolation is where reproductive structural differences prevent successful mating and reproduction

  • gamete isolation is where sperm of one species may not be able to fertilize the eggs of another species

• POSTzygotic barriers prevent a zygote from developing into a viable, fertile offspring

✦﹒there are two models that represent the rate of evolution—

• punctuated equilibrium, is evolution that occurs rapidly after a long period of stasis

  • stasis is periods of little to no change

  • changing ecological conditions are the stimulus for evolution

• gradualism is evolution that occurs slowly over a LONG TIME

  • ecological conditions change gradually too

  

╭ Other Resources :

:: amoeba sisters﹒﹒speciation covered by the one and only, amoeba sisters! its 7 minutes.

﹙✦﹚﹒all these are a must know! lots of memorizing :,,)

﹙7.11 - Extinction﹚

✦﹒extinction refers to the disappearance of a species

• can occur on a large scale at one time

  • caused by catastrophic changes to a ecosystem (ex: solar flares, sea levels, etc.)

  • species diversity decreases severely

  • Earth can recover from these events

• human activity can drive changes that cause extinctions (climate change, poaching, habitat loss, etc.)

• an ecosystem’s diversity can be determined by extinction and speciation rates

  • high species biodiversity → high speciation levels, low extinction levels

✦﹒extinction provides newly available niches

• niche describes the role an organism plays within its environment

  • roles include producer, decomposer, scavenger, consumer, etc.

• extinction leaves an open niche for another species to occupy

  • leads to rapid speciation rates and adaptive radiation

﹙✦﹚﹒this is like the only section that talks about niches but trust its gonna come back so understand niches and also generally what extinction is and what it does