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5.2 Natural selection Cheat Sheet by

Cheat sheet for IB Biology HL Chapter 5.2 (2016 syllabus)

Natural selection

Natural selection increases the frequency of charac­ter­istics that make indivi­duals better adapted and decreases the frequency of other charac­ter­istics leading to changes within the species.
According to the theory of natural selection posed by Charles Darwin and Alfred Wallace, the organism which is most responsive to change is most likely to survive.
Key components of natural selection
Inherited variation
There is genetic variation within a population that can also be inherited.
There is a struggle for survival as species tend to produce more offspring than the enviro­nment can support.
Enviro­nmental pressures lead to differ­ential reprod­uction within a popula­tion.
Indivi­duals with beneficial traits will be more likely to survive and pass these traits on to their offspring.
Over time, there is a change in allele frequency within the population gene pool.


Species tend to produce more offspring than the enviro­nment can support
The Malthusian dilemma, proposed by Thomas Malthus, states that population multiply expone­nti­ally, while food resources multiple linearly. This means that a stable population will eventually outgrow its reoousrce base, leading to compet­ition for survival.
When there is an abundance of resources, a population will grow according to its biotic potential (expon­ential J-curve). With more offspring, there are less resources available to other members of the population (envir­onm­ental resist­ance). This will lead to a struggle for survival and an increase in the mortality rate (causing population growth to slow and plateau).

Malthusian dilemma

Allele frequency

Indivi­duals that are better adapted tend to survive and produce more offspring while the less well adapted tend to die or produce fewer offspring
The variation that exists within a population is genetic and determined by alleles.
Alleles encode for the phenotypic polymo­rphisms of a particular trait and may be benefi­cial, detrim­ental or neutral.
Due to natural selection, the proportion of different alleles will change across genera­tions (evolu­tion).
As beneficial alleles improve reprod­uction prospects, they are more likely to be inherited by future genera­tions.
Detrim­ental allies result in fewer offspring and are less likely to be present in future genera­tions.
If enviro­nmental conditions change, what consti­tutes a beneficial or detrim­ental trait may change, and thus allele freque­ncies in the population are constantly evolving.

Natural Selection on Daphne Major

Antibiotic resistance

Antibi­otics are chemicals produced by microbes that either kill or inhibit the growth of bacteria (bacte­ricidal and bacter­ios­tatic respec­tiv­ely).
In a bacterial colony, over many genera­tions, a small proportion of bacteria may develop antibiotic resistance via gene mutation.
If antibi­otics are used to treat these infect­ions, it acts as a selective pressure and causes the antibiotic resistance gene to become more prevalent.
An example of antibiotic resistance is the evolution of Staphy­loc­occus aureus, which evolved to be methic­illin resistant, and infections cannot be treated by that antibi­otic.


Natural selection can only occur if there is variation among members of the same species
Mutation, meiosis and sexual reprod­uction causes variation between indivi­duals in a species
Natural selection needs variation, as it allows for differ­ent­iation for survival.
The three main mechanisms for genetic variation in a species are mutation, meiosis and sexual reprod­uction.
A gene mutation is a change in the nucleotide sequence of a section of DNA coding for a specific trait.
Meiosis promotes variation by creating new gene combin­ations via either crossing over or indepe­ndent assort­ment.
Crossing over
It involves. the exchange of segments of DNA between homologous chromo­somes during Prophase I.
Indepe­ndent assortment
The orient­ation of each bivalent during Metaphase I occurs indepe­nde­ntly, meaning different combin­ations of chromo­somes can be inherited when the bivalents separate in Anaphase I.
Sexual reprod­uction
As meiosis results in geneti­cally distinct gametes, random fertil­isation by egg and sperm will always result in different zygotes.
For mutations and meiosis, refer to Unit 3: Genetics
For sexual reprod­uction, refer to Unit 11.4: Sexual reprod­uction


Adapta­tions are charac­ter­istics that make an individual suited to its enviro­nment and way of life
Indivi­duals that reproduce pass on charac­ter­istics to their offspring
Adapta­tions are features of organisms that aid their survival by allowing them to be better suited to their enviro­nment.
Classi­fic­ations of adapta­tions
Physical differ­ences in biological structure.
Differ­ences in patterns of of activity.
Variations in detection and response by vital organs.
Differ­ences in molecular compos­ition of cells and enzyme functions.
Variable changes that occur across the lifespan of an organism.
Biological adapta­tions have a genetic basis and may be passed to offspring when the parents reproduce.

Adaptive radiation

Changes in beaks of finches on Daphne Major.
Adaptive radiation is the rapid evolut­ionary divers­ifi­cation of a single ancestral line and occurs when members of a single species occupy a variety of distinct niches with different enviro­nmental condit­ions.
Daphne Major is a volcanic island that forms part of the Galápagos Islands and is the habitat of a variety of bird species, known as Darwin's finches, subfamily Geospi­zinae.
These finches shoed adaptive radiation and marked variation is beak size and shape according to diet. (Smaller beaks - smaller seeds, larger beaks - larger seeds).
In 1977, an extended drought changed the frequency of larger beak sizes within the population by natural selection.
The dry conditions result in plants producing larger seeds with tougher seed casings.
Between 1976 and 1978 there was a change in average beak depth within the finch popula­tion.
Finches with larger beaks were better equipped to feed on the seeds and this produced more offspring with larger beaks.


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