Multiple Loci and Selection

Intro to Evolution (Dr. Reed)

Evolution at multiple loci

there are many traits that are controlled by a single gene

the majority of traits are controlled (or at least influenced) by multiple loci - polygenic traits

Continuous variation in Polygenic traits

the presence of polygenic traits results in more continuous variation in traits

Mendel’s pea plants had direct genetic effects and discrete phenotypes

Darwin’s theory hypothesized gradual changes to continuous traits

Polygenic traits allow for this

Epistasis

epistasis is the expression of an allele affected by another allele

the effects are not additive, but interactions

• for example: a lab woth the B allele is black, but only if they have the E allele at another locus

Additive effects

Phenotypes often are affected by multiple loci

the combinations of alleles at these loci then affect the trait

AbCDeF has different phenotype than aBCDEf

selection can favor allele separate or in “groups” where the allele itself isn’t selected but the phenotype is

this can result in a phenotype out side the original range

so population can have a new phenotype without mutation

Linkage

genes and alleles often are inherited randomly

if one inherits allele ABC that has no effect on if they inherit D or d

 ✧ this is not always the case

Haplotype: a set of alleles that are generally inherited as a group

• described by one half of alleles (example: aBc)

This grouping of alleles can be due to several factors

the genes may be physically linked

the are found close on the same chromosome, so crossing over and recombination are uncommon

instead of being independently assorted the alleles are passed as a group

Linkage disequilibrium

alleles can be inherited in non-random patterns for reasons other than physical linkage

we assess this by testing the observed frequency of the haplotype compared to frequency if the alleles were independently passed down

Equation: D = h_AB - f_Af_B

• if the value for D is high we can conclude there is linkage disequilibrium

linkage disequilibrium can be the result of several things, only one of which is related to evolution

• can appear due to migration

  • not so much in contiguous population

  • but insular (island) populations can give the appearance of linkage disequilibrium due to insertion of alleles into population

• Mutation also can give the appearance of linkage disequilibrium

  • same idea as with islands, a new/rare mutation can indicate linkage disequilibrium

• Genetic drift gives the appearance of linkage disequilibrium for the opposite reason

  • alleles are lost in small populations

Linkage disequilibrium due to Selection

selection can favor, or disfavor, different haplotypes

a favored haplotype will appear in the population more than expected and disfavored less than expected

so not selection on a sinlge allele but group

Hardy Weinberg and Linkage disequilibrium tell us if an allele or haplotype are different than we would expect given random assortment. T or F?

True

Consequences of linkage

Loci that are linked don’t work like a single locus

traits that are good or bad can be affected by selection becuase of their linkage

the linkage means that the alleles do not inpendnetly assort

So A is associated with b and a with B

• if A is selected against and if the selection is strong enough then b may be lost too (this is called selective sweep or genetic hitchhiking, alleles are lost because they are linked to a deleterious allele)

Fitness and Selection

Fitness is a measure of the evolutionary success of an individual

• it is expressed in two ways:

  • absolute fitness: the number of offspring an individual has in a lifetime (some use “grandchildren” instead of direct offspring

  • relative fitness: the absolute fitness of an individual divided by the fitness of the individual with the most offspring (varies from 0 to 1, where 1 is the most fit individual)

Selection

selection then acts on the individual to increase or decrease survival and reproduction

this then increases or decreases the fitness of the next generation based on the traits of the individuals

there are two(ish) types of selection:

• artificial selection

• natural selection

• sexual selection

Artificial selection

is differential survival and/or reproduction in individuals based on human decisions

this is often associated with domestic/agricultural plants and animals

the general model is that humans allow individuals with desirable characteristics to breed

• this may increase the prevalence and exaggeration of theser characteristics

• often associated with ag but does apply in natural systems as well

Natural Selection

is differential survival and/or reproduction in individuals naturally

reproduction and survival is different among individuals due to differences in phenotype

this requires a few things:

• genetic/phenotypic variation

• heritability of phenotypes

• differential reproductive success

Heritability

in the simple case we can see dominant/recessive traits that are 100% heritable

there are methods to determine heritability that rely on regression methods

one can experimentally demonstrate the heritability of a trait

• beach mice bread with a different species and the F1 generation built a characteristic complex burrow

Differential reproduction

differential survival impacts reproduction

increases in fitness are not always obvious, but any change is a balancing act

having longer leaves may be good, but if they get too long they interfere with flowers

• this interaction “fine tunes” adaptation (these are called fitness peaks)

the fitness benefits change based on environmental conditions

in one location the peak may be different, or may be a fitness valley

• this is one way that subspecies arise

• a genetically and morphological distinct form of a species

Natural Theory and Selection

‘The majority of mutations at the molecular level are neutral’

Important for genetic diversity and produces phenotypic diversity

The benefit of a mutation may change over time as well

• the first wings was not for flight, later the wings was used in flight (exaptation)

Exaptation

a trait that had one function but later gains another

Evolutionary tradeoffs

new structures, functions or chemicals do not evolve de novo

they come about from modifying an existing structure

this means that the new structure must provide more of a fitness benefit than losing the structure costs

Paenarthrobacter ureafaciens KI72

is a strain of bacteria that can metabolize nylon

Nylon

is an anthropogenic substance

Humans and vitamin C

cannot synthesize vitamin C (ascorbic acid) because of a mutation to the GULO gene

but it may be derived to eliminate a parasite that required ascorbic acid

generally, vitamin C deficiency is not a problem

They Why of Adaptation

assigning a reason for an adaptation is difficult/risky

the benefit of a change may have been in the past and no longer applies

often called the Ghost of Selection Past