57 terms

EEB 120

Directional Selection
selection favors
one extreme, trait distribution shifts in one
Stabilizing Selection
favors intermediate values (disfavors
both extremes), trait distribution
Disruptive Selection
favors both extremes (disfavors
intermediate values), trait
distribution splits
Gene flow
movement of alleles
within or between populations
• may be facilitated by movement of
adult individuals, larvae, or gametes
• but alleles can also just move through
large through populations over
successive generations even with little
movement of individuals (typically
more slowly in this context)
FST = (HT - HS) / HT
FST compares actual heterozygosity of a hypothesized subpopulation
to the expected heterozygosity based on allele frequencies of the
entire population

HS = observed heterozygosity of subpopulation
HT = expected heterozygosity based on allele freq. of entire pop.

When FST = 0, there is no population subdivision
When FST = 1, population subdivision is complete
mating throughout a population is
usually non-random due to geographic barriers, distance, behavior, or
other factors
Reduces genotypic diversity in small wild populations
(e.g. following a bottleneck or founder effect)
identical by descent
increased homozygosity caused by inheritance
of exact same copy of rare allele
inbreeding depression
deleterious recessive alleles combine more often than usual
inbreeding coefficient (F)
probability that both copies of an allele will
be identical by descent
Balancing Selection
two or more alleles are maintained in the population by selection for heterozygous genotype (selection that equally favors both homozygous genotypes could theoretically have a similar effect)
Negative frequency-dependent selection
fitness of alternate
genotypes increases as each becomes more rare in the population
conspicuous traits (usually coloration) serve as warning for predators to avoid venomous/ toxic prey
Selective Sweeps
When a highly beneficial allele arises in a population, it can "sweep" to fixation (or near fixation) rapidly
homologous genes separated by a speciation event (A1 & A2)
homologous genes separated by gene duplication (A & B), paralogs together forms a gene family
synonymous substitution (aka silent)
substitutions that do not alter the amino acid sequence of a protein, but changes in DNA. because these substitution do not affect the protein an organism produces, they are less prone to selection and often free from selection completely.
non-synonymous substitution
changes in amino acid and DNA.
-can affect the phenotype and are therfore more subject to selection
Neutral Theory
-predicts that most DNA evolves in a steady, clock-like fashion (constant rate of mutation)
- describes the pattern of nucleotide sequence evolution under the forces of mutation and random genetic drift in the absence of selection
-predicts that the neutral mutations will yield nucleotide substitutions in a population at a rate equivalent to the rate of mutation, regardless of the size of the population
amount of DNA (in picograms) in the haploid nucleus of a species (i.e., a measure of genome size)
C-value paradox
outdated term refers to the apparent paradox of the lack of correlation between genome size and organismal complexity (presumed number of genes)
C-value enigma
updated term reflects our awareness of functional non-coding DNA, but we still don't understand why its abundance varies so widely among eukaryotes
Species concept
a set of criteria used to determine whether individuals belong to the same species or not
Biological Species Concept
a species consists of populations of organisms that can and do produce viable offspring when given the opportunity and do not interbreed with other populations
Phylogenetic Species Concept
species are defined by genetic
distinctiveness, specifically by monophyletic groups separated by relatively long branches
Post-zygotic isolation
individuals can and do mate, but they produce offspring with decreased fitness or no offspring at all
Pre-zygotic isolation
individuals either avoid mating or are unable to produce a zygote due to genetic incompatibilities
Intraspecific variation
Individuals of the same species look very different
Interspecific variation
different species may go unnoticed
Phylogenetic species concept
species are defined by genetic distinctiveness, specifically by monophyletic groups separated by relatively long branches
Cryptic species
Look the same and may be recognized as one species
Reproductive isolation
when 2 populations either cannot or do not interbreed when given the opportunity
Allele Fixation
-When two populations of the same species stop interbreeding with each other
-Drift and/or selection happens
-Different alleles become fixed/ lost in each population= lineage sorting
-As time passes, populations continue to diverge until each one is a new species
Allopatric speciation
Allopatric= " different place" -> physical separation sends each population down its own evolutionary path
individuals colonize a new area
new geographic barrier separates individuals
-Plate tectonics
-Island formation
-Climate (glaciers range shifts)
Reproductive isolation
established as different alleles become fixed/ host in each mating population
-either pre or post zygotic
Prezygotic isolation
individuals either avoid mating or are unable to produce a zygote due to genetic incompatibilities
Post-zygotic isolation
individuals can and do mate, but they produce offspring with decreased fitness or no offspring at all
selection against hybridizing behavior between individuals from diverging populations
Host shift
resulted in reproductive isolation in both space and time
-a population's response to: natural selection
-adaptive phenotypes are often easy to observe and make intuitive sense
Regulatory network
entire biological circuit that controls gene expression
Promiscuous protein
protein with more than one function
Gene duplication
mutation that creates an entire new copy of a gene
Gene recruitment
existing gene gains a new function
Cis-regulatory element
region of DNA in the viscinity of a protein- coding gene that helps control its expression
region of DNA near 5' end of a gene that initiates transcription
Transcription factor
protein that binds to enhancer or promoter regions of DNA, up- or down- regulating gene expression
when one copy of a duplicated gene gains a new function while the other maintains its original function
the process by which, looking back through time, the genealogy of any pair of homologous alleles merges in a common ancestor.
incomplete lineage sorting
describes the case when the history of a gene differs from the history of species carrying the gene
separate lineages can independently arrive at the same character state.
Maximum parsimony
a statistical method for reconstructing phylogenies that identifies the tree topology that minimizes the total amount of change, or the number of steps, required to fit the data to the tree
noncoding stretching of DNA containing strings of short (1-6 base pairs), repeated segments. The number of repetitive segments can be highly polymorphic, and for this reason microsatellites are valuable genetic characters for comparing populations and for assigning relatedness among individuals (DNA fingerprinting).
Purifying selectiion
-removes deleterious alleles from a population.
-also known as negative selection
-common form of stabilizing selection
molecular clock
-a method used to determine time based on base-pair substitutions
-use the rates of molecular change to deduce the divergence time between two lineages in a phylogeny
-calibrated with other markers of time, such as fossils wit known age and palcement