midterm 2 Flashcards
what two factors contribute to one’s phenotype?
- their genotype (set of alleles possessed by the individual)
- the environment in which the individual lives
what is population genetics?
the study of patterns of genetic variation in natural populations
gene pool
all the alleles present in all individuals in a population or species
species
group of individual that are capable, through reproduction, of sharing alleles with one another
– exchange genetic info through interbreeding to produce fertile offspring
populations
interbreeding groups of organisms of the same species living in the same geographical area
two sources of genetic variation
- mutation
- recombination
– result in new alleles
somatic mutations
occur in the body’s tissues in nonreproductive cells
–affects only the cells descended from one cell in which the mutation originally arose –> only affects that one individual
germ-line mutations
occur in reproductive cells
– passed on to the next generation & appear in every cell of an offspring
neutral mutations
have little to no effect
deleterious mutations
harmful effect on an organism
advantageous mutations
improve their carriers’ chances of survival or reproduction
- can increase in frequency in a population until eventually they are carried by every member of the species
–result in species that are better adapted to their environment
allele freqeuncy
of an allele x, is the number of x’s present in the population divided by the total number of alleles
fixation
-when a population exhibits only one allele at a particular gene
-one alleles replaces all other alleles in a population
genotype frequency
proportion in a population of each genotype at a particular gene or set of genes
polymorphisms
any genetic difference among individuals that are present in multiple individuals in a population
what does it mean to say an allele is “fixed” in a population?
if there is only one allele for that gene in the population (frequency is 1)
evolution
change in the frequency of an allele or a genotype from one generation to the next
-only occurs if specific forces act on the population
hardy-weinberg equilibrium
describes the situation in which evolution does NOToccur
- to determine whether evolutionary forces are at work, we need to determine whether a population is in HWE
HWE conditions to be in equilibrium
- no gene flow
- very large population size
- no natural selection
- no mutation
- individuals mate at random
genetic drift
a change in the frequency of an allele due to the random effects of a small population size
nonrandom mating
occurs when individuals do not mate at random
-affects genotype frequencies but not allele frequencies
frequency of AA in HWE
p^2
frequency Aa in HWE
2pq
frequency of aa in HWE
q^2
relationships when HWE conditions are not met
allele frequencies can be determined from genotype frequencies, but genotype frequencies cannot be determined from allele frequencies
primary mechanisms of evolution
1.natural selection
2. genetic drift
3. migration
4. mutation
5. nonrandom mating
natural selection
results in allele frequencies changing from generation to generation according to the allele’s impact on the survival & reproduction of individuals
adaptations
the exquisite fit of organisms to their environment
- darwin suggested natural selection brings about adaptation
-natural selection increases the frequency of advantageous alleles, resulting in adaptation
observations natural selection depends upon
- there is variation among individuals in a species
- some of this variation is heritable
- individuals often compete for resources in nature
- genetic variation results in some individuals that are more likely than others to survive & reproduce
fitness
a measure of the ability of an individual to survive & reproduce in a particular environment
-the extent to which the individual’s genotype is represented in the next generation
-one’s fitness is higher than the other’s if they leave more surviving offspring
modern synthesis
current theory of evolution that combines Darwin’s theory of natural selection & Mendelian genetics
-multiple genes per trait could account for patterns of continuous variation
positive selection
natural selection that increases the frequency of an advantageous allele
negative selection
reduces the frequency of deleterious allele
what happens when a deleterious allele is recessive
natural selection is inefficient at eliminating it
-negative selection will only act against it when it is present as a homozygote (and the deleterious phenotype is expressed)
balancing selection
natural selection can maintain an allele at some intermediate frequency between 0 and 1
-acts to maintain two or more alleles of a given gene in a population
heterozygote advantage
form of balancing selection in which the heterozygote’s fitness is higher than than of the homozygotes, resulting in selection that ensures both alleles remain in the population at intermediate frequencies
stabilizing selection
selects against extremes & therefore maintains the status quo
-acts in favor of intermediate phenotypes
-graph looks like bell curve
directional selection
leads to a change in a trait over time
-selection in favor of one extreme against another
-graphs are kinda in parallel
-tends to occur when the environment changes
disruptive selection
acts in favor of extremes & against intermediate forms
-acts against intermediates between the 2 groups
-graph looks like 2 flipped images
selective pressure
the set of environmental conditions & biological that result in some organisms surviving & reproducing better than others
-in a stable environment, these pressures are constant
artificial selection
-form of directional selection
-humans select for traits
sexual selection
-acts in the opposite direction as natural selection
-promotes traits that increase an individual’s access to reproductive opportunities
intrasexual selection
involves interactions between individuals of one sex, as when members of one sex compete with another for access to the other sex
intersexual selection
involves the interaction between males & females, as when females choose from among males
genetic drift
the random change in allele frequencies from generation to generation
-dramatically affects small populations
-marked loss of genetic variation across the entire genome
-does not lead to adaptations
population bottleneck
-genetic drift is a result of this
-occurs when an originally large population is reduced to just a few individuals
founder event
-genetic drift is a result
-occurs when a few individuals start a new population
migration
movement of individuals from one population to another
-results in gene flow
-consequence is the homogenization of populations, reducing genetic differences between them
gene flow
the movement of alleles from one population to another
nonrandom mating
individuals choose mates according to their genotypes –> certain phenotypes increase and others decrease
-just redistributes alleles already in the gene pool & does not add new alleles to the population
inbreeding
-form on nonrandom mating
-mating occurs between close relatives
-increases the frequency of homozygotes & decreases that of heterozygotes
inbreeding depression
if an alleles is a deleterious recessive mutation, a reduction in its fitness occurs due to homozygosity
species are reproductively isolated from other species
on a graph, dots–representing individual organisms–fall into overlapping clusters
-the fact that clusters are distinct implies that species are biologically real
biological species concept (BSC)
species are groups of actually or potentially interbreeding populations that are reproductively isolated from other such groups
-offspring must be fertile & capable of passing their genes onto their own offspring
-ability to exchange genetic material by producing fertile offspring
-does not apply to asexual or extinct organisms –> BSC is based on the exchange of sexual genetic info
morphospecies concept
holds that members of the same species usually look alike
cryptic species
consist of organisms that had been traditionally considered to belong to a single species because they look similar, but turn out to belong to two species because of differences at the DNA sequence level
hybrid offpspring
the offspring produced by a cross; sometimes applied specifically to interbreeding two closely related species
niche
a complete description of the role a species plays in its environment – its habitat requirements, nutritional & water needs, and the like
ecological species concept
the idea that there is a one-to-one correspondence between a species & its niche
-it is impossible for 2 species to coexist in the same location if their niches are too similar b/c competition inevitably leads to extinction
factors that cause reproductive isolation
prezygotic & postzygotic
phylogenetic species concept
members of a species all share a common ancestry & a common fate
prezygotic isolating factors
act before the fertilization of an egg
-prevent fertilization from taking place
-barrier to gene flow
postzygotic isolating factors
come into play after fertilization
-result in the failure of the fertilized egg to develop into a fertile individual
-barrier to gene flow
geographic isolation
a form of prezygotic isolation in which individuals are separated in space
ecological isolation
a form of prezygotic isolation in which individuals are separated on the basis of where they live or what they do in the environment
behavioral isolation
a form of prezygotic isolation in which individuals only mate with other individuals on the basis of specific courtship ritual, songs, and other behaviors
temporal isolation
a form of prezygotic isolation in which individuals are reproductively active at different times
gametic isolation
a form of prezygotic isolation in which there is incompatibility between the gametes of different individuals
mechanical isolation
a form of prezygotic isolation in which individuals are unable to mate, for example, because of structural incompatibility of genitalia
genetic incompatibility
genetic dissimilarity between two organism that is sufficient to act as postzygotic isolating factor
hybrid inviability
form of postzygotic isolation factor where embryo forms but does not fully develop
hybrid sterility
form of postzygotic isolation factor where offspring are produced but are sterile
speciation
by-product of the genetic divergence of separated populations
-occurs when 2 populations are no longer able to produce viable, fertile offspring
-the development of reproductive isolation between populations
-caused by the accumulation of genetic differences between populations
partially reproductively isoloated
two populations that have genetically diverged but not yet diverged far enough for full reproductive isolation
allopatric speciation
-occurs when populations are geographically separated from each other
-accumulate population-specific traits
subspecies
allopatric populations that have yet to evolve to even partial reproductive isolation but that have acquired population-specific traits
dispersal
-manner in which a population can become allopatric
-some individuals colonize a distant place far from the population source
vicariance
-manner in which a population can become allopatric
-geographic barrier arises within a single population, separating it into two or more isolated populations
peripatric speciation
-specific kind of allopatric speciation in which a few individuals from a mainland population (central population of a species) disperse to a new location (island population) remote from the original population & evolve separately
adaptive radiation
a bout of unusually rapid evolutionary diversification in which natural selection accelerates the rates of both speciation & adaptation
co-speciation
a process in which 2 groups of organisms speciate in response to each other & at the same time
how can a mutation become fixed in a population
through natural selection if its advantageous or through genetic drift if its neutral
sympatric speciation
speciation that occurs when populations are in the same geographic location
-natural selection (disruptive selection) must act strongly to counteract the homogenizing effect of gene flow
instantaneous speciation
caused by hybridization between 2 species in which the offspring are reproductively isolated from both parents
altruistic
self-sacrificial behavior in which an individual’s actions decrease its own fitness while increasing that of another individual
group selection
selection caused by the differential success of groups rather than individuals
evolutionarily stable strategy
a type of behavior that cannot readily be driven to extinction by an alternative strategy
reciprocal altruism
the exchange of favors between individuals
kin selection
a form of natural selection that favors the spread of alleles promoting behaviors that help close relatives
phylogeny
shows both the evolutionary history & relatedness of groups or organisms
phylogenetic tree
a branching diagram representing a hypothesis about the evolutionary relationships among groups of organisms
-built by comparing their anatomical, physiological, or molecular features
nodes
the point where a branch splits, representing the common ancestor from which the descendant species diverged
what makes two species considered to be closest relatives
if they share a common ancestor not shared by any other species or group
sister groups
a group of species that is more closely related to another group of species than any other group of species
how is closeness of a relationship determined in a phylogenetic tree
by looking at how recently two groups shared a common ancestor, which is indicated by a node, or branch point on the tree
-follow nodes from the tips of the branches to the root of the tree
taxa
a named taxonomic group at any rank, such as a species, a genus, or a family
monophyletic groups/clades
all members share a single common ancestor that is not shared with any other species or group of species (ex: amphibians)
paraphyletic groups
includes some, but not all, descendants of a common ancestor
polyphyletic groups
groupings that do not include the last common ancestor of all members
-share traits that evolved independently by convergent evolution
genus
closely related species
family
closely related genera
order
closely related families
class
closely related orders
phylum
closely relate classes
kingsom
closely related phyla
domains
3 largest limbs of the entire tree of life (Eukarya, Bacteria, and Archaea)
characters
the morphological, physiological, or molecular features that make up an organism
character states
characters have several observed conditions
- a character can be present or absent
- or there can be multiple character states
convergent evolution
character state may have evolved independently in the two groups as an adaptation to similar environments
homologous
characters that are similar because of descent from a common ancestor
analogous
similarities due to independent adaptation by different species
-result of convergent evolution
synapomorphies
a shared derived character; the basis of cladistic phylogenetic reconstruction
outgroup
a branch that splits off closer to the root of the tree and is outside the ingroup
parsimony
choosing the simpler of two or more hypotheses to account for a given set of observations
fossils
remains of once-living organisms, preserved through time in sedimentary rocks
trace fossils
a track or trail, such as a dinosaur track or the feeding trails of snails, left by an animal as it moves about or burrows into sediments
transitional fossils
helps us understand important steps in evolution
molecular evoution
change in DNA or amino acid sequences over time
what is the extent of sequence difference between 2 speices
a function of time 2 species have been genetically isolated from each other
molecular clock
the correlation between the sequence differences among species & the time since the common ancestry of the species is known
