23 Flashcards
how does natural selection act on individuals
each individual’s combination of traits affects its survival and reproductive success compared to other individuals
____ are selected; ____ evolve
individuals; populations
microevolution
evolutionary change on its smallest scale; change in the genetic makeup of a population from generation to generation
darwin didn’t have a satisfactory explanation for
how the heritable variations required for natural selection appear in populations or how organisms transmit these variations to their offspring
model proposed by gregor mendel
particular hypothesis of inheritance
stated that parents pass on discrete heritable units that retain their identities in offspring
darwin considered the raw material for natural selection to be
“quantitative” characters – those characteristics in a population that vary along a continuum
mendel and other early geneticists worked only with
discrete “either-or” traits
geneticists later determined that
quantitative characters are influenced by multiple genetic loci and that the alleles at each of these loci follow Mendelian patterns of inheritance
population genetics
the study of how populations change genetically over time
population genetics gave rise to ____
modern synthesis
modern synthesis
a comprehensive theory of evolution that integrated ideas from many other fields
population
a localized group of individuals that are capable of interbreeding and producing fertile offspring
populations of the same species may be isolated from one another, thus exchanging genetic material only rarely. such isolation is common for populations
confined to different,. widely separated islands or lakes
individuals near the population center are more likely to breed with
members of their own population than other populations and thus on average are more closely related to one another than to members of other populations
gene pool
the aggregate of genes in a population at any one time
the gene pool consists of
all alleles at all gene loci in all individuals of the population
if only one allele exists at a particular locus in a population,
that allele is said to be FIXED in the gene pool, and all individuals are homozygous for that allele
if there are 2 or more alleles for a particular locus in a population,
individuals may be either homozygous or heterozygous
when there are 2 alleles at a particular locus, the convention is to
use p to represent the frequency of one allele and q to represent the frequency of the other allele
at loci that have more than 2 alleles,
the sum of all allele frequencies must still equal 1
hardy-weinberg theorem was derived in the year
1908
the hardy-weinberg theorem describes
the properties of gene pools that aren’t evolving
the hardy-weinberg theorem states that
the frequencies of alleles and genotypes in a population’s gene pool remain constant from generation to generation, provided that only mendelian segregation and recombination of alleles are at work
the hardy-weinberg theorem describes how
mendelian inheritance preserves genetic variation from one generation to the next in populations that aren’t evolving
the hardy-weinberg theorem lays the groundwork for
undeerstading long-term evolutionary changes that darwin, lacking knowledge of genetics, couldn’t have envisioned
the preservation of genetic variation provides the opportunity for
natural selection to act over many generations
the allele frequencies in all the gametes produced by the population
will be the same as in the original population
if the individuals in a population _______ and _______, this population will ______ and ________
donate gametes to the next generation at random and also mate at random;
have the same allele frequencies from one generation to the next, and its genotype frequencies can be predicted from the allele frequencies
a population’s allele and genotype frequencies would remain constant if
a population were in hardy-weinberg equilibrium and its members continued to mate randomly generation after generation
a population ________ for its allele frequencies to remain constant
doesn’t need to be in hardy-weinberg equilibrium
raw material of evolutionary change
genetic variation
conditions for hardy-weinberg equilibirum
- extremely large population size
- no gene flow
- no mutations
- random mating
- no natural selection
genetic drift
chance fluctuations in allele frequencies from one generation to the next
the smaller the population, the greater
the role played by genetic drift
gene flow
transfer of alleles between populations; genetic additions to and/or subtractions from a population resulting from the movement of fertile individuals or gametes
gene flow can
alter allele frequencies
why do mutations modify the gene pool?
b/c they introduce or remove genes from chromosomes or change one allele into another
if individuals preferentially choose mates with certain genotypes,
random mixing of gametes doesn’t occur
departure from the conditions for H-W equil. usually results in
evolution
while natural populations rarely if ever are in true H-W equil., in many populations
the rate of evolutionary change is so slow that these populations appear to be close to equilibrium
the hardy-weinberg equation can be used to
estimate the percentage of the population carrying the allele for an inherited disease
PKU
phenylketonuria, a metabolic disorder that results from homozygosity for a recessive allele. left untreated, it results in mental retardation and other problems
to use the H-W equation, we must assume that
we must also neglect
people don’t choose their mates on the basis of whether or not they carry this a and don’t generally mate w/ close relatives
any effects of gene flow from other populations, and differential survival and reproductive success among genotypes
which 2 processes produce the variation in gene pools that contributes to individual differences?
mutation and sexual recombination
____ and ____ originate only by mutations
new genes and new alleles
mutations
changes in the nucleotide sequence of DNA
most mutations occur in
somatic cells and are lost when the individual dies
only mutations in __________ can be passed to offspring, and _______
cell lines that produce gametes,
only a small fraction of these spread through populations
point mutation
change of one base in a gene
most point mutations are
probably harmless
why are most point mutations harmless?
much of the DNA in eukaryotic genomes doesn’t code for protein products.
and because the genetic code is redundant, even point mutations in genes that code for protein may have little effect because they don’t alter the protein’s amino acid composition
changes in what type of DNA regions can have profound effects?
noncoding regions of DNA that regulate the expression of genes
what type of chromosomal mutations are almost certain to be harmful?
however, when such mutations ______, their effects on organisms may be neutral
chromosomal mutations that delete, disrupt, or rearrange many loci at once
leave genes intact
duplications of chromosome segments are
nearly always harmful
smaller pieces of DNA are often introduced into a genome through
the activity of transposable elements
if a duplicated segment doesn’t have severe effects, it
can persist over generations, providing an expanded genome with new loci that may take on new functions by further mutations and subsequent selection
exons
coding portions of genes
new genes may also arise when
exons are shuffled within the genome
______ appear to have played a major role in evolution
beneficial increases in gene number
mutation rates
tend to be low in animals and plants, averaging about one mutation in every 100,000 per generation
mutations can rapidly generate genetic variation in
microorganisms and viruses with short generation spans
HIV has a generation span of
about 2 days
an RNA genome has
a much higher mutation rate than a typical DNA genome
in sexually reproducing populations, ____ is far more important than _____ on a generation-to-generation time scale in producing the variations that make adaptation possible
sexual recombination; mutation
nearly all phenotypic variations based on genetic differences result fro m
recombinational shuffling of the existing alleles in the gene pool
bacteria and many viruses can also undergo recombination, but
they do so less regularly than animals and plants and often in ways that allow them to cross species barriers
the ability of pathogens to ______, combined with their _______, makes them especially dangerous adversaries
evolve rapidly through extensive recombination; high mutation rates
although new mutations can modify allele frrequencies,
the change from one generation to the next is likely to be small
what does recombination do?
it reshuffles alleles but doesn’t change their frequencies
effect of nonrandom mating
it can affect relative frequencies of homozygous and heterozygous genotypes but by itself usually has no effect on allele frequencies
the 3 major factors that alter allele frequencies annd cause most evolutionary change are
natural selection, genetic drift, and gene flow
result of natural selection in terms of alleles?
selection results in alleles being passed to the next generation in proportions different from their relative frequgencies in the present generation
over time, genetic drift tends to
reduce genetic variation through losses of alleles from the gene pool
2 situations that can increase the likelihood that genetic drift will have a large impact on a population are referred to as
the bottleneck effect and the founder effect
bottleneck effect
a sudden change in the environment (fire/flood) may drastically reduce population size. in effect, the survivors have passed thru a restrictive “bottleneck” and their gene pool may no longer be reflective of the original popoulation’s gene pool
genetic drift may continue to have substantial effects on the gene pool for many generations until
the population is large enough that chance fluctuations have less effect
founder effect
when a few individuals become isolated from a larger population, this smaller group my establish a new population whose gene pool isn’t reflective of the source population. these founders represent a distinct gene pool w/ different allele frequencies from those of the parent population
gene flow tends to
reduce differernces between populations
discrete characters often are determined by
a single gene locus with different alleles that produce distinct phenotypes
most heritable variation consists of
quantitative characters
heritable quantitative variation results from
the influence of 2 or more genes on a single phenotypic character
morphs
different forms that result when individuals differ in a discrete character
phenotypic polymorphism
a population is said to display phenotypic polymorphism for a character if two or more distinct morphs are each represented in high enough frequencies to be readily noticeable
genotypic polymorphism
the existence of 2 or more distinct alleles at a given locus in a population’s gene pool
population geneticists measure the number of polymorphisms in a population by
determining the amount of heterozygosity at both the level of whole genes (gene variability) and the molecular level of DNA (nucleotide variability)
average heterozygosity
measured as the average percent of the loci in the genome that are heterozygous
nucleoitde variability is measured by
comparing the nucleotide sequences of DNA samples from 2 individuals and then averaging the data from many such comparisons
why does average heterozygosity tend to be greater than nucleotide variability
a gene can consist of thousands of bases of DNA. a difference at only one of these bases is sufficient to make 2 alleles of that gene dififerent and increase average heterozygosity
based on measurements of nucleotide variability, humans have
relatively little genetic variation compared to most species. 2 humans differ by only about 0.1% of their bases
geogrphic variation
differences between the gene pools of separate populations or population subgroups
most species exhibit _____ variation
geographic
cline
a graded change in a trait along a geographic axis
in some cases, a cline may represent
in other cases,
a graaded region of overlap where individuals or neighboring populations are interbreeding.
a gradation in some environmental variable may prorudce a cline
experimental studies of clines have confirmed that both __ and __ play a role in the geographic differences of phenotype
genetic variation; environment
fitness
the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals
relative fitness
the contribution of a genotype to the next generation compared to the contributions of alternative genotypes for the same locus
natural selection acts on __, not __
phenotypes, not genotypes
the entity that is subjected to natural selection
the whole organism
the relative fitnes of a particular allele depends on
the entire genetic annd environmental context in which it is expressed
natural selection can alter the frequency distribution of heritable traits in 3 ways:
directional, disruptive, stabilizing selection
directional selection favors individuals that
deviate from the average
directional selection is most common when
a population’s environment changes or when members of a population migrate to a new habitat with different environmental conditions than their former one
disruptive selection can be important in
the early stages of speciation
b/c most eukaryotes are diploid, a considerable amount of genetic variation is hidden from selection in the form of
recessive alleles
heterozygote protection maintains a huge pool of alleles that might not be favored under present conditions but
some of which could bring new benefits when the environment chages
balancing selection includes (2)
heterozygote advantage and frequency-dependent selection
balanced polymorphism
a situation in which two different versions of a gene are maintained in a population of organisms because individuals carrying both versions are better able to survive than those who have two copies of either version alone;
stable frequencies of 2 or more phenotypic forms in a population
balancing selection occurs when
natural selection maintains balanced polymorphism
heterozygote advantage
if individuals who are heterozygous at a particular gene locus have greater fitness than the homozygotes, natural selection will tend to maintain 2 or more alleles at that locus
frequency-dependent selection
in which the fitness of any 1 morph declines if it becomes too common in the population
neutral variation
differences that appear to confer no selective advantage
pseudogenes
genes that have become inactivated by mutations; where genetic “noise” is free to accumulate in all parts of the gene
sexual selection
natural selection for mating success. can result in sexual dimorphism
sexual dimorphism
marked differences between the sexes in secondary sexual characteristics, which aren’t directly associated w/ reproduction (ex: differences in size, color, ornamentation)
intrasexual selection
selection within the same sex. direct competition among individuals of one sex for mates of the opposite sex. usually more obvious in males
intersexual selection
aka mate choice. in which individuals of one sex (usually females) are choosy in selecting their mates fro the other sex. in many cases, the female’s choice depends on the showiness of the male’s appearance/behhavior
