AP Bio Chapter 23

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 two alleles of that gene different and increase the heterozygosity

Why can't natural selection fashion perfect organisms?

- evolution is limited by historical constraints -adaptations are often compromises -chance and natural selection interact - selection can edit only existing variations

Natural selection can alter the frequency distribution of heritable traits in three ways:

-directional selection -disruptive selection -stabilizing selection

As with allele frequencies, all of these genotype frequencies sum to _

1

the relative fitness of most reproductively successful variants is set at _ as a basis for comparison

1

state at which the allele frequencies are the same from one generation to the next, but its genotype frequencies can be predicted from the allele frequencies

Hardy-Weinburg Equilibrium

the _______ _______ is measured as the average percent of these loci that are heterozygous.

average heterozygosity

_______ ____ occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population, a state called _______ _____.

balancing selection; balanced polymorphism

Darwin and other realized over time that _______ would eliminate the differences between individuals

blending

Two situations that can increase the likelihood that genetic drift will have a large impact on a population are referred to as the _________ ____ and the ________ _____

bottleneck effect; founder effect

_______ _____ 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

directional selection

_______ ____, such as the red, pink and white colors of our hypothetical wildflower population, can be classified on an ____-__ Basis.

discrete characters; either or

both ______ and ________ characters contribute to variation within a population

discrete; quantitative

What type of selection is this: a population of black-bellies seed cracker finches in Cameroon displays two distinctly different beak sizes. Small billed bird feed mainly on soft seeds, whereas large-billed bird specialize in cracking hard seeds. It appears that birds with intermediate size bills are relatively inefficient at cracking both types of seeds, thus have lower relative fitness

disruptive selection

___________ ____ occurs when conditions favor individuals on both extremes of a phenotypic range over individuals with intermediate phenotypes

disruptive selection

However, some noncoding regions of DNA __ _________ the expression of genes. Changes in these regulatory regions of DNA can have _____ ______

do regulate; profound effects

the contribution an individual makes to the gene pool of the next generation relative to the contributions of other individuals

fitness

It is important to remember that only the _____ _____ of variation can have evolutionary consequences as a result of _____ _______

genetic component; natural selection

Of course, it is possible that ____ ____ that appear to be neutral may actually influence ______ and _______ success in ways that are difficult to measure. Furthermore, a variant allele may be ______ in one environment but not in another

genetic differences; survival; reproductive; neutral

An example of this _______ _____ occurs at the locus in humans that codes for one of the peptide subunits in hemoglobin. In homozygous individuals, a recessive allele at that locus causes sickle-cell disease. However, heterozygotes are protected against the severest effects of malaria

heterozygote advantage

Mutation rates tend to be ____ in animals and plants. But in ____________ and ______ with short generation spans, mutations can rapidly generate genetic variation.

low; microorganisms; viruses

Intrasexual selection is most obvious in ____

males

three major factors that alter allele frequencies and cause most evolutionary change are ________ ______, ______ ______, and _____ ____

natural selection; genetic drift; gene flow

Most of the DNA base differences between humans that are down in untranslated parts of the genome appear to confer no selective advantage and therefore are considered _______ ____

neutral variation

We now know that selection results in alleles being passed to the next generation in proportions different from their _____ _______ in the present generation.

relative frequencies

Recombination _______ alleles, but does not change their _________.

reshuffles; frequencies

Sexual selection can result in ______ ________

sexual dimorphism

What are the five conditions for Hardy-Weinberg

1. Extremely large population size 2. No gene flow 3. No mutations 4. Random mating 5. No natural selection

Example of ____ ____: suppose, that near our original hypothetical wildflower population their is a newly established wildflower population consisting of primary of _____-______ individuals. Insects carrying pollen from these plants may fly to and pollinate plants in our ______ ____. The introduced C^W alleles will ______ our original population's _____ ______ in the next generation

Gene flow: white flowered; original population; modify; allele frequencies

_______ proposed just such a model: the particulate hypothesis of inheritance, which states that parents pass on _____ ____ ____ (genes) that retain their identities in offspring.

Mendel; discrete heritable units

most heritable variation consists of _________ _____ that vary along

Quantitative characters

______ ______ that are less favorable than their dominant counterparts, or even ________ in the current environment, can persist because they are _______ in heterozygous individuals

Recessive alleles; harmful; propagated

The _________ that sex provides: one explanation is that the process of ______ recombination and ________ generate the genetic variation on which _______ ______ acts. The assumption is that natural selection sustains ___, in spite of its reproductive drawbacks, because genetic variation enables ______ adaptions to an ever-changing __________

advantage; meiotic; fertilization; natural selection; sex; future; environment

Every time a female chooses a mate based on certain appearance or behavior, she perpetuates the _____ that _______ her to make that choice, allowing a male with an especially showy _______ to pass on his _____ to offspring

alleles; influences; phenotype; alleles

The gene pool consist of all ____ at all the gene _____ in all individuals of a _________.

alleles; loci; population

_______ events affect the subsequent evolutionary history of populations. Thus, not all alleles present in the founding population's ____ ____ are better suited to the new _______ than alleles "left behind"

chance; gene pool; environment

sex provides for "_______ the ____" and varying them among _____.

changing; locks; offspring

the smaller the sample, the greater the chance of _______ from the _____ ____.

deviation; predicted result

Because environmental factors are likely to _____ from one place to another, ______ ____ can contribute to geographic variation.

differ; natural selection

Because most eukaryotes are ______, a considerable amount of genetic variation is hidden from _________ in the form of _______ ____.

diploid; selection; recessive alleles

In _______-_______ _____, the fitness of any one morph declines if it becomes too common in the population

frequency-dependent selection

the heritable component of height is the result of such _______ _______ for alleles at the several loci that influence height

genetic polymorphism

the preservation of _____ ______ provides the opportunity for ______ _____ to act over many generations

genetic variation; natural selection

Although population geneticist often refer to the relative fitness of a ________, it is important to remember that natural selection acts on _________ not ________.

genotype; phenotype; genotype

Selection favors certain ______ _____ through differential _______ ______

heritable traits; reproductive success

Balancing selection includes ________ ______ and ______ _______ selection

heterozygote advantage; frequency dependent

Although the fitness advantage to the ________ is much _____ than the disadvantage to the homozygotes, there are so many more ___________ than _________ that the aggregate benefit of the allele in the population _________ its aggregate harm

heterozygotes; smaller; heterozygotes; homozygotes; balances

the asexual condition would _________ in frequency because all of the females' offspring would be ________ that are able to give birth to more reproductive daughters in turn.

increase; daughters

individuals of one sex (usually females) are choosy in selecting their mates from the other sex

intersexual selection

a direct completion among individuals of one sex for mates of the opposite sex

intrasexual selection

Each species has a _______ of descent with modification from a long line of _________ forms. Evolution does not scrap ________ _____ and build each new complex structure from scratch; it co-opt existing structures and adapts them to new ___________.

legacy; ancestral; ancestral anatomy; situations

the _______ _____ of neutral variations are not affect by ______ _____; overtime some neutral alleles increase in ______ and other ______ through genetic drift

neutral variation; natural selection; frequency; decrease

in the bottleneck effect, certain alleles may be _________ among the survivors, others may be __________, and some may be _______ all together. Genetic drift may continue to have _____ ______ on the gene pool for many generations until the population is _____ enough that chance ________ have less effect

overrepresented; underrepresented; eliminated; substantial effect; large; fluctuations

directional selection shifts the frequency curve for some ______ ______ in one direction or the other by favoring individuals that _________ from the average

phenotypic character; deviate

the study of how populations change genetically over time

population genetics

genes that have become inactive by mutations

pseudogenes

In __________ genetic "_____" is free to accumulate in all parts of the gene

pseudogenes; noise

Sexual reproduction, As a mechanism of ______ ______ ________, it is far inferior to asexual reproduction.

rapid population expansion

the ______ the recessive allele , the ______ the degree of protection from _______ ________

rarer; greater; natural selection

the latent variation is exposed to natural selection only when both parents carry the same ________ _____ and combine ___ copies in one ______. This happens only ____ if the frequency of the recessive allele is very ___.

recessive allele; two; zygotes; rarely; low

the contribution of a genotype to the next generation compared to the contributions of alternative genotypes for the same locus

relative fitness

thus the _______ ____ of a particular allele depends on the _____ genetic and environmental _____ in which it is expressed.

relative fitness; entire; context

Nevertheless, ___ is maintained in the vast majority of eukaryotic species, even those that can also reproduce _______. it must enhance _________ _______ somehow, for otherwise natural selection would act in favor of alleles that promote ______ _________.

sex; asexually; reproductive success; asexual production

marked differences between the sexes in secondary sexual characteristics, which are not directly associated with reproduction

sexual dimorphism

_______ ______ is far more important than mutation on a generation-to-generation time scale in producing the variations that make adaptation possible

sexual recombination

natural selection for mating success.

sexual selection

In many cases of intersexual selection, the female's choice depends on the ______ of the male's _______ or behavior. However, because females have ____ ____ to mate than males, a female only gains an advantage over other females if she chooses a mate that enables her to ______ more ________

showiness; appearance; few chances; produce; offspring

the distinctions in sexual selection include ____, _____, and ________

size; color; ornamentation

What type of selection is this: the birth weights for most human babies lie in the rang of 3-4 kg; babies who are much smaller or larger suffer higher rates of mortality

stabilizing selection

_________ ________ act against extreme phenotypes and favored intermediate variants

stabilizing selection

this mode of selection reduces variation and maintains the status quo for a particular phenotypic character.

stabilizing selection

on the other hand, alleles or other pieces of DNA that contribute nothing to an organism's _________ may be perpetuated in individuals whose overall ______ is _____

success; fitness; high

Thus, many factors affect both ________ and ________ and contribute to evolutionary fitness

survival; fertility

But ________ is necessary for reproduction, and _________ increases fitness if it means that long-lived individuals leave more ________ than those who die sooner

survival; longevity; descendants

when individuals who are heterozygous at a particular gene locus have greater fitness than the homozygotes, natural selection will tend to maintain ____ or more _____ at that ___. This is called ______ _____

two; alleles; locus; heterozygous advantage

If there are ____ or more alleles for a particular locus in a population, individuals may be either ____________ or ________

two; homozygous; heterozygous

When there are _____ alleles at a particular locus, the convention is to use _ to represent the frequency of one allele and _ to represent the other allele.

two; p ; q

Individual ________ occurs in populations of all species

variation

the entity that is subjected to natural selection is the ____ _____.

whole organism

Relative fitness is ______ for a sterile plant or animal, even if it is robust and outlives other members of the population

zero

However, in examining __ gene loci in a representative sample of the seals, researchers found __ ________: for each of the __ _____, there was only ___ ______. In contrast, populations of the southern elephant seal, a closely related species that was not so severely _______, show considerable genetic ______ at these loci

24; no variation; 24 genes; one allele; reduced; variation

______ ____ can also produce allele frequency differences between populations through the cumulative effect of _______ _____ in frequencies rather than ______ _____

Genetic drift; random fluctuations; natural selection

The Hardy- Weinberg theorem describes how ________ ________ preserves genetic variation from one generation to the next in _______ that are not evolving.

Mendelian inheritance; populations

One reason it is important to understand the bottleneck effect is that human actions can sometimes create severe _________ for other species. For example, in the 1890s, hunters reduced the population of northern elephant seals in California to about __ ________. Since then, the mammal has been protected and the population has rebounded to over ________.

bottleneck; 20 individuals; 30,000

Only mutations in ____ _____ that produce gametes can be passed to ____________, and only a small fraction of these spread through _________.

cell lines; gametes; populations

Each allele has a _________ (proportion) in the population.

frequency

the aggregate of genes in a population at any one time is called the population's ____ ____

gene pool

If such a duplicated segment does not have sever effects, to can persist over __________, providing an expanded ________ with new ____ that may take on new ________ by further mutations an subsequent selection.

generations; genome; genes; functions

by introducing or removing _____ from chromosomes or by changing one ____ into another, mutations can modify the ____ ____

genes; allele; gene pool

In other cases, a ________ in some environmental variable may produce a ______.

graduation; cline

We can define evolutionary change on its smallest scale or ____________, as change in the genetic makeup of a population from generation to generation

microevolution

by the mid-20th century population genetics also gave rise to what is called the _______ _____, a comprehensive theory of evolution that integrated ideas from many other fields

modern synthesis

A Change of as little as one base in a gene - _____ _______ - can have a significant effect on the _______. But most point mutations are probably _________.

point mutations; phenotype; harmless

a localized group of individuals that are capable of interbreeding and producing fertile offspring.

populations

Thus, the equation for Hardy Weinberg equilibrium states that a locus with ___ alleles, the ______ genotypes will appear in the following proportions: _____________

two; three; p^2 + 2pq +q^2 = 1

_______ and many ______ can also undergo recombination, but they do so less _______ than animals and plants and often in ways that allow them to cross ____ ______.

Bacteria; viruses; regularly; species barriers

it is not possible to predict how mutations will alter ____ and what its _____ will be. Most mutations occur in ______ _____ and are lost when the individual _____.

DNA; effects; somatic cells; dies

Differential survival and reproductive success of individuals carrying different genotypes will alter _____ ______

allelic frequencies

According the the Hardy Weinberg theorem, the _______ ________ in all the gametes produced by the population will be the _____ as in the original population

allelic frequencies; same

Still, individuals near the population center more likely to _____ with members of their own ________ than other ________ and thus on average more closely related to another more than to members of other ________

breed; population; populations; population

some examples of geographic variation can occur as a ______, a graded changes in a trait along a geographic axis

cline

Chromosomal mutations that _____, _______, or ______ many loci at once are almost certain to be harmful. However, when such mutations leave the gene ______, their effects on organisms may be ______.

delete; disrupt; rearrange; intact; neutral

gene flow tends to reduce ________ between ________.

differences; populations

Over time, _____ tends to reduce _____ _____ through such losses of allele from the ___ ____

drift; genetic variation

Gene _________ is an important source of variation. ________ of chromosome segments, are nearly always harmful. But _____ _____ of DNA are often introduced into a genome through the activity of ________ elements.

duplication; duplication; small pieces; transposable

A sudden change in the ________, such as a fire or a flood, may drastically reduce the size of a ______. IN effect, the survivors have passed through a restrictive "_______", and the gene pool may no long be ______ of the original population's _____ ____; This is called the ________ _____

environment; population; bottleneck; reflective; gene pool; bottleneck effect

departure of the five conditions of Hardy- Weinberg usually results in ________. While ______ ______ rarely if ever are in true Hardy- Weinberg equilibrium, in many populations that rate of evolutionary change is so _____ that these populations ______ to be close to equilibrium

evolution; natural populations; slow; appear

When a few individuals become isolated from a larger population, this smaller group may establish a new population whose gene pool is not reflective of the source population: this is called the ______ ______

founder effect

the ______ ____ can occur when a few members of a population _________ a new location. these founders pass through an "________ ______" and represent a distinct gene pool with different _______ ______ from those of the parent population

founder effect' colonize; isolation bottle neck; allele frequencies

the repeated shuffling of population's gene pool over the generations cannot, in itself, change the _______ of one ______ relative to another

frequency; allele

the Hardy Weinberg theorem states that the ________ fo alleles and _______ in a population's gene pool remain _______ from generation to generation, provided that only ________ _________ and _________ of alleles are at work

frequency; genotypes; constant; Mendelian segregation; recombination

one reason is that much of the DNA in eukaryotic ________ does not code for _____ ______. And because the genetic code is ____________, even point mutations in genes that code for proteins may have _____ ____ because they do not alter protein's amino acid composition.

gametes; protein products; redundant; little effect

A population may gain or lose alleles by _____ ____, genetic additions to and/ or subtractions from a population resulting from the movement of fertile individuals or gametes

gene flow

_____ _____, the transfer of alleles between populations, can alter allele frequencies

gene flow

If it is extensive enough, ____ _____ can amalgamate neighboring populations into a _____ _____ with a common ___ ___.

gene flow; single population; gene pool

such beneficial increase in ____ _____ appear to have played a major role in evolution.

gene number

Heritable quantitative variation results from the influence of two or more _____ on a single ___________ character

genes; phenotypic

If individuals preferentially choose mates with certain __________, including close relatives, random mixing of _______ does not occur

genotypes; gametes

Most species exhibit _______ _______, differences between the gene pools of separate population subgroups

geographic variation

In some cases, cline may represent a ____ ____ of overlap where individuals of neighboring populations are __________.

graded region; interbreeding

In contrast, _____ ______ in the human population does not show phenotypic polymorphism because it does not consist of ______ and ______ morphs. - heights vary along a _________.

height variation; distinct; separate; continuum

Population geneticist measure the number of polymorphisms in a population by determining the amount of ___________ at both the level of ____ _____ (gene variability) and the molecular level of ____ (nucleotide variability)

heterozygosity; whole genes; DNA

Nonrandom mating can affect relative frequencies of __________ and ________ genotypes but by itself but usually has no effect on _____ _____.

homozygotes; heterozygotes; allele frequencies

Ideas about ________ from Darwin's time could not explain how inherited _______ are maintained in _________

inheritance; variations; populations

the founder effect probably accounts for the relatively high frequency of certain ______ _____ among isolated ____ ________

inherited disorders; human populations

populations of the same species may be _______ from one another, thus changing _____ ______ only rarely. Such __________ is common for population confined to different, widely separated _____ or _____.

isolated; genetic material; isolation; islands; lakes

The Hardy-Weinberg theorem lays the groundwork for understanding ____-____ __________ changes that Darwin could not have envisioned

long-term evolutionary

When individuals differ in discrete characters, the different forms are called _______

morphs

New genes and new alleles originate only by _________ which are changes in the nucleotide sequence of DNA

mutations

Although new ________ can modify allele frequencies, the change from oner generation to the next is likely to be ______

mutations; small

Of all the factors that can change a gene pool, only ____ ______ is likely to adapt a population to its environment.

natural selection

Nucleotide variability is measured by comparing the ______ _______ of DNA samples from two individuals and then __________ the data from many such comparisons

nucleotide sequences; averaging

If only _____ allele exists at a particular locus in a population, that allele is said to be _______ in the gene pool, and all individuals are ___________ for that allele

one; fixed; homozygous

Organisms reflect thousands of generations of ____ ______, and a single mutational change is about as likely to ______ the genome as blindly firing a gunshot through the hood of a car is likely to improve engine performance

past selection; improve

The Hardy Weinberg equation can be used to estimate the _________ of the population carrying the allele for an _______ ______

percentage; inheritable disease

a population is said to display __________ __________ for a character if two or more distant morphs are each represented in high enough frequencies to be readily noticeable.

phenotypic polymorphism

Nearly all _________ variations based on genetic differences result from recombinational shuffling of the _______ _____ in the gene pool.

phenotypic; existing alleles

Not all _________ variation is heritable. Phenotype is the cumulative product of an inherited _________ and a multitude of ________ _______.

phenotypic; genotype; environmental factors

For example, in our imaginary wildflower population, white flowers might be more visible to ____-____ _______ resulting in more white flowers being _____. And perhaps red flowers are more attractive to _______, increasing the opportunity for red flowers to produce _________. These differences in survival and reproductive success would disturb _______ -________ ______: the frequency of the C^W allele would ____ in the gene pool and the frequency of the C^R allele would ______.

plant eating insects; eaten; pollinators; offspring; Hardy Weinberg equilibrium; decrease; increase

a population is not considered ________ if it consists primarily of a ____ _____ and other morphs are extremely _____

polymorphic; single morph; rare

Darwin considered the raw material for natural selection to be "_____________" characteristics. - those characteristics in a population that _____ long a ______, such as fur length in mammals or the running speed of animals fleeing from predators

quantitative; vary; continuum

Provided that gametes are contributed to the best generation at ________, allele frequencies will be ________

random; unchanged

if a population were in Hardy- Weinberg equilibrium and its members continued to mate ___________ generation after generation, Allele and genotype frequencies remain ______

randomly; constant

Similar deviation from the expected result -which occur because ____ _____ are ____ in size rather than _______- explain how the allele frequencies can ______ __________ from one generation to the next. Such fluctuations are called ______ _____

real populations; finite; infinite; fluctuate unpredictably; genetic drift

In rare cases, chromosomal _________ may even be beneficial. For example, the translocation of part of one chromosome to a different chromosome could link ______ that together have some heightened _____ _____

rearrangement; genes; positive effect

Because of _____-_______, pea populations are far from _____-_______ _____. All the ___________ plants produce only _______ while about half the offspring of the heterozygous plants are ________. It does not take many generations before almost all the plants in the population are _______. But even though such species do not mate __________, they still produce their gametes at ____ from their ____ _____

self-fertilization; Hardy Weinberg Equilibrium; homozygous; homozygous; homozygous; randomly; random; green pools

discrete characters often are determined by a _____ ____ ____ with different alleles that produce distinct ________.

single gene locus; phenotype

the ______ the population, the greater the role played by chance fluctuations in _____ ______ from one generations to the next, known as _____ _____

smaller; allelic frequencies; genetic drift

on rare occasions, however, a mutant allele may actually make its barer better _____ to the ___________, enhancing ________ ____. this is more likely when the environment is _________ and mutations that were once selected ______ become ______

suited; environment; reproductive success; changing; against; favorable

________ are always present, and as Darwin realized, variations that are heritable are the raw material for _______ _____. In addition to the differences that we can see or hear, populations have extensive genetic variation that can be observed at the _______ ____. For example a person's ___ ____

variations; natural selection; molecular level; blood group