MCQ EVOLUTION
(A)
All organisms have the capacity to grow and reproduce.
(B)
All organisms use oxygen for cellular respiration in mitochondria, an organelle
that was once a free-living prokaryote.
(C)
Different species have specific traits and adaptations that allowed them to
succeed in diverse environments.
(D) All organisms share a genetic code that allows one organism to express a gene from another organism.
2.
Which explanation is least consistent with evolutionary theory regarding why
humans lack a tail?
(A)
Humans didn’t need a tail.
(B)
Primate ancestors without tails were more fit than those with tails.
(C)
In the population of the common ancestor of apes and humans, those that had
shorter or no tails left behind more offspring than those with full-sized
tails.
(D)
Tails were a disadvantage to our ancestors.
3. Which of the following statements is an accurate conclusion based on the data and modern evolutionary theory?
(A)
As the number of resistant strains increases, a greater volume of antibiotics
is required to kill them.
(B)
An increase in the prevalence of resistant bacteria directly caused an increase
in the use of antibiotics.
(C)
Lowering the volume of antibiotic use would result in a decreased prevalence of
resistant strains.
(D)
It is highly probable that increased use of antibiotics directly caused an
increase in the prevalence of resistant bacterial strains.
4.
The greyhound dog was originally used to hunt the fastest of game, fox and
deer. The breed dates to ancient Egypt. Early breeders of greyhound dogs were
interested in dogs with the greatest speed. Breeders carefully selected the
fastest dogs from a group of hounds. From their offspring, the greyhound breeders
again selected those dogs that ran the fastest. By continuing selection for
those dogs that ran faster than most of the hound dog population, they
gradually produced dogs that could run up to 64 km/h (40 mph). This application
of artificial selection most resembles:
(A)
stabilizing selection.
(B)
genetic engineering.
(C)
directional selection.
(D)
disruptive selection.
5. Which of the following true statements is the best single piece of evidence that nonhuman apes such as chimpanzees, bonobos, and gorillas are more closely related to humans than any other mammal?
(A)
Apes and humans have similar anatomy. They both have nails instead of claws,
lack a tail and full facial hair, and have forward-facing eyes.
(B)
Apes and humans have very similar behaviors and social structures, and they
both have the ability to use tools.
(C)
Apes have 24 pairs of chromosomes and humans have 23.
(D)
Humans have a greater percentage of DNA sequences in common with apes than with
any other group of organisms.
6.
Which of the following organisms would be considered the most fit?
(A)
a young female bear with five baby cubs and three males competing to mate with
her
(B)
an old, male bear whose two offspring each have two offspring
(C)
a young male bear with three offspring who has just killed an unrelated bear
before he was able to reproduce
(D)
an adolescent female bear who has three mature males (each with two offspring)
competing to mate with her
7.
In northern elephant seals (Mirounga angustirostris), only a few large males
compete for the much smaller females. Each of these males will father many
offspring by many females. Most of the males in the population do not get the
opportunity to breed. From a genetic point of view:
(A)
the rate of genetic drift would be higher than expected for a population of
equal size.
(B)
the frequency of heterozygotes in the population is higher than expected for a
population of equal size.
(C)
males and females will eventually become reproductively isolated due to the
dramatic difference in body size.
(D)
small males will devise a new reproductive strategy.
8.
Which of the following is not a mechanism by which new genes are generated in
organisms?
(A)
Existing genes are modified by mutations in its DNA sequence.
(B)
Two or more segments of a gene are recombined.
(C)
Gene duplication and mutation create new alleles.
(D)
DNA nucleotide chains are randomly assembled by DNA polymerase without a
template strand.
9.
Which of the following most accurately states the effect of selection on the
frequency of dominant versus recessive alleles?
(A)
Dominant alleles are subject to selection even at low frequencies.
(A)
Lethal, recessive alleles’ frequencies reach zero if selected against for a
long enough time.
(B)
Dominant alleles are more affected by negative than positive selection.
(C)
Recessive alleles are more subject to selection than dominant alleles because
there is only one genotype for the recessive phenotype.
10.
Fragments from a small meteorite that fell to Earth approximately one hundred
million years after Earth formed were found to contain more than 80 amino
acids. Which of the following true statements is the best evidence that the
amino acids were not contaminants from organisms from Earth?
(A)
The proportions of amino acids found in the meteorite were similar to those
produced in the Miller–Urey experiment (showing that organic molecules can be
synthesized from inorganic precursors).
(B)
The amino acids were present in equal amounts of D- and L-isomers (mirror image
enantiomers), but organisms on Earth can only make and use L-isomers (with rare
exceptions).
(C)
There are only 20 amino acids that make up proteins in organisms.
(D)
Molecules exposed to extreme conditions often undergo chemical changes.
11.
Which of the following true statements does not support the idea that RNA was
likely the first genetic material?
(A)
Cells typically contain about 8 times more RNA than DNA.
(B)
RNA plays a central, informational role in protein synthesis.
(C)
RNA can replicate by Watson–Crick base pairing rules.
(D)
Single-stranded RNA molecules can take on a variety of three-dimensional shapes
and can carry out several catalytic functions.
12.
When planning a fossil hunt for marine fossils of the Devonian period (354
million years ago), which of the following is the most crucial aspect of
planning?
(A)
Make sure you pack the equipment used for radioactive dating.
(B)
Locate sedimentary layers of 354 million years ago in coastal regions.
(C)
Find sites where sedimentary rocks of approximately 354 million years ago are
exposed and accessible.
(D)
Collect fossils of various ages and place them in order of relative age.
13.
Which of the following hypotheses is not supported by the evidence that follows
(hypothesis: evidence)?
(A)
Rapid evolution/punctuated equilibrium: different compositions of taxa are
represented above and below the iridium rich layer.
(B)
Mass extinction: many more fossils are present directly below compared to
directly above the iridium rich layer.
(C)
Iridium layer poisoned some plant life: the composition of plant biomass in the
taxa above the iridium rich layer is different from the composition of plant
biomass below the iridium-rich layer.
(D) Iridium-rich cloud blocked out the sun: greater biomass of plant life directly above the iridium layer.
14.
Which of the following is the most likely explanation for the higher allele
frequency despite its deleterious effects?
(A)
One or more bottlenecking events caused an increase in the frequency of the
Tay-Sachs allele.
(B)
Because the population is a subset of a larger population, the Tay-Sachs allele
frequency appears higher.
(C)
The heterozygous case confers resistance to tuberculosis.
(D)
The Tay-Sachs allele is a transposon.
15.
Many bacteria obtain resistance to an antibiotic (such as ampicillin) by
carrying a plasmid that contains a gene that confers resistance to the antibiotic.
The plasmid is passed on to other bacteria through conjugation and can be
passed onto offspring during binary fission. If bacteria already resistant to
ampicillin were not exposed to ampicillin, which of the following would most
likely be the effect over several generations?
(A)
The gene for the ampicillin would eventually mutate to confer resistance to the
new antibiotic.
(B)
The plasmid containing the gene for ampicillin resistance would disappear from
the population.
(C)
Transmission of the plasmid containing the ampicillin resistance would increase
as those bacteria without the plasmid would be killed.
(D)
The transmission of the plasmid conferring ampicillin resistance would decrease
but would not disappear from the population.
(A)
Organisms with an odd number of chromosomes are sterile.
(B)
Hybrid sterility occurs only when the nonhybrid populations or species have a
different number of chromosomes.
(C)
The genes for hybrid sterility are not expressed in nonhybrid individuals.
(D)
The genes that contribute to hybrid sterility do not affect the fertility of
the parent species; the effect likely stems from the interaction in hybrid individuals.
17. Which of the following best explains why reproductive isolation is necessary for speciation?
(A)
Pre- and post-zygotic mechanisms prevent different species from interbreeding.
(B)
Gene flow is needed to prevent speciation.
(C)
Isolation introduces different selective pressures, which differentially affect
disparate gene pools or allow genetic drift to fix or eliminate certain
alleles.
(D)
Adaptive radiation isolates populations by eliminating gene flow.
18. Which of the following is a reasonable prediction?
(A)
Predation by Crenicichla should select for an overall increase in body mass of
the guppies.
(B)
Predation by Crenicichla should favor the evolution of early sexual maturation
and reproduction.
(C)
Lack of predation by Crenicichla should favor the production of many, smaller
offspring earlier in life.
(D)
Lack of predation by Crenicichla should select for guppies that can choose when
to reproduce based on environmental conditions.
19.
Which of the following is an acceptable method to estimate reproductive effort
(reproductive investment)?
(A)
weight of embryos relative to the weight of the mother
(B)
weight of embryos relative to the weight of the father
(C)
the average weight of an individual embryo relative to the weight of the mother
(D)
the total number of embryos that live to sexual maturity
20.
In mammals, Drosophila, and many other animals, a greater number of new
mutations enter the population through sperm as compared to the number that
enter the population through eggs. Which of the following most directly and
accurately accounts for this observation?
(A)
For each fertilization event, there are millions of sperm for each egg.
(B)
More cell divisions occurred in the germ line before spermatogenesis as compared
with oogenesis in individuals of the same age.
(C)
Eggs are larger and less subject to mutation.
(D)
Sperm are produced at a faster rate than eggs, increasing the probability that
DNA polymerase will make a mistake that is uncorrected by proofreading.
21.
Which of the following hypotheses is a reasonable explanation for the jaw and
tooth development in gnathostomes?
(A)
Loss of Hox gene expression is one of the genetic factors involved in the
appearance of the oral jaw.
(B)
The loss of the pharyngeal jaw is due to loss of Hox gene expression.
(C)
Development of teeth requires Hox gene input regardless of the presence or
absence of a jaw.
(D)
Jaws could not evolve until teeth evolved.
22.
A 2009 study (by Fraser) found that tooth number and position were highly
correlated in the oral and pharyngeal jaws of the cichlids in Lake Malawi. This
observation taken in context with the previous information suggests that:
(A)
the two sets of teeth share the same regulatory network.
(B)
development of the first teeth (in agnathans) was under Hox control, but the
network came under different control later in the evolution of gnathostomes.
(C)
the development of the two sets of toothed jaws require the same gene
expression but in different areas.
(D)
Hox gene expression indirectly regulates the development of the jaw through the
development of teeth and associated structures.
23.
Several phylogenies have been proposed for the cichlids. Which of the following
methods of phylogeny construction would be the least useful
in
the construction or evaluation of a phylogeny?
(A)
Maximum parsimony: the phylogeny that requires the minimum number of
evolutionary changes assumed for all traits is preferred.
(B)
Morphological, molecular, developmental, and if applicable, behavioral data are
considered.
(C)
The complete sequence of the genomes of all the species involved are compared.
(D)
Mathematical models are applied to the data to identify the tree that most
likely produced the observed data.
24.
The amount of genetic variation within populations of hunter-gatherers in
southern Africa is much greater than in non-African populations. The genetic
divergence among these African populations as compared to non-African
populations suggests that:
(A)
the environment in Africa has not changed much in the past 200,000 years.
(B)
populations in southern Africa have been subjected to more inbreeding.
(C)
the members of the population that initially left Africa were all related by a
common ancestor not more that 3 or 4 generations prior to their departure.
(D)
the populations that colonized the rest of Earth carried the genetic
information from their populations of origin almost intact and few genetic differences
have occurred since then.
25.
Refer to the age schedules of reproduction. Semelparous species have a life
history in which individuals, especially females, reproduce only once.
Iteroparous species have a life history in which individuals reproduce more
than once. In which of the following situations would semelparous reproduction
be suboptimal?
(A)
The rate of reproductive output increases exponentially with body mass.
(B)
The rate of body mass growth decreases as the individuals increase in size.
(C)
Reproduction is likely to fail in some years and be successful in others.
(D)
The probability of survival decreases with increased body size.
ANSWER
1. (D) The passing
down of DNA from one generation to the next is the mechanism by which all life
is linked. Changes in DNA sequences (mutations) produce new genes and alleles,
which can be inherited by offspring through reproduction. The mechanism by
which genes are used inside cells is so similar across diverse forms of life
that when genes from one organism are introduced into the genome of a different
organism, they may be expressed: for example, genes for spider silk expressed
by goats. Although all life-forms have the capacity for growth and
reproduction, this fact alone supports but is not evidence that all organisms
are linked by lines of common ancestry. That different species have specific
traits and adaptations is a result of evolution through natural selection, but
again, this fact alone is not direct evidence of common ancestry. The true
statements of choices A, B, and particularly C actually require the model of
natural selection to make sense.
2. (C) Natural selection works by reproduction, heritable variation, and selection. Humans lack tails not because tails weren’t needed, but because either having one was a disadvantage and/or not having one was an advantage for our ancestors. The lack of selective pressure for or against a particular structure can produce a vestigial structure (our wisdom teeth and appendix are typically classified as vestigial structures). A vestigial structure is present in a rudimentary form. Typically, it has been reduced in size, and perhaps complexity, from a more functional structure in the ancestor. Over the course of many generations the structure has become smaller and less functional but has not been selected against enough to “completely disappear.”
3. (D) A graph like this is suggestive of a cause-and-effect relationship but is not proof. There are two things compared over time. Importantly, the two y-variables, for the most part, increase and decrease concurrently (with a reasonable time lag between changes in drug use and changes in prevalence of resistant bacteria as the bacteria need time to adapt.). For example, the increased drug volume from 1986–1990 correlates with an increase in the prevalence of resistant bacteria. However, the reduction of drug volume in 1985 and 1991 is accompanied by a decrease in the prevalence of resistant bacteria. Although the relationship is not perfectly correlated (e.g., in the year 1984, a decreased drug volume corresponded to very little change in resistance), the general trend shows that as drug volume increased, the prevalence of resistant strains increased with some lag time, and when drug volume decreased, the prevalence of resistant strains decreased. This graph alone allows us to say nothing of a causal relationship between the two factors but knowledge of evolutionary theory and antibiotic resistance does allow us to select the correct answer.
4. (C)
Directional selection is the type of selection, whether from the environment or
by humans (who are technically part of the environment), in which one extreme
of the phenotypic continuum is selected for (or against).
5 (D) Although all
the choices are ways to compare, contrast, and attempt to relate organisms by
common ancestry, DNA sequence comparisons are the “gold standard.” Convergent
evolution can produce similarities in anatomy and behavior (choices B and C). Convergent
evolution only makes the organisms being compared appear to be related (e.g., a
shark and dolphin look quite similar, but the shark is a cartilaginous fish and
the dolphin is a mammal). Importantly, organisms do not become more related as
time passes; they can only maintain their degree of relatedness or through
future generations become more distantly related through divergence.
6. (B) Fitness of an organism is not measured by its reproductive potential, but by differential reproductive success. Only offspring that have been produced matter. In the case of choice D, no offspring have been produced yet. Choice B, the old bear, has 2 offspring, but each offspring has 2 offspring, so there are 6 bears in later generations carrying his genetic material. The bear in choice A has 5 cubs, so she’s the next most fit. The young male with 3 cubs gets the bronze. How do you measure fitness? Fitness is defined with respect to a genotype or a phenotype. The greater the contribution made to the gene pool of the next generation, the greater the fitness of the genotype or phenotype.
7 (A) The changes in allele frequency due to genetic drift are higher in smaller populations (see the graph above question 23). In the northern elephant seal, the effective population size is smaller than the actual population size because most of the males do not sire offspring. The “effective” size is determined by the number of individuals who actually reproduce because the next generation is composed only of the genetic information put there by reproducing individuals. Also noteworthy is that the population of northern elephant seals does not engage in random mating, and so it does not meet the conditions of Hardy-Weinberg equilibrium. (See answers 14, 22, 23, and 51 for more information on genetic drift.)
8. (D) Choices A,
B, and C are all mechanisms by which new genes and alleles can be created by
the modification of pre-existing genetic information. Choice D is incorrect
(therefore, the correct answer to the question) because DNA sequences are not
assembled “from scratch” (de novo) in cells; they can be made only from a
template or a modification of an existing sequence.
9. (A) An allele
is considered dominant when the phenotypes of the heterozygote and homozygous
dominant genotypes are identical. Because dominant alleles cannot be masked in
the heterozygote condition as recessive alleles can, they are more prone to
selection than recessive alleles.
10 (B) It is
often easier to think “positively,” in other words, asking “What evidence would
support contamination by amino acids from Earth?” may be easier to answer than
“Which of the following is evidence that amino acids did not contaminate the
meteorite?”. If the amino acids were contaminants from Earth, they would be
present almost exclusively as L-isomers, the specific isomer (specifically,
enantiomer) of all the amino acids that make up proteins in living things. When
amino acids are synthesized by abiotic means, they are produced in a racemic
mixture (50% D, “right-handed” enantiomers and 50% L, “left-handed”
enantiomers, except for glycine, which doesn’t have D- or L-isomers).
11 (A) The fact
that cells contain a greater mass of RNA than DNA is not evidence that RNA was
the first genetic material. Water is the most abundant molecule in the cell,
for example, but that does not support its role as a molecule of inheritance.
The information contained in genes is used to make functional RNA molecules
such as tRNA and rRNA and proteins.
12 (C) It is crucial to know where the rocks
of the appropriate age are exposed and accessible. Radioactive dating is not
typically done on-site. You want to know the location of the rocks of the age
you’re studying before you get there. Choice B is tempting but what is coastal
now was not necessarily coastal 354 million years ago, so knowing where the
rocks that were coastal back then are now, and making sure you can get to them,
is the most crucial aspect of planning of the four things listed.
13. (D) If the
iridium-rich cloud layer blocked out the sun, plant matter would be expected to
decrease. The iridium layer settled to Earth, but depending on the depth of the
layer, the deposition rate, and some other factors, a reduction in plant life
is expected over the time the iridium was in the atmosphere. When the
atmosphere cleared, the plants that survived would be expected to produce an
increase in plant biomass over time, but they would have to “rebuild” the
biomass they lost in the initial reduction. After an extinction event, diversity
tends to increase slowly, at least at first. The hypothesis that the sun was
blocked out and caused the mass extinction, or was significant, is not
generally accepted. It appears that the impact, the temperature fluctuations
that ensued, and the concurrent massive volcanism all conspired to make Earth
uninhabitable for about half the kinds of organisms living on it. The exact
causes of the massive extinction has not been conclusively determined, but it
is generally agreed that the meteor played a large part in it. The
End-Cretaceous extinction (also called the K/T extinction) ended the lineages
of the nonavian dinosaurs, the ammonoids, rudists, many families of
invertebrates and planktonic protists, and most marine reptiles. Approximately
155 families of marine animals and 47% of all genera became extinct.
14 (A) The high
carrier frequency of the allele in Ashkenazi Jews is probably caused by a
combination of founder effect, genetic drift, and differential immigration
patterns. Genetic analysis links the bottlenecks to the 70 CE (AD) diaspora and
the Black Death in 1348. It was hypothesized that Tay-Sachs carriers may have
had a resistance to tuberculosis, but evidence to support the hypothesis is
inconclusive. Grandparents of Tay-Sachs carriers die from proportionally the same
causes as grandparents of noncarriers. In other words, carriers are not less
likely to die from tuberculosis. This supports the null hypothesis: that the
allele confers no advantage to heterozygotes.
15(D) Transmission of the plasmid to daughter cells requires time and energy in replication of the plasmid. Bacteria that transmit the plasmid are not conferring any immediate benefit to their offspring. Over time, there would be fewer bacteria carrying the plasmid relative to those not carrying the plasmid, because those without the plasmid would take longer and require more energy to reproduce, although by not that much. The plasmid for ampicillin resistance does not confer any other benefit or have any other deleterious effect. So it is unlikely to disappear completely from the population, especially in the “several generations.”
16 (D) Although the genes for hybrid sterility are best known in Drosophila and not well-characterized in most other species, they are not thought to have any effect on the nonhybrid organism that possesses them. The genes in diverging populations likely differ enough that their recombination in the offspring creates sterility or inviability because of disharmonious interactions between the different sets of genetic information supplied by the parents. Alleles at different loci within the same population are said to be co-adapted. Their harmonious combination has presumably been selected for. The gene pool of this population, and any population, is a co-adapted gene pool.
17 (C) If there is no genetic differentiation between two (or more) populations, speciation cannot occur. Reproductive isolation allows two (or more) gene pools to diverge. If there is gene flow between populations (in other words, they are not reproductively isolated), their allele frequencies to differ enough to maintain separate, distinct populations. In order for speciation to occur between two populations (according the biological species concept, see the box under answer 24), the two populations must not be able to interbreed, or at least not significantly (so choice B is totally incorrect). Choice A states two mechanisms that keep different species from interbreeding after they have already speciated. Choice D is incorrect because adaptive radiation is the result of the lack of gene flow, not the cause of it.
18 (B) Guppies from the streams where Crenicichla live are smaller, mature faster, and reproduce more frequently than those that live in streams where Crenicichla are absent. This hastened development and reproduction result in a higher reproductive effort for the guppies that live in Crenicichla-dominated streams but is selected for because those guppies that have a longer development and reproductive schedule are less likely to live long enough to reproduce. In a population where the predator is present, the “cost” of living long enough to delay reproduction (and the risk that it requires) does not offset the benefit of delayed reproduction (lower reproductive effort). In Crenicichla-dominated streams, guppies that reproduced at a younger age clearly left behind more offspring than guppies that delayed reproduction, as demonstrated by the decreased size and increased rate of maturation in later generations.
19(A) Reproductive effort is the proportion of energy and other resources that an organism allocates to reproduction as opposed to its own growth and maintenance. In the case of the guppies, the reproductive effort is measured by comparing the relative weights of the embryos to their mother. By itself, the embryo weight is not a true indicator of the effort the mother puts into creating the embryo. For example, a mother of low mass would put a larger percent of total resources than a higher mass mother into producing an embryo of the same mass. Maximizing resources is a trait that will typically be selected for, so a lower reproductive effort for the same quality of offspring would be selected for.
The male germ cells divide by mitosis to produce diploid spermatogonium. It is these spermatogonia that continuously divide to produce a steady supply of spermatocytes, the cells that undergo meiosis. The average male will produce 525 billion sperm over the course of his lifetime.
A note on the word accurate . . . Accurate means “correct, true or widely accepted as true.” An explanation that directly accounts for an observation would be one that provides a reason or cause that directly precedes the phenomenon it attempts to explain. (See the following box.)
An illustration of DIRECT versus ULTIMATE causation
(and consequences) The direct cause, or reason, for why you need to breathe
oxygen is to supply your electron transport chain with the final electron
acceptor (oxygen) it requires. Oxygen deprivation prevents the electron
transport chain from functioning (a direct consequence), and for humans, more than
a few minutes of oxygen deprivation is typically lethal (the ultimate
consequence).
Why was oxygen used? Nitrogen was also in the
atmosphere. However, oxygen is more highly electronegative, so the movement of
electrons from glucose to oxygen results in a large decrease in their free
energy, which can be transformed into chemical energy in the cell. Also, the
double bond between the two oxygen atoms in the O2 molecule is easier to break
than the triple bond between the nitrogen atoms in the N2 molecule. And so on .
. .
Ultimate causes are often related to evolution, natural selection, and the laws of chemistry and physics as applied to biology.
21(A) The pharyngeal arches are embryonic structures that differentiate into many parts of the head and neck in vertebrates. The exact structure a particular arch develops into depends on the species. In agnathans (jawless fish), the 1st pharyngeal arch gives rise to teeth, and their development is under the control of the Hox genes. (Vertebrate teeth originated in the posterior pharynx of jawless fish over 500 million years ago)
The development of jaws in vertebrates does not require Hox gene expression. A reasonable hypothesis is that the absence of Hox gene expression in the first pharyngeal arch is what allowed the oral jaw to appear many, many years later.
None of the other answer choices make sense given the information in the question. This question lends itself well to the process of elimination. See the box after answer 6 for suggestions on when and how to use it.
22. (B) Development of first teeth was under Hox gene regulation in agnathans (in the pharynx) but this network came under different control during the evolution of the teeth in gnathostome oral jaw. Because most gnathostomes do not possess pharyngeal teeth (some sharks and bony fishes do), the cichlids are an interesting case to study. They possess secondary jaws (the pharyngeal jaws) where the first vertebrate teeth evolved and have teeth where the first vertebrate jaw evolved (oral jaw). This suggests that the similar patterns of dentition (teeth) in the two jaws appear to be controlled by a common gene regulatory circuit.
23(C) All phylogenies and cladograms are hypotheses that are revised when new data or techniques for analyzing data become available. Sequencing the entire genomes of all the species in a phylogenic tree is an unrealistic goal. Genomes are complex and not fully understood, so the excess sequence information would not necessarily be useful.
When biologists
compare sequences between species, they choose homologous sequences and need to
calibrate the “molecular clock” of the particular regions they are comparing.
Some genes or pseudogenes accumulate sequence changes faster than others, and species
with longer generation times usually have a slower molecular clock because they
produce less generations in a given period of time.
24.(D) Genetic
similarities and differences among human populations have been used to trace
the dispersal of humans from Africa throughout the rest of the world. The
amount of variation within human populations is lower the farther they are from
Africa, as would be expected from successive colonizations.
The amount of DNA
sequence variation within human populations decreases with increasing distance
from Africa. Fossils of modern humans date back 195,000–170,000 years ago in
Africa. Fossils of modern humans outside of Africa are not found until 80,000
years ago. The amount of genetic variation within populations of
hunter-gatherers south of the Saharan desert, particularly in southern Africa,
is much greater than in non-African populations. This is expected if colonizing
populations carried only a sample of genes from the ancestral population
(similar to the founder’s effect). The genetic divergence of some of these
African populations suggests they have been diverging for up to 150,000 years.
Genetic difference among non-African populations are mostly quite small.
25 (C) Iteroparous and semelparous refer to the age schedules of reproduction. If survival contributes to fitness only for as long as reproduction continues, why not start reproducing as early in life as possible and continue to reproduce for as long as possible? As with all processes shaped by natural selection, the answer is trade-offs. The cost of reproduction is generally high, and early reproduction is correlated with lowered subsequent reproduction. Risks associated with a substantial reproductive effort at an early age are an increased risk of death, decreased growth, and decreased subsequent fecundity. Suppose you decide to invest in fixing up your home before selling it. This is expected to increase the value and therefore the selling price of the house. However, if the house loses value (for whatever reason), the cost of the repairs may never be recovered. Selling the house as soon as possible greatly increases the chance that you’ll walk away with a payout, but you may not get the best price for the house.
Reproduction requires
that the reallocation of energy and resources used for growth, maintenance, and
self-defense be used for the reproductive effort. Delaying reproduction until
you are large enough, for example, may pay off greatly later. However, every
day that reproduction is delayed presents another opportunity for another
factor, like a predator, parasite, or random event like a volcano or mudslide,
that may kill you. Certain traits make semelparous (“big bang” reproduction) an
advantage for a particular species.
• The rate of growth
of body mass declines as an individual grows larger.
• The probability of
survival increases with body mass.
• There is an
exponential relationship between body mass and reproductive output.
Iteroparity is an
advantage if greater fecundity can be achieved by deferring reproduction.
Repeated reproduction is more likely to evolve if adults have high survival
rates from one age class to the next and if the rate of population increase is
low. A major advantage of iteroparity is that the risk of failure is spread out
over many bouts of reproduction.
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