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CHAPTER 23
REVIEW QUESTIONS
23.1
Stabilising selection occurs when environmental conditions do not change and there is
selection against the extremes of variation within the population. With change in
environmental conditions, one of the extremes of variation within the population may exhibit a
survival advantage and as a consequence these individuals are selected for – the norm is
changed in the direction of that particular trait and progressive selection occurs.
23.2
Changes in the individual cannot bring about changes in the species – only changes in the
population. Thus mutations in gametes produced by individuals can be passed on to future
generations and with time enough individuals may exist with these mutations to influence the
population as a whole, i.e. evolution can then occur within the species.
23.3
A particular genotype is not always expressed in the individual. The expression of any
particular genotype may be determined by complex interactions between several genes ot the
gene and its environment as well as the genotypic composition (homozygous or heterozygous,
whether or not the alllele is dominant or recessive). It is the actual phenotype that is expressed
in the individual and this determines abilities (structural or chemical). Thus natural selection
acts upon the phenotype and not the genotype.
23.4
Those individuals that are better able to reproduce and rear offspring will contribute a greater
proportion of their genes to the next generation than individuals without this ability. Any
feature, therefore, that aids the survival, in the particular set of environmental conditions, of
the individual and its offspring, will be retained in the gene pool. The feature is therefore an
adaptation for the environment which, because it gives the individuals an advantage, is
selected for.
23.5
A gene pool is the sum total of all the genes present in all the individuals of a population.
23.6
a.
b.
c.
d.
The population must be large enough to make it highly unlikely that chance alone could
significantly alter the gene frequencies.
Mutation must not occur or there must be mutational equilibrium.
There must be no immigration or emigration.
Reproduction must be totally random.
23.7
Genetic drift is an evolutionary change in a small population due to the random loss of an
allele.
23.8
Because the population is very small, there is little variaton in the gene pool. Thus if a number
of the animals succumb to a pathogen there could be an even further decrease of specific
alleles in the gene pool. This reduces the genetic variability in the population even further,
making them susceptible to environmental change.
23.9
Within any population, some organisms could have genetic resistance to any particular
chemical, e.g. antibiotic. As new antibiotics are developed new populations will occur which
are resistant to each type – the non-resistant individuals will die, leaving only the resistant
forms to reproduce. The resistant alleles must already exist in the population for this to occur.
23.10
Different ranges in the distribution of a population may experience varying abiotic conditions
which would lead to the prevalence of a variation in the phenotype for a specific trait. This
difference is termed clinal variation. The cline refers to the trait – thus within a population
there may be many clines, depending on the environmental differences at different parts of the
range.
Geographic isolates refer to populations of a species which have become geographically
isolated from the main gene pool of the total population.
Hybrid belts are regions of the common gene pool where two populations experiencing
different environmental conditions and thus adaptations intermingle.
23.11
A species is a group of interbreeding natural populations which are reproductively isolated
from other such groups.
23.12
Yes – there are as many clines as there are differences in environmental conditions.
23.13
Races are groups of natural populations within a species that differ genetically and are
geographically isolated from each other but which could interbreed if the ranges were to again
overlap.
23.14
a.
b.
Divergent evolution: one ancestral species gives rise to two or more species which grow
increasingly unalike with time.
Convergent evolution: two different species which are unrelated come to resemble each
other with time usually due to similar environmental selection pressures. They will not
become one species since they originally had an entirely different gene pool – the
convergence refers to similarities in physical features rather than reproductive
compatibility.
Allopatric species are populations which are reproductively isolated as a result of a
geographic barrier.
Sympatric species are populations which are reproductively isolated as a result of
intrinsic genetic barriers.
23.15
Some form of geographic barrier separates two populations of a species.
Clinal variations already existing in the two populations means there is an initial genetic
difference.
Different chance mutations would occur in the two populations.
Exposure to different environmental conditions would favour progressive selection of different
traits.
23.16
Convergent evolution can occur when two unrelated species, in different areas, occupy a
similar niche in near identical environmental conditions, e.g. the gerboa of the Northern
African deserts and the desert hopping mouse of central Australia.
23.17
The answer is variable and can be selected from any of the factors given (with examples) in
23.3.6.
23.18
If their ranges again overlap, i.e. they become sympatric, and they are unable to interbreed, i.e.
reproductive isolating mechanisms have evolved. Even if their ranges do not overlap, breeding
in artificial conditions or studies of niche and reproductive reuirements in their natural
environment could detect the occurrence of a reproductive isolating mechanism and thus the
separation into two species.
23.19
Coevolution refers to the simulataneous evolution of two or more species that are
interdependent, the changes in one species, creating selection pressures on another.
23.20
In coevolution it is the diurect selective forces exerted by interacting species on each other that
result in the adaptation. In convergent evolution, the selective forces may be either biotic or
abiotic, direct or indirect.
23.21
The more closely an organism mimics a distasteful or harmful species, the less the chance of it
being eaten by predators. Thus there is a direct interaction between model, mimic and predator
which creates the selective forces.
23.22
Many examples have been cited in this and other chapters in the text, e.g.:
Koala and gum trees: the eucalypt produces toxic oils and prussic acid – and adaptation to the
selection pressure of herbivorous animals. The koala is able to detoxify or tolerate these
chemicals.
Mimicry of flowers to look like their pollinating insect: the flower of certain orchids mimic the
shape, colour and odour of a female digger wasp. The male wasps are attracted to the ‘super
female’ and attempt to copulate with it, thereby bringing about cross-pollination.
Release of pollen by the shooting star (Dodecatheon conjugens) in response to frequencies of
sound related to those of the buzzing of bees.
The production of a mild laxative in the fruit of the rainforest pioneer shrub (Witheringia
solanacea), which is eaten by its main dispersal agent, black-faced solitaires (Myadestes
melanops).
23.23
The evolution of parthenogenesis probably resulted from genetic changes in one female
individual. Because only one individual is involved, there can be rapid reproduction, the
offspring of which are all clones of the original mutant. These animals have become
reproductively isolated from sympatric populations. Thus divergence of this population from
sympatric species can therfore be very rapid.
23.24
Plants.
23.25
Regulatory genes control the activity of structural genes. Thus a change in regulatory genes
can result in rapid evolution.
23.26 Species remain constant for long periods of time. A some particular point associated with
changing environmental selection pressures, or genetic drift, sudden and dramatic change (e.g.
mutation of one or more regulatory genes, dvelopment of parthenogenesis etc.) leads to the
development of new species.
23.27
In the hominids there has been a change in the structure of the chin, position of the pelvis and
foot structure.
23.28
The stage of their evolution coincided with the Miocene Ice Age that resulted in dry conditions
in Africa and contraction of rainforests. These conditions were not conducive to fossil
formation and thus there is a large gap in the fossil record between the early Miocene and start
of the Pliocene.
23.29
Similarities DNA and serum proteins (1.1% difference in nucleotide sequence).
Behavioural similarities.
Developmental rates.
23.30
Australopithecus matured much slower than than apes. Showed distinct handedness (skull
fractures in kills); use of developed tools; upright stance.
23.31
In a harsh environment, the ability to communicate past information to project forward plans
was probably significant in survival. Foresight and the ability to communicate would have
enhanced leadership and thus hunting success by the group. This added a further selection
pressure to the increased development of memory centres and logical reasoning centres, i.e. to
enlargement of the brain.
23.32
The multiregional theory proposes that there were many early migrations of Homo erectus out
of Africa. Each population evolved adaptations to their specific environmental conditions to
produce racial differences. There was, however, adequate gene flow between different
populations to maintain single species rank.
The single-origin theory proposes that modern humans evolved fairly recently in Africa and
then colonised the world as Homo sapiens sapiens. Thus modern racial features are recent and
therefore genetically superficial.