Download CHAPTER 23

CHAPTER 23
EXTENDING YOUR IDEAS
1.
Tongue rolling dominant.
Of 5000 individuals, 1,800 non-rollers, i.e. homozygous recessive.
(p + q)2 = 1
a.
b.
c.
d.
e.
f.
g.
2.
a.
b.
q2 = 1,800 ÷ 5000 × 100 = 36%
q2 = 0.36
q = 0.6
p+q = 1
p = 1–q
= 0.4
p2 = 0.42 = 0.16
i.e. 16% homozygous dominant.
2 pq = 2 × 0.4 × 0.6 = 0.48
i.e. 48% heterozygous.
Highly likely since it is a large population but conditions of Hardy–Weinberg b, c and d
are not
usually met in human populations and these frequencies could change.
If the four conditions are met and there is a change in gene frequency over successive
generations
in a population then there is a case for progressive selection and thus evolution.
Graph A: stabilising selection
Graph B: progressive selection
Graph C: disruptive selection
i. The mean of the characteristic in the population will shift towards the right.
ii. Two different gene pools will develop, each responding to different environmental
selection pressures. This could lead to other changes in the gene pool of each
population and eventually speciation.
.3.
Most of the species of rock wallabies are geographically isolated from each other. Even if they
are in the same general area, they may still be geographically isolated from populations of
their own and closely related species, since rocky outcrops, their habitat, are not continuous
within an area but separated by plains. Each group is a tight social entity, thus even if there
were migration from one population to another, it is highly unlikely that it would integrate
within the group. Thus the evolution of rock wallabies is most likely to have been a result of,
and to continue to result from, allopatric speciation.
4.
a.
green ants
adults

crab spider
b.
c.
larvae

lycenaenid butterfly larvae
i. The green ant larvae are prey to the butterfly larvae.
ii. The crab spider is a mimic and predator of the adult green ant.
The adult green ants are aggressive protectors of their nests and can pierce and inject
formic acid into large animals. This has put selection pressure on the lycaenaenid
butterfly to evolve adaptations that allow its larvae to prey on the ant larvae (smooth,
tough skin and ability to press themselves firmly against the nest wall, thus preventing
penetration of the adult ant’s mandibles) and to escape from the adult ants as they
emerge from the pupae (dense covering of easily detachable scales which clog the ant’s
mandibles). In order to escape detection and attack by the adult ants, whilst preying on
them, selection pressures have been placed on the crab spider to evolve adaptations.
These have included structural and behavioural mimicry of the ant, attack from the rear,
the production of a fast-acting venom and the ability to drop by a thin silk thread out of
reach of the ants.
5.
The gene pool of the two populations is different as a result of environmental selection
pressures (with the black, heat-absorbing allele predominant in the cold north, and the red
allele predominant in the warmer south of Britain). There would be selection pressures against
the alternative allele in either of the habitats.
6.
Variable answer which should re-examine knowledge of human control of parasites, pests and
pathogens, the ecological effects of these control measures and side-effects on people;
resistant forms and selection pressures changing the structure of the gene pool of a population.
7.
The relative proportion of ebony flies in the cool conditions is always lower than in warm
conditions. In warm conditions the relative proportion of ebony flies remains greater than of
wild type (greater than 65%), indicating that they are not normally found in the experimental
range of warm conditions (the wild type is the most usually encountered phenotype). The
relative proportion of ebony in the cool conditions is lower than that for the wild type (less
than 50%) suggesting that the flies would be found in cooler areas such as that of the
experimental condition. The presence of both phenotypes is an adaptive advantage to the fly
to counter seasonal changes in temperature or in distribution of the flies.
8.
This statement assumes that there was a direct evolution from chimpanzees to humans and
that this relationship is recent. In reality, both animals evolved from an ancestral animal, each
adapting to their specific habitat and the environmental changes experienced throughout their
long evolutionary history.
9.
Human evolution occurred during a period of great environmental change. These changing
conditions could have placed such strong positive selection pressures on individuals with
larger brain sizes, that there was rapid evolution in this direction.
10.
Discussion should include data on relative brain sizes of each species:
H. habilis
580–750 mL
H. erectus
800–1100 mL
H. sapiens
1250–1370 mL
An increase in brain size is associated with an increase in the cerebral cortex which is the
centre of integration and coordination of all but hindbrain activities (mostly autonomic
responses), correlating incoming information with past experience. Thus the memory
capabilities as well as the ability to utilise these memories in projecting future actions
increases with brain size. Increased coordination of responses could result in finer motor
movements. Overall, the behaviour patterns exhibited by the three species would increase in
complexity with increasing brain size.
11.
Fossil records show that upright posture was a characteristic of the earliest protohuman,
Australopithecus afarensis. The increase in brain size and complexity, as demonstrated by
skull size and internal texture of the skull, has been progressive throughout the evolution to
modern humans. Increase in brain size and complexity is associated with increased
intelligence.
12.
Variable answer that should show:
 Understanding of the two theories.
 Critical analysis of the data explaining why it fits one theory better than the other.
 Logical sequencing of ideas leading to a valid conclusion.
13.
Possible alternatives to the suggested demise of the human species:
 Some males with a low number of repetitive sequences survive and continue to mate
and pass this on to male offspring – unlikely given the data.



A mutation could arise that suppresses replication of repetitive elements and so gene
loss in the Y chromosome. Males with this gene are able to produce viable sperm and
so the mutation becomes the predominant form.
Female mutations could occur so that diploid eggs are able to begin development
without the stimulus of fertilisation, resulting in an all-female population and
reproduction by parthenogenesis. This reduces the chance factor in pregnancy – all
ovulations could theoretically result in pregnancy unless artificially suppressed or
another mutation relates pregnancy to environmental conditions.
With increased technology the human species could be artificially maintained by
cloning of existing female cells – this could control the population size to set limits.