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. The population must be large enough to make it highly unlikely that chance alone could significantly alter the gene frequencies.
b. Mutation must not occur or there must be mutational equilibrium.
c. There must be no immigration or emigration.
d. 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. 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.
b. 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.