Year 12 Biology
Unit 4 Area of Study (AOS) 1 in Year 12 Biology focusses on the question, How are species related? To answer this question, students must develop an understanding of the processes involved in the evolution and how to determine the relatedness between species. The School Assessed Coursework (SAC) for this AOS was a Media Response essay. In the lead up to the actual SAC, the Year 12 Biologists were provided with opportunities to practice the task by reading articles or watching documentaries and responding to a stimulus. Their goal was to successfully link the concepts from the course to the examples of evolution in the media. One such practice SAC was based on the David Attenborough documentary “Madagascar: Episode 1 – Island of Marvels”. Students watched the documentary through the Woodleigh subscription to Clickview, and they had just one hour to write an essay responding to the following:
“Madagascar is an island off the coast of Africa, which has been isolated for around 88 million years. It is home to around 10,000 endemic species. Using at least one example from the documentary, explain how the Island may have developed so many endemic species.”
I received many excellent essays, and a credit to the English department, all with an excellent understanding of coherence and cohesion. What follows is an exemplar written by Jussy P:
As the “oldest Island in the world,” according to David Attenborough’s ‘Madagascar’ documentary, Madagascar is home to many species which are found solely on the island – thus giving them the term ‘endemic,’ which denotes their sole existence in one particular location. As acknowledged by David Attenborough, the development of its many unique species is likely to be both due to the geographical isolation of such species (separated spatially from other continents by vast stretches of the ocean) and unique environments on Madagascar, which introduce unique or at least very particular environmental selection pressures on the organisms that live on the Island. Off the coast of Africa, Madagascar is thought to have originated as part of the single continent ‘Pangaea,’ before continental drift caused earth’s landmass to separate into different continents. Being the oldest Island has accounted for the significant evolution many species of organisms have undergone in Madagascar, thus allowing them to be very different from their ancestors and other descendants living in other locations, such as Africa, despite sharing a common ancestor with species endemic to Madagascar.
Many of the species endemic to Madagascar are thought to have descended from organisms in Africa, such as tenrec – with as many as 32 species within the family Tenrecidae living in Madagascar, according to Attenborough. Attenborough believes that individuals belonging to ancestral species of modern tenrecs endemic to Madagascar washed over from Africa in a chance event, for instance, a natural disaster such as a cyclone or storm. Such a chance event has caused geographic isolation (and lack of gene flow between these populations – lack of interbreeding and therefore lack of maintenance of similar gene pools) of a sub-population of tenrecs, subjecting them to different environmental selection pressures due to the unique environments of Madagascar. This is likely to have led to speciation, through different changes in the gene pools of tenrec populations in Madagascar and Africa, due to different environmental selection pressures that caused different characteristics (controlled by different alleles) to increase the change of individuals’ survival long enough to reproduce. This would have ultimately caused speciation of tenrecs in Madagascar from populations in Africa, as an event of allopatric speciation, leading to gene pools so different that even if placed together, individuals of tenrec species in Africa and Madagascar are unlikely to mate and produce fertile offspring.
Evidence for the evolutionary relationships of tenrecs (which led to the theory that they were washed over to Africa by chance) may have originally come from structural features of tenrecs in Madagascar, which were similar to the features of certain species found in Africa. Considering certain features of these species, despite their different locations, to be homologous, scientists are likely to have originally considered structural similarities in features of such organisms to be derived from common ancestry. However, this method of determining evolutionary relationships through the similarities in structural appearances (morphology) of organisms has been proved by later molecular homology techniques (including the analysis of similarities in nucleotide sequences of DNA and amino acid sequences of proteins) to not always be attributable to common ancestry. Take Attenborough’s discussion of Tenrecs, for example; Despite their spines and similar size to hedgehogs, which has commonly allowed them to be mistaken for this different species, tenrecs are only very distantly related to hedgehogs, with most types of tenrecs developing in isolation on the shores of Madagascar and having evolved for a relatively long time in isolation – therefore having only very distance evolutionary relationships to hedgehogs. Such similarities between these species are much more likely to be due to convergent evolution, in which similar environmental selection pressures have led to the development of similar features that have similar functions but aren’t due to common ancestry. This can be seen in the spines of tenrecs and hedgehogs. Despite being similar in appearance and playing similar roles, warding off predators and allowing them to camouflage on forest floors, both organisms would have developed such traits through random mutations occurring in their genes which coded for longer and longer spines. Having high adaptive value due to allowing protection and camouflage, the alleles for longer spine length would have allowed individuals with long-spine alleles to have survived longer than those with shorter spines, allowing them to survive long enough to reproduce and allow their alleles to increase in allele frequency within the population. Eventually, all individuals in both species of hedgehog and tenrec species will have come to possess this trait of having a long spine independently. This process of environmental factors contributing to changing allele frequencies in populations due to specific environmental selection pressures is referred to as natural selection.
The number of endemic species in Madagascar, as acknowledged by Attenborough, is likely to be due to the unique and diverse environments within Madagascar. Separated by mountain ranges along the middle of the Island, Madagascar has huge forests on the western side, and “lush woodland, drenched in the rain” on the eastern side. This rainforest environment is home to many different types of lemur – as many as 38 different species which have great diversity – with some being the size of “mice” while the Indri lemur is the size “of a small child.” The diversity in lemur populations may be due to the varied environments of Madagascar. Different lemur species may have been separated due to prezygotic isolating mechanisms (mechanisms which isolate gene pools of different species which prevent the formation of a zygote through interbreeding) such as geographic isolation, for instance, if different populations of lemur occupied different environments within the diverse environments of Madagascar. The differences in environmental selection pressures may have allowed different traits to provide survival advantages in these different environments, for instance, small lemurs in some rainforest environments may have allowed them to evade their predators – leading to increased frequency of alleles which contribute to the phenotype of a smaller body size (along with environmental factors).
Meanwhile, in a different population of lemur, random germline mutation - changes in nucleotides or larger changes in chromosomes of an organism’s genetic material, within gametes - may have caused the introduction of new alleles allowing increased muscular size or strength in the legs, allowing an increase in jumping abilities. This may have increased the chance of escape from predators by lemurs, thus causing the predecessors of Indri lemurs to develop impressive jumping skills, as Attenborough discusses. Therefore, due to these varied landscapes, which may have resulted in geographical isolation of populations of different species and therefore speciation, Madagascar has developed many different unique species that are found exclusively on the Island. Being the oldest Island on the planet, the length of time that organisms have been isolated on Madagascar is likely to have contributed to the relative amount of evolution through mutation that has allowed the development of such diverse species, as well as genetic drift (change in allele frequencies within a population due to random events, such as death of individuals) – which may have originally been especially prevalent in small populations of species washed over from Africa. Due to the great variation in the habitats in Madagascar, niche partitioning (ecological isolation) may contribute to the continued separation of different related species on Madagascar, for instance, with different lemur species occupying different ecological ‘roles,’ such as their method of finding food, shelter, etc. The length of time different populations of lemur have had isolated gene pools due to such prezygotic isolating mechanisms is likely to have contributed to their very different characteristics.
Attenborough’s documentary highlights Madagascar as home to many endemic species. Madagascar’s unique habitats, varied environments, and the length of isolation of organisms on the Island are likely to have led to the development of so many unique species found solely on the Island. Y12 Jussy P
Head of Science and Director of Sustainability