YEAR 11 DEPTH STUDY TASK 3 - Biological... · Web view2020. 4. 27. · As its name boasts, ... They are known to feed on Cacti, Herbs and Grasses. Distinct from other Giant Tortoises,

Adnan Shah

11_BIO2

YEAR 11 DEPTH STUDY TASK

Biology Depth Study Task – Adnan Shah

PART A

Enquiry Question: “How do changing selection pressure and adaptations increase the organisms ability to survive and allow for speciation to occur”

The aim of this scientific report is to show how changing selection pressures and adaptations increase the organisms ability to survive and allow for speciation to occur via analysing the change observed in species that originally inhabited the Galapagos Archipelago.

Abstract: “In this task, students are presented with a scenario in which they travel to the Galapagos islands. The students will investigate types of adaptations, the process of natural selection and the evidence for evolution, using Galapagos species as examples.”

HISTORY OF THE GALAPAGOS

The Galapagos Archipelago is a volcanic group of 19 islands, with most of the islands possessing a very distinct conical shape which is believed to be directly related to the volcanic activity found within the island cluster. All of the islands house a volcano, except the largest island named Isabella, meaning the soil composition found in Isabella would differ to that of the other islands.

Figure 1: Volcanic Ash Soil (https://www.worldatlas.com/articles/why-is-volcanic-soil-fertile.html)

The archipelago lies on the Nazca Plate, a perpetually moving plate that formed the chain of islands as a result of its geological movement. The islands are thought to have originally been formed some 5-10 million years ago as a result of mantle pluming, where columns of hot rock about 100 kilometres in diameter rise to the top of the ocean due to its low density as a result of its extreme


heat. (www.geo.cornell.edu, Last revision 2 Oct 1997 by W. M. White) The cause of the mantle pluming was a result of the interactions of plate tectonics.

The islands are perpetually increasing in height due to volcanic activity, and the formation and layering of volcanic rock is the reason the island displays such high slopes, with heights ranging from a few meters above sea level to more than 5000 feet above sea level. (www.galapagos.org, date: unknown).

The Galapagos islands are located near the equator. The islands are distributed on both sides of the equator, result in differentiating climates and biomes across all islands. Every single island in the Galapagos differs in environment, thus resulting in different ecosystems resulting in variation of flora and fauna between the ecosystems. For example, as stated before, Isabella would house a different soil composition, lacking the nutrients the other 18 islands possess from volcanic formation. This would therefore mean that flora would differ on Isabella, then, for example, flora on Santa Cruz demonstrating one of many abiotic factors seen in the islands.

Figure 2: Map of the Galapagos Islands (http://www.galapagosisland.net/galapagos_islands/map.html)

In the last 200 years, an estimate of 50+ volcanic eruptions have occurred within the island chain, some of which threatening the local flora and fauna. Due to its volcanic activity and unique climates and nature across all islands, the island chain provides a dynamic and constantly changing environment for its inhabitants, creating selection pressures for the original inhabitants of the Galapagos, forcing them to either adapt in order to increase the organism’s ability to survive via speciation, or fall to the selection pressures and ultimately die.


http://www.galapagos.org/

http://www.geo.cornell.edu/

It is therefore hypothesised that due to perpetually differing environments seen in the Galapagos, the original inhabitants faced selection pressures which have caused them to evolve and speciate, increasing their ability to survive as they have evolved to adapt to these selection pressures, satisfying natural selection.

Our first association with the word ‘evolution’ would be the surname ‘Darwin’. Charles Darwin, in his trip to the Galapagos Islands noticed that unique creatures were similar from island to island, but perfectly adapted to their environments.

(www.galapagosislands.com, date: unknown).

He based the thought that these “specialized” animals came from the same lineage and possess the same common ancestors.

Darwin observed the differentiation in species seen across different islands, and collated his data on the population of species to draw conclusions, eventually leading to the theory of Darwinism; that is the theory of biological evolution.

The next section of this report will explain Darwinism in essence by comparing species found on the Galapagos Islands.

SPECIES COMPARISON

This section of the scientific report aims to compare and contrast the distinguishing structural, physiological and behavioral adaptations of three pairs of closely related species.

Tortoises

As its name boasts, the Galapagos Island Giant Tortoise is the largest living species of tortoise. They only exist in one of two locations; with one being the entirety of the Galapagos islands. It is one of the most widespread and longest living species of tortoises on the island. The Galapagos island tortoises cover the Galápagos giant tortoise complex, with all tortoise species evolving from the same original ancestor.

This section will discuss two species of the Galapagos Giant Tortoise Complex.

SPECIES ONE : Chatham Island giant tortoise (San Cristobal) [Chelonoidis chathamensis]


http://www.galapagosislands.com/

SPECIES TWO: James Island Giant Tortoise (Santiago Island) [C. darwini]

The James Island Tortoise is a result of a variation of the original Galapagos Island Giant Tortoise; a species from the same complex. The James Island Tortoise is only found on the island of Santiago (James Island in English). They are a diurnal terrestrial species which live in deciduous forests, evergreen montane forests, and humid grasslands (Chelonoidis darwini :Arteaga A, Guayasamin JM 2019) all of which Santiago possesses as part of its unique climate. They are known to feed on Cacti, Herbs and Grasses. Distinct from other Giant Tortoises, the James Island Tortoise features a carapace that is shaped intermediate to domed and saddleback shapes. Refer to figure 1,2 & 3 below.

The saddle-back shaped carapace is an apaptove trait of the Galapagos Giant Tortoise complex. Simply put; the saddleback carapace came after the rounded carapace.

The Carapace of James Island tortoise features this unique shape so as to help the organism reach food sources. The tortoise requires the slight saddleback to extend their neck in order to reach the height of the cactuses of which they feed on. The slightly rounded shape is for the tortoise to be allowed to swivel its head around to feed on herbs and grasses which are located on the ground. This example of the unique shell, unfound in any other species of tortoises in the Galapagos shows evolution working in synergy with natural selection. As a result of biotic factors between the James Island Tortoise and the flora which it feeds on, selection pressures were created where the tortoise


Figure 1 :

The James Island Tortoise (C. Darwini)

This image shows the unique carapace of the C. Darwini, neither round nor saddle backed, but rather a combination between both.

Figure 2:Typical dome-shaped shell

Courtesy of https://www.amnh.org

Figure 3: Typical saddleback-shaped shell

Courtesy of https://www.amnh.org

had to change its carapace shell in order to reach the cactuses. This selection pressure led to a structural adaptation where the shell adopted its unique shape to allow the James Island tortoise to feed on the cactuses, increasing their ability to survive to the selection pressure.

On the other hand, the Chatham Island Giant Tortoise is also a species from the same complex as the James Island Tortoise. They are also a diurnal terrestrial species that are only thought to inhabit the grasslands and dry shrublands of San Cristobal. The Chatham Island Tortoise is only found on the island of San Cristobal (Chatham Island in English), the fifth largest island in its archipelago. The Island is comprised of three extinct volcanoes which have contributed to the superior soil fertility found on the island. San Cristobal is the only island in the chain to have access to a continual freshwater resupply from the rain. The freshwater together with the islands soil fertility allows the flora on the island to thrive and cover the whole island. Water-storing plants such as cactuses are unseen on the island.

Unsurprisingly, the Chatham Island Tortoise feeds only on the shrubs and herbaceous vegetation (Chelonoidis chathamensis: Arteaga A, Guayasamin JM 2019), unlike the James Island Tortoise who also feeds on the cactuses present. Unlike the James Island Tortoise, who features an intermediate carapace, the Chatham Island Tortoise possesses a very rounded carapace. Refer to fig 4 & 5.

This is as a result of differing climates between Santiago and San Cristobal. Due to the availability of lush herbaceous and shrub-type vegetation, the Chatham Island tortoise doesn’t need to extend its neck to reach higher vegetation like cactuses, whereas the James Island Tortoise does. The Chatham Island tortoise doesn’t need the headspace provided by the saddleback shape, as it can simply swivel its head around to reach the shrubs it feeds on. Simply put, the Chatham Island tortoise hasn’t faced the selection pressure that the James Island Tortoise has, therefore it inhibits the initial shell of its ancestors; the dome-shape.


San Cristobal and its lush green lands

(https://www.google.com/url?sa=i&source=images&cd=&ved=2ahUKEwjmg4KwloTjAhVMfX0KHZEOASgQjhx6BAgBEAM&url=https%3A%2F%2Fwww.tripadvisor.com%2FLocationPhotoDirectLink-g297530-d10837630-i250974647-Ecuagringo_Fishing-San_Cristobal_Galapagos_Islands.html&psig=AOvVaw1ZsTFbKqDm2DlDjCYi9TeO&ust=1561536346276021)

Figure 4: The James Island Tortoise in its Grassland habitat

The rounded shell of the Tortoise is seen in this figure.

Figure 5: The James Island Tortoise in its Grassland habitat

The rounded shell of the Tortoise is also seen in this figure.

Distinguishing between the James Island tortoise and the Chatham Island tortoise has allowed the possibility to see adaptations at work. The clear difference between the structural features of the two tortoises shows the adaptations the James Island tortoise has undergone to possess a carapace different to that of the Chatham Island Tortoise.

On the same wavelength, it is important to note that both species have experienced a huge decline in their populations.

Figure 6. Tortoise Population Decline

As seen in figure 6, the Tortoise population has decreased significantly. It is believed that the rate of decline has direct relations with the extremely slow rate of movement the Tortoises possess, making them vulnerable to predators. However, the slow pace allowed the turtle to have a slower metabolism, which is the reason as to why they can live so long. (Why do giant tortoises live so long? : ANDREW LASANE MAY 17, 2016)

The tortoises slow metabolism can be linked back to their carapace, most notably in the case of the Chatham Island Tortoise. Dome-shape carapace tortoises usually always inhabit islands with high peaks, and mountains. Such is the case for the Chatham Island Tortoise who reside in San Cristobal. In these high peaks, the unstable nature and the fluctuations of the elevation in the ground means that the tortoises can flip over as a result of carelessness. Refer to Fig 7 & 8.

The lungs of the Galapagos tortoise are positioned in such a way that if the tortoise flips, due to its immense

weight, they will collapse and consequently kill the tortoise. This become an abiotic pressure for the James Island tortoise, and the


Figure 7 – Topographic Map of San Cristobal

The yellow-red area on the right side of the map shows an unsteady elevation of the ground.

Figure 8 – Distribution of the Chatham Island Tortoise in San Cristobal

The tortoises are seen to be living in exactly the “yellow-red area” discussed in figure 7, showing their unpredictable and unsteady habitat.

species had responded with by adopting a much flatter circular shaped carapace. (Seen in Fig 8) This helped the tortoise flip over with the geometrical properties of the carapace just in time so that their lungs would not be collapsed.

This factor also poses a threat for the James Island Tortoise, but Santiago possesses flatter landscapes and less risk of lung-collapse due to flipping-over for the species. Rather, the James Island Tortoise possesses a behavioural adaptation, where the tortoise chooses to move much slower than species possessing the dome-shaped carapace. The James Island Tortoise does this so as to reduce the possibility of becoming inverted, responding to the selection pressure differently to its Chatham Island counterpart and thus increasing the species chance of surviving the selection pressure.

Marine & Land Iguanas

The Galapagos Land Iguanas is a land reptile of the family iguanidae. They’re ancestors arrived to the Galapagos presumably through a vegetation raft. Land iguanas are large – up to 3 feet long and weighing up to 30 pounds. Their diets consist of shrub and plant vegetation, including fallen fruits and cactus pads.

The Galapagos Marine Iguana is a special type of iguana from the same family of the land iguana, igaunidae. The Galápagos marine iguana, or marine iguana, is a species of iguana that is only found on the Galapagos Islands. Just about every rocky shoreline of the Galapagos is home to these iguanas. Marine Iguanas are a species of the Land Iguana that have evolved to adapt to feed on aquatic food sources. They feed almost exclusively on Algae and the large males are known to dive to attain the algae, making them a marine reptile. They mainly live on the rocky shores of islands but are also known to be found on marshes, mangrove and beaches.

A recent study that analysed the Mitochondrial DNA and Nuclear DNA of both the Galapagos Land and Marine Iguanas concluded that the two diverged about 4.5 million years ago; an example of divergent evolution.

(Study conducted by:MacLeod, A.; A. Rodríguez; M. Vences; P. Orozco-terWengel; C. García; F. Trillmich; G. Gentile; A. Caccone; G. Quezada; S. Steinfartz (2015))

The binomial nomenclature of the two species of Iguanas that will be contrasted are listed below:

Species 1 – Galapagos Land Iguana (Conolophus subcristatus)


Figure 8: The circular shaped carapace of the Chatham Island tortoise

Species 2 - Galapagos Marine Iguana (Amblyrhynchus cristatus)

The Galapagos Land Iguana are a species of iguana thought to have evolved from the Ctenosaura, a genus of lizard native to Central America. It is widely accepted that the original ancestors of the Conolophus subcristatus arrived to the Galapagos islands via a raft of vegetation. Land iguanas live in the drier areas of islands, such as deserts and Rocklands. The first and most obvious adaptation the Land Iguanas possess is their earth-like colour skin; a mixture of red and yellow hues. Charles Darwin described their morphological qualities as:

“ugly animals, of a yellowish orange beneath, and of a brownish-red colour above: from their low facial angle they have a singularly stupid appearance."

[Darwin, Charles (1989), The Voyage of the Beagle: Charles Darwin's Journal of Researches, New York: Penguin Classics, p. 401]

Their unique colour allows them to blend in with the landscape of their habitat to hide themselves from their predators. The main predators of the Land iguana are thought to be introduced exotic species, such as feral dogs cats and pigs, as well as birds of prey. The idea that the only predators of the Land Iguana are introduced species has sparked a debate about its adaptations to skin colour. Most probably, when the predators of the land iguanas were introduced, the iguanas faced an abiotic selection pressure, being predators. By chance, Land Iguanas chose to variate from its parent species by adopting a complexion that matches their habitats, hence why they possess the yellow-red colour they have today; to match with their habitats and mask them from predators. The most adapted Iguanas passed on their complexion traits to their offspring, allowing a new species to be born. This example of a theoretical adaptation of the Land Iguana shows how the species evolved to adapt to selection pressures in nature, transcending with natural selection.


https://en.wikipedia.org/wiki/Charles_Darwin

The Land Iguana possesses very interesting behavioural adaptations in contrast to its ancestors. Conolophus subcristatus possesses a symbiotic relationship with Darwin’s Finches, where they raise themselves up to the finches to allow them to remove ticks from their bodies, benefitting both animals. (animalcorner.co.uk) This behaviour of the Land Iguana is a behavioural adaptation where the iguanas have learned of the benefits of establishing a symbiotic relationship with the finches, allowing them to freely feed on ticks on the iguanas body who could otherwise harm the iguana.

On the other side of the spectrum is the relatively new Marine Iguana, Amblyrhynchus cristatus. The marine iguana is thought to have diverged from the land iguana some 4.5 million years ago. It is hypothesized that land iguanas came out of events on certain islands where they were forced and stranded onto marine environments due to trees falling and barricading their exits. Due to these events, the original land iguana had to adapt to its new environment, ultimately evolving into the Marine Iguana through millions of years of evolution.

The most obvious contrasting quality of the Marine Igauna to its predecessor is the Marine Igaunas unique complexion. Due to its new environment, the marine iguanas ancestors develop a new way of hiding from new predators in the marine environment, manifesting a new biotic selection pressure. By chance, the marine iguana developed a black-rock type of complexion. This new trait of the Marine Iguana species allowed them to blend in with their new environment, masking them from predators.

See Figure 9 below.

The difference in complexion between the two species of iguana shows evolution at work. The marine iguana has diverged from the land iguana and adopted the black-type complexion in order to respond to the selection pressure of predators; ultimately enabling the marine iguana to hide from its natural predators and increase the species ability of surviving the selection pressure as a whole.

On the note of structural features, the marine iguana possesses a larger lateral tail than its land iguana predecessor, which allows it to swim underwater flawlessly, responding to the abiotic factor of adapting to a new environment: the aquatic environment. They feed almost exclusively on algae and henceforth must swim and dive to retrieve it. The tail is a evolutionary trait gained as an adaptation as a result of the abiotic selection


The Galapagos Land Iguana in contrast with its environment.

(GoGalapagos.com : DATE UNKNOWN)

Figure 9: The Marine Iguana Upon a rocky shoreline.

The complexion of the Marine Iguana is very close to that of the rocks they spend their time on, masking them form their predators

pressure of the marine iguanas environment. They also possess a special jaw and tooth structure adapted to perfectly crop algae off rocks, and sharp claws that allow the marine iguana to climb and grip rocks in wave-washed tidal areas. (au.expeditions.com, DATE:UNKNOWN) All the marine iguanas structural features differ heavily from that of the land iguana, showing how fauna respond to different environments on the Galapagos through adaptations, increasing the organisms ability to survive.

Structural features aside, the Marine iguana possesses a very unique physiological adaptation that differs them from the land iguana. The marine iguana has a unique desalinisation system unseen in many reptiles. The marine iguana rids itself of excess salt through special glands in its nostrils, allowing it to stay in homeostasis. These glands are unseen in its Land Iguana counterpart. The Marine Iguana’s example of its unique physiological adaptation shows its response to its abiotic environmental selection pressures, the same trend seen in its structural features, and henceforth allowing the marine iguana to increase its ability to survive in its environment.

Cormorants

Cormorants are a family of 40 species of aquatic birds called Phalacrocoracidae. Most species of the family are flight-capable birds, meaning they have the correct bodyweight to wingspan ratio for flight, however the endemic Galapagos species is the only species of the family to have lost the ability to fly, hence dubbed the “Flightless Cormorant”. Like all cormorants, the Flightless Cormorant features webbed feet and sturdy legs that help propel it through the water to seek prey of fish, and other small marine creatures. However, unlike other cormorants, the Flightless cormorant feeds deep on the seafloor no more than 200 meters out from shore, whereas other cormorants feed in deeper waters, but practice shallower diving. (en.wikipedia.org, DATE:UNKNOWN)

The two species of Cormorants that will be compared are listed below.


Phalacrocorax carboGreat Cormorant

One of the most wide-spread species of cormorants; found all across the northern hemisphere.

Phalacrocorax harrisiFlightless Cormorant

A species of Cormorant endemic to the Galapagos Islands

The Great Cormorant is a large black bird found widespread across the northern hemisphere, including the Galapagos. Its bodyweight-wingspan ratio is more than sufficient for flight, allowing for widespread migration of the species. The Great Cormorant feeds atop the sea, estuaries, freshwater rivers and lakes; anywhere there is an abundant supply of fish. It is a perfectly adapted bird of prey, hence why it is so widespread. The great cormorant nests in nests made of sticks near trees, on the ledges of cliffs, and on the ground on rocky islands that are free of predators. (en.wikepedia.org, DATE:UNKNOWN)

On the other end, the Flightless Cormorant is one of the very unusual species of Cormorant birds, simply because it is the only species that is unable to fly. This is due to the birds short-stubby wings, which meant that the bodyweight-wingspan ratio would change drastically and un-proportionately, disallowing the Flightless Cormorant’s flight capabilities.. Refer to figs 11 & 12.


Figure 11 : Great Cormorant Wingspan

The gigantic wings of the great cormorant are seen in this image

Figure 12 : Flightless Cormorant Wingspan

A visual difference is seen between the stubby wings of the flightless cormorant versus the great cormorant’s large wings.

It is largely accepted and theorized that that because there were no large predators that posed a risk to the Flightless Cormorants on the islands when they arrived in the Galapagos, the ability to fly became not just unnecessary, but a disadvantage to the birds. The physiology that makes flying possible are metabolically expensive to all bird species, therefore Cormorants that were able to feed without having to fly were more likely to survive and pass on their traits to their offspring, resulting in a new species of Cormorant; the Flightless Cormorant. The Flightless Cormorants ancestors response to this selection pressure was to develop smaller wings and acquire traits that allowed it to be an effective and specialized deep-sea bird of prey. Losing the ability to fly enabled the Flightless Cormorants to gain specific traits for swimming.

Examples of acquired traits the Flightless Cormorant features that differs it from the Great Cormorant are denser bodies with less surface area, stronger and denser bones, high leg muscle density, and fully webbed feet. All these traits allow the Flightless Cormorant to plunge to the seafloor to seek its favourite food of eels and fish.

One study found that the number of cilia-related genes, CUX-1 was abnormally high in the flightless birds in contrast with other Cormorant species. The gene allows for narrower and denser bone growth. Humans born with this gene have shorter limbs, narrowed chests and stunted rib cages - all of which is seen in the Galapagos cormorants. (http://newsroom.ucla.edu, DATE:UNKNOWN)

All the traits the Galapagos Flightless Cormorants possesses that differ it from the Great Cormorant are clearly observed to be for diving and swimming, showing how the Flightless Cormorant has adapted to its unique environmental abiotic factor physiologically, and structurally, increasing the organisms ability to survive in its environment, satisfying natural selection.

Sub-Question: Outline how an accumulation of micro evolutionary changes can drive evolutionary changes and speciation over time, using one Galapagos species as an example.

Microevolution (evolution on a small-scale) refers to the changes in allele frequencies within a single population. An allele frequency refers relative frequency of an allele appearing in a population, usually expressed as a percentage. Allele frequencies in a population may change due to four


http://newsroom.ucla.edu/

fundamental forces of evolution: natural selection, genetic drift, mutations and gene flow. Mutations are the ultimates source of new alleles in a gene pool.Two of the most relevant mechanisms of evolutionary change are: Natural Selection and Genetic Drift. Natural selection usually predominates in large populations whereas genetic drift does so in small ones.

Natural Selection leads to an evolutionary change when some individuals with certain traits in a population have a higher survival and reproductive rate than others and pass these features onto to their offspring. Evolution acts through natural selection where reproductive and genetic qualities that prove advantageous to survival prevail into future generations. The cumulative effects of natural selection process have giving rise to populations that have evolved to succeed in specific environments. Natural selection operates by differential reproductive success (fitness) of individuals.

The Darwin’s Finches adaptive radiation diagram describes the way the finch has adapted to take advantage of feeding in different ecological niches. Refer to fig 12

Darwin's finches are a classical example of an adaptive radiation. Their common ancestor arrived on the Galapagos about two million years ago. During the time that has passed

the Darwin's finches have evolved into 15 recognized species differing in body size, beak shape, song and feeding behaviour. (Uppsala University. (2015, February 11)

Changes in the size and form of the beak have enabled different species to utilize different food resources such as insects, seeds, nectar from cactus flowers as well as blood from iguanas, all driven by natural selection.

All species of Darwin's finches are closely related, having derived from a common ancestor. They live in the largely undisturbed environment in which they evolved, and none have become extinct as a result of human activity. Populations of the same species occur on different islands, and in some cases, they have different ecologies. Closely related species occur together on the same island and


Figure 12 : Adaptive Radiation in Galapagos Finches.

The diagram shows how different species of finches have adapted to take advantage of ecological niches, henceforth spectating.

differ. Thus, considering populations across the entire archipelago, we can see all stages of the speciation process, from start to finish, at the same time.

The finches represent a model of adaptive radiation (production in a short period of time of many species from one species occupying different ecological niches).

The association between beak size and diet is obvious when comparing the species that have contrasting structures. It is less obvious when comparing populations of the same species on different islands. Different populations of the sharp-beaked ground finch feed in different ways on different foods with beaks of differing size and shape. On the high islands of Santiago, Fernandina, and Pinta they have relatively blunt beaks and feed on arthropods and molluscs, as well as fruits and seeds in the dry season. On the low island of Genovesa, where their beaks are much smaller, they are more dependent on small seeds, as well as on nectar and pollen from plants. On the low island of Wolf, they exploit seabirds. First, they kick the eggs until the eggs fall and crack, enabling the finches to open them and consume the contents. They also inflict wounds at the base of the sitting boobies' wing feathers and consume the blood. On this island the finches' beaks are long.

Birds with large robust beaks do not probe Opuntia flowers or poke at eggs. Instead, the beak of this finch is a tool for tearing bark and crushing twigs and small branches. These examples illustrate some of the ways that Darwin's finches vary in beak morphology and are versatile in their feeding habits. This versatility is brought on by ecological opportunity and driven by food scarcity in the dry season and in dry years.

When the environment changes, some of the variants in each population survive while others die. This is known as adaptation. Birds with small beaks and small body size suffered selective mortality during a severe drought.

Concluding, micro-evolutionary changes between a species can result in evolutionary changes via repeated breading of two partners with the favoured allele. This is seen in the Galapagos Finch population where an accumulation of offspring with the favoured allele has resulted in a whole new species of bird, showing how microevolution airy changes can drive evolutionary changes ad allow for speciation to occur.

Sub-Question: Analyse an example of divergent evolution and convergent evolution in the Galapagos and draw a flow chart to illustrate the process by which it occurred.

Evolution can have several different patterns. The two of which will be discussed in this report are divergent evolution and convergent evolution.

Divergent Evolution

Divergent evolution is the archetype of evolution. It is the most commonly known form of evolution and occurs when closely related species diversify to new environments, or habitats. It involves one species splitting into two, diverging away from one another. Diagram A represents this trend.


A well-known example of divergent evolution has been discussed earlier in this report: the divergence of the Marine Iguana from the Land Iguana.

From the Conolophus subcristatus (Land Iguana) diverged the Amblyrhynchus cristatus (Marine Iguana) approximately 4.5 years ago, a recent study concluded.

The Amblyrhynchus cristatus is a fairly new and unevolved species as it is a monotypic genus; having only one species in its genus.

The flow chart below shows the divergence of the land iguana into the marine iguana.

Convergent Evolution

The second and lesser-known pattern of evolution is convergent evolution. Convergent evolution begins to occur as two species of different ancestry (non-monophyletic) begin to evolve and share analogous traits as a result of both species facing the same selection pressure. Diagram B shows convergent evolution.

An example of convergent evolution is seen in the Darwin’s Finches population of the Galapagos islands due to introgressive hybridization and selection. A study found conducted and summarised in abstract by Grant PR1, Grant BR, Markert JA, Keller LF, Petren K concluded that the mean morphological features of the cactus finch (Geospiza scandens) and the medium ground finch (Geospiza fortis) had begun to converge due to both birds facing the same selection pressures and due to birds between the two species mating – resulting in genes flowing predominantly from Medium Ground Finch to the Cactus finch.

The morphological study concluded quantitatively that the two species converged in beak shape by 22.2% and in body size by 45.5%.

The ecological selection pressures can only be explained by both birds suffering consequences due to El Niño events, causing them both to acquire the same morphological properties.


The flow chart below shows a hypothetical series of events that have/could happen in future, showing the convergence of both Geospiza scandens and Geospiza fortis species.

Sub Question: Analyse secondary data on modern selection pressures in the Galapagos Islands, including biotic and abiotic factors and those caused by humans to make predictions about the future pathway of evolution

Today, species on the Galapagos suffer different selection pressures than they have in historical times. The bulk of these selection pressures are now abiotic, caused chiefly by human processes which change the biophysical environments, ecosystems, and biodiversity, caused directly or indirectly by humans. These pressures also include global warming and environmental degradation. (en.wikipedia.com)

An indirect biotic factor caused by humans are the introduction of exotic species of prey. Never before seen to the existing Galapagos species, these new species became experts at killing the unwary Galapagos species. Introduced dogs, cats, pigs and birds of prey are now the main predators of many Galapagos Species. As galapagosconservation.org.uk puts it –

“Invasive species are animals, plants, pathogens or fungi that thrive outside of their native range, subsequently interrupting and damaging the balance of flora and fauna within the local ecosystem. They usually have the ability to grow quickly, causing harm to the original environment in many different ways.”

The fact that they can feed and grow populations so quickly has created a disruption in the balance of natural flora and fauna in the Galapagos Environment. Statistically speaking, there are now


greater than 1500 introduced species in the Galapagos. Biosecurity is now a big focus in the Galapagos.

However, these statistical measures may be exaggerated as the website is from a pro-change organisation. Realistically speaking, many of the >1500 are harmless, however some have been posed as a dire threat to the Galapagos species due to their ability to ruin whole populations of species.

Of the 13 introduced mammal species the problematic ones are cats, dogs, and goats. aboutgalapagos.nathab.com says that the introduced goat species, first brought in by sailors for food, have destroyed massive swaths of habitat over time, turning them into desert lands. This inversely creates selection pressures for the native Galapagos species who inherited the now destroyed lands, showing an example of a modern selection pressure in the Galapagos.

aboutgalapagos.nathab.com also mentions an integral example of a Galapagos species unable to cope to rapidly changing and increasing selection pressures. The population of the Galapagos Mangrove Finch has decreased from the thousands into the hundreds over time, with the critically endangered species estimated to have only 100 finches in the wild. The main cause of their ever-decreasing population is an introduced ectoparasitic fly named Philornis downsi. These flies lay their eggs in the defenceless Mangrove Finches nests so that once hatched, the Philornis downsi larvae may feed on the hatchlings of the Mangrove Finch eggs, severing the ability for the population to increase. It has become virtually impossible for most populations of Mangrove Finch to reproduce with healthy offspring that will continue to give birth to other hatchlings, which in theory could re-lift the population.

Graph One shows the probability of three Mangrove Finch populations persisting.

(Graph courtesy of Jorge Rodríguez-Matamoros)

As seen in the trends of the graph, two mangrove finch populations have shown in a decline in probability of persistence, which is very strongly linked to the selection pressure of battling the exotic fly species in a way that their hatchlings become invincible to the larvae.


https://www.researchgate.net/profile/Jorge_Rodriguez-Matamoros

This example of the decline of the Mangrove Finch population is a strong example of the modern abiotic and biotic selection pressures affecting the Galapagos Species today.

The 30,000 human inhabitants of the Galapagos receive upwards of 200,000 visitors every year. This creates a huge demand and pressure on resources which is increasing environmental threats. Humid zones, which are areas of high plant biodiversity, have been teared down and reduced to monocultures regions for agriculture, severely affection plant biodiversity and habitats, as well as food sources for Galapagos Species. Adversely, the ability of an ecosystem to respond to environmental changes without suffering damage decreases, ultimately meaning that Galapagos Organisms will find it harder to cope with and adapt to selection pressures as a result of anthropogenic activity.

The future of the pathway of evolutions will be largely determined by humans. Invasive species extermination efforts are underway and working miraculously, helping bring back the balance of flora and fauna originally found in the Galapagos. Evolution of Galapagos species in future will, with certainty, be in response to invasive fauna affecting endemic species.

Concluding, in future, species will evolve to adapt to the selection pressures put onto them by the introduction of these exotic species, increasing the resilience of the biological culture.

PART B

EXTENDED RESPONSE

QUESTION : How did the environmental conditions in 1997 impact on finches resources and hence their population?

The Grants study of the evolution of Darwin's finches on the Galapagos Islands took place on the island of Daphne Major. The pair chose this island as the finches had little to no natural predators or competitors. Rather, the pressures they were subject to was the erratic weather of Daphne Major, and hence, the availability of food. Their research was conducted on the medium ground finch, who’s diet was comprised of mostly seeds. The first major event that affected the Finches food was the 1977 drought, where the island didn’t receive rain for 551 days. This is shown in figure 1.

Figure 1 : Seasonal Rainfall in the Galapagos

Northwestern University (2009)

The wet season of 1977 experienced only 25 cm of rainfall, in contrast to other wet seasons which received upwards of 200cm. This was followed by a complete drought during the dry season


Temperatures during the drought stayed relatively constant, meaning temperature was not a contributing factor towards the availability of foods This is illustrated in figure 2. However it was the lack of water that affected the availability of foods for the finches.

Figure 2 : Seasonal Temperature of the Galapagos


As illustrated in the graph, temperature stayed relatively constant in 1977 compared to previous/continuing years.

As the drought continued, plants begun to wither and the natural source of foods for the Finches perished with them. Plants including Spurges, whose seeds were an ideal food source for the finches had dried out and died. The lack of small seeds for the Finches to feed on grew scarce, and finches with smaller beaks who were accustomed to this diet fell to starvation, whilst large beaked finches took advantage of their beak depth and began to crack open larger seeds as a source of nutrition.

Figure 3. Finches on Daphne Major from 1973-1976


The Grants studies of the 1977 finch population found that due to the drought, the finch population had been severely impacted, with 50 finches perishing from the wet season 1976 to the dry season 1977. This is illustrated in Figure 3. Through comparatively studying characteristics of the finches such as beak size, tarsus length, wing length and body weight. However, the Grants concluded that out of the four, the beak depth and size of the finches was the factor that most impacted the population during the drought.

As discussed above, the medium finches with a lower beak depth could not eat the seeds from the plants they relied on previously because they had dried out and no longer produced seeds due to a lack of water during the 1977 drought. However, there were


alternate foods that the medium finches with larger beaks could capitalise on. Some plants continued to grow in the drought and produce seed bearing fruits for the for the Finches to feed on. There was only one problem: these seeds were larger in size than the Finches usual diets, meaning that only finches with larger beaks could take advantage of using their beak depth to eat alternate foods because they could crack them open.

The Grants begun to take measurements of the four factors mentioned above. As previously stated, the first and most integral factor to the survival of the birds was beak depth. They found that the average size of a finches’ beak was bigger in 1978 than in 1977, essentially showing that the surviving finches had bigger beaks then the dead finches. Non surviving finches had a beak depth ranging from 7.5mm to 11.25mm, with a range of 3.75, whereas surviving finches had a beak depth ranging from 8mm to 11.25mm, with a range of 3.25. This is illustrated in the graphs below.

This meant higher range meant that the dead finches possessed greater variation of beak depths within their species, whereas the living finches possessed less variation; suggesting that they were specialized as a species to resist the lack of resources. This is also supported by the standard deviations (SD) of both data sets, with the non survivors possessing a greater SD (0.88) than the surviving finch population (0.84).

The standard error (SE) of both sets of data suggests the information is relatively accurate with the SE of the non-surviving finches data set being approx. 0.125 and the surviving finches data set being approx. 0.119.

When the data sets for both populations are averaged, the outcome is clear. Surviving finches clearly had a greater beak depth than non-surviving finches purely due to the fact that only finches with larger beaks were able to withstand the selection

pressures created by the drought. The fittest of the finches made in through – and in the case of the


1977 drought, being fit meant having a larger beak size. The death of the finches with smaller beaks allowed natural selection to reduce the frequency of the small beak alleles from the gene pool of the medium ground finches, hence ensuring all offspring will be fit to survive the selection pressures. The case of the beak sizes meant that the population of medium ground finches after 1977 would have a lower chance of giving offspring with small beaks opposed to giving birth to offspring with larger beaks.

An adverse effect of beak size of the medium ground finches was bodyweight. The Grants found that Finches who survived the drought possessed on average, a higher body mass than the finches who perished. This is most likely because of the lack of resources for the Finches with small beaks to feed

on. The lack of available foods that the small beaked finches could capitalise on caused them to starve and lose body mass due to processes of metabolism, while the large beaked finches who were fit persisted through the drought and were able to retain their weight by feeding on larger seeds, allowing their population to survive and retain a healthy body

mass.

Concluding, through the analysis of secondary data from the Grant’s study and Northwestern Universities 2009 studies, it is clearly seen that due to the drought on Daphne Major in 1977 the population of the Medium Ground finch was affected. The fittest Finches whom possessed larger beaks were able to feed on larger seeds, leaving the smaller beaked finches who couldn’t feed on larger seeds to perish, eliminating small beak alleles from the gene pool of the Medium Ground Finch and hence allowing future populations of the species to possess larger beak sizes to ensure survival if another drought is to come.


Non Survivors

Survivors

15 15.5 16 16.5 17 17.5

Average body mass of surviving Finches vs non surviving Finches.

Body mass (g)

REFERENCES:

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https://animalcorner.co.uk/animals/galapagos-great-frigate-bird/

https://www.google.com/url?sa=i&source=images&cd=&ved=2ahUKEwjmg4KwloTjAhVMfX0KHZEOASgQjhx6BAgBEAM&url=https%3A%2F%2Fwww.tripadvisor.com%2FLocationPhotoDirectLink-g297530-d10837630-i250974647-Ecuagringo_Fishing-San_Cristobal_Galapagos_Islands.html&psig=AOvVaw1ZsTFbKqDm2DlDjCYi9TeO&ust=1561536346276021



http://bguile.northwestern.edu/env/islandenvironment.html


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