Theme 1 Flashcards
(32 cards)
What is the importance of understanding natural extinction rates?
Natural extinctions rates tell us what rate we should expect to see species disappearing. By comparing that with real data of how many are observed to be going extinct, or are at high risk of extinction, we can evaluate the effects which we are having on nature. Today, the extinction rate is around 20 times greater than the background extinction rate - this is indicative of a mass extinction event.
What is the background rate of extinction? How does this compare to mass extinctions in the grand scheme?
The background rate of extinction is an estimation of the rate of species extinctions we would expect to see out with extinction events (i.e. under normal levels of environmental pressure). Natural rates of extinction are 0.1-1 extinction per million species per year. This background rate accounts for 95% of all extinctions, and the average species lasts 1-4 million years. Although the rate of extinction is much greater during mass extinction events, they only account for 5% of the total number of extinctions in biological history.
How do mass extinctions drive adaptive radiations? Give an example of this.
The sudden loss of organismal diversity can provide “ecospace” for other organisms to diversify and occupy the newly available niches. For example, after the cretaceous-tertiary boundary event, around 70% of all species died out (most of them being dinosaurs). Where very large and competitive organisms were no longer outcompeting the smaller mammals at the time, and this resulted in the ecological release of mammals. A rapid expansion in disparity of mammals allowed them to occupy many newly available niches by virtue of this freed up ecospace.
Explain the concept of local negative entropy, and how this is key to all life.
Entropy is a measure of how randomly energy is distributed in a system - net entropy of any reaction must always increase (2nd law of thermodynamics). This is not necessarily the entropy of the system, but is the entropy of the universe as a whole.
In order for organisms to survive, they must have highly organised systems, which is not consistent with increase in entropy. Therefore, they must create local negative entropy, whereby the biological system has a decrease in entropy (via processes such as chemical synthesis or mechanical work), but the entropy of the universe still increases due to the dissipation of energy from the biological system. This dissipation includes kinetic energy from mechanical work, excretion of bi-products, release of heat, etc.
What are the requirements/tradeoffs for multicellular life?
The conditions for multicellular life to be viable are highly specific - there are many limiting factors such as sufficient nutrients, intracellular conflict, sufficient oxygen, etc.
They require adaptations such as cell-cell adhesion, cell specialisations, germ-soma separation, and most importantly, alternation of life cycles via a unicellular intermediate.
This unicellular intermediate (zygote) is vital in purging inter-cell variation, facilitating sexual recombination, restoration of telomere length, and mitigation of the impact of deleterious mutations in the somatic cell line.
Discuss the differences in early embryonic development between protostomes and deuterostomes (and cindarians).
From an embryological perspective, all higher (bilateral) animals can be divided into two categories based on the development of their blastula - protostomes and deterostomes. Cnidarians are non-bilateral multicellular animals (such as jellyfish). The blastula is the hollow ball of cells which develops from a zygote. It gastrulates (invaginates) to give rise to the inner endoderm (central tract), and outer ectoderm (outer surfaces such as skin). There are two gastrulations which occur to form blastophores - one for the mouth and one for the anus. Bilateral animals also possess a third layer of tissue between these two - the mesoderm, which gives rise to musculature and internal organs between the digestive tract and skin.
In protostomes, the mouth blastophore invaginates first, but in deuterostomes, the anal blastophore invaginates first. Deuterostomes include echinodermata and chordata. Protostomes include all other bilateral life apart from cnidarians.
What are HOX genes?
HOX genes are a subset of homeobox genes, and are responsible for determining the body plan of a multicelllar organism. They are extremely simple and conserved across all life. Very profound changes can be produced from very small changes to HOX genes. They are transcription factors.
What is lagerstatten?
A good layer of sediment for fossilisation - shows fossil evidence which can be dated back hundreds of millions of years.
What were the major drivers of the Cambrian explosion?
The Cambrian explosion was a huge explosion in abundance and diversity of multicellular life which occurred 490-540 MYA. Potential drivers for this explosion include expansion of the shallow seas alongside the break-up of the supercontinent Pangea, end of a recent ice age, and an increase in atmospheric oxygen. However, these processes may have been too gradual to explain the very rapid explosion in life. It is more likely because of the ecospace created by the Ediacaran mass extinction, and the emergence of predator-prey dynamics creating an arms race.
How can modularity in body plans influence evolutionary radiations? Give an example of this.
Modularity is genetic independence between groups of phenotypic traits. Group of traits which interact with each other at a genetic level are known as modules, and whilst there is a lot of interplay between traits within modules, there is very little between traits of separate modules. This means that the evolution of one trait may avoid a cascading effect where it affects many other traits, helping an organism to avoid tradeoffs.
For example, domestic dogs show variations in their snouts/jaws which far exceeds the variation exhibited in wild dogs, wolves, and coyotes combined. This variation in facial structure has evolved extremely rapidly, with the help of selective breeding. This required a genetic predisposition to profound changes in jaw phenotype with negligible effects on cranial phenotype. Through experimentation, it was found to be true that domestic dogs have separate genetic modules for jaw and cranial shape.
Explain how biologists can determine the genetic basis of particular phenotypes.
Once morphological variations have been quantified (e.g. by principal component analysis), GWAS can be used to determine which allelic changes correspond to specific morphological variations. This is used to inform researchers of likely candidate genes for the variation. Body plan genes are highly conserved across species, which means that model species can be appropriate for experimenting on if the species in question would raise ethical concerns. By interfering with transcription or translation pathways of a particular gene/gene product, causation of certain genotypes regarding their phenotypic influence can be inferred. Morpholinos are molecules which are designed to block the activity of certain genes, and these are commonly used to evaluate the outcomes.
Outline the different geographical modes of speciation.
1) Allopatric speciation:
Occurs when a geographical barrier such a mountain or flowing water physically separates a population into two more groups, and different environmental pressures cause alternative adaptations until the populations, if/when re-introduced, ca no longer interbreed successfully. This requires a large amount of space and time for speciation to occur, and can occur under low or high levels of selection. Extrinsic barriers usually cause no gene flow at all between the divided groups. Allopatric speciation can be tested by measuring sexual isolation against geographic distance.
When the population is split into two or more roughly equally sized groups, this is known as vicariance. An example of this would be the different compliments of species on either side of the Amazon river.
When a small population separates from the main population and diverges this is known as peripatric speciation. These smaller populations will be subjected to a much stronger effect from genetic drift (due to bottlenosing). Evidence for peripatric speciation involves observing isolated, smaller populations having distinct genetic profiles. An example of peripatric speciation is Darwin’s Galapagos finches.
2) Parapatric speciation:
The population splits due to a lack of gene flow across a large area with abutting environmental differences. Restricted gene flow between populations (due to a cline or small geographical barrier) results in reproductive isolation. Space and time must be intermediate, and so must be selection pressure.
For example, water spring salamanders exist in a large extended body of water, but separate lineages develop in caves adjacent to the main body of water - gene flow is restricted to the interface between the cave and the surface.
It is difficult to infer this speciation mechanism, as many examples in nature can be explained as allopatric speciation and then re-introduction.
3) Sympatric speciation:
Reproductive isolation occurs without the influence of any geographical barrier. There is full potential for gene flow within the populations, but a high degree of selection against the mean trait value is imposed (disruptive selection). This means that intermediate phenotypes become a fitness valley, and extreme traits become fitness peaks. A mechanism of assortative mating gradually produces reproductive isolation.
Outline the differences between ecological and genetic speciation paradigms.
The ecological and genetic speciation paradigms consider mechanisms for reproductive isolation from an ecological or genetic perspective rather than from a geographical perspective.
Ecological speciation is the process by which barriers to gene flow evolve between populations as a result of ecologically-based divergent selection.
Contrary to this genetic modes of speciation are where fundamental genetic differences result in divergence (bottlenecks/founder effects, genetic drift hybridisation, etc). These can be slow (e.g. genetic drift), or rapid (e.g. polyploidy).
An example of an instantaneous, non-ecological speciation is observed in frogs. One species is diploid, and another is tetraploid - they may be able to produce a triploid hybrid with intermediate characteristics.
Describe pre and postzygotic incompatibility.
pre/postzygotic incompatibility are types of reproductive isolation, caused by the evolution of genetic differences due to natural selection. Prezygotic incompatibility means that the two genetically distinct organisms will choose not to mate with each other, whereas postzygotic incompatibility means that if/when they do mate, the resulting zygote will not be viable, and no fertile offspring will be produced.
In nature, prezygotic incompatibility is usually observed before postzygotic, meaning that as organisms speciate, they will usually choose not to mate before they become genetically incompatible.
Describe pre and postzygotic incompatibility.
pre/postzygotic incompatibility are types of reproductive isolation, caused by the evolution of genetic differences due to natural selection. Prezygotic incompatibility means that the two genetically distinct organisms will choose not to mate with each other, whereas postzygotic incompatibility means that if/when they do mate, the resulting zygote will not be viable, and no fertile offspring will be produced.
In nature, prezygotic incompatibility is usually observed before postzygotic, meaning that as organisms speciate, they will usually choose not to mate before they become genetically incompatible.
In sympatric pairs of Drosophila, prezygotic isolation occurs at lower genetic distances than in allopatric pairs, due to reinforcement.
Outline the differences between adaptive and evolutionary radiations.
Adaptive radiations are associated with ecological differences and particular geographic conditions, whilst evolutionary radiations are associated without much ecological partitioning or sexual selection. Adaptive radiations have a particular adaptive diversification component, but evolutionary radiations generate extreme levels of species richness over much longer time-scales (finer differences).
Adaptive traits are tested for using comparative methods, which involve comparing historical and contemporary similarities and differences of the trait across different lineages. We can also test whether a trait is adaptive based on its fitness consequences which can be done through experimentation. Convergent evolution among different lineages is good evidence of an adaptive trait.
Describe the origin and key adaptations of mammals.
The oldest mammalian fossils are around 220 million years old. Mammals are considered air-breathing vertebrates with no larval form. They possess hair, have a large, four-chambered heart and high metabolic rate, expanded hard and soft palates, specialised teeth and jaw musculature, and are usually viviparous (embryos are nourished by a vascular placenta). Limbs are typically held under the body for rapid and efficient locomotion, requiring endothermy. They excrete urea rather than uric acid. Mammals also typically have three middle ear bones, mammary glands, and highly developed sense of smell, as well as RBCs with no nuclei.
Mammals also possess a two-layered skin, the outer layer (epidermis) contains a protective cornified keratin-layer, and the inner contains hair follicles, glands, and more.
Hair is produced by keratin, and can be modified in many ways (hooves, horns claws, etc) for a huge variety of uses.
Mammals have 5 main types of glands - sweat glands, eccrine glands, apocrine glands (pheromones and reproduction), scent glands, and sebaceous glands. Mammary glands are a particular type of apocrine gland which excrete milk to nourish offspring. They are believed to have first evolved to confer innate and adaptive immune protection to offspring. They first produce colostrum, a thick milk which contains immune and growth factors, antibodies, nutreints, a gut microbiome, and endocrine competence, as well as a laxative.
Mammals evolved from synapsid reptiles. The evolution of lactation is believed to predate the appearance of mammals - synapsid reptiles first evolved into cynodontia, then mammaliaformes, then mammalia, and these pre-mammal lineages were able to secrete a primitive form of milk fro their cutaneous glands.
Describe the three main subgroups of mammals.
Mammals are divided into three subgroups - prototheria, eutheria, and and theria.
Prototheria are considered the most primitive mammals, and are oviparous monotremes. They are only found in Australia and New Guinea, and there are 5 living species (Platypus and echidna x4). Their urinary, defaecatory, and reproductive systes all open into a single duct (cloaca). They lack nipples (milk secreted into depression in mother’s belly), have many “reptilian” characteristics, adults lack teeth. They have 10 sex chromosomes, lower body temperatures and metabolic rates than other mammals. They lay cleidoic, amniotic eggs.
Theria are marsupials. They are believed to have evolved in South America and crossed antarctica to what is now Australia, reaching here around 50 MYA (eutheria disappeared from here around 55 MYA). They give birth to highly altricial young. Male penises have no urinary function, and females have two lateral vaginas with separate uteri, both of which open externally to a cloaca. They lack a corpus callosum between their brain hemispheres (as do prototherians). The embryo develops within an embryonic capsule, which it then hatches from and crawls to the nipple. They then exhibit rapid forelimb and facial development, but slow hind limb and brain development. Nipples can be found in the marsupium or open to the environment.
Eutheria are placenta mammals. The oldest eutherian fossil was found to be 160 million years old. They have highly developed allantoic placentae, as well as complex milk and prolonged parental care. They show a huge diversity of dietary and ecological specialisations and body forms. Most are homeothermic, endothermic, but some are poikilothermic (hibernate or torpor). They have the absence of epipubic bones found in all other mammals (possible that these bones are harmful in pregnant placentals.
What are the four main clades of eutherians? How did they come to be?
Eutherians can be subdivided into laurasiatherians, euarchontoglires, xenarthrae, and afrotherians.
Laurasiatherians evolved on the supercontinent Laurasia, after it split from Gondwana when Pangea broke up.
Euarchontoglires probably split from the Laurasiatherians 85-95 MYA, developing in the Laurasian island group which later became Europe.
Xenarthrae were restricted to South America until the great American interchange. All Xenarthan megafauna died out in the Pleistocene.
Afrotheria evolved on Africa when it separated 105 MYA, and is a sister taxon of Xenartha.
Explain what a key innovation is, and give examples.
Key innovations are evolved traits which allow a rapid increase in the rate of speciation and success of a lineage.
One example of a key innovation is amniotic eggs - these eggs have a membrane surrounding the fetus, which allows them to be laid on land or held within the mother (resulted in the evolutionary success of terrestrial reptiles, birds, and mammals).
Describe the three main groups of amphibians.
1) Caecilians are legless amphibians with dermal scales, a blunt head for digging, vestigial eyes, and have retractable sensory tentacles. The earliest fossils to have been found date back to the early Jurassic. There are roughly 160 species, most found near the equator, and they exhibit both internal and external fertilisation.
2) Caudata (salamanders and newts) have reduced skulls well developed tails, cylindrical bodies, and some species are lungless. They are long-lived and have a slow rate of development. They have a slightly more temperature distribution than caecilians (mostly on the northern hemisphere), and have been traced back to the middle Jurassic. They exhibit both internal and external fertilisation, and there are over 400 known species.
3) Anura (frogs and toads) have a short/no tail as adults, long muscular hind limbs, and shortened vertebral column. They are very widespread, found in almost every global habitat. There are over 6775 species, and they exhibit mostly external fertilisation. There are believed to be so many anura species due to radiation during the tertiary period (following the K/T mass extinction). There are two major subgroups of anura - archaeobatrachia and neobatrachia. 88% of all neobatrachian species emergences coincide with the K/T event (for example, arboreal frog species emerged at this time, and massively diversified).
Describe key innovations of modern amphibians
A wide variety of reproductive strategies have resulted in high rates of cladogenesis (emergence of new lineages), potentially due to reduced gene flow with other frog lineages, and increased potential for spatial segregation and speciation. For example, Darwin’s frog incubates its young the mamle’s vocal sacs (rather than in a water-pool). Another example, seen in Pristimantis frogs, is direct development - here, eggs are laid on land, and they hatch as “froglets” rather than tadpoles.
Another key innovation of many amphibians are defensive toxins, which allow them to avoid predation. Different species produce a range of toxins in different ways - for example, poison dart frogs produce their toxins by processing their prey after they consume them. Some species of salamander are highly toxic, and produce their poison endogenously.
Describe the possible drivers and consequences of the End Ordovician mass extinction event.
The End Ordovician extinction was believed to be due to intense glaciation (ice age). This extinction event occured over a period of around 30 million years, ending around 434 MYA. During the Ordovician period, ocean invertebrates were diversifying - cephalopods emerged, predatory sea scorpions and giant orthocones, as well as coral, molluscs, trilobites, and the first vertebrate fish all emerged (as well as terrestrial plants). The continents were all located in the southern hemisphere, and Antarctica, Australia, and Africa formed the supercontinent Gondwana. A major ice cap developed over most of the continental area and into the sea.
One theory as to the cause of this global cooling was a major drop in atmospheric CO2 due to the weathering of silicate rocks (forming calcium carbonate), and volcanism. The sea level dropped due to the increased ice, and many coastal organisms died out. Next, subsequent global warming took place, CO2 was released, and biodegradation by marine organisms caused anoxia.
Describe the possible drivers and consequences of the Late Devonian mass extinction event.
Devonian period lasted around 60 million years, and ended roughly 354 MYA - it was known as “a greenhouse age” and “an age of fish” due to its wetness and warmth, and marine life, respectively. Lobe-finned fish emerged at this time (later became amphibians), reef ecosystems dominated by coral and sponges developed, as well as diversification of cephalopods, the first terrestrial vertebrates emerged, as well as many invertebrates (possibly flying insects), and lush forest developed with vascular plants and deep roots. An abrupt drop in average temperature from global cooling, as well as oceanic volcanism, and the potential contribution of two meteorites resulted in lowering of sea levels and ocean anoxia. It is also possible that cosmopolitan species started to expand, pushing others out of their niches, and reducing the overall speciation rate.
Another theory is that the forests’ roots broke through rock substrates, creating nutrient-rich soils. When these were weathered, the nutrients moved to downstream aquatic ecosystems, resulting in huge algal blooms which eventually disintegrated through aerobic decomposition, causing anoxia. The extinction event is split into two spikes - the Hangenberg extinction spike and the Kellwasser extinction event. 70-80% of ocean species went extinct, mostly in the tropics (reef communities suffered most)
