Test 2 Flashcards
What is geographical variation?
Tendency of pop’s of same species to differe according to their geographical location.
Best criterion for defining dif species…
REPRODUCTIVE ISOLATION - dif species coexist in same locality remain distinct bc they do not interbreed.
If there were no barriers to interbreeding b/w members of dif species, diversity of life would…
Not exist, there would be something approaching a continuum of forms.
When barriers to interbreeding b/w formerly separate species break down,
highly variable offspring are produced (continuum of forms).
How do species become distinct?
Barriers:
- difference in habitats
- difference in time of breeding of species
- features of organisms (esp details), e.g. not producing right smell/sound during mating
- behavioural patterns
- chemical menas of detecting pollen from wrong species and rejecting it (plants)
- sperm and egg incompatibility (thus unsuccessful fertilization)
Separate species never mate.
FALSE. Sufficiently closely related species will occasionally mate, esp if no choice of member of their own species is given.
First generation hybrids
usually fail to develop - die at early stage of development, or survive (rare) and are usually sterile - do not produce offspring that could pass genes onto future gens. E.g. mules (donkey and horse hybrid). Complete inviability/sterility of hybrids isolates the 2 species.
Is inviability/infertility of hybrids direct product of natural selection?
Unlikely bc there can be no advantage to an individual producing inviable/sterile offspring if hybridized with dif species. It is advantageous to avoid mating to produce hybrids. When hybrids survive well, it is unclear where the advantage is.
What is likely the way that barriers to evol changes occurred?
After pop’s became isolated from e/o by being geographically or ecologically separated
Geographical variation
Tendency of pop’s of same species to differe according to their geographical location, in ways that are adaptive (often)
Example of geographical variations in humans
Minor physical difs like skin pigmentation and stature, also found in many other species of animals and plants
In a species that consists of a set of local pop’s, there is usually
Some migration of individuals b/w dif locations
Amount of migration is the same for all organisms
FALSE. Varies enormously - slow for snails, birds/flying insects high.
What kind of force is migration?
Homogenizing - opposes tendency for local pop’s to diverge genetically by selection or gen drift
What is an example of strong selection causing adjacent pop’s to differ?
Lead/copper contaminated soil near mines - metal tolerant plants vs regular plants to which the waste is toxic
Dif’s between geographically separate pop’s of same species do not necessarily need dif types of selection
TRUE - e.g. dif molecular paths to moleria resistance, or dif most common blood types in dif countries/continents
Intermediate stages in development of reproductive isolation of species are needed in theory of evolution.
TRUE - e.g. humans are same species throughout the world, though with small different physical features. Some orgs will be part of same species but look entirely dif - all those in between them on the “spectra” of the species are intermediates (e.g. Canada vs Colombia (Bogota) flies - look very dif but can still successfully reproduce so same species, only dif bc of location)
Males of fly hybrids are fertile.
FALSE, only females are.
Each alteration in genetic composition of one pop must either be favioured by selection in the pop, or…
Have a slight effect on fitness that it can spread by gen drift
If a variant is spreading in a pop bc it has an advantage in adapting the pop to its local envi, its spread will not be impeded by any harmful effects when combined (in hybrids) w genes from a dif pop which it never naturally encounters
True
There IS selection to maintain compatibility of mating behaviour bw individuals from geographically or ecologically separated pops or to maintain harmonious interactions that allow normal development, bw genes that have COME TO DIFFER in dif pops
FALSE
1st geen male hybrids are sterile
TRUE
By testing the … of male offspring of cross bw hybrid femaile and non-hybrid male we can study the genetic basis of the hybrid male sterility. e.g. done with flies
fertility
Once two related pops have become completely isolated from each other, their evol fates will FOREVER be independent of one another, and will tend to diverge over time
TRUE
Important cause of irrevesible divergence bw 2 separate species is
Natural selec - 2 closely related species may look almost identical but not be able to reproduce w each other
Same time is needed to become reproductively isolated for all species
FALSE. Could be quick, like with galapagos finches, could be slow.
It is impossible for slow evolving species to abruptly transition to a new form
FALSE. Could very well happen, as seen in fossil record, and usually end up being recognized as new species.
Traits will not change greatly once a species living in a stable envi has had time to adapt to it
TRUE
It is usually impossible to tell from fossil record whether an observed sudden evol change implies the origin of a new species or simply involves a single lineage evolving in response to envi changes
TRUE. Still rapid change though as intense selection produces profound changes in trait in 100 generations or less
What do species mean when there is ASEXUAL reproduction as in single-celled bacteria?
- criterion of interbreeding is meaningless
- for classification, biologists use arbitrary measures of similarity based on characters of practical importance (e.g. composition of bacteria cell walls) OR increasingly on DNA sequence difs
- sufficiently similar individuals that cluster together based on related characters are classed as same species
- thus, NOT based on interbreeding patterns as in sexual reproduction
Technique that is increasingly used to make inferences ab relationships bw species
DNA sequences
Types of possible changes in DNA sequences
Due to insertions and deletions (however rare in genes that code for proteins as they have major effect on sequence of amino acids in protein and make it non-functional). MOST CHANGES due to changes to individual letters of sequence.
Comparing the numbers of letters in DNA by which the sequences of same gene differ bw a pair of dif organisms, one can do what?
Quantify their level of divergency precisely, which is difficult to do with morphological similarities and difs.
Knowing the genetic code, we can see what?
Which of the differences alter the protein sequence corresponding to gene in question (replacement changes) and which do not (silent changes). The more difs at sequence level, the more distantly related organisms are.
Patterns of relationships revealed by sequence comparisons ARE in broad agreement with what is expected from the times at which the major groups of animals and plants appear in fossial record, as expected in …
The theory of evolution
Which types of sequence changes are more common?
Silent, bc often have little or no effect on biological functions while replacement changes actually affect functioning of org. Most changes to AA sequence of protein impoair its function to some extent, so never contribute to evol difs in gene sequences that accumulate bw species as natural selection quikcly eliminates them
Some AA sequence evol is driven by selec acting on occasional favourable mutations
TRUE
How does silent mutation spread through pop?
Through random changes in frequencies of alternative variants (gen drift) in finite pops. Since effect of gene is NEUTRAL, genes of next gen will be drawn randomly from parental pop. Frequency of mutant gene in progeny pop will not be same as its frequency in parents pop due to random factors (some producing more offspring while others none, and it being very unlikely for all individuals to produce exactly same number of offspring). Over gens, there will thus be continual random fluctuations in composition of pop, until eventually the silent gene is present in all (FIXATION) or disappears entirely from pop. In small pop, gen drift is fast so it wont take long for all members of pop to become the same. Process takes much longer in larger pop.
Input of new neutral variants by mutation and changes in variant frequencies by drift determine variability in the pop.
How to determine effect of gen drift in a pop?
Examination of DNA sequences of same gene from dif individuals from pop reveal variability at silent sites.
A selectively neutral variant that is initially very rare has some chance to…
spread throughout whole pop and replacing alternative variants (although has much greater chance of being lost). Gen drift thus leads to evol divergence bw isolated pops, even without any selection promoting changes.
Gen drift is a fast process
False. It is very slow, the rate of which depends on rate at which new neutral mutations arise, as well as rate at which gen drift leads to replacement of one version of a gene by a new one.
What does rate of DNA sequence divergence bw pair of species depend on?
Only on the rate of mutation per DNA letter (the frequency with which a particular letter in a parent is mutant in the copy that is passed to an offspring). EXPLANATION: if no selec is acting, nothing affects number of mutational difs bw a pair of species except rate at which mutations appear in sequence and amount of time since the species’ last common ancestor.
A small pop has more new mutations per gen
FALSE. Large pop has more bc there are more individuals in which a mutations might happen.
Gen drift happens faster in a small pop
TRUE
Opposing effects of population size cancel out exactly, so mutation rate determines rate of divergence. Which 2 effects are discussed?
of mutations per gen vs rate of gen drift.
Neutral changes accumulate in a gene as time goes on at a rate that depends on the gene’s mutation rate. Known as what?
Molecular clock principle. Changes in genes are likely to take place in much more clock-like fashion than changes in traits subject to selection. Not very precise - its rates can change over time within the same lineage and bw lineages. Use of mol. clock allows biologists to roughly date divergence bw species for which there is no fossil evidence.
Rates of morphological changes depend strongly on…
Environmental change. Variable rates and reversals of direction can occur.
How to calibrate molecular clock
One needs sequences from closest available species whose divergence dates are known. This method has been used to date timing of split bw lineage giving rise to modern humans and one leading to chimpanzees and gorillas for which no independent fossil evidence is available. Dates of 6 ot 7 million yrs have been estimated with considerable confidence.
Which molecular clock observation is consistent w experimental measurements of mutation rates?
That the rate of neutral sequence evolution depends on mutation rate and the is thus clock is exceedingly slow, since the rate at which single letters in the DNA change by mutation is very low
A mol clock can also be applied to… and how?
Amino acid sequence of proteins. Protein sequences evolve slower than silent DNA difs and are therefore useful for the task of comparing species that diverged a long time ago. It is impossible to count exactly the number of DNA mutations between such species, so scientists interested in reconstructing the times of divergence bw major groups of living forms use data from slowly evolving molecules. Dates are rough but estimates from many dif genes can improve accuracy.
Two major and largely unsolved problems in evolution are…
- basic features of living cells
- origin of human consciousness
In the evol of orgs and their complex machinery, many aspects are…
modified (adapted) versions of pre-existing structures
Name of process that creates minor changes to make their possessors survive/reproduce better than others
Tinkering
What ultimately produces large evol changes?
Succession of small changes to a structure that already works but can be improved.
Evol of what molecule is sometimes posed as an especially difficult problem, and how did they likely evolve?
Proteins. They probs started as short chains of a few amino acids that could cause reactions to go a bit faster, and were successively improved as they evolved.
Protein sequences during evolution started off…
catalysing reactions better than when no protein is present, and then got successively better over evolutionary time. With this, there is no need to worry about the many millions of potentional non-functional sequences that will never exist.
How does knowledge about protein function support protein evolution explanation?
Part of protein that is essential for its chemical activity is often only a very small part of its sequence - typical enzyme has just a handful of AA’s that physically interact w the chemical that is to be changed by the enzyme - the rest of the protein mostly provides a scaffold that supports the structure of the part involved in the interaction. THUS functioning of protein depends critically on only relatively small set of AA’s so new function could evolve by a small number of changed to the sequence of protein.
Explain the possible evolution of successive enzyme reactions.
Many useful chemicals were probs present in the envi of early orgs. As life evolved, these became scarce. An organism that could change a similar chemical into the useful one would benefit and so an enzyme could evolve to catalyze that change. The useful chemical would now be synthesized from the related one. THUS a short biosynthetic pathways, with a precursor and product, would be favoured. Like so, pathways could evolve - backwards from their end-products - to build up the chemicals organisms need.
Why were there questions about the evolution of successive enzyme reactions?
One might thinkt hat, even if the end products are useful, it would be impossible to evolve such pathways since evolution has no foresight and cannot build up a chain of enzyme reactions until its function is complete.
Two sources of evidence for complex adaptations evolving in steps are…
We need evidence for intermediate stages in the evolution of such characters, so:
- fossil record
- present-day species that show intermediate stages bw simple and more advanced stages (only source of info on features that do not fossilize)
Major divisions of multicellular animals (molluscs, arthropods, vertebrates) nearly all appeared rather suddenly in what era?
Cambrian. With sudden change, there is virtually no fossil evidence concerning their ancestors so no proof of intermediate forms in evolution for these (does NOT mean that intermediates did not exist). DNA sequences are studied to see similarities, but we have no info on what they looked like, probs bc they were soft-bodied and thus unlikely to fossilize.
What animal feature does not fossilize? Give 2 examples of it.
Flight
- mistaking limbs for wings as wings adapted from limbs in bats
- flying squirrels having same bones just extra flaps of skin for the wings
How could have a vertebrate eye evolved, when a lens is apparently useless without a retina, and vice versa?
A retina is not useless w/out a lens. Many types of invertebrate animals have simple eyes, with no lens. Such animals do not need to see clearly - it is enough to perceive light and dark in order to detect predators. Starting with a simple ability of cells to detect light through rhodopsin protein, a series of steps followed, in which increased light-capturing abilities evolve step by step, leading eventually to focusable lens that produces a sharp image.
How are single-celled eukaryotes capable of detecting and responding to light?
By means of receptors composed of a cluster of molecules of the light-sensitive protein rhodopsin (it is used in all animal eyes, and is also found in bacteria).
Ageing in multicellular orgs may appear to be a severe difficulty for the evol theory by contradicting the idea that natural selection causes the evol of adaptation. Answer to of those that support the evol theory is what?
Adaptation is never perfect. Ageing is partly an unavoidable consequence of cumulative damage to the systems necessary for continued survival, and selection likely cannot prevent this.
Why must ageing demand an evolutionary explanation?
Some organisms live longer than others (e.g. mice vs humans) and are able to replenish the traits that become diminished with age - e.g. reptiles teeth will renew from time to time, while a mammal will eventually starve to death bc is unable to eat w teeth that have worn out. Single-celled orgs like bacteria reproduce simply by dividing into daughter cells and the lineages of cells produced by the divisions have persisted over billions of years - they continually break down damaged components and replace them with new ones, they can continue to propagate indefinitely, given that harmful mutations are removed by selection.
Given that all individuals have some risk of death from accidents, diseases and predation, and the chance of death from each is age-INDEPEDENT, the chance of surviving decreases as age increases. Therefore, selection favours…
Selection favours survival and reproduction early rather than late in life simple bc, on average, more individuals will be alive to experience the good effects. The greater the mortality due to accidents, diseases and predation, the more strongly will selection favour improvements early in life, relative to later on, bc few individuals can survive to late ages if the death rate from these external causes is high. Thus, mutations with harmful effects will be most strongly selected against if htey express their effects early in life.
The argument of selection favouring survival/reproduction early and not late in life suggests what?
That ageing evolves bc of the greater selective value of variants w favourable effects on survival/fertility early in life, compared with variants that act late.
The 2 main ways in which natural selection might work to cause ageing and brief explanation:
- keep early-acting mutations rare in populations, while allowing ones with effects late in life to become common (many common human genetic diseases are due to mutations whose harmful effects appear only LATE in life, e.g. Alzheimer’s)
- variants that have beneficial effects early in life will be more likely to spread through the population than those whose good effects come only in old age. Improvements to early stages of life can evolve, even if these benefits come at the expense of harmful side-effects later on (e.g. higher levels of some reproductive hormones may enhance women’s fetility early inl life, but at the risk of later breast and ovarian cancer, confirmed by experiments, OR fly experiment w breeding of a pop only with old individuals, offspring age slower but have reduced reproductive success early in life)
What does the evol theory of ageing predict?
Species with low externally caused death rates should evolve low rates of ageing and longer life spans, compared with species with higher external death rates.
Explain the relationship between body size and the rate of ageing.
Strong relationship. Smaller species tend to age much faster than larger ones and reproduce earlier. This probably reflects the greater vulnerability of many small snimals to accidents and predation.
Why are striking difs in rates of ageing seen bw animals w dif mortality rates in the wild, even bw species w similar body sizes?
The difs often make sense in terms of differing risks of predation. E.g. many flying creatures are notable for longevity, which makes sense bc flying is a good defence against many predators.
How are humans an example of evolution of a slower rate of ageing compared to chimpanzees?
Our closest relatives rarely live beyond 50 yrs and start reproducing earlier in life. Humans have thus probably substantially reduced their rate of ageing since diverging from our common ancestor w apes and postponed reproductive maturity. These changes are probs due to increased intelligence and ability to cooperate which reduced vulnerability to external causes of death and reduced the advantage of reproducing early.
How can industrialization be linked to natural selection in present day society?
It has led to a dramatic decline in mortality rates among adults, changing the relative advantages of early vs late reproduction by altering the natural selec affecting the ageing process in human pop’s.
Mass extinction in evol history have repeatedly affected what groups of animals more?
Large, slow breeding species (like mammoths) have been more severely affected than small, fast breeding ones, like mice.
What is the cause behind the current mass extinction event?
Human activies causing loss of/damage to natural environments are causing the loss of species such as elephants and rhinos.
Explain the natural selection behind the ageing process in human pop’s.
Consider Huntington’s disease, caused by rare mutant gene. It affects ppl in their 30s or later. In a pop w high mortality bc of disease and malnutrition, few individuals survive to their 40s and carriers of disease have on average only slightly fewer offspring than unaffected individuals. In industrializes societies, with low mortality, ppl often have children later in life, in age when disease could appear, so affected ppl have fewer children, on average, than unaffected individuals. If current conditions continue, selection will gradually reduce the frequencies of mutant genes w effects expressed late in reproductive life, and survival rates of older individuals will increase.
Explain sterile social castes.
- In social wasps, bees and ants, some of the females in a nest are workers who do not reproduce. They represent a small minority within the colony (often just single queen). Worker females look after queens’ offspring and maintain and provide for nest.
- In termites (other main group of social insects) both males and females can behave as workers.
- In advanced social insects, there are often several dif castes which perform dif roles and are distinguished by difs in behaviour, size and body structure
Majority of inhabitants of a social insect nest have been found to be
sterile (e.g. naked mole rat - may be several dozen inhabitants of next and only single reproductive female. if she dies, there is a struggle to replace her among the other females but one wins). Many dif species have system of social castes - from insects to rats.
Explain the social group system of species, aka PRINCIPLE OF KIN SELECTION. Applies not only to sterility but certain animals like birds and wild dogs not mating/hunting but staying back and looking after the young.
Members of a social group are generally close relatives, often share same mother and father. A genetic variant that causes its carriers to forgo their reproductive success to help raise its relatives may help the relatives’ genes pass to next gen, and the genes are often the same as the helper individual’s own genes bc of the relatedness (same mother or father means half of same genes is present). If sacrifice by sterile individuals results in sufficient increase in numbers of surviving and reproductively successful relatives, the increase in # of copies of the worker gene can outweigh the decrease due to their own lost reproductive success. The increase needed to outweigh the loss is smaller the closer the degree of relationship.
What is the development as a member of a particular caste of worker controlled by? What is that factor controlled by?
- Environmental cues, such as the amount/quality of food provided to the individual while a larva.
- Ability to respond to the cues is genetically determined. A certain genetic variant might confer the potentiality of a sterile member of an ant colony to develop as a soldier (e.g.) so bigger jaws, and not a worker. Depending on whether soldiers make the colony better fit in the envi/pop (i.e. colony is better defended against enemies and better reproductivity on average), the success of this variant will increase in the colony. If the reproductively active members of the colony are close relatives of the workers, the genetic variant that induces some workers to become soldiers will be transmitted by the colony thru queens and males founding new colonies. THUS, selection can act to increase the representation of the variant among colonies in the species.
How is the differentiation of cells into dif types during development analogous to the differentiation of castes in social insects? How does this idea illuminate the evol of multicellular orgs from single-celled ancestors?
The cells produced from the egg and sperm fusion remain associated and most lose their ability to become sex cells and thus to contribute to the next gen. Since the involved cells are genetically identical, this would be advantageous if survival and reproduction were sufficiently increased in the group of associated cells, compared to single celled alternative. The non reproducing cells sacrifice their own reproduction for the benefit of the community of cells.
The serious consequences for orgs when cells regain the ability to divide without regard to the organism is manifested as…
Cancer.
What makes the origin of the basic features of living cells and the origin of human consciousness unique events in the history of life?
- We cannot use comparisons among living species to make firm inferences ab how they might have occurred.
- Lack of any fossil record for early history of life or for human behaviour means we have no direct info about the sequences of events involved.
- Guesses cannot be prevented, but they cannot be tested like the other evolutionary ideas.
Explain the ideas behind the case of the origin of life.
- aim of current research is to find conditions resembling those of the early history of the earth, which allow the purely chemical assembly of molecules that can then replicate themselves. Once such self-replicating molecules have been formed, its easy to imagine how competition bw dif types of molecules could result in the evol of more accurate and faster replicating molecules (i.e. NATURAL SELECTION acting to improve them)
- chemists have shown that the basic chemical building blocks of life (sugars, fats, AA’s, constituents of DNA and RNA) can be formed by subjecting solutions of simpler molecules (of the type that are likely to have been present in the oceans of the early earth) to electric sparks and UV irradiation. There is limited progress in showing how these can be assembled into more complex molecules that resemble RNA and DNA and esp in getting such molecules to self replicate
- once the above goal is achieved, the question of how to evolve a primitive genetic code that allows a short RNA or DNA sequence to determine a simple protein sequence must be solved
- many ideas, no definitive solutions to problem yet
Explain the ideas of the evolution of human consciousness.
- difficult to state the nature of the problem clearly bc consciousness is hard to define precisely. It is probs a matter of degree, not kind, so that in principle there is little difficulty in imagining a gradual intensification of self awareness and ability to communicate during the evol of our ancestors
- We know nothing of the details of the selective forces driving the evolution of human mental and language abilities
- Biologists are making rapid progress in understanding the functioning of the brain. There is little doubt that all forms of mental activity are explicable in terms of the activities of nerve cells in the brain. These activities must be subject to control by genes that specify the development and functioning of the brain. Like any other gene, these will be liable to mutation, leading to variation on which selection can act. This is not just a hypothesis - mutations have been found which lead to deficiencies in specific aspects of speech of their carriers, leading to identification of a gene involved in the control by the brain of this aspect.
What would likely be regarded as the strongest criterion for possession of true consciousness?
Language ability. Note: it develops gradually, and is seen even in animals (e.g. teaching parrot to talk). Gap bw ourselves and higher animals is more apparent than real.
What is the chief difference bw the modern view and Wallace/Darwin view on evolution?
Two advances mean that the process of evol through selection acting on random mutations of the genetic material is now much more credible than it was at the beginning of the 20th century:
1. we have a much richer body of data demonstrating the action of natural selection at every level of biological organization, from protein molecules to complex behaviour patterns
2. we also now understand the mechanism of inheritance, which was a mystery to Darwin and Wallace
What is the major force guiding the evolution of structure, functions, and behaviours?
Natural selection
Genes lay down only the potential range of traits that an organism can exhibit.
TRUE. This knowledge of heredity and our understanding that natural selection drives the evol of organisms’ physical and havioural characteristics does NOT imply rigid genetic determination of all aspects of such characteristics.
- The traits which are actually expressed often depend on the particular environment in which an organism finds itself.
- In higher animals, learning plays major role in behaviour, but the range of behaviour that can be learned is limited by the animal’s brain structure, which is in turn limited by the animal’s genetic makeup. Applied across species - no dog will learn to talk or humans cant smell rabbit at a distance.
Among humans, there is strong evidence for the involvement of both genetic and environmental factors in causing differences in mental characteristics.
TRUE.
The characteristic which we regard as most human, such as our ability to talk and think symbolically, as well as the feelings that guide our family and social relationships, must reflect…
a long process of natural selection that started tens of millions of yrs ago, when our ancestors started living in social groups. This does NOT mean that all details of people’s behaviour are genetically controlled, or that they represent characteristics that increase human fitness.
There are facts in the theory of evolution that suggest that evolution will eventually stop.
FALSE. There is nothing in the theory of evol by natural selec to suggest that progress in evol is inevitable, and of course bacteria are still one of the most abundant and successful forms of life.
Give examples of evolutionary reduction of complexity.
- cave dwelling species that have lost their sight
- parasites that lack the structures and functions needed for independent existence
Natural selec cannot foresee the future, and merely accumulates variants that are favourable under prevailing conditions.
TRUE. Increased complexity may often provide better functioning, as in the case of eyes, and will then be selected for. If fuction is no longer relevant to fitness, its not surprising that the structure concerned will degenerate.
Natural selec cannot foresee the future, and merely accumulates variants that are favourable under prevailing conditions.
TRUE. Increased complexity may often provide better functioning, as in the case of eyes, and will then be selected for. If fuction is no longer relevant to fitness, its not surprising that the structure concerned will degenerate.
Sources of microevolution include:
- natural selection
- genetic drift
- mutation
- migration
What is defined by a species?
- phenotypic similarity
- sympatric (fairly easy to identify within a region), but problems arise from gradual differences across regions (allopatric)
- genetic similarity also used to identify and define species (phylogenetic species concept)
The 2 main species concepts are:
- taxonomic (or morphological): based primarily on distinct measurable differences
- biological: based on inter-fertility among individuals
- BUT, concepts vary among groups of organisms: No universal species concept
Key points of the biological species concept (most USEFUL species concept we have, leads to best research on speciation) are…
- focuses on the PROCESS
- geographic isolation alone is NOT sufficient
- isolation does NOT have to be absolute (question of what cutoff?)
- must be possibly interbreeding IN THE WILD
- doesnot apply well for bacteria, asexuals, highly self-fertlizing species or fossils
Where does speciation occur?
- often called geographic speciation due to involvement of geographical isolation
- allopatric speciation much more common and easier to evolve
Allopatric vs. sympatric isolation
- allopatric: 2 separate species merge (bc closeness)
- sympatric: part of one population gets into another and then speciation occurs
Stages when reproductive isolation (RI) can occur…
- pre-zygotic RI barriers: finding a compatible mate and mating, fertilization
- post-zygotic RI barriers: development and growth of zygote (F1), adult survival and reproduction (F1)
- growth, survival, reproduction of offspring (F2)
Pre-zygotic barriers prevent mating or fertilization, so no zygote gets formed. Could be caused by:
- geographical, ecological
- temporal, behavioural (mate recognition)
- mechanical (gentical structure compatibility)
- cellular (sperm-egg compatibility)
Post-zygotic barriers prevent proper functioning of zygotes once they are formed. Caused by… and what are the types?
- caused by combinations of genes with low fitness in the HYBRID
- types: INTRINSIC (inviability, sterility, or abnormal development of hybrids), EXTRINSIC (ecological mismatch of hybrid phenotype to environment)
- CANNOT be directly favoured by natural selection (rather arise as an indirect byproduct of evolution acting separately in dif populations)
Example of INTRINSIC post-zygotic isolation is…
- the mule (it is a sterile hybrid cross of MALE DONKEY and FEMALE HORSE)
- hinny (sterile hybrid of MALE HORSE and FEMALE DONKEY)
Example of EXTRINSIC post-zygotic isolation is…
- mullerian mimicry in Heliconius butterflies
- poorly adapted hybrids
- hybrids have aberrant colour patterns: higher predation risk and lower mating success
Is adaptive evolution REQUIRED for speciation? Also termed ECOLOGICAL SPECIATION.
- local adaptation by dif populations CAN lead to reproductive isolation AND speciation (bc distinct evolutionary responses to dif selective pressures. Local adaptation NOT absolutely necessary, but ACCELERATUREs population divergence and evolution of reproductive isolation)
- much current research aims to determine the BIOTIC and ABIOTIC agents of selection and the underlying “SPECIATION GENES”
What are the answers to some NAGGING DOUBTS by early researchers?
1. How and why do organisms DIVERGE and DIVERSIFY?
2. How can we explain patterns of VARIATION across species?
3. Can the evolutionary processes of mutation, selection, drift explain HOW we got millions of diverse species on Earth?
- populations diverge GENETICALLY as a result of evolutionary forces (mutation, natural selec, gen drift): become RI, foten as an INCIDENTAL byproduct of evol change within pops
- allows speciation to occur: pops continue evolving and adapting INDEPENDENTLY, driving MACROevol diversification
Adaptive radiation
- evolution of ecological and phenotypic diversity within a RAPIDLY multiplying lineage as a result of speciation
- originates from a SINGLE common ancestor
- process results in an array of many species
- species differ in traits allowing EXPLOITATION of a range of habitats and resources
Four features commonly identifying adaptive radiation are…
- recent common ancestry from a SINGLE species
- phenotype-environment CORRELATION
- trait UTILITY
- RAPID speciation
What causes adaptive radiations?
- ECOLOGICAL OPPORTUNITY (abundant resources, few competitors, often encountered on oceanic islands/their aquatic counterparts, like African rift lakes)
- ORIGIN OF A KEY INNOVATION (e.g. toepad in Anoles, floral nectar spur in Columbines)
- HIGH RATES OF SPECIATION CHARACTERIZE THE CLADE (test by comparing island to mainland clade, e.g. Darwin’s finches 7 Hawaiian honeycreepers also radiated on mainland, whereas Galapagos mockingbirds have NOT radiated on islands/continents)
Hybridization
- exchange of GENES b/w species as a result of OCCASIONAL inter-species mating (sometimes can reverse speciation process to MERGE 2 groups into one)
- varies across tree of life (common in plants and fish, RARE in mammals)
- can result in COMPLEX patterns of variation (can be evolutionarily significant for speciation, especially by POLYPLOIDY)
Polyploidy
- an organism, tissue, or cell w/ MORE than 2 COMPLETE sets of HOMOLOGOUS chromosomes
2 types of polyploidy
- ALLOPOLYPLOIDY (e.g. AA x AA create AA AA): arises from DUPLICATED karyotype following hybridization b/w species, MOST COMMON type of polyploidy
- AUTOPOLYPLOIDY (e.g. AA into AA AA): arises from DUPLICATED karyotype within a species (e.g. non-disjunction)
Evolutionary significance of polyploidy
- polyploids are REPRODUCTIVELY ISOLATED from their diploid parents (hence a form of SYMPATRIC speciation)
- polyploids exhibit NOVEL PHENOTYPES (allows exploitation of NEW habitats)
- polyploids often show GYBRID VIGOR due to heterozygosity, particularly in ALLOPOLYPLOIDS
- Polyploid origin for approx 50% of flowing plants (many crops plants and invasive species)
Speciation IS a UNIdirectional process
FALSE. NOT unidirectional - i.e. species can merge and diversify throughout time.
Carolus Linnaeus, the “father” of taxonomy, proposed what?
- binomial nomenclature
- hierarchical system of classification: (kingdoms, phyla, classes, orders, families, genera, species)
Purpose of biological classification?
- name is a KEY to shared info on an organism
- therefore has PREDICTIVE power
- enables INTERPRETATION of ORIGINS and EVOLUTIONARY HISTORY
Taxon
- a single names taxonomic unit at ANY level
Taxonomy
- theory and practice of CLASSIFICATION and NAMING
Systematics
- study of BIODIVERSITY and EVOLUTIONARY RELATIONSHIPS among organisms
In a phylogenetic tree, what does each represent: outgroup taxon, ingroup taxa, terminal nodes, terminal branches, internal nodes, internal branches?
- outgroup taxon: furthest taxa of tree
- ingroup taxa: all inner taxa of tree (before the furthest one)
- terminal nodes: taxa
- terminal branches: accumulated evolutionary change
- internal nodes: common ancestors, speciation
- internal branches: accumulated evol change
WHY conduct phylogenetic analysis?
- understand history of life
- understand large scale patterns of evol
- understand how many times traits evolved, how fast, under what conditions
- PRACTICAL: where/when parasites spread, which flu strain was MOST successful last year, what the driver mutations are as SARS-COV-2 evolves
Monophyletic groups in phylogenetic trees:
- Includes the COMPLETE SET of species derived from a common ancestor
- a SINGLE ancestor gave rise to ALL species in that taxon and NO species in any OTHER taxon
- much preferred because they link taxon names to evolutionary history
Paraphyletic group in phylogenetic trees.
- Contains SOME, but not all species derived from a common ancestor
- a taxon whose members are derived from TWO or more ancestral forms, NOT common to all members
Species that share a MORE common ancestor tend to be more similar
TRUE. Descent with modification.
A critical step in RECONSTRUCTION of phylogenetic history is…
the identification and distinction of ANCESTRAL (trait SHARED with common ancestor) and DERIVED (trait that DIFFERS from ancestral trait in a lineage) traits.
Homology vs. homoplasy
- homology (similarity of traits due to SHARED ANCESTRY)
- homoplasy (similarity of traits as a result of CONVERGENT EVOLUTION)
Examples of homologous structures
Human and fish skeleton
Convergent evolution
- the independent evol of structures that RESEMBLE one another and perform SIMILAR functional roles due to SHARED ECOLOGY of unrelated organisms
2 examples of convergent evolution
- convergent evol of succulence and spiny growth form in desert environments (cactus family vs. spurge family vs. milkweed family all developed similar traits/looks)
- cichlid fishes of the African great lakes: independent evolutionary radiations in 2 lakes, similarity in form indicates convergence in FEEDING strategies
WHY is molecular biology relevant to evolution and phylogeny?
- all life is related through branching descent
- common genetic code is EVIDENCE that all life IS related
- evolutionary relationships among species are reflected in their DNA and proteins (learning about protein function in ONE species can tell us about its function in OTHERS, e.g. mice, rats, flies, worms vs. humans)
Species relationships from DNA sequences
- genes or parts of a gene can be SEQUENCED for dif species
- species can be ASSESSED for changes in sequence of nucleotides
- changes can be used to INFER relationships in a phylogeny
KEY INNOVATIONS in using phylogenies to understand the origin and evol of traits
- origin of a NOVEL trait resulting in ADAPTIVE RADIATION
- carriers of the trait can EXPLOIT new resources or sets of habitats
- usually associated w/rapid evolutionary diversification (e.g. adaptive radiation)
Origin of traits associated w/increased diversification rate. What is the diversification equation?
- key innovation could increase speciation OR decrease extinction to influence NET diversification
- equation: diversification = speciation - extinction
What is the “sister group” comparison?
- with one phylogenetic comparison, it is difficult to say a key trait IS involved
- instead, REPLICATE comparisons of MULTIPLE groups adds more evidence
Why does herbivory associate with HIGHER diversification rate?
- COEVOLUTION b/w insects and plants likely drives HIGHER rates of speciation in herbivores
Features associated with greater diversification:
- species with more SEXUAL SELECTION
- ANIMAL POLLINATION in plants
- increased DISPERSAL
- increased RANGE SIZE
Lamarck’s view on evolution…
ALL organisms have an inherent tendency to BECOME more complex
Only SOME lineages have evolved greater complexity
TRUE. Eukaryotes (plants, fungi, animals) and bacteria (great complexity) vs. archaea.
Major transitions in evolution book. Main idea?
- greater complexity arises from greater ‘cooperation’ amongst PREVIOUSLY independent units
- talks about origin of: cells, chromosomes, genetic code, sexual reproduction, eukaryotes, multicellularity, colonies (e.g. nonreproductive castes)
- SMALL number of events led to major changes in how inheritance worked
- previously independently evolving units MERGED, leading to higher-level complexity and specialization through DIVISION OF LABOUR
What is the ‘unit of selection’?
- most phenotypic traits we study in orgs arose due to selection that increases FITNESS of individuals (may OR may not be good for species - e.g. sexual selec for showy coloration may increase chance of extinction bc of higher predation. ALSO, individual selection is USUALLY stronger than group selection)
- traits that are “good for species” but that REDUCE FITNESS of individual CANNOT be favoured by INDIVIDUAL SELECTION
When is cooperation adaptive?
- HIGH RELATEDNESS (genes that lead to helping relatives can spread via natural selec)
- RECIPROCAL ALTRUISM (in cases where orgs repeatedly encounter each other, mutual cooperation can lead to HIGHEST fitness)
- BUT, cooperation SOMETIMES breaks down (e.g. selection for ‘cheaters’)
Why are genes seen as the ultimate target of selection?
- genes are the unit of inheritance, so ultimately target of selection is the GENE
Cheater organism and selection
- cheaters use common resources at expense of others
- e.g. mitochondria that promote their own replication even if it comes at expense of overall fitness of organism
- cheaters are not necessarily as efficient
- cheaters ARE better at invading the next generation, they do this at expense of whole organism, and across many generations, so cheater DNA loses out to wildtype DNA at POPULATION LEVEL
- ability to survive in stressful environments can foster tolerance to cheating, inadvertently prolonging persistence of cheater genotypes
- could be useful in understanding diseases associated with disfunction of organelles (for e.g.)
Selection on INDIVIDUAL organisms IS NOT a form of cooperation.
- FALSE
- genomes are composed of UNRELATED genes and alleles that have been inherited from dif places
- segregation, recombination and random mating ensures that they are MOSTLY passed on independently
- yet genes typically persist by improving fitness of the “group” as a whole
How do genomes (esp higher levels of organization) stay so cooperative?
- many features of individual orgs prevent competition WITHIN an individual (prevents evol within individuals, align fitness interests)
- ensures that many genes succeed by ENHANCING fitness of the organism
How do INDIVIDUAL genomes stay so cooperative?
- MITOSIS AND MEIOSIS (ensures that alleles dont compete within an individual, FAIR REPRESENTATION of gene variants among daughter cells)
- DEVELOPMENT AND MULTICELLULARITY (starting from a SINGLE CELL prevents initial competition among cell lineages)
- UNIPARENTAL INHERITANCE OF ORGANELLES (chloroplasts and mitochondria replicate asexually, PREVENTS COMPETITION within cells of dif organelle genomes)
- when CHEATING ALLELES spread, strong selection on rest of genome for SUPPRESSION of cheating
How do alleles spread through a population?
- by increasing individual fitness
- positive natural selection on alleles
Cheating a fair meiosis by alleles. Driven by what 2 factors?
- meioitic drive
- over replication
Meiotic drive
- if an allele can bias its own transmission (i.e. by enhancing its own transmission, 1 allele has higher probability to spread), then it can spread to higher frequency even while REDUCING individual fitness
- “selfish” genetic element relative to organism’s fitness interests
Meiotic drive can RAPIDLY ELIMINATE alleles that have…
higher INDIVIDUAL fitness
How does meiotic drive cheat Mendel’s Law of Segregation?
- expected ‘fair’ meiosis does NOT occur bc majority of offspring possess allele combo that has enhanced transmission
Transposable elements: cheating Mendel’s Laws through over-replication
- over replication creates self-replicating segments of DNA
- transposable elements’ replication separated from cellular replication
- ensure their OWN over-representation in offspring (cheating of Mendel’s laws)
- transposable elements can make up greater than 50% of DNA in genomes of some species
Most common DNA in genomes often is selfish
TRUE
How do genomes not ‘explode’ from transposition?
- alleles arising elsewhere in genome that SILENCE transposable elements will be FAVOURED by individual selec (e.g. mechanisms controlling DNA and histone methylation)
- transposition-selection balance
What may have evolved as silencing mechanisms?
- piRNAs
- RNA interference
Mutations in genes for DNA methylation leads to…
rampant activation of transposable elements
What is transposition?
a form of mutation that can disrupt a gene
Transposition-selection balance
- transposition increases transposable element abundance
- natural selec against harmful effects on the org reduces abundance of chromosome copes with MOST transposable elements
- overall abundance results from a balance b/w these opposing forces
Mitochondrial transmission
- lack of mitosis and meiosis by plastids sets up potential for spread of SELFISH elements
How do mitochondria stay cooperative?
- uniparental plastid inheritance strongly reduced competition WITHIN individuals
- consistent w/ hypothesis that it evolved to MAINTAIN cooperation (e.g. active exclusion of sperm mitochondria at fertilization, that is, only egg mitochondria is passed down to offspring)
Uniparental inheritance creates new conflict. Discuss maternal vs. biparental inheritance in mitochondria.
- MATERNAL inheritance of cytoplasmic genome vs. BIPARENTAL inheritance of nuclear genome
- mitochondrial mutations that enhance MATERNAL fitness can spread, even if cost is severe to MALE fitness
Cytoplasmic male sterility in plants
- new mutations in mitochondria that make HERMAPHRODITIC plants “male sterile” can spread
- “male sterile” hermaphrodites = “female” bc they favour mitochondrial transmission
- can reduce fitness of plant as a whole
- leads to evolution of nuclear ‘restorer’ alleles that re-enable fertility through pollen (ARMS RACE co-evolution of CMS (cytoplastic male sterility) and restorer genes)
- mitochondrial mutations that make a HERMAPHRODITE plant into a “male sterile” female WILL spread, even if they REDUCE fitness of individuals
- “male sterility” DECREASES fitness through pollen (might increase fitness through ovules bc re-allocation of resources to ovules, but NOT double that of the hermaphrodite)
What is a hermaphrodite?
- an individual animal/flower that has BOTH male and female reproductive organs
How do collections of cells stay cooperative?
- starting from a SINGLE CELL reduces competition within individuals
- separation of GERMLINE with limited numbers of cell divisions inhibits transmission of SELFISH cell lineages
- tumor SUPPRESSORS, other features inhibit unregulated cell division
- BUT somatic mutation is inevitable in long-lived multicellular organisms (some of those somatic mutations might be selectively FAVOURED WITHIN an individual)
Cancer can also be called a selfish cell lineage evolving where?
- WITHIN an individual
- spreads commonly in tissue that is relatively undifferentiated (evolved resistance to treatment/immune system)
- illustrates the “short-sightedness” of evolutionary process
How do INDIVIDUAL GENOMES stay so cooperative?
- many features ensure that variance in fitness WITHIN an individual is minimized
- ensures that many genes succeed by enhancing the fitness of the organism (‘group’)
- BUT countless ways to evade cooperation
- presence of strong selection on rest of genome (‘policing’) seems essential to maintain higher-level cohesion
What are the different levels of selection?
- alleles
- organelles
- tumors
- individuals
Applied evolution concepts and examples
- agricultural relevance (pesticide and herbicide resistance)
- evolutionary medicine (evolution of resistance to antibiotics, evolution-proof vaccination)
- global chance and evolution (adapt or go extinct)
How do pest/herbicides work?
- we use chemicals to combat pests and pathogens
- we create strong selective pressure for resistance (fitness advantage to resistance genotypes arising from mutation and gene flow)
- weedy plants have repeatedly evolved resistance to herbicides (SUPERWEEDS!!) - can we design “evolution-proof” solution??
Where does resistance come/evolve from?
- pre existing genetic variation in the pop
- gene flow (‘epidemic spread’ of resistance from one region to next)
- new mutations (in very large populations new, simple mutations may be introduced at a HIGH rate)
What is the wisest management strategy to dealing with superweeds?
Depends on how resistance tends to EVOLVE
__________ weeds have more pre-existing resistance variation than _____________ weeds.
outcrossing, selfing.
Can herbicide resistance be stopped?
- multi herbicide treatment (makes new adaptation LESS likely, requires more complex adaptation)
- rotation of dif kinds of herbicides (weeds regularly hit by dif selection pressures)
- BUT could select for GENERALIZED resistance
Malaria and mosquitos, what is the major prevention strategy?
- insecticide in major prevention strategy
- strong selective pressure on mosquitoes has led to rapid evolution of resistance
What is a possible solution to malaria mosquito problem that may be “evolution-proof”?
- tailor insecticide application to knowledge of mosquito generation times and spatial distributions
- goal: minimize selection for mosquito resistance while still reducing malaria transmission
Multi-drug ‘cocktails’ SLOW evolution of HIV resistance.
TRUE.
- single mutations unlikely to confer resistance to MULTIPLE DRUGS with dif mechanisms of action
- lower viral loads make multiple mutations less likely
Cancers cannot evolve drug resistance
FALSE. Mutations to many dif genes lead to drug-resistant tumors.
Evolutionarily informed cancer treatment
- strong, prolonged selec pressures using same chemotherapy drugs (may not be best solution bc selects for resistance)
- cycling drugs, multidrug cocktails, lower doses of drugs (ethical considerations make tests of theory for human application challenging, but could a be better option)
