evolution iii - speciation & macroevolution

Question 1

what are the 5 different species concepts?

Answer 1

1. genetic
2. biological
3. phylogenetic
4. morphological
5. ecological

Question 2

LO: what is the genetic species concept?

Answer 2

in the genetic species concept, there is a genetically distinct group of natural population of organisms that share a common gene pool. this deals with the genetics of speciation & genetic isolation: a population is defined as a group of genetically compatible interbreeding natural populations that is genetically isolated from other such groups.

data from mitochondrial & nuclear genomes is used to identify species, the extent to which the integrity of the gene pool is protected, and the nature of hybridisation.

limitation: common gene pool of a species may change due to directional selection & interbreeding beetween 2 different species

Question 3

LO: what is the biological species concept?

Answer 3

the biological species concept defines a species as a population or group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring, but are unable to produce viable, fertile offspring with members of other populations. (members of a biological species are reproductively compatible

each species has a gene pool that is separate from that of other species - this concept hinges on reproductive isolation, the existence of biological factors, or reproductive isolation mechanisms (RIMs), that impede members of 2 species from producing viable, fertile hybrids.

new species evolve when formerly interbreeding populations become reproductively/genetically isolated from one another

Question 4

RIMs (read through & know the different types)

Answer 4

prezygotic barriers

• habitat isolation (includes geographic isolation) occurs when species occupy different habitats, so they never come into contact with each other
• temporal isolation occurs when species have different mating/flowering seasons/times of day, or become sexually mature at different times of year
• behavioural isolation occurs when sexual attraction between males & females of different animal species is limited due to differences in behaviour or physiology
• physiological isolation includes mechanical & gametic isolation. mechanical isolation is when morphological features like size & incompatible genitalia prevent 2 members of different species from interbreeding. gametic isolation is when gametic transfer take place, but gametes fail to attract each other/fuse or because the male gametes are inviable in the female reproductive tract of another species

postzygotic barriers

• hybrid inviability occurs when the egg fertilized by the sperm of a different species fails to develop past the early embryonic stages
• hybrid sterility occurs when the interspecies hybrid survives, but is sterile

hybrid breakdown - when the F1 interspecies is viable & fertile, but succeeding generations beceome increasingly inviable, usually due to the formation of less fit genotypes by genetic recombination

Question 5

what are the advantages & limitations of the biological species concept?

Answer 5

advantage: focus on reproductive barriers, since speciation occurs by the evolution of reproductive isolation. (speciation: the process by which one species splits into 2 or more species) -> such analysis distinguishes groups of individuals that are sufficiently different to be considered separate species -> important for studying how species originate

limitations:

• there is no way to evaluate the reproductive isolation of fossils
• the biological species concept does not apply to organisms that reproductive asexually, such as prokaryotes and self-fertilising species. (in prokaryotes, genes can be transferred between distantly related prokaryotes)
• in the biological species concept, species are designated by the absence of gene flow, but there are pairs of species that are morphologically & ecologically distinct, yet gene flow occurs and rare hybrids may be formed.

Question 6

LO: what is the phylogenetic species concept?

Answer 6

the phylogenetic species concept defines a species as the smallest group of individuals that share a common ancestor, forming one branch on the tree of life. members of a species descend from a common ancestor, and hence have a shared & unique evolutionary history.

the phylogenetic history of a species can be traced by comparing its morphological characteristics or molecular sequences such as DNA sequences with those of other organisms -> can distinguish groups that are generally similar yet different enoguh to be considered separate species

advantages:

• can distinguish groups of individuals that are sufficiently different to be considered separate species
• reveals the existence of species that appear so similar they cannot be distinguished on morphological grounds, so the phylogenetic distinction is confirmed by reproductive incompatibility through the biological species concept

limitations:

• difficult to determine the degree of differences required to indicate separate species

Question 7

LO: what is the morphological species concept?

Answer 7

the morphological species concept characterizes a species by body shape or other structural features - organisms are classified as the same species if their anatomical traits appear to be very similar. used to distinguish between many species because there is little to no info about their mating capabilities

advantages:

• can be applied to all organisms
• can be useful even without info on the extent of gene flow

limitations:

• difficult to decide how many morphological characters to consider when characterizing individuals
• difficult to analyse quantitative traits that vary in a continuous way among members of the same species
• members of the same species may sometimes look very different, while members of different species may look remarkably similar to each other

Question 8

LO: what is the ecological species concept?

Answer 8

the ecological species concept view a species in terms of its ecological niche that is within its native environment, focusing on unique adaptations to particular roles in a biological community.

species from different evolutionary branches may come to resemble one another if they have the same ecological niche (convergent evolution), while species which are distributed over a wide geographical range may show considerable phenotypic differences (divergent evolution)

advantages

• can accommodate both asexually reproducing & sexually reproducing organisms, unlike the biological species concept
• particularly useful in distinguishing between bacterial species that reproduce asexually, as bacterial cells of the same species are likely to use the same types of resources and grow under the same types of conditions

limitation:

• does not take into account the species’ morphology & reproductive compatibility

Question 9

define speciation

Answer 9

speciation is the evolution/origin of species. evolution occurs whenever the inherited characteristics of a population or of a species change over a period of time. when these changes lead to the formation of one or more new species, speciation has taken place

Question 10

what are the 4 stages of speciation?

Answer 10

1. single population
2. barrier develops (geographical & ecological isolation)
3. differentiation due to different selection pressures
4. barrier disappears (reproductive isolation)

Question 11

describe the mechanism of speciation

(LOs only mentioned needing to allopatric and sympatric speciation)

Answer 11

STAGE 1: SINGLE POPULATION

• the ancestral species is a single species, can interbreed with e/o and and are reproductively isolated from other species

STAGE 2: BARRIER DEVELOPS

• subgroups of the ancestral species become separated from one another by a barrier, and are geographically isolated (if the barrier is physical)
• members still can interbreed if brought together - there is no particular advantage or disadvantage for interbreeding between separated populations

STAGE 3: DIFFERENTIATION

• the external barrier that isolates the populations from one another interrupts gene flow between the geographically isolated populations.
• variation arising within 1 population is not shared with the other via interbreeding
• now divided, each population experiences a slightly different environment that delivers different selective pressures -> different characters are successful in each location. through successive generations, the accumulation of character driven by local environmental conditions leads to the genetic distinctiveness of each separate population -> due to natural selection & changes in the gene pool, each population becomes more adapted to its own environment
• each population undergoes independent evolution, and if the barrier persists and enough time passes, the separate populations become genetically distinct

STAGE 4: BARRIER DISAPPEARS

• the barrier disappears and the different populations come into contact
• if RIMs are sufficiently weak to allow free interbreeding across reunited populations, and such matings produce offspring that are not at fitness disadvantage compared to offspring derived from matings between individuals within either population, then speciation halts, and a single species remains
• but if RIMs that developed during allopatric speciation put offspring from matings between 2 populations’ individuals at a selective disadvantage, speciation continues. over time, they diverge substantially such that they are reproductively isolated and are unable to produce VFO
• there must be accumulation of sufficient RIMs, adaptations & genetic diversity for a new species to be formed.

Question 12

LO: what are the 2 modes of speciation

Answer 12

• allopatric speciation (new species that arise in geographically isolated areas)
• sympatric speciation (usually through formation of polyploid organisms)

Question 13

LO: what is allopatric speciation?

the most common method of speciation

Answer 13

allopatric speciation refers to the formation of new species when one population becomes geographically separated from the rest of the species and subsequently evolves by natural selection and/or genetic drift. (both natural selection & genetic drift results in changes in allele frequencies in a population, but only in natural selection is the change in allele frequency adaptive.)

any physical barrier that prevents individuals of different populations from meeting will inevitably prevent them from interbreeding by blocking gene flow.

the different selection pressures of the new environment to which the population is exposed to further accentuates the divergence caused by genetic drift

eg galapagos finches

Question 14

LO: what is sympatric speciation?

Answer 14

in sympatric speciation, a new species evolves within the same geographic region as the parental species OR geographically overlapping populations

sympatric speciation occurs in at least 2 ways:

• polyploidy
• ecological isolation

Question 15

what is polyploidy?

Answer 15

polyploidy describes instances in which organisms possess more than 2 of the haploid chromosomes set

2 types of polyploidy:

• autopolyploidy (>2 sets of chromosomes, all derived from a single species)
• allopolyploidy (>2 sets of chromosomes, from 2 or more species)

Question 16

how can autopolyploidy arise?

not in LOs but can 大概 know

Answer 16

an autopolyploid contains more than 2 sets of chromosomes, all derived from a single species

1. a failure of cell division after chromosome duplication could double a cell’s chromosome number in non-disjunction, causing the formation of a tetraploid (4n) cells
2. if one of the 4n cells gives rise to a tetraploidplants, flowers produced will have diploid gametes through meiosis
3. if self fertilization occurs, the resulting tetraploid zygotes will develop into plants that can produce fertile tetraploid offspring by self pollination or mating with other tetraploids
4. tetraploids cannot produce VFO by mating with a parent plant because fusion of 2n gamete with n gamete will produce 3n offspring, which will be steril as the odd number of chromosomes cannot form homologous pairs & separate in meiosis -> formation of a tetraploid plant is an instantaneous speciation event, in which a new species that is reproductively isolated from its parent species, is formed in just one generation

autopolyploids can also be formed in the lab

Question 17

how can allopolyploidy arise?

Answer 17

an allopolyploid contains more than 2 sets of chromosomes from 2 or more species.

1. when haploid gametes from 2 different species combine, the resulting hybrid is normally sterile as its chromosomes are not homologous and cannot pair during meiosis. (eg haploid for 1 species is 2, haploid for the other is 3. resulting sterile hybrid will have 5 chromosomes)
2. but the hybrid may reproduce asexually, as many plants do.
3. subsequent errors in cell division may produce chromosome duplications that result in a diploid set of chromosomes (from 5 chromosomes, double to 10 chromosomes). now, chromosomes can pair in meiosis, and haploid gametes will be produced. a fertile polyploid species has formed.

if a population of allopolyploids becomes established, various selection pressures can cause 1 of the following 3 outcomes:

• the new species may not compete successfully against species that are already established, and become extinct
• the allopolyploid individuals may assume a new role in the environment, and so coexist with both parental species
• the new species may successfully compete with either or both parental species. if it has greater fitness than either or both parental species for all parts of the original range of the parent(s), the hybrid species may replace the parent(s)

e.g. bread wheat

even if haploid number of the 2 gametes from diff species are the same, the hybrid may still be sterile as the chromosomes are not homologous and still cannot pair in meiosis!!!!

Question 18

LO: what is biological classification

Answer 18

biological classification is the act of systematically arranging organisms into groups based on particular shared characteristics and their similarities. it may not take into consideration evolutionary relationships between species.

hierarchy:

1. Domain
2. Kingdom
3. Phylum
4. Class
5. Order
6. Family
7. Genus
8. Species

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Question 19

read through should be can alr

why is biological classification important?

Answer 19

1. to create order out of chaos, helping in the study & referencing of organisms
2. reveals natural evolutionary relationships and is a guide to the study of evolutionary pathways
3. helps with the identification of newly discovered organisms
4. forms a basis for an international system of nomenclature

Question 20

define macroevolution

Answer 20

macroevolution is the large scale phenotypic changes in populations that generally warrant their placement in taxonomic groups at the species level or higher.

studies of macroevolution seek to discover & explain major changes in species diversity through time, such as during adaptive radiation and mass extinction

Question 21

what is adaptive radiation?

Answer 21

adaptive radiation is the evolutionary diversification of many related species from one or a few ancestral species in a relatively short period of time

adaptive radiation occurs because of

1. new resources (ecological opportunities)
2. new ways to exploit resources (evolutionary novelties)

Question 22

how do ecological opportunities trigger adaptive radiations?

Answer 22

• ecological opportunities mean the availability of new/novel types of resources that have driven a wide array of adaptive radiations
• adaptive zones are ecological opportunities that were not exploited by an ancestral organism. when an adaptive zone is empty, a colonizing species will encounter no competitors, & rapidly diversify that leads to the efficient use of available resources. the species develops adaptations that help them thrive, and while occupying the available adaptive zone, succeeding generations diversify into new species, leading to rapid speciation from the ancestral species

eg anolis lizard

Question 23

how do evolutionary novelties through morphological innovation trigger adaptive radiation?

Answer 23

• the evolution of a key morphological trait - that allowed descendants to live in new areas, exploit new sources of food, or move in new ways - have triggered many important diversification events in the history of life
• evolutionary novelties originate through modifications of pre-existing structures. new designs arise from structures already in existence. usually, these evolutionary novelties are variations of pre-existing structures called preadaptation that originally fulfilled one role but subsequently changed in a way that was adaptive for a different role

Question 24

what is extinction?

Answer 24

extinction is the end of the lineage, and occurs when the last individual of a species dies. the loss is permanent; once a species goes extinct, it never reappears

when species become extinct, their adaptive zones become vacant, and surviving species are presented with new evolutionary oppotunities and may diverge, filling in some of the unoccupied zones.

2 rates of extinction:

1. uniform/background extinctions - the continuous low-level extinction of species. members of taxonomic groups are lost gradually over long time periods without abrupt lost of large numbers
2. mass extinctions - each period of mass extinction has been followed by a period of adaptive radiation of some surviving species. mass extinctions may occur because of major changes in climate, changes in environment due to catastrophe and habitat destruction/competition

Question 25

LO: what is the link between microevolution & macroevolution?

Answer 25

- microevolution is the evolutionary event seen close up, and is concerned with **patterns of change within a population or species**. it is a short term perspective. - macroevolution is an evolutionary event seen zoomed out the **overall pattern**, the **origin of species** and higher-level taxa, like families or classes. it is a longer term view. - big changes characterize macroevolution, while microevolution focuses on populations - microevolutionary processes explain macroevolution. mechanisms that bring about microevolution like **natural selection, mutation, genetic drift & gene flow** account not only for **genetic variation within populations**, but also for the **origin of new species and higher level taxa** as well. mass extinctions create new ecological opportunities that can be exploited by surviving organisms to evolve, and bring about rapid adaptive radiations forming many descendant species within a short period of time. - **macroevolution thus occurs as a result of microevolution - and macroevolution cannot take place without microevolution**