evolution iii - speciation & macroevolution Flashcards
what are the 5 different species concepts?
- genetic
- biological
- phylogenetic
- morphological
- ecological
what is the genetic species concept?
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
what is the biological species concept?
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
RIMs (read through & know the different types)
prezygotic barriers
- habitat isolation (includes geogrphic 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 physology
- 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
what are the advantages & limitations of the biological species concept?
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.
what is the phylogenetic species concept?
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
what is the morphological species concept?
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 their 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
what is the ecological species concept?
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
define speciation
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
what are the 4 stages of speciation?
- single population
- barrier develops (geographical & ecological isolation)
- differentiation due to different selection pressures
- barrier disappears (reproductive isolation)
describe the mechanism of speciation
(LOs only mentioned needing to allopatric and sympatric speciation)
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.
what are the 2 modes of speciation
- allopatric speciation (new species that arise in geographically isolated areas)
- sympatric speciation (usually through formation of polyploid organisms)
what is allopatric speciation?
the most common method of speciation
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
what is sympatric speciation?
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
what is polyploidy?
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)
how can autopolyploidy arise?
not in LOs but can 大概 know
an autopolyploid contains more than 2 sets of chromosomes, all derived from a single species
- 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
- if one of the 4n cells gives rise to a tetraploidplants, flowers produced will have diploid gametes through meiosis
- 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
- 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
how can allopolyploidy arise?
an allopolyploid contains more than 2 sets of chromosomes from 2 or more species.
- 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)
- but the hybrid may reproduce asexually, as many plants do.
- 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 indidviduals 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 ithas greater fitness than either or both parental species for all parts of the original range of the parent(s), the hybrid species may repalce 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!!!!
what is biological classification
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:
- Domain
- Kingdom
- Phylum
- Class
- Order
- Family
- Genus
- Species
read through should be can alr
why is biological classification important?
- to create order out of chaos, helping in the study & referencing of organisms
- reveals natural evolutionary relationships and is a guide to the study of evolutionary pathways
- helps with the identification of newly discovered organisms
- forms a basis for an international system of nomenclature
define macroevolution
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
what is adaptive radiation?
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
- new resources (ecological opportunities)
- new ways to exploit resources (evolutionary novelties)
how do ecological opportunities trigger adaptive radiations?
- ecological opportunities mean the availability of new/novel tes 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
how do evolutionary novelties through morphological innovation trigger adaptive radiation?
- 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
