Species Flashcards
how are new species discovered?
- through work on museum collections & actually classifying their samples
- through phylogenetic/genetic studies
- through insights from traditional knowledge
- through fieldwork
argument for species being real?
- Discreteness argument: variation in nature is noticeably discontinuous, statistically the variation identifies clusters of organisms
- Accumulating evidence argument: long recognized species mostly stand up to further scientific study
- Origin of species argument: species arise by speciation, so species must be real because speciation is a real process.
define Species Concept
a species concept is a generalization of the ideas/criteria that we think identify what a species is defined as.
what do all species concepts have in common?
they all fundamentally describe units that are reproductive communities
- include forces that establish and reinforce boundaries between species
- the history of connections among individuals and populations
- traits distinguish different species
define Biological Species Concept (BSC)
A species consists of groups of actually or potentially interbreeding populations of organisms that are reproductively isolated from other such groups
- emphasizes reproductive barriers that prevent or reduce gene flow between sets of populations
what is BSC challenged by?
- asexual reproduction – asexual species are not addressed under this concept
- hybridization – how much interbreeding is allowed between species before they’re considered “one” species?
- potential vs actual interbreeding – what if populations would actually interbreed but they are geographically separated?
define Phylogenetic Species Concept (PSC)
Describes a species as a monophyletic set of populations (if sexual) or lineages (if asexual). On a phylogenetic tree, species would be the units at the tips of the tree.
what is PSC challenged by?
- “species” that originate more than once?
- levels of monophyly - what distinguishes a species from other taxonomic levels or populations
- hybridization
define Ecological Species Concept (ESC)
A species is a lineage (or closely related set of lineages) which occupies an adaptive zone minimally different from that of any other lineages in its range and which evolves separately from all lineages outside its range
- essentially described species as groups that occupy different ecological niches.
- can be applied to both asexual and sexual groups
what is ESC challenged by?
it is unclear how different niches have to make a distinction between species
define Genotypic Cluster Species Concept (GCSC)
Sympatric species are clusters of genotypes circumscribed by gaps in the range of possible multilocus genotypes between them
- species are characterized as having distinct trait clusters
- involves assessing patterns of genetic/trait variation, specifically looking for evidence of distinct clusters
- incorporates evidence for maintenance of distinct gene pools in face of potential or actual gene flow
what is GCSC challenged by?
- asexual lineages
- levels of differentiation - how are populations different from species?
define Evolutionary Species Concept (EvSC)
A lineage of ancestor-descendant populations of organisms which maintains its identity from other such lineages (in space and time) and which has its own evolutionary tendencies and historical fate.
why is it important to have a widely accepted definition of “species”?
the establishment of legislation to protect species requires a clear definition of what constitutes a species in order to protect affected populations in question
define Reproductive Isolation
- reproductive isolation prevents or reduces interbreeding between sets of populations through the action of one or more isolating barriers.
- it provides the mechanisms that prevent species from merging i.e. maintains the separation of species
- it defines the reproductive community within which interbreeding brings about species cohesion
Reproductive Isolation vs. isolating barriers
Reproductive isolation is the reduction in gene exchange between populations/species due to the action of one or more isolating barriers
An isolation barrier is a specific mechanism that acts at a specific stage to reduce or prevent gene flow.
Isolating barriers are evaluated by comparing the success of hybridization events to the success of intraspecific mating.
some examples of isolating barriers
- mating season differences
- geographic separation
- incompatible anatomy
- hybrid sterility
- and more
at what stages can isolating barriers act?
- Pre-mating barriers
- Post-mating, pre-zygotic barriers
- Post-zygotic barriers
what are pre-mating barriers?
these reduce the probability of mating (gamete exchange)
- geographic isolation
- behavioural isolation (like birds not recognizing other bird species’ songs)
- mechanical isolation (male and female reproductive parts are not compatible)
what are post-mating, pre-zygotic barriers?
these influence after mating but before zygote formation
- failure of fertilization (incompatible sperm and eggs)
- gametes may not survive the reproductive system
what are post-zygotic barriers?
these focus on hybrid fitness in viability and fertility, over one or many generations. They can be
Intrinsic - independent of the environment
- hybrid sterility
- hybrid inviability (won’t survive early life)
Extrinsic - dependent on the environment
- hybrids more likely to not survive the environment (does worse than both parents)
- hybrid sterility due to environmental factors (can’t attract a mate)
BDM incompatibilities (post-zygotic isolating mechanism)
Bateson-Dobzhansky-Muller Incompatibilities are predicted to arise as a byproduct of overall divergence when populations are isolated.
define Total Reproductive Isolation
measures the overall strength of the various barriers than contribute to isolation between a pair of species
- ranges from 0 (no isolation) to 1 (no gene flow)
Why are not all isolating barriers equivalent?
because earlier acting barriers (like pre-mating barriers) can prevent loss of reproductive effort and energy that comes from choosing the wrong mate and producing less fit offspring.
how to estimate Total Reproductive Isolation (IT)
Total isolation (IT) is evaluated as (1-IT) which is the amount of gene flow that is POSSIBLE given the action of a set of isolating barriers.
(1-IT) = (1-I1)(1-I2)…(1-IN)
- this is because isolating barriers act in sequence like a series of sieves, reducing the effects of reproductive interactions on gene flow between species
- If NO individuals make it through all the barriers, IT = 1
define Speciation
the process by which a new species is formed
what is the Allopatric Speciation Model
A population becomes divided into two isolated ranges (barrier to gene flow), then divergence occurs producing 2 separate species.
- mutation, drift, selection and gene flow now occur independently in these 2 isolated populations. So, they become more differentiated from each other.
how does reproductive isolation arise in allopatry?
isolating barriers accumulate as a byproduct the longer populations have been separated, and the less likely they are to interbreed.
define Secondary Contact and its outcomes
when previously separated populations come back together. This can result in
- the species re-merging
- the species form a hybrid zone
- selection favours stronger reproductive isolation i.e. Reinforcement
Why does reinforcement involve a strengthening of pre-zygotic, rather than post-zygotic, isolation?
Alleles that prevent inter-specific mating are more likely to be inherited by non-hybrid offspring than alleles that permit inter-specific mating.
define Reinforcement
the strengthening of pre-zygotic barriers to avoid formation of less fit/unfit hybrids under secondary contact of populations.
how can speciation happen with gene flow still occuring?
Sympatric speciation is most likely to occur if there is
- strong selection to overcome gene flow, like ecological selection (shift in habitat preference and resource use)
- rapid evolution of reproductive isolation, preferably through pre-zygotic barriers
define Homoploid hybrid speciation
the hybrid species is at the same ploidy level as the parents
define Hybrid Zone (how does it differ from hybrid species?)
a hybrid zone is where there is ongoing gene flow between the two parental species to generate hybrids. These hybrids are often less fit than parents,
a hybrid species must be genetically stable and be isolated from their parents.
Hybrid zones are much more common than hybrid species formation.
why is hybrid speciation rare?
- parents have to interbreed without merging
- hybrids must be viable and have some fertility
- hybrids must be/become reproductively isolated from their parents, as quickly.
- hybrids must overcome/recover from low fertility and propagate themselves.
define Polyploid hybrid speciation
when a novel polyploid is produced through hybridization that is highly reproductively isolated from its parents.
- the number of genome copies increases in the hybrid
- tends to be the most rapid form of speciation because within ~2 generations, the polyploid will be reproductively isolated from its parents
- makes it difficult initially for the polyploid to find compatible mates because its requires same ploidy level to produce offspring
define Autopolyploidy
when the polyploid arises from within a single diploid species
define Allopolyploidy
when the polyploid is the product of interspecific hybridization, followed by genome doubling
Models of speciation that include gene flow usually include this mechanism
disruptive selection based on resource use
if hybrids produced between two species are completely sterile, what is the value of Total Isolation between these species?
Total Isolation = 1
what is Total Isolation?
The impact of all isolating barriers on reproductive isolation
