Topic 8: Speciation Flashcards
Darwin envisioned speciation as a ____ event
branching
New species arise through the process of
speciation
Speciation
an ancestral species splits into two or more descendant species that are genetically different from one another and can no longer interbreed
Macroevolution
broad patterns of evolutionary change above the species level over geologic time scales
Examples of macroevolution
origin of new traits like wings, new groups like flowering plants, extinction
Speciation occurs between
microevolution and macroevolution
Macroevolution is the cumulative event of:
gradual compounding of small changes, cumulative new genetic variations, and speciation and extinction events
What is a species
a group of actually (or potentially) interbreeding individuals that produce viable, fertile offspring, and are reproductively isolated from other species
Capacity for ______ between populations of a species holds a species together genetically
gene flow
Limits of BDC (biological species concept)
cannot be applied to follis or asexual organisms, emphasizes the absence of gene flow when it can still happen (think grolar bears)
Other definitions for species
ecological species, morphological species, phylogenetic species,
morphological species
defines a species by structural features.
ecological species concept
defines a species as a set of organisms adapted to a particular set of resources, called a niche, in the environment
phylogenic species
defines a species as the smallest group of individuals on a phylogenetic tree
EBC emphasizes reproductive _____
isolation
Reproductive isolation is caused by
caused by biological factors (barriers) that impede two species from interbreeding and producing viable, fertile offspring (hybrid)
Reproductive isolation can be caused by (two types):
prezygotic barriers (before fertilization) postzygotic barriers (post-fertilization)
Ways prezygotic barriers block fertilization
- impeding different species from attempting to mate
- preventing the successful completion of mating
- preventing fertilization if mating is successful
Prezygotic reproductive barriers
habitat isolation, temporal isolation, behavioral isolation, mechanical isolation, gametic isolation
Habitat isolation
two species encounter each other rarely because they occupy different habitats even if not separated by different barriers
Temporal isolation
species that breed at different times of the day/seasons/ years
Behavioral isolation
courtship rituals and other behaviors unique to a species are effective barriers
Mechanical isolation
morphological differences can prevent successive mating (autonomically incompatible)
Gametic isolation
gametes of one species may not be able to fertilize eggs of another species
Postzygotic barriers prevent a hybrid zygote from developing into a viable fertile adult by:
- reduced hybrid viability
- reduced hybrid fertility
- hybrid breakdown
Reduced hybrid viability
genes of the different species may interact to impair the hybrid’s development or survival (results in death)
reduced hybrid fertility
hybrids come out sterile due to chromosomes of different numbers or structure
hybrid breakdown
some 1st-generation hybrids are vigorous and fertile, but when these hybrids mate with one another or with either parent species, offspring of subsequent generations are feeble or sterile
Allopatric speciation
occurs when gene flow between geographically isolated subpopulations is interrupted
barrier
stops the ability of a population to disperse
geographic isolation arises by
dispersion and vicariance
dispersal
results in the establishment of a new and distant population
vicariance
the range of a species is split by a change in the environment creating two subpopulations
productive isolation between subpopulations generally increases as the distance between them ____
increases
Allopatric speciation
the evolution of a new species following geographic isolation of two or more subpopulations of an ancestral species
As subpopulations diverge, they may evolve traits that act as:
prezygotic and/or postzygotic barriers to reproduction
The majority of speciation events occur due to
allopatric speciation
Sympatric speciation
the evolution of a new species from an ancestral species while both continue to inhabit the same geographic region
For sympatric speciation to occur the subpopulations occupying the same geographic region must become ___ ___ from each other.
reproductively isolated
Overlapping subpopulations can become reproductively isolated from each other by:
- chromosomal errors during meiosis or hybridization of closely related species
- natural selection for reproductive isolation
Polyploid speciation
occurs when changes in the number of chromosomes sets create genetically distinct descendants that are reproductively isolated from parental forms
Polyploidy is common in
plants
Hybrid speciation occurs when:
interbreeding between two related species creates genetically distinct descendants that are reproductively isolated from the parent species
Polyploids have different number of ____ than their parental forms
chromosome sets
Polyploids can only interbreed with individuals of the same ____
ploidy
polyploids arise from
- hybridization of related species to form allopolyploids
- chromosomal error during meiosis forming autopolypoids
An ____ is a species with multiple sets of chromosomes derived from the hybridization of different species
allopolyploid
Allopolyploid hybrids have ____ the
number of chromosome sets than the
parent species
double
Autopolyploids can arise due to:
by genome doubling or by fusion of 2n gametes (failure of cell division during meiosis and creates gametes with double the number of chromosomes)
Hybridization between different taxa is a trigger for:
polyploidization forming allopolyploids
Homoploid hybrid speciation
Hybrid speciation can also occur with no change in chromosome number (ploidy)
Homoploids
arise when hybridization produces novel combinations of genes that can adapt hybrids to new habitats that cause them to be reproductively isolated from parental populations
Habitat differentiation increases:
reproductive isolation between subpopulations
Natural selection for _______ between subpopulations
can occur after the appearance of
new ecological niches
reproductive isolation
Sexual selection
natural selection for reproductive isolation between subpopulations
Sexual selection leads to
evolution of traits that increase an individual’s access to members of the opposite sex.
Sexual selection often leads to
sexual dimorphism
Sexual dimorphism
marked differences between the sexes in secondary sexual characteristics
Darwin separated sexual selection
and natural selection because:
some sexually selected traits would act against naturally selected ones
Sexual selection is natural selection for
_______
mating success.
Sexual selection can drive:
sympatric speciation
Sympatric speciation occurs when
a reproductive barrier isolates a subset of a population without geographic separation from the parent species
Sympatric speciation can arise following
- Chromosomal errors during meiosis or hybridization of closely related species
- Natural selection for reproductive isolation or following habitat differentiation
Hybrid zone
a region where members of a different species interbreed and produce hybrids
Hybrids result from
the mating between species with incomplete reproductive barriers
Hybridization can result in ____
speciation
Outcomes of hybrid zones
reinforcement of hybrid barriers, stability of hybrid barriers, and fusion of hybrid barriers
reinforcement of reproductive barriers
occurs when hybrids are less fit than parent species resulting in the selection for prezygotic barriers
Where reinforcement occurs, reproductive barriers should be stronger for _____ than _____ species
sympatric than allopatric
stability of hybrid barriers
Continuous formation of hybrid individuals
Continuous gene flow from outside the hybrid zone can:
overwhelm natural selection for increased reproductive isolation inside the hybrid zone
Fusion ____ reproductive barriers
weakens
If hybrids are as fit as parents there will be ____
gene flow between species and possibly the parent species will fuse back to one species
Speciation is caused by
accumulation of genetic differences between populations
How genetic drift is important in allopatric speciation:
Genetic drift is important in allopatric speciation when a new population arises following the dispersal of a few individuals to an isolated location
Incomplete reproductive isolation between related species allows for ______.
hybridization
Selection for increased reproductive
isolation is strongest when
during sympatric speciation when hybrids between the sympatric populations are of lower fitness, i.e. reinforcement of reproductive barriers
Two patterns for the rate of speciation observed in fossil records
gradual, punctual models
Gradual speciation model
species diverge slowly by natural selection over many generations
punctuated speciation model
new species diverge quickly from a parent species but change little for the remainder of their existence
What causes speciation to happen
- gene flow between populations is interrupted
- population diverges genetically to become reproductively isolated
- there is no event that causes gene flow to resume
