Speciation Flashcards
Species
an evolutionary independent population or group of populations
Speciation
a splitting event that creates two or more distinct species from a single ancestral species
Gene Flow
- AKA migration
- the transfer of alleles from one population to another
- makes populations increasingly similar
Reproductive Isolation
- behaviors or processes that prevent gene flow between populations
- creates opportunity for divergence
Species Concept
a set of criteria used to define and differentiate species
Biological Species Concept
-species are reproductively isolated
- do not interbreed in nature
- or if the do, offspring are not viable/sterile
- cannot be applied to asexual or extinct species (fossils)
- isolating mechanisms (pre/post zygotic)
Morphological Species Concept
- based on differences in size, shape, or other morphological features
- can be applied to asexual or extinct species (fossils)
- morphospecies
Issues with the Morphological Species Concept
- species with different morphs; polymorphic species
- cryptic species
- the singing of birds attracts
- which traits to use?
- how different do traits need to be?
Phylogenetic Tree
- representation of the evolutionary relationships among organisms
- AKA phylogeny/evolutionary tree
- made up of hierarchies of monophyletic groups (clades)
- can be applied to asexual and extinct species
Monophyletic Group or Clade
-a group that consists of one ancestor and all its descendants
Phylogenies
nested hierarchies of clades
Phylogenetic Species Concept in practice
- gather data from multiple populations
- use data to construct a phylogeny
- analyze for monophyletic groups
Difficulties with the Phylogenetic Species Concept
- we do not have good phylogenies for many groups of species
- yet, this is changing with advancements in DNA sequencing
Isolating Mechanisms
- Mechanisms that prevent the formation of a zygote
- pre zygotic (before zygote formation) or post zygotic (after zygote formation)
Prezygotic Mechanisms
- Temporal isolation
- Habitat isolation
- Behavioral isolation
- Mechanical isolation
- Gametic isolation
Temporal isolation
- breeding at different times
- e.g. flowers coming out at different times and corals sending out sperm at different times
Habitat isolation
- preferences for living or mating in different habitats
- may never come into contact during mating periods
Behavioral isolation
- individuals behave in such a way that they reject or fail to recognize potential mating partners
- not attracted to each other
For example:
- the frogs songs
- the displays of birds
- pollinators may favor different floral shapes or food (nectar)
- behavior of pollinators may isolate plants
Mechanical isolation
- differences in size and shape of reproductive organs make mating impossible
- genital lock-and-key
- most common in insects
Gametic isolation
- eggs of one species do not have appropriate chemical signals for sperm of another species
- or sperm is not able to attach and penetrate the egg
Post Zygotic Isolation
-mechanisms that prevent zygotes from passing on alleles
- hybrid inviability
- hybrid sterility
Hybrid Inviability
- hybrids do not survive to sexual maturity
- they die during development or soon after birth
Hybrid Sterility
- hybrids survive to maturity but do not produce gametes
- are not fertile
- have an intermediate phenotype that keeps them from reproducing
How do isolating mechanisms evolve?
Allopatric speciation:
the most common mode of speciation because geographic isolation is so effective at reducing gene flow
Allopatric Speciation
- the most common mode of speciation because geographic isolation is so effective at reducing gene flow
- opportunity to evolve isolating mechanisms
- routes to allopatry: dispersal and vicariance
Dispersal
- movement to a new habitat
- across unsuitable habitat or barrier
- the likelihood that speciation is negatively correlated with connectivity between patches
Vicariance
- the physical splitting of a population into smaller, isolated populations by a physical barrier
- e.g. rising mountains, change in river course, break up of land…
-likelihood of speciation is positively correlated with the strength of the barrier
How do populations diverge when isolated?
- Natural Selection
- Genetic Drift
- Mutation
Natural Selection in relation to divergence
- populations in different environments experience different selection pressures
- thus, natural selection could favor different traits/alleles
Genetic Drift in relation to divergence
- random changes can create fixation/loss of different alleles
- smaller populations due to dispersal or vicariance
- greater affect of drift
- fixation/loss of different alleles in two populations
Mutation in relation to divergence
- random
- different alleles created
- new alleles can then diverge due to natural selection or genetic drift
Evolution by Natural Selection
-populations could have different ecologies
including:
-temperature rainfall, soil…
-different communities (species interactions)
- thus, potentially different selection pressures in each population
- natural selection would then favor different alleles in each population
Evolution by Genetic Drift
- Smaller population due to dispersal or vicariance
- greater affect of drift
- fixation/loss of different alleles in two populations
Allopatric speciation by vicariance
- speciation that begins with geographic isolation
e. g. the amazon river changes over millenniums
Secondary Contact
- when allopatric populations come back into contact with each other
- become sympatric
When isolated populations come into contact, speciation is complete when…
- they no longer recognize each other as potential mates
- or produce inviable/infertile offspring
When isolated populations come into contact, population fusion occurs when…
- random mating among individuals
- hybrids are fit
Reinforcement
- natural selection for traits that prevent interbreeding among populations
- selection of traits that increase reproductive isolation between populations after secondary contact
Fitness of hybrids
- some gene flow, but hybrids have lower fitness than parental forms
- partially sterile or inviable
- intermediate phenotypes reduces realized niche or ability to attract mates
Collared and pied flycatchers
- divergence during glacial cycles
- expansion since last glacial cycle has caused secondary contact in central Europe
- in sympatry, hybridization is lower than expected with random mating (2-7% of breeding adults are hybrids)
- hybrids have reduced fitness
- females mostly sterile
- males have reduced fertility
-since hybrids have reduced fitness, natural selection should favor traits that favor conspecific mating (=reinforcement)
- in sympatry, there is increased divergence in male phenotypes
- male pied flycatchers are brown/white
-also greater differences in UV color and wing patch size
Hybrid Zone
- defined geographic area with hybridization that results in fertile offspring
- dispersal limitation and/or natural selection maintains a defined zone
- the two species are diverged enough to be called different species by at least one species concept
Hybrid speciation
- when hybrids are fit and become reproductively isolated from parental species
- homoploid hybrid speciation: # of chromosomes does not change
- intermediate phenotypes promote isolation
- occupy different niche (sunflowers in sand)
- not recognized as potential mates
Sympatric Speciation
- no geographic isolation: sympatric individuals live in the same geographic area
- divergence: new population begins to diverge due to selection
- genetic isolation: eventually the two populations become genetically isolated from one another in single populations
- single populations experience disruptive selection for extreme phenotypes
- trait under disruptive selection must be somehow tied to mating preferences
- e.g. small with small; big with big
-e.g. apple maggot fly
Disruptive selection
- phenotype higher and lower than mean has high fitness
- mean could stay the same
- genetic variation is increased
Disruptive Selection in Sympatric Speciation
-single populations experience disruptive selection for extreme phenotypes
- trait under disruptive selection must be somehow tied to mating preferences
- e.g. small with small; big with big
Apple Maggot Fly
- Native to the US, historically associated with hawthorn (type of crab apple)
- they lay eggs in hawthorn fruits
- Europeans brought apples to US
- Some individuals switched to laying eggs in apple fruits
- individuals flies tend to looks for mates and lay eggs around the same types of fruit they were born
- temporal variation in host availability has led to disruptive selection
- If intermediate phenotypes have lower fitness, reinforcement is expected
- an example of speciation by host shift
Speciation by host shift
- e.g. apple maggot fly
- tight species interaction with host; shift to new host creates reproductive isolation
- could be common in insects, but also found in other groups
Synapomorphy
A trait that is found in certain groups of organisms and their common ancestor but is missing in more distant ancestors
e.g. fur and milk-producing glands are synapomorphies that identify mammals as a monophyletic group
Polymorphic species
differing phenotypes
Cryptic species
differ in traits other than morphology
Subspecies
populations that live in discrete geographic areas and have distinguishing features such as coloration or calls, but are not considered different enough to be called separate species
Biogeography
the study of how species and populations are distributed geographically
Polyploidy
the condition of possession more than two complete sets of chromosomes
Autopolyploid
produced when mutation results in a doubling of chromosome number and the chromosomes all come from the same species
Allopolyploid
created when parents of different species mate, and then an error in mitosis occurs, resulting in viable, non sterile offspring with two full sets of chromosomes
Tree of life
- the most universal of all phylogenetic tree
- depicts the evolutionary relationships among all living organisms on earth
Branch
A line representing a population through time
Root
the most ancestral brach in the tree
Tip (terminal node)
- the endpoint go a branch
- represents a living or extinct group of genes, species, families, phyla, or other taxa
Outgroup
- a taxon that diverged prior to the taxa that are the focus of the study
- helps to root the tree
Node (fork)
- a point within the tree where a branch splits into two or more branches
- the node represent the most recent common ancestor of the descendent groups
