Speciation Flashcards
What is speciation?`
‘Evolution of reproductive isolation within an ancestral species, resulting in two or more descendant species. (Futuyma 2005)
What is anagenesis?
The evolution of a new species along a single lineage due to significant morphological changes
What is cladogenesis?
The evolution of more than one new species from the same common ancestor. It occurs in a branching pattern
3 reasons why speciation is important
- Speciation generates biodiversity
- Speciation events is the distant evolutionary past generates high level branching
- The study of speciation reveals the role of genetic processes and natural selection in evolution
What 3 methods have been used to demonstrate that species exist
Common sense
Folk
Statistical methods
What is the common sense argument for the existences of species
Species are real because we recognise organisms to be distinctly different , not in a continuum
Who was a significant advocate of the common sense species argument and what did he say
Dobzhansky 1937 noticed that there were discrete groups everywhere, in both animals and plants, and that this grouping must be a fundamental characteristic of organic diversity
Flaw of the common sense argument
Does not provide hard evidence for the existence of sp.
What does the folk argument for species involve
Involves surveying regions indigenous peoples and compare lists of animal and plant types with Linnaean species.
What are the 2 outcomes of a Folk vs Linnaean survey that provide evidence for the existence of species
- a one to one correspondence or
- the folk species are under-differentiated ie 2 or more Linnean species per folk species as untrained observers may not be able to tell cryptic species apart
What are the is the one outcome of a Folk vs Linnaean survey that provides evidence against the existence of species
If the folk species are over-differentiated
Does the Folk vs Linnean sruveys actually show
Remarkable concordance between folk and Linnean species ie folk species either matched or were undifferentiated Majnep & Bulmer 1977
What goals should a species concept fufill
Help with systematic classification
Corresponds with the distinct entities we see in nature
Help us understand how species arise
Represent evolutionary history of organisms
Apply to as many organisms as possible
What are the 4 most common species concept
Biological Species Concept
Morphological species concept
Phylogenetic
Genotypic Cluster
Pros for the morphological species concept
It is wildly applicable, including asexual and fossilised organisms
Cons for the morphological species concept
Can be arbitrary and subjective, how big a difference does there need to be to call it a new species.
Cant distinguish morphologically similar species who are phenogentically very different with different evolutionary histories
Name a cryptic species which shows how the morphological species concept fails
The pipistrelle bat was thought to be one species but differences in mitochondrial DNA showed that it was actually 2; the common pipistrelle and the soprano pipistrelle
Define the morphological species concept
A lineage thought to be evolutionary independent on the basis of size, shape of another morphological features
Define biological species concept
Groups of actual/potential interbreeding populations that are reproductively isolated from other such groups
Pros of the biological species concept
Stresses the importance of a total lack of gene flow/ being genetically isolated
Can be tested, can 2 chosen populations successfully interbreed
It is widely adopted and in the USA it is a legal definition
Cons to the biological species concept
Cannot be applied to fossils or asexual organisms
Testing the ability to interbreed is a time consuming process
Lots of currently defined species can hybridise and form fertile offspring
What is the phylogenetic species concept
a species is defined as the smallest monophyletic group of common ancestry
3 pros to the phylogenetic species concept
Applies to asexuals and fossils
Utilises ever increasing phylogenetic data
Separateness in the phylogeny reflects genetic isolation
3 cons to the phylogenetic species concept
It doesn’t create a stable system as the increasing data will encourage phylogenies to change
Difficult to form robust phylogenies, especially molecular phylogenies
Not clear which elements should be used to construct the phylogenetic tree
Example of the phylogenetic species concept in use
After comparing 4 nuclear genes, Roca et al. 2001
Forest v. Savannah Elephants show 58% of difference shown between African and Asian Elephants and distinguished the african forest elephant as it’s own species
Define Genotypic Cluster Species Concept
Species are groups that remain distinct in sympatry due to the morphological and genotypic gaps between them
3 Pros to Genotypic Cluster Species Concept
Combines elements of both the phylogenetic SC and the Biological SP
It’s pragmatic, doesn’t require speciation to occur in allopatry
aims to separate definition of species from hypotheses about how speciation occurs
2 cons to the Genotypic Cluster Species Concept
It still has elements of subjectivitiy such as the morphological SC e.g
Not widely used, used for butterflies and now occasionally in fish
Example of a species following the Genotypic Cluster Species Concept
Heliconius cydno and Heliconius melpomene hybribise in nature and in labs to form Heliconius heurippa. This butterfly has a hybrid genome and shows reproductive isolation as it doesn not mate with it’s parent species.
Give an example where the different species concepts don’t agree
Under MSC, Red Wolf is a species
Under BSC, it is not a species as it hybridises with the Coyote
Phylogenetic SC - suggests red wolves result from hybrids between the Gray Wolf and the Coyote as no part of its DNA is unique
What do all the species concepts agree on
They all agree that evolutionary independence is a key criterion in defining species
What is an isolating barrier
Any intrinsic process or phenomenon that forms a barrier to gene flow between populations aka anything that stops two different individuals fro successfully exchanging genetic material
What is a premating barrier
A product or byproduct of evolution that prevents hyrbidisation when F1 or F2 hyrbids have reduced fitness
What is a postmating barrier
A byproduct of evolution that prevents successful fertilization and development, even though mating may have occurred.
What are the two premating prezygotics isolating barriers
Ecological isolation and when potential mates meet but do not mate
Give an example of behavioural isolation
Female fireflies only respond to males who flash the the pattern of their own species
Give an example of pollinator isolation
aquilegia formosa is pollinated by humingbirds and has a different morphology to aquilegia pubescens which is pollinated buy hawkmoths
What are the 2 postmating, prezygotic isolating barriers
Mechanical mismatch/copulatory and gamete incompatibility
example of mechanical mismatch
There is incomplete mechanical incompatibilities in RI between river bluet and tule bluet which lead the females from being resistent over mating with heterospecific or hyrbid males
Give 2 non-specific examples of gametic incompatibility
sperm may not possess the correct enzymes to remove the protective coat from around the egg
pollen may not be able to grow towards the egg in different species.
what are the 3 types of post zygotic isolation
Hybrid inviability: hybrid embryos die during development, e.g. Wood Frog (Rana sylvatica) x Leopard Frog (R. pipiens)
Hybrid sterility e.g mules
F2 hybrid breakdown: e.g. Drosophila nasuta and D. albomicans produce fertiles F1’s but the F2’s are relativley infertile and the sex ratios are skewed.
What are the 3 main modes of speciation
Allopatric
Parapatric
Sympatric
Define Allopatric speciation and the two forms it comes in
Its when geographically isolated populations become reproductively isolated either with vicariance (exisiting pop gets divided into smaller pops) or through peripatric speciation when a small pop gets isolated from the larger parental population
Define Parapatic speciation
when geographically neighbouring populations become reproductively isolated
Define sympatric speciation
when geographically overlapping populations become reproductively isolated
3 ways in which peripatric speciation can occur
Founder events
Isolation of peripheral population.
Non-peripheral population could become isolated from rest (rare?).
Define parapatric speciation
two subpopulations of a species evolve reproductive isolation from one another while continuing to exchange some genes.
What are the two models of parapetric speciation and define them
Clinal model and Stepping stone model
Describe clinal model
the reduced gene flow in parapatric speciation produces a genotypic/ phenotypic gradient due to variations in evolutionary pressures e.g habitat differences which in turn will cause changes in allele frequencies between the different pops.
Describe the stepping stone model
Limited gene flow is overpowered by the influences of selection or drift between discrete populations leading to reproductive isolation. The smaller the discrete population the higher the rate of parapatric speciation.
What are the 4 criteria that should be met to support sympatric speciation
- the species must be largely or completely sympatric
- substantial reproductive isolation preferably based on genetic difference
- must be sister groups but genetic similarity can not be due to hybridisation
- The biogeographic and evolutionary history of the groups must make the existence of an allopatric phase very unlikely.
Why is the the acceptance of sympatric speciation controversial
Evidence in the wild is rare and frequently proven incorrect later on
What are the 4 main causes of speciation
Macromutation
Genetic drift
Natural Selection
Sexual Selection
What is macro-mutation
Speciation by polyploidy
What is polyploidy and name the 2 forms
doubling of chromosomes either through hybridisation (allopolyploidy) or spontaneous doubling within a species (Autopolyploidy)
How many plants are estimated to be polyploids
30-70%
What mode is genetic drift proposed to occur under and what is it then also called
Under peripatric mode , also known as founder effect speciation
How does speciation occur under genetic drift
A small subpopulation become geographically isolated from the parent pop and the becomes reproductively incompatible through genetic drift
What 2 forms does natural selection occur in
Direct - Reinforcement
Indirect - in allopatry or sympatry
Explain natural selection as a direct cause of speciation (reinforcement)
When there is selection for prezygotic isolation due to reduced fitness in hybrids
Explain natural selection as a indirect cause of speciation
Populations adapt to different environments or niches (in (i) allopatry or (ii) sympatry), reproductive isolation then follows as a by-product
What are the definitions of speciation rate
Time for speciation - the time required for
reproductive isolation to evolve
Biological speciation interval - the average time
between speciation events
What can the biological speciation interval be used to estimate
can be estimated from the amount of time between
the present number of species and the common ancestor assuming exponential growth and no extinctions
How do exticntions effect speciation rate estimations
Extinctions result in underestimations
How to work out diversity
Speciation - Extinction
How to work out diversification rate
speciation rate - extinction rate
What is the biological speciation rate and how is it expressed
average rate at which one species branches to produce two groups that are reproductively isolated.
Expressed as the no. of new species produced per existing species per time period (1 mya).
What are the two sources of information can phylogenies provide about diversification rates
the topological distribution of species diversity across branches, and the temporal distribution of branching events through time.
What do topological methods describe
They look at tree shape, how imbalanced it is. aka compare the observed difference in species diversity between two (or more) groups descended from a common node to a distribution of diversity differences generated under a stochastic model of diversification,
What do temporal methods compare
the distribution of branching times (based on branch length/duration estimates) to that generated under a null model of random diversification.
In macroeveolution, what do the branch lengths represent and how can they be interpreted
They are proportional to time and branch lengths can be interpreted independently of the topology, in order to identify temporal shifts in diversification.
What can cause a slowdown in diversification rate
caused by a decrease in speciation rate or an increase in extinction rate.
What can cause an increase in diversification rate
an actual increase in the speciation rate, or an illusory increase caused by a constant background extinction rate, d, that is high relative to the (constant) speciation rate, b
What does diversification require
Diversification requires extrinsic opportunity (new niches)
and intrinsic potential (ability to exploit new niches)
