Download Speciation

Evolution
Spring 2017
Modes of Speciation
• Classified by several criteria
• Geographic origin of barriers to gene exchange
• Genetic basis of these barriers
• Causes of evolution to those barriers
• These criteria are independent of one another
Speciation occurs in three geographic
settings
• Allopatric speciation  evolution of reproductive barriers in
populations that are prevented by a geographic barrier from
exchanging genes @ a more negligible rate
• Distinctions between allopatric speciation by…
• Vicariance (divergence of two large populations)
• Peripatric (divergence of a small population from a widely distributed ancestral
form
• Parapatric speciation  adjacent, spatially distinct populations,
between which there is some gene flow, diverge & become
reproductively isolated
• Sympatric speciation  evolution of reproductive barriers within
a single, initially randomly mating (panmictic) population
Allopatric Speciation
• Geographically separated
• Defined by the severe reduction of movement of individuals or
gametes not by geographic distance
Examples
Higher rate of gene flow
Lizards on islands
Low rate of gene flow
• Island species need large area for allopatric speciation to occur
• Widely distributed species may become fragmented into
allopatric populations if the habitat becomes fragmented by
climate changes
Origin of incompatibility
• Dobzhansky and Muller described that differences at two or
more loci have complimentary effects on fitness
• Fitness depends on the combined action of the ‘right’ alleles at both loci
• Idea supported by genetic data showing that reproductive isolation is
based on epistatic interactions among several or many loci
Ecological selection & speciation
• Occurs in two ways:
• Mutation-order-speciation: mutations @ different genes occur in each
pop, are selected for the same reason and confer DM incompatibility
• Ecological speciation: the evolution of barriers to gene flow caused by
divergent ecologically based selection
Sexual Selection & Speciation
• Closely related species
of animals are often
sexually isolated by
female preferences for
features of conspecific
males
• Some evidence
suggests that sexual
selection increases
diversity
• Sexual selection likely plays a role in the evolution of gametic
isolation, especially between species of broadcast-spawning
marine inverts
Reinforcement of reproductive isolation
• Many biologists have supposed that reproductive isolation
evolves, in part, as an adaptation to prevent the production of
unfit hybrids
• This enhancement of prezygotic barriers that Dobzhansky
envisioned is often called reinforcement of prezygotic isolation
Peripatric speciation
• Ernst Mayr coined founder effect speciation…later dubbed
peripatric speciation
• Isolated populations with restricted distributions, in locations
peripheral to the distribution of probably parent species, are
often highly divergent from those parent species, to the point of
being classified as different species or even genera
• Initially ‘founder effect’ would be evident
• Lead to ‘snowballing’ genetic change that might result would yield
reproductive isolation
Speciation with Gene Flow
• Occurs if reproductive isolation evolves while the incipient
species are exchanging genes
• Includes parapatric & sympatric speciation, as well as cases in which
incipient reproductive isolation between formerly allopatric pops is
reinforced
• The DM incompatibility example
• For substantial isolation to occur, the alleles @ these loci must be in
strong linkage disequilibrium
Parapatric speciation
• Can theoretically occur if gene flow between populations that
occupy adjacent regions with different selective pressures is
weaker than divergent selection for different gene combinations
• Another possibility is that populations separated by distance
can evolve reproductive incompatibility, even though the
species is distributed throughout the intervening region
• Parapatric pops.
Sympatric Speciation
• Highly controversial
• Difficulty any model must overcome is how to reduce the frequency of
the intermediate genotypes that would act as a conduit of gene
exchange between the incipient species
• Models of sympatric speciation postulate disruptive ecological
selection
• Includes nonrandom mating models
• Assortative mating = individuals prefer mates that match their own phenotype
• Trait preference = different genes control a female preference and a male trait
• Evidence of SS
• Does not occur easily
Polyploidy
• Polyploids are rare among sexually reproducing animals
• Autopolyploid = formed by the union of unreduced gametes
from genetically and chromosomally compatible individuals that
may be thought of as belonging to the same species
• Allopolyploid = polyploid derivative of a diploid hybrid between
two species
• Several conditions – self-fertilization, vegetative propagation,
higher fitness that the diploid – might enable a new polyploid to
increase and form a viable population
Recombinational speciation
• Hybridization sometimes gives rise not only to polyploid
species, but also to distinct species with same ploidy as parents
• Certain genotypes may be fertile but reproductively isolated from
parents
• These genotypes may then increase in frequency, forming a distinct
population
• Also called hybrid speciation
How fast is speciation?
• Time for speciation (TFS) is the time required for (nearly)
complete reproductive isolation to evolve once the process has
started
• Biological Speciation Interval (BSI) the average time between
the origin of a new species and when that speices branches
(speciates) again
• BSI includes not only the TFS, but also the “waiting time” before the
process of speciation begins again
• Diversification rate, (R), or increase in species # per unit time =
the difference between the rates of speciation (S) and extinction
(E)
• R can be estimated for a monophyletic group if the age of the
group (t) can be estimated and if we assume that the # of
species (N) has increased exponentially according to ….
• N = eRt
Consequences of Speciation
• Most importantly is increased diversity
• Remember each branch in phylogenetic tree represents a speciation
event
• Speciation stand @ the border between…
• Microevolution (genetic changes within & between populations)
• Macroevolution (the evolution of higher taxa in all their glorious
diversity)
• Evolutionary change in
body size in various
terrestrial vertebrates
in relation to the
elapsed time between
samples
Model of how speciation might facilitate long-term evolutionary change in morphological and other
phenotypic characters
Summary
• Most common mode of speciation is allopatric speciation, in
which gene flow between populations is reduced by geographic
or habitat barriers, allowing genetic divergence by natural
selection and/or genetic drift
• Vicariant allopatric speciation, a widespread species becomes
sundered by a geographic barrier, and one or both populations
diverge from the ancestral state
• Reproductive isolation between allopatric populations appears
to evolve as s side effect of divergent ecological or sexual
selection
• Prezygotic isolation evolves mostly while populations are
allopatric, but may be reinforced when the populations become
parapatric or sympatric
• Peripatric speciation (founder effect speciation) is a hypothetical
form of allopatric speciation in which drift in small peripheral
pops initiates rapid evolution…reproductive isolation is a byproduct of that evolutionary change
• Speciation by polyploidy in plants is very common
• Time required for speciation to proceed to completion is highly
variable
• Shorter for some modes (polyploidy) than others (drift & neutral alleles)
• The process of speciation may require 2 to 3 My on average
• Speciation is the source of diversity of sexually reproducing
organisms, and it is the event responsible for every branch in
their phylogeny
• Does not stimulate evolutionary change in morphological characters,
as suggested by punctuated equilibria