Evolution - Christian Brothers Universityfacstaff.cbu.edu/~jmoore25/EvolutionLectures/Ch18.pdf ·...

EvolutionSpring 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

Lizards on islands

Higher rate of gene flow

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 gameticisolation, 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 Dobzhanskyenvisioned 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 polyploidspecies, 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 by-product 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