ECOLOGICAL REPRODUCTIVE BARRIERS AND DIFFERENTIATION BY
RANDOM GENETIC DRIFT
5 Agents of Evolution:
a) Mutation
b) Migration/ Gene flow
c) Non-random mating
d) Genetic drift
e) Selection
Populations diverge over time - Speciation results from the interaction of
evolutionary forces:
Mutation: random process introduces the neutral allele
Selection: alters allele frequencies as a consequence of environment-specific adaptation
Random genetic drift: randomly fixes alleles in a population
Mating system variation: alters the strength of random genetic drift. Why sexual selection
accelerates speciation.
In the absence of another force: migration
Modes of Speciation:
Allopatric: geographically isolated. Formation of new species.
Sympatric: small sub-population diverges from parent population without geographic isolation.
Conditions that accelerate speciation:
Geographic Isolation – “Allopatry”: Island-Mainland barrier/ Geographic Barrier/ Extinction of
intermediate populations – all make exchange of genes (migration) very unlikely.
Population splits, no gene flow. Do populations diverge – speciation? After populations become
allopatric, either populations become sympatric again and interbreed (speciation has not occurred)
or populations become sympatric again but do not interbreed (speciation has occurred).
Speciation can occur in sympatry (species together):
Reproductive isolation: prevention of gene exchange between species. Results when reproductive
barriers evolve between populations.
Barriers to reproduction (gene flow):
a) Pre-zygotic (pre-mating): act before hybrids are formed, impede mating/prevent fertilisation.
Habitat Isolation: species that use different portions of the environment do not hybridise because
they don’t encounter each other. Sometimes associated with geographical isolation. E.g.
silverswords, Bufo toads
Temporal Isolation: species that breed at different times of the day, season, or year, do not
exchange genes. E.g. Lactuca flowering time, frog mating seasons
Behavioural Isolation: species that do not recognise each other’s courtship behaviours and rituals,
do not exchange genes. E.g. cicadas, many bird, insect and other species, pollinator isolation in
plants.
b) Post-zygotic (post-fertilisation): act after hybrids are formed, prevent hybrids from developing
into viable and/or fertile adults.
Hybrid Inviability: hybrid embryos often die early after fertilisations – may be malformed and/or
frail, may be maladapted to parental environments. E.g. goat/sheep hybrids, Rana hybrids
Hybrid maladaptation: hybrid offspring may be maladapted to parental environments. E.g. plant
hybrids, mating calls
, Hybrid sterility: hybrid offspring may have malformed reproductive development, are often sterile in
F1 or F2 generations. E.g. donkey/horse hybrids (mules), many other plants and animals.
Conditions that accelerate speciation:
Strong divergent sexual selection:
- Behavioural isolation can result from divergent sexual selection on different mating repertoires.
- Differentiation in mating signals between populations can result in failure to recognise potential
mates.
- e.g. Male Guppies: high variable male morphology, morphological differences possibly generated
by geographical variation in female choice. Appears to be speciation in progress.
Why does sexual selection accelerate speciation?
Random Genetic Drift and Mating System
- Random Genetic Drift is the random change in allele frequencies from one generation to the next
that is caused by the finite size of the breeding population of parents.
- By chance, some parents have more offspring than other parents. By chance, some parents have
fewer offspring or no offspring at all.
- Occurs in all countable, finite populations. The strength of RGD is proportional to (1/2N) in a diploid
population and to (1/N) in a haploid population.
- RGD is STRONGER in small populations (0<N<500)
If N=20 haploids, then (1/2N)=0.05. RGD is always twice as strong for a haploid population of N
individuals than a diploid population of the same size.
- RGD is WEAKER in large populations (500<N)
If N=2000, then (1/2N)=0.00025.
Evolutionary Consequences:
1) WITHIN populations RGD DECREASES GENETIC VARIATION: makes a population genetically less
variable and the individuals in the population more homozygous. The ultimate outcome: ALL
HERITABLE VARIATION IS LOST. The population becomes either p*=0.0 (allele is lost) or p*=1.0 (allele
is fixed).
2) BETWEEN populations RGD INCREASES GENETIC VARIATION: makes two populations become
genetically different from one another.
RANDOM GENETIC DRIFT
5 Agents of Evolution:
a) Mutation
b) Migration/ Gene flow
c) Non-random mating
d) Genetic drift
e) Selection
Populations diverge over time - Speciation results from the interaction of
evolutionary forces:
Mutation: random process introduces the neutral allele
Selection: alters allele frequencies as a consequence of environment-specific adaptation
Random genetic drift: randomly fixes alleles in a population
Mating system variation: alters the strength of random genetic drift. Why sexual selection
accelerates speciation.
In the absence of another force: migration
Modes of Speciation:
Allopatric: geographically isolated. Formation of new species.
Sympatric: small sub-population diverges from parent population without geographic isolation.
Conditions that accelerate speciation:
Geographic Isolation – “Allopatry”: Island-Mainland barrier/ Geographic Barrier/ Extinction of
intermediate populations – all make exchange of genes (migration) very unlikely.
Population splits, no gene flow. Do populations diverge – speciation? After populations become
allopatric, either populations become sympatric again and interbreed (speciation has not occurred)
or populations become sympatric again but do not interbreed (speciation has occurred).
Speciation can occur in sympatry (species together):
Reproductive isolation: prevention of gene exchange between species. Results when reproductive
barriers evolve between populations.
Barriers to reproduction (gene flow):
a) Pre-zygotic (pre-mating): act before hybrids are formed, impede mating/prevent fertilisation.
Habitat Isolation: species that use different portions of the environment do not hybridise because
they don’t encounter each other. Sometimes associated with geographical isolation. E.g.
silverswords, Bufo toads
Temporal Isolation: species that breed at different times of the day, season, or year, do not
exchange genes. E.g. Lactuca flowering time, frog mating seasons
Behavioural Isolation: species that do not recognise each other’s courtship behaviours and rituals,
do not exchange genes. E.g. cicadas, many bird, insect and other species, pollinator isolation in
plants.
b) Post-zygotic (post-fertilisation): act after hybrids are formed, prevent hybrids from developing
into viable and/or fertile adults.
Hybrid Inviability: hybrid embryos often die early after fertilisations – may be malformed and/or
frail, may be maladapted to parental environments. E.g. goat/sheep hybrids, Rana hybrids
Hybrid maladaptation: hybrid offspring may be maladapted to parental environments. E.g. plant
hybrids, mating calls
, Hybrid sterility: hybrid offspring may have malformed reproductive development, are often sterile in
F1 or F2 generations. E.g. donkey/horse hybrids (mules), many other plants and animals.
Conditions that accelerate speciation:
Strong divergent sexual selection:
- Behavioural isolation can result from divergent sexual selection on different mating repertoires.
- Differentiation in mating signals between populations can result in failure to recognise potential
mates.
- e.g. Male Guppies: high variable male morphology, morphological differences possibly generated
by geographical variation in female choice. Appears to be speciation in progress.
Why does sexual selection accelerate speciation?
Random Genetic Drift and Mating System
- Random Genetic Drift is the random change in allele frequencies from one generation to the next
that is caused by the finite size of the breeding population of parents.
- By chance, some parents have more offspring than other parents. By chance, some parents have
fewer offspring or no offspring at all.
- Occurs in all countable, finite populations. The strength of RGD is proportional to (1/2N) in a diploid
population and to (1/N) in a haploid population.
- RGD is STRONGER in small populations (0<N<500)
If N=20 haploids, then (1/2N)=0.05. RGD is always twice as strong for a haploid population of N
individuals than a diploid population of the same size.
- RGD is WEAKER in large populations (500<N)
If N=2000, then (1/2N)=0.00025.
Evolutionary Consequences:
1) WITHIN populations RGD DECREASES GENETIC VARIATION: makes a population genetically less
variable and the individuals in the population more homozygous. The ultimate outcome: ALL
HERITABLE VARIATION IS LOST. The population becomes either p*=0.0 (allele is lost) or p*=1.0 (allele
is fixed).
2) BETWEEN populations RGD INCREASES GENETIC VARIATION: makes two populations become
genetically different from one another.
