321 Flashcards
directional selection
- Favouring one extreme over the other
disruptive selection
favouring both extremes over intermediate phenotypes
niche partitioning
stabilizing selection
- Favouring intermediate phenotypes over both extremes
fitness
- Average lifetime contribution of individuals of a particular genotype to the population after one or more generations
- Includes not only the number of offspring born but also the number that survive
= Reproductive success
absolute fitness
Number of offspring produced over a
Lifetime
relative fitness
(w) average contribution of individuals of a given phenotype to the population relative to the fitness of the genotype with the highest fitness
* w11 = relative fitness of genotype
* W11 = absolute fitness of genotype
fitness determined by
viability, mating success, fecundity, fertilization success
viability
Probability of survival through reproductive age; does not affect fitness after last age of
last reproduction
mating success
Number of mates obtained by an individual; variation is the basis of sexual selection
fecundity
Average number of viable gametes per female; fertility of mating may depend on maternal genotype or genotypes of both partners
fertilization success
An allele may affect gamete’s ability to fertilize an ovum
coefficient of selection (s)
measure of the strength of selection that favours the allele of interest
sex
Process that combines genetic material from more than one individual.
Sex is not reproduction but a precursor to reproduction.
ADVANTAGES TO ASEXUALITY
▪ Avoids two-fold cost of producing males.
▪ No need to locate mates,
▪ advantage at low density.
▪ Maintains coadapted gene complexes,
▪ advantage in stable environments.
DISADVANTAGES TO ASEXUALITY
- Accumulate deleterious mutations
▪ Muller’s Ratchet. - Time delay in acquiring optimal multi-locus genotypes in changing environments.
▪ Clonal interference
▪ Ruby-in-the-rubbish-effect - Slow rate of evolution
▪ Allows sexually reproducing antagonists (parasites, competitors, and predators) to get the upper hand. - Selective sweeps can eradicate all variation from a population.
MULLER’S RACHET AND MUTATIONAL MELTDOWN IN ASEXUAL POPULATIONS
- An asexual genome cannot produceoffspring better than itself, except by rare back mutation.
- The ratchet advances when the best class leaves no offspring, or if all of its offspring have acquired new deleterious mutations.
- A mutational meltdown begins when the mutation load is so great that the populations is unable to replace itself.
key benefit of sex
-recombination provides a mechanism for eliminating deletrious mutations and possible genomic repair. children can have higher fitness than parents
▪ Many modern theories that provide an
explanation for the advantage of sex
incorporate an idea originally proposed by
Weismann more than 100 years ago
sex
allows natural selection to proceed more
effectively because it increases genetic
variation.
sexual selection
▪ Special form of selection that accounts for
many elaborate traits and behaviors in species.
▪ Arises from differences in the ability to find
and mate with members of the opposite sex.
▪ Only occurs when access to one or the other sex is limiting, ie., when there is competition for mates or offspring.
Why does SEXUAL SELECTION occur?
▪ Sexual selection can cause evolution of traits
that decrease survival if a reproductive
advantage compensates for the cost!!
2 forms of sexual selection
▪ Intrasexual selection: direct competition for mates between members of the same sex,
usually male-male competition (“combat”).
▪ Intersexual selection: differences in attractiveness to the opposite sex, usually non-random mate choice by females.
anisogamy
differential investment in gametes
sexual selection is directly related to
relative investment in offspring production
the sex that invests in more in offspring production…..
has fewer reproductive opportunities
the sex that invests more
ahould be more choosey, become a limiting resource to the opposite sex
limitations on reproduction success differ for the sexes
- Females are limited by fecundity & resources
- Males are limited by the number of mates they
can obtain
batesman principles
greater variance in reproductive success among males and females
since male gametes are not as limiting . male reproductive success
increases linearly with increasing numbers of mates. sexual selection is higher on males
Bateman’s Principles:
Males have:
- greater variance in reproductive success,
- greater variance in mating success
- greater slope in the relationship between
reproductive and mating success
The asymmetric nature of sexual selection often leads to dramatic
sexual dimorphism in
characters directly related to male-male competition and/or female choice.
- Primary sex traits:
organs that aid directly in reproduction
(gonads, genitalia)
– present at birth and enlarge during adolescence
- Secondary sex traits:
traits that don’t play a direct role in
reproduction (physiological or ornaments)
– Often not expressed in immature individuals
Modes of selection
intrasexual selection, intrasexual selection,
intrasexual selection
- Darwin’s hypothesis
1. exaggerated 2nd sex traits are favoured by sexual selection because they
increase male mating success
2. Exaggerated traits decrease survival
▪ Sexual selection can be very strong and often
opposes natural selection.
▪ This can lead to exaggerated and sometimes
maladaptive development of male traits.
sexual dimorphism
intrasexual selection
- Male-male competition
– Combat
– Resource guarding
– Alternative mating strategy - Sperm competition
- Infanticide
▪ Sneakers
males not directly
engaging in competition for
mates may gain extra-pair
copulations.
(e.g., small “Jack” salmon)
alpha
largest, dominant, guides harem of females
beta
same size as females, slips inside and mates
gamma
smallest, uses speed to slip by alpha
sperm competition
male-male .post compilation competition. intrasexual
infanticide
male-male.
* Decrease fitness of other males.
* Causes females to become
fertile more quickly.
* Sexual selection gives
advantage to selfish genes even
if they have –ve effects on other
sex
REASONS FOR FEMALE CHOICE OR PREFERENCE
direct benefits:
▪ Females may benefit from increased nutrition,
provisioning, or paternal care that increases their
reproductive output or the quality of their offspring.
REASONS FOR FEMALE CHOICE OR PREFERENCE
indirect benefits:
▪ Good Genes Hypothesis: Genetically superior mates
produce fitter offspring.
▪ Sexy Son Hypothesis: Females that mate with preferred
fathers produce sons that will have high mating success.
Nuptial Gifts
Male Hanging Flies present their female partners with insect food items. The size of the gift is correlated with the duration of copulation and the number of sperm transferred.
▪ How can we explain female preferences when there are no direct benefits?
good genes model
good genes model
FISHERIAN RUNAWAY SEXUAL SELECTION
An alternative to the “Good Genes” Hypothesis:
▪ Assortative mating within a population
between males with the most exaggerated trait
and females with the strongest preference can
lead to a genetic correlation between trait
genes and preference genes. The female
preference genes will “hitchhike” onto the
successful male genes.
FISHERIAN RUNAWAY CAN LEAD TO MALADAPTIVE TRAITS
▪ When the trait and the preference are genetically
correlated, then the trait can evolve way beyond the
point where it indicates overall genetic quality.
▪ Runaway of the male trait can proceed to a point of
exaggeration where it actually decreases male
fitness.
▪ The runaway process leads to a situation where the
only benefit to female choice is that her sons inherit
the most attractive state of the trait. This is in direct
contrast to the “Good Genes” Hypothesis and has
been referred to as the “Sexy-son” Hypothesis.
ALTERNATIVE HYPOTHESIS FOR
THE ORIGIN OF FEMALE
PREFERENCE
Sensory Bias
▪ Preexisting preferences for certain traits
may be hardwired in females and lead to the
development of exaggerated traits in males.
female preference should evolve first
followed by the evolution of the male trait
polygyny
1 male multiple females
polyandry
1 female, multiple males
sexual conflict
asymmetry between the sexes in the
potential evolutionary costs and benefits of any particular
mating event.
coevolution
two or more species that
1) exert selective pressures on eachother and 2) evolve in response to eachother
important feature of coevolution
selective environment is constantly changing
sequential evolution
changes in one species influences the other but not the reverse
mutualism
both benefit
abiotic and biotic constraints on Nitrogen fixation
1.energentically expensive
2.nutrient colimination (may be ultimate control over N supply in some systems)
3.herbivory( high N content of N fixing plants makes them preffered forage)
Partner diversity in legume/rhizobia interactions
Mutualistic relationships such as legumes and rhizobia have been stable for
millions of years.
These relationships are vulnerable to invasion by exploitative or cheater
genotypes that reciprocate less or not-at-all in a relationship.
Invasions by exploiters can be prevented if individuals could identify and
exclude or sanction less beneficial partners, and thereby stabilize a
mutualistic interaction.
If these mechanisms are effective, it is expected that genetic variation in
partner quality would be reduced, but in legume/rhizobia interactions
diversity is often observed.
“Why should a symbiont benefit its host when it would gain immediate advantage by injuring it?” (Maynard Smith 1989)
Possible post-infection sanctions against rhizobial defectors include:
- direct attack on non-fixing bacteroids (e.g., by acid hydrolases),
- limiting the C supply to non-fixing bacteroids or whole nodules
- limiting nodule O2 supply
When does coevolution occur?
Selective pressure will be strongest when there is a close ecological relationship
“Close” ecological relationship = usually specialists rather than generalists
Coadaptation
Reciprocal adaptations of two species
cophylogeny
congruent phylogenies due to cospeciation - strong evidence for co evolution
phylogenetic reconciliation analysis
tests the hypothesis that two phylogenies are more different than expected by chance
Types of coevolution:
-Specific: one species interacts closely with another. Changes in one
species induce adaptive changes in the other, and vice-versa
-Diffuse: selection imposed reciprocally by one interacting species on
another is dependent on the presence or absence of other species
Plant-herbivore coevolution
- Plants have no opportunity to “flee” or to “hide”.
- Evolved defenses reflect that.
- Herbivore activity is harmful to plants
- Plants are able to evolve effective defences
- Herbivore activity, growth, reproduction and evolution have been
guided by plant defences
monophagous
feeds on plants in 1 species, specialists
oligophagus
feed on many plant but from the same family
polyphagous
can feed on plants from many families . generalists
Antagonistic interactions:plant herbivore interactions
Plant-herbivore coevolution
- Plants produce toxic “secondary” chemicals (not directly involved in
growth and reproduction such as terpenoids, alkaloids and phenols)
that reduce herbivory - Some herbivores have evolved to detoxify the toxic chemicals.
– herbivores may specialize on the hosts whose defenses they have overcome
– plants may evolve new defenses, and the cycle continues (Red Queen)
Plant-insect coevolution
Host specificity is determined by the chemical defenses of the plant
-Four major chemical classes of plant defenses against herbivory (indicated by colors)
-These chemical classes do not correspond to plant clades (top)
-The bottom figure shows beetle phylogeny with branches coded for the chemical type of the host
-The phylogenies are incongruent because host switching can occur as long as the beetle switches to a new host with chemical defenses to which it is already adapted
Evolutionary arms race
promotes diversity via antagonistic interactions
* Herbivores selective pressure increases
plant defenses
* Plant is free from herbivores
* Until new group of herbivores
circumvents defense
* → new defenses
Geographic mosaic theory of coevolution
Interactions coevolve as constantly changing geographic mosaics
* Coevolution is prominent in some areas (coevolutionary hotspots) but
not others (coevolutionary cold spots).
* The outcome of an interaction can vary between areas and gene flow
can affect the outcome of interactions
arms race
coevolving species have to constantly improve to meet each new adaptation with a better adaptation of their own
escalation
coadaptations become increasingly powerful, yet species are not any better yet adapted because the selective landscape is constantly changing
fig wasp mutualism
figs send smells to wasps and they are wasp shaped
ants and insects that produce honeydew
many different insects provide ants with honeydew. in return ants will defend
coevolution as an explanation for diversity
-speciation events
-may explain sympatric speciation
-
speciation
the process by which one genetically- cohesion population splits into two or more reproductively- isolated populations
cladogenesis
the branching or splitting of a lineage
anagenesis
evolutionary change within a lineage, resulting in differences between sister lineages
issues with biological species (ecotype)
-within population variation
sibling species
-reproductively isolated populations
-difficult to distinguish by morphological traits
-recognized by ecological, behavioural and chromosomal differences
Ring Species
geographic variation + reproductive isolation
lIssues with biological species (hybrids)
- Individual formed by mating between unlike
forms, usually genetically differentiated
populations or species - If hybrid individuals are less viable or less
reproductively successful than non-hybrid
individuals, the two species are considered to
be good (distinct) species
Hybrid zone
- Region where genetically distinct
populations come into contact &
produce at least some offspring of
mixed ancestry - Carrion crow (Corvus corone)&
hooded crow(C. cornix) sometimes
considered as two subspecies of a
single species; hybrids with various
intermediate plumage patterns
found in central Europe, but
because of limited gene exchange
they are mostly considered
separate species
natural selection
-post mating/ post zygotic
-hybrid inviability
-ecological
-developmental
sexual selection
pre mating/ pre zygotic
-behavioural
-ecological
-temporal
gametic selection
post mating/pre zygotic
-gametic isolation
-copulatory behavioural isolation
issolating barriers to gene flow
pre mating isolation
mate recognition
prezygotic barriers
habitat isolation, behavioural isolation, temporal isolation, mechanical isolation, gamete isolation
habitat isolation
populations live in different habitats and do not meet
behavioural isolation
little or no sexual attraction between males and females
temporal isolation
mating or flowering occurs at different seasons or times or day
mechanical isolation
structural differences in genitalia
gametic isolation
gametes fail to attract eachother
postzygotic barriers
reduced hybrid fertility, reduced hybrid viability, hybrid breakdown
reduced hybrid viability
hybrid zygotes fail to develop or failure to reach sexual maturity
reduced hybrid fertility
hybrids fail to produce functional gametes
hybrid breakdown
offspring of hybrids have reduced viability or fertility
extrinsic
reduced survival due to ecological factors . reduced postzygotic survival
intrinsic
interactions between genes. genetic incompatability
modes of speciation
allopatric, peripatric, parapatric,sympatric, speciation by polyploidy
allopatric
divergence of two large populations, gene pool large, initially similar environments. evolution likely slow but speciation in time.
-ecological:as a result of divergent natural selection
-gene conflict at secondary contact
-sexual selection : female choice by location
-reinforcement:discrimination and fitness
peripatric
divergence of a small population from a widely distributed ancestral form
-dispersal
-founder effect (small subset of gene pool) speciation by genetic drift
-rapid evolution more likely
-extreme environment
parapatric
two species or populations having contiguous (=sharing a common border; touching) but non overlapping geographic distributions
-neighbouring populations
-low gene flow between them
-evolution of reproductive isolation
-range expansion leads to sympatry
Sympatric
- Initially, a single randomly mating
population - Evolution of reproductive barriers
within the same geographic area - Still debated as to how common
this is
speciation by allopolyploidy
allo=different
poly=many
ploidy=number of sets of chromosomes
speciaition
-birth of a species
-evolution of reproductive isolation withiin an ancetral species, resulting in two or more descendant species
extinction
death of a species
background extinction
long lasting rate at which taxa become extinct
mass extinction
a highly elevated rate of extinction of a taxa; extends over a relatively short geographic interval
mass extinction
worldwide cooling
volcanic eruptions
changes in ocean chemistry
extinction and speciation
-organisms that speciate readily also tend to have short species duration
-high speciation and high extinction rates go together
why might extinction rates decline
does natural selection cause species to be less vulnerable
-natural selection cannot prepare for novel environmental conditions
-extinction resistance does not carry over
other hypothesis
-more family now so less likely to lose a family
-some clades are more voliatile (high turnover)
-evolve new families while lose others
-leaving less volitile taxa-long lifespan and low extinct %
GOOD GENES MODEL
ELABORATED MALE TRAITS MAY BE
INDICATORS OF HERITABLE GENETIC
QUALITY (I.E. FITNESS).
▪ Some males may have a heritable trait that reduces
viability.
▪ Only males with “Good Genes” can survive despite the
disadvantage.
▪ Females that mate with these males will have offspring
with higher fitness
cospeciation and coevolution
Cospeciation is a form of coevolution in which the speciation of one species dictates speciation of another species and is most commonly studied in host-parasite relationships.
Introgression
movement of genes from one distinct gene
pool into another which can transmit specific traits from one
group to another, or homogenization of two gene pools.
Consequence of Secondary Contact
▪ No hybridization (no hybrid zone)
▪ Hybridization but no introgression (narrow hybrid zone)
▪ Limited introgression (wider hybrid zone)
▪ Reunification of the gene pool or extinction
