Exam 4 Flashcards
What is prezygotic isolating mechanism?
Live in the same place, but do not encounter each other
What are the prezygotic mechanism?
Habitat isolation
Temporal isolation
Behavioral isolation
Mechanical isolation
Gametic incompatability
What are the postzygotic mechanism?
Zygote mortality
Hybrid inviability
Hybrid infertility
What is reinforcement?
The process by which two populations begin to diverge in allopatry but complete the process of species in sympatry when matings between individuals in these populations produce hybrids with reduced fitness
What is postzygotic mechanism?
prevent the hybrid zygote from developing into healthy and fertile adults. There are three likely cases that will occur to ensure that the hybrid does not reproduce.
What could happen is secondary contact occurs when two formerly
allopatric populations meet?
Three outcomes are possible:
1. No interbreeding occurs
• isolating mechanisms in place – speciation
=completed.
2. Introgression (complete inbreeding)
• no isolating mechanisms in place – populations merge completely
3. Partial interbreeding occurs:
Some isolating mechanisms in place – a
hybrid zone forms (but hybrids are less fit).
reinforcement should occur to “complete”
the process by the evolution of additional prezygotic barriers
Can postzygotic isolation be possible?
Yes it reduced viability or fertility of interspecific hybrids
What are some patterns of partial postzygotic isolation?
In some species, male hybrid offspring can be
produced, but females cannot.
• In other species, female hybrid offspring can
be produced by males cannot.
• Haldane summarized this pattern due to the
mode of sex determination: If among hybrid offspring “one sex is absent, rare, or sterile, that sex is the [heterogametic] one
What are the sympatric speciation mechanism?
Host shifts
Polyploid speciation
What is speciation without physical isolation?
Sympatric speciation
Why is sympatric speciation difficult to understand?
because gene flow should be higher under such conditions compared to, for
instance, geographically isolated species or across a geographic cline.
What is an example of sympatric speciation? (Host shift)
Parasitic apple maggot flies would inject eggs in hawthorn tree (fruit) but have shifted to apples in the past 150 years.
What are consequences of host shift? (Using apple maggot flies)
Reproductive isolation resulting from host specificity:
Flies emerging from hawthorn fruits are attracted to hawthorn
during mating. Those emerging from apples are attracted to apples
Reproductive isolation resulting from “allochronic” mating periods:
Apples ripen 3-4 weeks earlier than hawthorns.
Selection favors incompatible genotypes in the two races:
- Hawthorn fly larvae must develop rapidly so that they can pupate and reach diapause before the onset of winter.
- Apple fly larvae must sustain a longer diapause period before
emerging in the spring.
- This leads to different optimal developmental processes
What are the two types of reproduction?
Asexual and sexual
What are the three common components of sex?
–Crossing over (recombination)
–Independent orientation and Reduction division (meiosis)
–Fusion of gametes (syngamy)
Did sex evolve to facilitate reproduction?
No
What does sexual reproduction increase?
Genetic variation by:
1. Recombination that produces new chromosomes variant
2. Haploid gametes are produced by meiosis
3. Gametes fuse, and diploid is restored in zygote.
What is cost of sex?
Twofold cost of sex: it takes two individuals to produce the same number of progenyin sexual reproduction as opposed to asexual reproduction.
Cost of sex also impact what part of reproduction?
Meiosis: twofold cost of meiosis
Asexuality is twice eas efficient at transmitting genes and should be strongly favored by selection. Sexual only transmits 50% of their alleles to their offspring.
Cost of sex: whats another point of meiosis?
Meiosis is more complicated than mitosis
– Requires more energy (more stages)
– More error-prone
Cost of sex: what does sit say about mating?
Sex incurs the difficulties of finding a mate
– Asexual organisms don’t have to find a mate
– For sexual individuals, finding a mate is costly
(encounter, energy, predation, mate recognition)
Cost of sex: what about gene complexes?
Sex splits up gene complexes
- In asexual lineages, linked groups of coadapted
gene complexes remain linked within the genome
- With sex, linked groups of gene complexes can be
split apart
Cost of sex: what about fitness variation?
Sex can generate fitness-reducing variation
-In a diploid sexual lineage, 50% of the offspring produced
by a pair of heterozygotes will be homozygous
(and hence, less fit)
Cost of sex: another downside to sex? (Think bacteria/virus)
Sex can lead to sexually transmitted diseases
How common is sex?
42,000 vertebrate species described: 0.01% asexual (22 species of fish, 23 amphibians, 29 reptiles, NO ASEXUAL
birds or mammals.
Almost no taxa composed of entirely species that reproduce asexually.
On a phylogeny, asexual species are rare and
short lived
Why is sex so common?
Theory for evolution of sex
1. Long-term advantages
- Fisher-Muller Hypothesis
- Muller’s Ratchet
2. Short-term advantages
- Lottery model
- Red Queen hypothesis
What is the fisher-mullee hypothesis?
Response to natural selection is proportional to
existing variation
• Sexual reproduction produces more variation than
asexual reproduction
Therefore, sexual populations can evolve faster
if and when selection pressures change
What is fisher Muller model?
Muller’s example:
Fisher-Muller Model
- Population with 2 loci: a and b
- All individuals start with genotype ‘ab’ but AB is more fit
- Favorable mutations:
a → A
b → B
Asexual: takes twice as long to mutate twice
Sexual: gets to population quickly by mating
What are the conditions in which fisher-mullee hypothesis doesn’t work?
Small population size or low mutation rate
– Either A or B will spread through the population
to fixation before the other allele mutates
– Therefore populations will result in either aB or Ab
(depending on which mutated first)
- The chances of mutating to AB are then the same as for
asexual populations
What is Muller’s ratchet?
Purging deleterious alleles through sex
In an asexual population:
• Mildly deleterious mutations accumulate over time.
• The more mutations in an individual, the less fit it is.
• If there is only a small number of individuals with no mutations, there is a good chance these individuals might not reproduce because of chance (drift).
Long-term Advantages of Sex
This results in everyone in the population having at least one mutation. The “Ratchet” has clicked! Mean fitness has decreased
How does sex reverse Muller’s ratchet?
Recombination gets rid of deleterious alleles through sex.
Thus, in a sexual population:
• Two individuals with mutations could mate to produce
offspring with NO mutations
• Each generation, new mutation-free individuals would
be produced
• Therefore the ratchet cannot operate
What are some cons to Muller’s ratchet theory?
It seems to act too slowly to prevent asexual populations
from out-competing sexual rivals before themselves
going extinct.
- It would require greater mutation rates than are generally observed, and/or that mutations have a synergistic effect.
That is, the deleterious effects of a pair of mutations would
have to be multiplicative, rather than additive.
What are the short term advantages of sex?
Lottery model
Red queen hypothesis
What is the lottery model of evolution of sex?
If the environment is patchy (i.e. heterogeneous) then different genotypes do well in different patches
- By producing genetically diverse offspring, the chances are better that at least some offspring will match some patches (assuming offspring disperse)
- Sibling competition: If two siblings are identical they compete for identical resources. This is less likely to happen if offspring are
diverse.
Lottery: Better to buy 100 different numbers
than 100 of the same numbers than 100 of the same number.
Can species switch between asexual and sexual reproduction?
Yes, Many species have the capacity to switch between sex and asex.
In such species, sex usually occurs as a response
to a change in the environment.
Why is asexual most common in harsh environment? (Deserts and alpine regions)
Less energy required , don’t have to worry about a mate
What kind of reproduction is factored by abiotic features?
Asexual reproduction because they can track changes
Biotic features favors which type of reproduction?
Sexual reproduction
When one species adapts to its competitors, etc., the
environment for the competitors changes, thus
imposing new selection pressures on the competitors.
This in turn changes the environment for the first
species, imposing new selection pressures on it.
What type of organism play importance in Red-Queen hypothesis? (Pro sexual reproduction)
Parasites pose one of the most important selective forces
Why?
- Parasite loads are costly
- Most parasites are host specific - high degree of coevolution
- Parasites have very short generation times in relation to host;
this allows them to evolve more quickly than the host
Therefore, one way to escape parasitism is to make
offspring as different from parents as possible
