lec18: sex, reproductive systems, and evolution

Question 1

what are the 3 systems of reproduction?

Answer 1

1. reproductive system
2. sexual system
3. mating system

Question 2

draw all the modes of reproduction considering the systems. (asexual, sexual, dioecious, hermaphrodite, cross-fertilization, self-fertilization)

Answer 2

drawing

Question 3

asexual vs sexual

Answer 3

• asexual: making clones of themselves (violation of Mendel’s law)
• sexual (species that have separate sexes like us)

Question 4

dioecious vs hermaphrodite

Answer 4

• dioecious: have both male and female reproductive functions within the same organism
• hermaphrodite: can have diff mating systems

Question 5

cross-fertilization vs self-fertilization

Answer 5

• cross-fertilization: can make with others
• self-fertilization: mate themselves (it’s different from asexual)

Question 6

sexual reproduction:
1. ___ parent(s) contribute genetic material
2. yes/no meiotic reductive division
3. yes or no fusion of gametes?

Answer 6

1. 2 parents contribute genetic material to offspring
2. yes meiotic reductive division to form gametes
3. fusion of gametes to create offspring

organisms undergo meiosis; so they go from having 2 copies of their genes in somatic cells to the germline cells where there’s a random choice of one or the other copy in the gametes and then the gametes fuse to create offspring

Question 7

asexual reproduction:
1. ___ parent(s) contributes genetic material
2. yes/no meiotic reductive division
3. yes or no fusion of gametes?

Answer 7

1. 1 parent contributes genetic material
2. no meiotic reductive division (meiosis is not occurring)
3. does not involve fusion of gametes. offspring are genetic replicas (clones) of parents

Question 8

what was Darwin puzzled about sexual reproduction?

Answer 8

why sexual reproduction evolved? how could this have been favoured by natural selection? why not go through the process of asexual reproduction? Why organisms bother with sexual reproduction cause it doesn’t seem the greatest strategy compared to asexual reproduction?

Question 9

T or F? do different reproductive systems occur in different environments? does it depend on environment? (2 ex)

Answer 9

yes for example the water fleas where the sexual ones live in warmer turbulent water and asexual ones live in cooler calm water.

many perennial plants (like water hyacinth) reproduce through both sexual AND asexual reproduction. it is the worst invasive species because it can create massive numbers of clones of itself over a short time frame.

Often, the type of reproduction that they undergo depends on their environmental conditions or the point in their growth cycles.

Question 10

what are the costs/ disadvantages of sex (sexual reproduction?) (6)?

Answer 10

1. time and energy to find and attract mates
2. increased energetic costs of mating
3. increasing risk of predation and infection (by having to go out and finding a mate)
4. cost of producing males
5. 50% less genetic transmission
   - (compared to asexual females, sexual females contribute only 50% of her gene copies to the next generation)
6. break up of adaptive gene combinations
   - segregation, recombination
   - (sexual reproduction, because it creates new combinations of genetic variation, can break up good combinations that were already there built up by natural selection)

Question 11

what is the paradox of sex

Answer 11

why has sexual reproduction evolved if it seems like such a bad evolutionary strategy (costs of sex) relative to asexual reproduction

Question 12

draw a diagram and explain the “50% less genetic transmission for sexual reproduction”,”the 2-fold cost of meiosis” demonstrating sexual vs asexual and explain. what does this transmission bias favours?

Answer 12

diagram. this transmission bias favours asexuals in competition with females

Question 13

explain the heterozygous (Aa: adapted to neither) and homozygous alleles (adapted to dry/aa or wet/AA habitat) in relation to sexual and asexual reproduction and what does it tell us about both in terms of its combinations being favourable or unfavourable

Answer 13

AA: adapted to wet habitat
aa: adapted to dry habitat
Aa: adapted to neither habitat (It doesn’t excel in either condition but may be more of a generalist or have adaptations that allow it to survive in a range of environmental conditions, without a preference for wet or dry habitats)

• asexual reproducing parent will continue to make homozygous individuals
• sexual reproducing parents will produce heterozygotes and maybe it will adapt to neither habitat
• asexual reproduction maintains favourable combinations of alleles; (which means if the parent is well-adapted to its environment, its offspring are likely to be similarly well-adapted) (HOWEVER, While asexual reproduction is efficient for maintaining successful traits in stable environments, it may lack the ability to generate novel combinations of alleles that could be advantageous in changing conditions)
• sexual reproduction can continually re-create unfavourable combinations of alleles (sexual reproduction is creating bad combinations every generation. natural selection will act against those heterozygotes. asexuals will keep the around the types that have been favoured by natural selection. sexual reproduction will keep creating these bad combinations over and over again. that doesn’t seem like something that should be favoured by natural selection)

Question 14

what are the hypotheses for the advantages of sex (the possible benefits of sexual reproduction that could outweigh the disadvantages)? (2) + provide a diagram considering harmful (-) and beneficial (+) mutation of 2 diploid individuals in a population

Answer 14

1. bringing together favourable mutations (can bring together new combinations and potentially that could be favourable)
2. is effective at eliminating harmful mutations (a big reason why genetic variation or bringing new combinations together might be beneficial is if environments are often variable. sexual reproduction can shuffle up the genetic material to potentially produce combinations that are now favoured in this changing environment)

diagram shows that sexual reproduction:
- independent assortment
- recombination

Question 15

what are the benefits of genetic variation in variable environments? (hint: lottery models… / spatially… / temporally… )

Answer 15

1. “lottery models” given environmental unpredictability (sexual reproduction is creating a bunch of different lottery tickets, different combinations, and one ends up being the wining one in this changing environment)
2. spatially heterogeneous environments (refer to areas where the physical and ecological characteristics vary across space)
   - tangled bank hypothesis
3. temporally heterogenous environments (refer to areas where the conditions and factors affecting the ecosystem change over time)
   - red queen hypothesis

heterogenous or varying environments in both space and time, that might favour sexual reproduction

Question 16

what does the hypotheses for the advantages of sexual reproduction tell us about “models” and “experiments”

Answer 16

many theoretical models but only limited experimental evidence

Question 17

explain how combinations of mutations brought together more rapidly by sex and compare it with asexual reproduction. (provide diagram)

Answer 17

asexual: from the ancestral side to new environment, A popped up and it spreads. B and C also popped up but only one of these 3 can spread at once because we have a clonal population. so B and C gets wiped out and now that the whole population is A, we have to wait around for B to reappear in that A genetic background and then the the AB combination will fix. AC popped up too but you can’t have both fixing at the same time (what we call clonal interference). these clonal genotypes, there might have different genotypes might have good ideas but they’re not sharing. so only one can spread to fixation before the next one so it takes more time for ABC to come together into one genotype than in a sexually reproducing population

sexual: where AB and C could pop up early and the ideas are being shared through sexual reproduction. and so under conditions that favour the bringing together of favourable combinations that are found in different individuals that will favor sexual reproduction

Question 18

the idea of sex being favoured, is it because it is inherently advantageous?

Answer 18

the idea is that sex is being favoured not because it is inherently advantageous, but because it’s creating the best combinations that will then spread through the population. so sexual reproduction can be beneficial for eliminating harmful mutations and spreading, bringing together favourable mutations

Question 19

based on advantage of sex in Evening Primrose (multiple sexual asexual transitions), looked at the genomes of these asexually reproducing and sexually reproducing lineages to ask, are the asexually reproducing lineages showing evidence of having more harmful mutations in their genome than the sexually reproducing lineages? if yes what is causing it? (GRAPH included) + what does this indicate about these asexual lineages

Answer 19

that’s what we’d expect if asexuals are less able to eliminate harmful mutation due to the lack of sexual reproduction. one way of looking at this is to look at proteins in the genome and look at how many cases these species show evidence of a protein that has become inactivated due to, for ex, a stop codon (will abort the production and translation of the protein), how many genes in the genome have stop codons in the middle of the gene that are basically rendering that gene non functional? we found that on average, the asexual lineages had significantly more stop codons in their genes than the sexual lineages. also, the asexual lineages seem to be evolving much faster in their proteins than the sexual lineages, which is another reason that they are accumulating harmful mutations

Asexual Oenothera have”
- more “premature” stop codon mutations (leads to dysfunctional proteins)
- higher rates of protein sequence evolution
- implies greater accumulation of deleterious mutations

this is an evidence that indicate these asexual lineages maybe be suffering the long term accumulation of harmful mutation that the sexual lineages are not

Question 20

what’s an example of an experiment made for the “theory predicts spatial heterogeneity in selection can facilitate evolution of sex” and the (graph of this experiment showing how) “higher rates of sex maintained in population evolving in heterogeneous habitats”; for each (experiment + the graph) what does it tell us

Answer 20

homogeneous (AA) and (BB) conditions vs heterogeneous (AB) condition

the prediction under these evolutionary models was that under this varying environmental conditions, that should favour more sexual reproduction than if the environment was constant, where cloning yourself might be the best strategy

extra:
1. there’s genetic variation within this species to reproduce both sexually and asexually, and different individuals might reproduce more asexually or sexually. this experimental system examine whether organisms are likely to evolve more or less sexual reproduction under different conditions
2. talked about if environments are fluctuating, that might favour sexual reproduction because new combinations might have to be introduced that are the right combination in the new environment compared to the old environment. whereas if you’re in a constant environment, asexual reproduction might be the best strategy because you just keep things the same because natural selection has built up the optimal genotype

graph:
- sex declined rapidly over 12 weeks (70 generations) in homogeneous environments
- sex persisted at a much higher level with spatial heterogeneity
- the environments that were remained constant in the lab, the rate at which individuals were reproducing sexually evolved rapidly to go down from the starting rate, whereas in the heterogeneous (the varying environment), the rate of sexual reproduction stayed relatively high.
so this is consistent with this idea that if environments are changing recurrently and rapidly, then that can select for shuffling up our genetic material every generation whereas in a long term constant environment, that might favour primarily asexual reproduction

Question 21

what are the 3 points of macroevolutionary history of asexuality

Answer 21

1. asexuality by parthenogenesis
2. asexuality by clonal propagation
3. asexual species are usually at the tips of phylogenies

Question 22

asexuality by parthenogenesis in terms of trophic

Answer 22

• sporadically distributed across the animal kingdom
• more common in invertebrates, rare in vertebrates

Question 23

asexuality by clonal propagation in terms of trophic

Answer 23

• much more common in plants
• few species (if any) are exclusively asexual (so almost all species, if not all, reproduce at least somewhat sexually in nature when we look at eukaryotic organisms)

Question 24

asexual species are usually at the tips of phylogenies (meaning that asexual species are typically recent).

1. so does that mean it indicates that it has a lower or higher extinction rate+ why
2. low or high chance of long-term evolutionary persistance + why

Answer 24

• macroevolutionary pattern indicates a higher extinction rate (don’t seem to survive for very long over evolutionary time)
• low chance of long-term evolutionary persistence - probably due to extremely low genetic variation and accumulation of deleterious mutations

so over the long evolutionary time, they might have a higher rate of extinction because they are more likely to accumulate harmful mutations and they’re likely to have less genetic variation and we talked about that having genetic variation and effective natural selection is key for adaptation and potentially for a species to survive over the long term specially as climate change or environments change across the landscape

Question 25

does that mean that asexually reproduction for millions of years (ancient asexuality) is not possible?

Answer 25

it is but very very rare case of ancient asexuality such as Bdelloid rotifers that had no sex for millions of years. males are unknown

Question 26

outbreeding

Answer 26

mates are less closely related than random
(random mating)

Question 27

inbreeding

Answer 27

mates are more closely related than random

Question 28

is there a continuum or discrete between outbreeding and inbreeding?

Answer 28

continuum where the rate of inbreeding vs outbreeding can continuously vary in populations

Question 29

outcrossing

Answer 29

mating with someone else where it can be either by outbreeding or inbreeding

Question 30

is outcrossing sexual or asexual

Answer 30

• sexual
• fusion of gametes from 2 parents where gametes derive from meiotic reductive division

Question 31

selfing (self-fertilization)

Answer 31

• mating with yourself (hermaphrodites)
• NOT asexual reproduction

Question 32

what is the most extreme form of inbreeding but NOT asexual reproduction

Answer 32

selfing (self-fertilization)

Question 33

what is the difference between asexual and selfing reproduction in terms of if the individual was heterozygous?

Answer 33

asexual is cloning while with selfing, organisms are still undergoing meiosis (it’s just happening within the exact same parent)

so if an individual is heterozygous for ex, and it’s self fertilizing, that’s a very different outcome from a asexual reproduction because an asexual that’s heterozygous will just stay heterozygous. but an individual that’s selfing that’s heterozygous will undergo mendelian segregation and produce homozygotes alongside heterozygotes

Question 34

does selfing have fusion of gametes?

Answer 34

yes from 1 parent
- gametes derive from meiotic reductive division

Question 35

what are the potentials for inbreeding (3)

Answer 35

• local population substructure enhances mating among relatives
• hermaphroditic organisms have potential for self-fertilization (most plants, many animals)
• in small populations, even random mating can lead to mating among relatives (also small population means there’s less genetic diversity)

Question 36

what are the things flowering plants do that’s actively evolving to avoid inbreeding (3)

Answer 36

1. large showy flowers attract pollinators
2. timing offset between male and female reproduction (avoids self-fertilization)
   - pollen vs ovule maturation within a flower
   - when male vs female flowers open
3. diverse morphological and physiological mechanisms to avoid selfing
   - self-incompatibility
   - eg. spacing of anther and stigma
   - eg. some plants have a molecular system in place that prevents them from mating with themselves even though they can and ensures they mate with others

Question 37

what are the inbreeding avoidance behaviours in animals (4)?

Answer 37

1. dispersal by one sex (one sex disperse further than others)
2. delayed maturation
3. extra pair copulation (in birds eg, there’s a lot of male and a female parent that stayed paired across the season, but there’s a lot of genetic evidence now that even under those mating systems, there’s a lot of extra pair of copulation, mating with other individuals beyond the pair and that is though to potentially be a mechanism to avoid inbreeding in case who you paired up with ends up being a close relative )
4. kin recognition and avoidance (mating with them)

Question 38

what are the genetic effects of inbreeding in population (or like what are the genetic consequences of inbreeding) (2)?

Answer 38

1. changes GENOTYPE frequencies where it increases homozygosity and decreases heterozygosity
   (if population inbreeds at a rate of 50%, what happens over time, it’s because they’re mating with close relatives, they end up with a higher fraction of homozygous genotypes and a lower proportion of heterozygous genotype. so you get a change in the frequency of genotypes, specially fewer heterozygotes and more homozygotes)
   - heterozygosity reduced by 50% per generation with self-fertilization
   - competition between homozygous genotypes (selection) and genetic drift of small populations can reduce P
2. does not directly change ALLELE frequencies (does not change polymorphism)
   (Allele frequency does not change during inbreeding it rather remains constant because there is no exchange of allele between two genetically related organisms)
   - homozygosity for deleterious recessive alleles

Question 39

draw a diagram/graph of “effect of inbreeding on rate of heterozygosity decline depends on mating patterns” with a trend line of mating with “self” “sibling” and “double 1st cousin”

Answer 39

over time, if you start with lots of heterozygosity, if you have inbreeding over time, you’ll get a reduction in the fraction of heterozygous genotypes and an increase in the fraction of homozygous genotype. this will be fastest with self-fertilization, but with mating among full siblings can also happen really rapidly and double first cousins for ex, this is the decay.. so mating among relatives reduces the fraction of heterozygotes in the population, increases the fraction of homozygotes.

Question 40

inbreeding depression (2)

Answer 40

1. the REDUCTION IN FITNESS of inbred offspring (of related individuals) compared to outcrossing offspring
   - lower viability (survival)
   - lower fertility (reproductive output)
2. strong inbreeding depression disfavours inbred offspring
   - thus favouring outcrossed mating systems
   (strong inbreeding depression should select against inbreeding and select for mechanisms that ensure mating with others)

Question 41

why can inbreeding reduce fitness considering in a case where you have 3 different diploid individuals and 3 different spots in DNA + mutation?

Answer 41

turns out that in any sexually reproducing population, harmful mutations are happening all the time, every generation. if they are found as heterozygous, it’s very unlikely to have an impact on your health. if they’re found as homozygous, they can be very detrimental. so these are recessive deleterious mutations. thats’ what these blue mutations represent, these individuals are OK because they’re not affected by these mutations because they’re in heterozygous form. if these individuals mated randomly, no problem because any of these mutations then will be found as heterozygous. but if they self-fertilized or if they inbred, now we’re going to have the exposure of these homozygous recessive mutations and that can lead to a reduction in survival and/or reproduction. this is why inbreeding occurs. we have these heterozygous rare mutations in populations that aren’t affecting our survival under randomating, but with high levels of inbreeding it can lead to a strong reduction in survival and reproduction

homozygosity of recessive deleterious alleles

Question 42

T or F: inbreeding depression causes reduced fitness meaning it selfing barely evolve.

Answer 42

False: yet selfing has evolved many times.
- one of the most common evolutionary transitions

so far we’ve gone through all the reasons why self-fertilization and strong inbreeding might be selected against and yet somehow repeatedly also been favoured by natural selection

comparing the pics of outcrossing flowers vs selfing ones; outcrossing plants are more showy and larger

Question 43

explain and draw a chart to determine what the total gene copies are for an outcrosser and selfer

Answer 43

• there’s an inherent genetic transmission advantage
• let’s take an outcrossing individual and when it produces offspring, it can produce seed and it’s transmitting one copy of its genetic material through seed and it can produce pollen and so in total (in units of pollens and seeds) it’s contributing 2 gene copies to the next generation
• but what if you have a new genotype in the population that self-fertilizes but it can also outcross through pollen?
• now it’s contributing 2 copies of its genetic material through seed. it’s both mother and father to its seed and it can still spread pollen to other individuals and it’s producing 3 copies and so once again we have sort of an inherent transmission advantage of the genotype that’s mating with itself, that’s reproducing on its own than the individual that’s reproducing by outcrossing
• so basically if the benefits outweigh the costs of inbreeding depression, then self-fertilization should spread and be selectively favoured
• and so depending on the conditions in nature, maybe sometimes inbreeding depression is lower, favouring the evolution of self-fertilization

Question 44

what a benefit of selfing when it comes to colonization. is it a good strategy for glaciation?

Answer 44

another benefit of self-fertilizing. if you’re a colonizing species in a new area, there not might be many mates or even pollinators around, and if you have mate with yourself you can colonize well. so fertilization might be a good strategy after glaciation for colonizing new habitats across the continents

Question 45

position the selfing and outcrossing in a “proportion of deleterious mutations in genome” scale

Answer 45

0.05 (outcrossing) —- 0.08 (selfing)

Question 46

understanding the frequency in nature of selfing and outcrossing:
over the short term (2)

Answer 46

selfing might be beneficial and favoured by natural selection because they can mate with themselves, they have this transmission advantage

1. if conditions are favourable selfing can spread via natural selection
   - lack of “reproductive assurance” due to rarity of pollinators or mates
   - transmission advantage from self + exported pollen
   - low inbreeding depression
2. BUT harmful effects of inbreeding depression encourage outcrossing

Question 47

understanding the frequency in nature of selfing and outcrossing:
over the long term (3)

Answer 47

1. selfing leads to low diversity and inefficient selection
2. can drive higher extinction rates in selfing species
3. macroevolutionary pattern of greater prevalence of outcrossing

can have less diversity and less efficient selection in selfing and that might drive higher extinction rates of selfers compared to outcrossers much like we saw with asexuals