Week 3: Lectures Flashcards
Key Points:
- Sexual reproduction can be costly
- Sexual reproduction can advantageous
to deal with environmental pressures
such as parasite stress - Sexual reproduction might help to purge
costly mutations - Genetic drift can occur independently
from natural selection - It’s costly but very beneficial for purging
negative variants and remerging
positive variants.
____% of Eukaryotes reproduce sexually (at least sometimes)
99.9%
Why sexual reproduction when there are many other forms of reproduction?
Name three
All forms of asexual reproduction.
(A) Parthenogenesis:
- Self-cloning, or asexual reproduction
where you pass on your own genes
(AB-AB).
- Where females can fertilize their eggs in
the embryo without any sperm.
- Always produces female’s offspring.
(A) Gynogenesis: - When a female produces an egg, which makes contact with sperm to active/fertilize it but no male genetic information is passed on. - A form of Pathogenesis (asexual reproduction). The Male paternal DNA dissolves before it can fuse with the egg. - In fish.
(A) Hybridogenesis:
- An unusual form of reproduction that is
found in hybrids between different
species. It involves the selective
transmission of one of the parental
genomes (where mothers DNA (AB)
Is separated from the male (BB) purged
while the AB is passed onto offspring.
Males get no benefit from mating with
the females because their DNA is trashed
and women are viewed as parasites to
males.
- Only in frog hybrids.
- A combination of asexual reproduction
and sexual reproduction.
Darwin’s (1862) thoughts on sexual selection…
Darwin (1862) came up with the theory of sexual selection but admitted that:
“We do not even in the least know the final cause of sexuality; why new beings should be produced by the union of the two sexual elements. The whole subject is as yet hidden in darkness.”
Why sexual reproduction?
what’s a common claim/theory which is disproved by bacteria?
- A common claim people make is that
sexual reproduction is used because it
is fun or pleasurable. However,
evolution doesn’t care about “pleasure”
that claim comes from a teleological
(functional) perspective not supported
by evolution.
Bacteria:
- Why do bacteria sexually reproduce if
they do not have the sensory capacity
to “enjoy” sexual reproduction? Joy
cannot be the answer.
Two main types of reproduction:
(A) Sexual
- Allows for genetic variation to occur
because it combines DNA from male
and female sex of the species and
therefore means the offspring can be
male or female.
- DNA is encoded in X chromosome,
which females have 2x of, and is
dominantly passed down from female
lineage. This means there are lots of
males in the species not really doing
anything because they are not able to
reproduce.
(B) Asexual
- Female genome clones their DNA and
produces female offspring with the
same DNA as them.
The evolutionary cost of sexual reproduction:
(4) costs
- The twofold cost of sex: the fact that
women spend 50% of their resources
making sons who can not reproduce
which for evolutionary purposes is a
waste of time. All they contribute to
reproduction is their genes which only
further diminishes females’ chances of
passing their genes onto their offspring.
Mathematically speaking mutations
which allow for asexual reproduction
would be more beneficial because it
allows for females to pass on double
the number of genes with every
generation (i.e., passing on genes is the
biggest evolutionary goal in life). - You spend time and energy away from
feeding or protecting yourself from
predators and instead you are looking
for a mating partner, evaluate your
potential mating partners, manage
conflict with same-sex competition
when trying to attract a mate from the
opposite sex, woo them enough to
mate with them (it’s exhausting!). - Risk of STI’s when mating.’
- Mendel’s Laws of segregation (50% of
genes come from male-female) and
independent assortment (alleles are
independently assorted; colour vs
shape). In a population where the
parents have homozygotic (A/B or a/b)
alleles that are well adapted or fit to the
environment engage in sexual
reproduction, due to Mendel’s laws,
they will produce offspring with
heterozygotic allele’s that are selected
against by natural selection. Random
variation can be beneficial, neutral or
maladaptive!
= Recombination Load
Benefits of Sexual Reproduction:
- It increases variation in offspring:
- Asexual reproduction maintains the
variation in aa, Aa, AA in the population
and passes the same pattern onto
offspring over generations (preserved).
- Sexual reproduction means that the
variation in aa, Aa, AA in the
population, via recombining (male-
female random mix of dominant and
recessive genes), increases variation in
phenotypes like height within the
population.
Curious cases of Hermaphrodite species:
- Hermaphrodite species can reproduce
asexually and sexually, (e.g., aphids,
yeast) switch to sexual reproduction
under stress.
- For example, aphids are asexual in the
spring/summer where they reproduce
asexually to reproduce as many of
themselves as they can when resources
in the environment are high (fast).
- The switch to sexual reproduction in the
autumn/winter when there is less light,
its colder, there is less food and more
predators (stress).
- This Paradox, where sexual
reproduction may be linked to
environmental stressors in changing
environments.
- Sexual reproduction is like the lottery, it
has no teleological purpose in natural
selection, but the more variants that it
produces (i.e., hetero and homo) the
more likely it is that you will get the
winning ticket (the correct random
variation of genes to be an adaptive fit
for the environment).
- Where sexual reproduction is beneficial
in passing on genes across generations
in unstable evolutionary environments
(environmental factors like food
resources, temperature, competing
species, parasites or disease.
- Negative Frequency Dependent
Selection/The Red Queen
Hypothesis).
- Negative frequency in the dependent
selection in the coevolution of hosts
and parasites.
- An interactive struggle between host
and parasite where species are
required to reproduce as fast as
possible to outcompete their
competitors.
- “Now, here, you see, it takes all the
running you can do just to keep in the
same place.
If you want to get somewhere else, you
must run at least twice as fast”
- The parasite adapts to the most
frequent allele in the population so to
avoid being outcompeted the host uses
sexual selection to increase variation in
the population and the new high fitness
allele that the parasite then has to
adapt to. When parasite load gets too
high, the host numbers die down, which
in turn, negatively impacts on parasites
till the host increases their variance,
their population goes back up and the
cycle repeats.
- COVID-19-Humans
Through vaccination we are reducing
the pathogens presence in the
population and increasing our fitness
(parasite/host in a one-to-one
relationship; 1 unit increase of fitness for
host is 1 unit lost for the parasite and
vice versa). Some researchers claim
that we will be stuck in an arms race
with coronavirus, and it will never go
away. Eradication of illnesses is VERY
rare!
- Potamopyrgus antipodarum (sea snails-
parasite)
- Under low parasite stress the snails’
percentage of clones in the population
increases but decreases again when
the parasite stress is high (parasite have
adapted).
- Wave pattern of up and down clone
percentages in the population.
- The wave of clones occurs in both high
and low parasite stress conditions but
the frequency in the population is much
higher in low parasite stress relative to
high parasite stress.
- Clones > Sexuals in low parasite stress
- Sexuals > Clones in high parasite stress
conditions (less sharp selection or
peaks of clones).
- This supports that sexual selection is
used to increase variants in the
population to allow them to adapt to an
ever-changing environment and
compete with the coevolution of
parasites.
- Inverse relationship between host and
parasite (increase in one cause
reduction in the other).
Do all parasites negatively impact the host?
- It may vary in degree but not in kind.
They leach resources or make us more
vulnerable.
Mutations:
Two types of mutations can occur:
(A) Somatic mutation
- In bodily cells, occur all the time like
cancer but is not easily passed on to
offspring (in non-reproductive tissue).
(B) Germ-line mutation
- What we will focus on where there are
mutations in the germ-line cells
(reproductive tissue in sexual
reproductive organs; m-f).
- Through sexual selection is passed
down randomly to approximately 50% of
the next generation.
Why do mutations occur?
Two reasons
§ We will focus on processing and
environmentally occurring mutations.
§ The processing way can lead to DNA
deletion or insertion. Where alleles
became misaligned during
recombination and attach at the wrong
point which causes a chromosome with
a log side and a short side
(chromosomes are not identical copies).
§ The environmental factors like the sun,
thymine dimer; when thymine bonds to
another thymine and not in its correct
place in the DNA sequence.
Environmental factors include radiation,
smoking, sun etc.
What are common mutations?
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(A) Insertion mutation (when a base gets
inserted into the DNA strand; G added)
(B) Deletion mutation (when a base gets
deleted from the DNA strand; A
removed)
(C) Duplication mutation (when base
sequence is repeated; C,T,C,T)
(D) Inversion mutation (when the
chromosome breaks in two places; the
resulting break is reversed and
reinserted into the chromosome).
(E) Substitution mutation (when one base
pair is switched with another one).
Are mutations random?
§ Two types of randomness: teleogical
and statistically.
§ Genetic mutations are teleological
random, but they may not be statistically
random because some are more likely
to occur than others based on basic
chemistry principles. For example,
thymine-thymine is common. Slips in
DNA sequences in long DNA
sequences make inversion mutations
more common. Mutations are not
purposeful but the underlying
mechanisms of mutation favour some
over the other.
Consider Allele’s:
sickel cell amenia
tuberculosis
§ Mutations are linked to alleles (homo
and hetero pairs) in a homo pair (same)
in hetero (different; one side basically
has a mutation one side doesn’t).
§ Thinking about sickle cell anemia, there
is a homo (most of the plates are sickle
shaped) and a hetero (only some of the
plates are shaped like sickles) type of
sickle cell. Not all mutations are bad. In
its homo form it is a deadly disease
(anemia; heart attacks) but in its hetero
form is resistant to malaria (mix of
sickle and normal cells is resistant to
malaria).
§ Hetero allele frequency overlaps with
malaria incidence over the globe.
§ Tuberculosis resistance through cystic
fibrosis is an example of how hetero
frequency changes in the population
can occur rapidly with high selective
pressures.
Homo form cystic fibrosis is a
rehabilitating disease but in its hetero
form can be resistant to tuberculosis.
White plagues occurred recently in
Britain and killed a lot of people 20% of
the population (recent selective
pressure) caused for genetic mutations
to be adaptive (cystic fibrosis in its
hetero form) to explain why its
incidence rates peaked in Britain at that
time.
Single Nucleotide Polymorphisms
§ A single nucleotide (a single base)
polymorphism is a variation at a single
position in a DNA sequence among
individuals (a single change!).
§ If more than 1% of a population does
not carry the same nucleotide at a
specific position in the DNA sequence,
then this variation can be classified as a
SNP.
§ Copy changes during reproduction I.e., a
common mutation that occurs at a
single place in the DNA sequence and can be
associated with vulnerability or resistance to
diseases (sickle cells and cystic fibrosis).
§ For example, the 5-HTLPR is unevenly
distributed across the globe the
long/short versions are important for
RNA (serotonin transportation) and has
been linked to personality traits like
neuroticism.
§ An example on how small genetic
mutations can manifest in behavioural
variations.
Purging harmful mutations
§ Mutations are random and come from
lots of sources. Most are so small and
ineffective, but they can accumulate
overtime.
§ A benefit of sexual reproduction is that
it can purge harmful mutations via
recombination which trades defective
traits with better fit ones.
§ In Asexual selection harmful mutations
accumulate and are not easily purge
from the gene pool.
§ For example, the minilobster who
reproduces asexually accumulated four
times as many mutations than sexually
reproducing minilobster’s; this explains
why asexual lineages do not last longer
than sexual reproductive strains.
§ Beetles: red line is sexual reproduction
and blue is monogamous sexual
reproduction and then inbreeding the
lines (good method to accumulate
mutations) the strand that had
accumulated variation through sexually
reproduction didn’t die out compared to
the monogamous beetles.
§ Sexual reproduction can be beneficial in
the long run to avoid the effects of
inbreeding and the accumulation of
mutations.
The invisible hand of probability
Genetic Drift:
§ Genetic Drift:
§ Statistical processes detached from
natural selection.
§ The phenomenon that through random
sampling it is possible for genetic
variants to disappear from the
population without selection, which will
then reduce genetic variability overtime
and the genetic fitness of the population.
§ 50/50, 60/40, 80/20, 100/0
§ Genetic drift as the ‘default’ option
Genetic Drift has HUGE effects on small
populations (it’s a problem for small
population which do not have many
variants; even positive adaptive variants
can be lost through genetic drift).
