1. S E X Why is sexual reproduction the rule
for nearly all living species?
Why is it so common,
and why did it evolve in the first place?

2. Reproduction: cloning vs. sex
Every organism must reproduce to pass on its genes
Reproduction can be asexual, if cells or organisms
clone themselves (mitosis)
- does not require a mate
- produces a perfect copy of the parent
Sexual reproduction is the production of male and
female gametes (meiosis)
- offspring are a genetic mix of two parents

3. Mitosis: cell duplication
In mitosis, a diploid cell replicates each
chromosome, then divides into two cells
Both daughter cells get identical copies
of every chromosome
By repeated divisions, one cell of an
organism could grow into a whole
copy  asexual reproduction

4. Asexual reproduction: attack of the clones
In asexual reproduction, an
organism creates an identical twin
copy of itself
A well-adapted organism can
reproduce itself exactly as-is
Avoids scrambling of genes,
as occurs with sex

5. Asexual
4 granddaughters per female

6. Asexual
Sexual
4 granddaughters per female
2 granddaughters per female

7. Population growth advantage
Asexual
Sexual
n = 2
n = 2
n = 8
n = 4
2-fold faster rate of population growth when you don’t
waste energy making “useless” males

8. Benefits of asexuality
 Well-adapted organisms can clone themselves, so
their good combinations of genes can stay together
 2-fold advantage in reproduction when you don’t
have to produce males
- males consume resources, but cannot make offspring
- seem like a waste of space, evolutionarily speaking
...So why do almost all organisms reproduce sexually?

9. Sex produces genetic diversity
A clone of a successful parent would thrive if the clone
faced identical challenges from the environment
- same predators, diseases, weather, etc.
However, conditions change from season to season,
meaning children may need different traits to survive
Alleles can be viewed as potential solutions to potential
problems that may face an organism
- sex “shuffles the deck” of alleles, and deals a new
hand for each generation

10. Sex: diversity by meiosis
Special mechanisms create genetic diversity, making
new combinations of alleles not present in either parent
In meiosis, a diploid cell
(2 copies of each chromosome)
divides twice
- produces 4 gametes
(sperm or eggs)
Gametes are haploid
(contain 1 copy of each
chromosome)

11. Sources of diversity #1: independent assortment
Homologous
chromosomes
Each gamete carries a
different combination
of parental chromosomes
One human egg may have
4 chromosomes from
grandma and 19 from
grandpa...
Crossing over
Recombination

12. Sources of diversity #2:
recombination
From mom
From dad
Homologous
chromosomes
Crossing over
Recombination
Crossing over moves some alleles from one chromosome
onto another, creating a new combination of alleles

13. Sex occurs in nearly all multi-cellular organisms
 Very few species only reproduce asexually
- a few micro-organisms (rotifers)
- parthenogenesis in some fish, amphibians
- some, like aphids, start as asexuals & then switch
 Fossil record shows most asexual species go extinct
 Recombination must therefore confer an advantage
on sexually reproducing species, allowing them to
out-compete asexual species

14. Cost of asexual reproduction #1:
Muller’s ratchet
In an asexual population, once a
bad mutation occurs, it is passed
on to all offspring of that line
Stumpy-arms mutation

15. Cost of asexual reproduction #1:
Muller’s ratchet
In an asexual population, once a
bad mutation occurs, it is passed
on to all offspring of that line
Stumpy-arms mutation
Square tips mutation

16. Cost of asexual reproduction #1:
Muller’s ratchet
In an asexual population, once a
bad mutation occurs, it is passed
on to all offspring of that line
Stumpy-arms mutation
Square tips mutation
 mutant alleles accumulate
every generation

17. “Turning the ratchet” Sometimes, by chance, all the
individuals with no mutations will
die off

18. “Turning the ratchet” Sometimes, by chance, all the
individuals with no mutations will
die off

19. “Turning the ratchet” Sometimes, by chance, all the
individuals with no mutations will
die off
All remaining adults will now pass
on the mutant alleles
Basically, things can only
get worse over time

20. Cost of asexual reproduction #2:
Background trapping
Consider our mutant starfish carrying 2 bad alleles:
Stumpy
arms
Square
tips
normal
body

21. Cost of asexual reproduction #2:
Background trapping
Consider our mutant starfish carrying 2 bad alleles:
Stumpy
arms
Square
tips
Big strong
body
Suppose a helpful mutation occurs...
This new, beneficial allele is trapped on a chromosome
with bad alleles, and can never escape from them

22. Selfing: 2 ways to get AA offspring once
you’ve inherited one A allele
AA parent A A A A
egg
sperm
egg
sperm AA AA
Way #1: get 2nd copy
of same A allele
Way #2: get copy of
other A allele
….but what’s the difference? You end up AA either way

23. Alleles have a chromosomal contextb B A A
Alleles are physically linked to neighboring alleles
- linkage alters their pattern of inheritance
- in other words, alleles aren’t inherited in a vacuum;
they usually come with their neighbors
In selfing, calculating F = identifying offspring that inherit
copies of the same allele from same ancestral chromosome

24. Offspring inherits 2 copies different chromosomes
Alleles have a chromosomal context
AA parent A A A A
egg
sperm
egg
sperm AA AA
Offspring inherits 2 copies
of same chromosome
Offspring inherits 2
different chromosomes
This is why selfing gives a coefficient of inbreeding F = 0.5

25. Asexual reproduction: pros and cons
Advantages: Disadvantages:
Well-adapted individuals (1) Muller’s ratchet:
can clone their good allele bad mutations keep piling
combinations up in every generation
(2) 2-fold advantage in (2) Background trapping:
reproduction when you don’t new beneficial mutations
have to waste effort making get stuck on loser
males chromosomes riddled with
maladaptive alleles