Lecture 14 &15: Evolutionary Forces and Sexual Selection
Sex ≠ Reproduction
Everything doesn’t reproduce sexually, some reproduce ASEXUALLY
First living things reproduced asexually
Bacteria (picture) is reproducing (binary fission) but it is NOT engaging in sexual
o The duplicated bacterium will be genetically identical to the parent
Plants are simultaneously engaging in sexual reproduction (flowers are transfer
pollen) and asexually (sending out vegetative runners, which sprout in the
ground and grow into a clone of the parent)
Some fish, such as the Guppy, reproduce parthenogenetically. A form of asexual
reproduction in females where the growth of embryos occur without fertilization
by a male.
Many things reproduce asexually, we can have reproduction without sex (sexual
Sexual Organisms can be Dioecious
Organisms can be sexual in many ways
In humans, and many other vertebrates, are called dioecious.
o Dioecious: Individual is male OR female
…. Or Monoecious
Simultaneously Monoecious: Have both functional eggs and sperm
(hermaphrodite). They can mate with each other.
Sequentially Monoecious: Can start off with one gender and can change into
another later in life. I.e. Born as female, by certain age or body size, can develop
into a male (clownfish).
Sex-advantage Model of Sex Change
In some species, Males have relatively little to gain by body size in order to gain
The relationship is MORE IMPORTANT for FEMALES. They have more to gain
from body size, since a bigger female can lay MORE eggs, where as males can
produce sperm with little cost.
In the clownfish example, the fish would be born as males first, and after
reaching a certain body size or age, the functions of males and females cross (Protandry: male reproductive organs come to maturity), they should change
genders, and start reproducing as females
Sometimes this can be the opposite. Females can have little to gain form body
size and males have more to gain. They are born as females and at female
maturity (Protogyny), they should change genders and start reproducing as
Why reproduce Sexually?
Well, many reasons NOT to
o Finding, attracting, competing for mates takes a LONG time
o Increased chances of acquiring Sexually Transmitted Diseases (STDs)
o Cost of meiosis Transmitting ONLY half of genome, while asexual
reproduction results in transmitting ALL of genomes (clone)
o Cost of males Females are the ones ONLY capable of giving birth. Only
females ultimately produce new offspring.
In diagram below, 2 females. One reproduce sexually, the other
asexually, both producing 4 offspring each. Asexual lineage takes
over in the number of offspring. Sexual lineage has to invest time
into finding a male.
Asexual (clonal) females should quickly outcompete sexual
females, unless sex improves offspring quality
So why do so many species do it?
Multi-cellular organisms majority reproduce sexually, require copulation
Asexual reproduction still requires courtship or copulation (new from evolution)
Sex New combinations of alleles Sex generates new combinations of alleles, genetically different from one
another, due to recombination among non-homologous chromatids,
independent segregation of chromosomes, into the gametes, ne combination of
Mutational explanations for sex
Over time, asexual populations accumulate harmful mutations
If you are in an asexual population, and reproduced clonally, over time, the
average number of mutations is going to increase
Sex continually recreates genotypes with fewer (or more) harmful mutations
Ruby in the Rubbish Hypothesis
Ecological Explanations for Sex
The Lottery Principle: Sex in Constant versus Changeable Environments
o In a constant environment, any female at reproductive age is probably
very adapted to the environment, and can clone herself, and clone will be
adapted to same conditions
o In an environment that is changing, a female would do better to produce
offspring that are genetically diverse (Sexually), offspring will be adapted
to the unpredictable environmental conditions.
Parasites place Negative Frequency-Dependant selection on host genotypes
o Parasites tend to infect the most common genotypes in a population,
since it’s the most ‘profitable’ prey.
Sex (producing diversity of offspring genotypes) favoured in parasite-rich
environments (Red Queen Hypothesis)
o You have to continue to