Transcript Structure of mating systems

Sex and Evolution
(Chapter 11)
You’ve seen, in the last lecture, how sexual selection can
lead to sexual dimorphism.
The example that begins the text chapter identifies an example
where genetics plays an important role in the dimorphism, as
well as being a major factor in the sex ratio and mating system
of a fly species, Cyrtodiopis whitei.
This species has stalked eyes; males have varying stalk
lengths. The species also has a biased sex ratio. Only around
1/3 of a population are males.
With fewer males to ‘service’ the females, any female who
could produce more male children would gain fitness. How
does a female select a male who will father more sons?
It turns out that male eye stalk length is an
indicator. Males with shorter eye stalks produce
mostly X bearing sperm, and father daughters,
due to defective sperm production.
Males with longer eye stalks produce normal
sperm ratios, i.e. 50% Y bearing and 50% X
bearing. They, therefore, father more sons.
In sexual selection within this species, females
choose males with longer eye stalks.
All this (sexual dimorphism, sexual selection,
etc.) assumes that reproduction is occurring
sexually.
However, asexual reproduction occurs widely in nature.
There are many ways to achieve asexual reproduction, and
they have differing genetic consequences.
1) It may occur by budding (as in corals) or by vegetative
reproduction (e.g. in plants, by runners (strawberries) or
horizontal rhizomes (in many goldenrods). Here every
offspring is genetically identical to its parent. The offspring
form a clone.
2) It may occur with partial meiosis. The first meiotic
division, with crossing over and recombination, produces
genetic variation. If there is no second division, the
resulting cells are diploid and can develop into mature
adults.
3) There can be full meiosis, with fusion of a pair of gametes
to restore the diploid number with genetic variation from
both meiosis and random gametes fusing. This is one form
of self-fertilization. This is asexuality in which only sex
(the mating of different individuals) is lacking.
Now let’s go back and think about sex. Fitness, in the
evolutionary sense, is measured by the number of copies of an
individual’s genes, relative to others parents, in the offspring
generation. What happens in sexual reproduction?
Meiosis and fertilization mean that each offspring carries ½
the genes of each parent.
In asexual reproduction (whichever form) each offspring
carries the complete genome of its parent. That is twice as
much of a contribution to parental fitness.
That difference in genetic contribution between sexual and
asexual reproduction is called the cost of meiosis.
There have been numerous attempts to study species that can
reproduce both asexually and sexually. The object has been to
show that females reproducing sexually have twice as many
surviving offspring (or more), and (whatever advantages sex
offers) they can make up for the cost of meiosis.
It has been a failure. Sexual reproduction produces more
offspring, but not twice as many.
So, why does sex exist and persist?
There is a single answer: variation among offspring.
This figure looks across generations
(over time). Recombination and
crossing over generate variation
among offspring of a single set of
parents. Both within and across
generation variation are important.
If the environment did not differ in time or space, then the
advantage of asexuality would predominate, and sex would
be rare among species.
The asexual parent was successful (grew, survived,
reproduced).
The adaptations that that the parent had would be equally
advantageous for the offspring if the environment remained
the same.
However, the environment varies over both time and space.
Can a parent predict the environmental conditions its
offspring will encounter?
So, through sexual reproduction (recombination, crossing
over) a variety of genotypes (and thus phenotypes) of
offspring are produced. At least some should achieve success.
There are two theoretical explanatory constructs:
1. An adaptation of the myth of Sisyphus. Sisyphus was
doomed to roll a large boulder uphill. He could never reach
the top. The boulder would roll back to the bottom, and
he’d have to try again. In evolutionary terms, since the
environment is constantly changing, selection can never
achieve a ‘perfect’ genotype, and only through sex can
evolution ‘keep trying’.
2. The Red Queen hypothesis. Quoting Lewis Carroll:
“Here, you see, it takes all the running you can do, to keep
in the same place.” Species, to succeed, need to evolve
relatively rapidly to succeed in the biological realm, since
those they interact with are evolving new defenses, attack
strategies, or whatever is important to their success.
If sex is the means to reproduce, what’s the best way to do it?
Think about all the problems of finding mates and achieving
a successful mating. Wouldn’t it be easier and more
successful if male and female functions could be combined
in a single individual? A number of species do things that way.
They are hermaphroditic.
Some are simultaneously male and female (typical in snails
and worms and many plants), and some are sequential – first
one sex, then the other – (plants and some fish).
Sometimes it is years before sex change occurs. In many
maples, they are male first (it’s less energetically demanding
to be male), then, when they’ve grown larger and stronger they
switch to being female. Under harsh, energetically demanding
conditions, they may switch back.
However, having both sexes can limit the energy committed
to and success of one sex or the other.
Evolution will inevitably select the sex strategy that
maximizes the sum of success from being male and being
female. When having both sexes produces a greater total
success than being only one sex, hermaphroditism is
advantageous.
If, on the other hand, if each sex interferes with the success
of the other one, the sum of successes will be less than could
be achieved by having individuals function as only one sex.
Hermaphroditism is selected against.
We can only surmise what factors in life history or
environment lead to different strategies, but there are many
examples of dioecious plants (separate sexes), suggesting that
there is some interference in fitness contributions between the
sexes…
Marijuana is dioecious, and female plants are more valuable;
grown in the absence of males, flowers of the plant are called
sinsemilla (meaning without seeds).
Here’s another example, with pictures of both female and
male plants. It’s a cycad, a relatively primitive gymnosperm.
Female
Male
Many plants have both sexes separately on the same
individual. They are monoecious; the sexes are separate (in
different flowers) on the same plant body. Here’s one
example, an oak:
In some cases male and female flowers appear simultaneously.
In others (like the arctic dwarf birch I study), flowers appear
sequentially. Sequencing flowers tend to prevent inbreeding.
Finally, some plants produce ‘perfect’ flowers, in which both
male and female parts occur together. Even in these flowers
the male and female parts may function simultaneously or
sequentially.
To ensure outcrossing, many plants carry self-infertility genes
that prevent inbreeding. These loci must be heterozygous (the
alleles at these loci coming from different individuals) for
embryos to survive.
Sex Ratios
When the sexes are separate, there is a ratio of the number of
males to the number of females. We typically think of that
ratio as at least approximately 1:1, but that isn’t always the
case.
When the ratio isn’t 1:1, there is a selective advantage to the
minority sex. As a result, the ratio tends to be driven back
towards 1:1.
This result is called frequency dependent selection.
As a result of differences in the survivorships of the two sexes,
the sex ratio may also change with age in a population.
Think of the human population. There is a primary sex ratio.
It is the number of male and female embryos conceived in a
population. The secondary sex ratio is the ratio of the number
of male and female babies born.
We can assume that the primary sex ratio in humans is 1:1. But
we’d be wrong. The primary sex ratio is somewhere between
1.079 (1948 Carnagie Institute data) and 1.2-1.6:1. Why? One
favorite explanation: Y-bearing sperm are lighter (Y is smaller
than X) and more motile.
The secondary sex ratio is 1.06:1; 106 male babies for every
100 female babies in the U.S. and Canada. Even so, there is
higher in-utero mortality in males than females.
As you’ll see in the demography lectures, mortality through
maturation (and even later in life) is also higher in males than
in females.
The tertiary sex ratio, the ratio at the time of reproduction, is
generally slightly female-biased.
By middle age and beyond the ratio is even more female
biased – among older adults there are more unmated females
than males.