Transcript Lecture 9

Outline
 Introduction
 Evolution
to Ecology
and Natural Selection
 Physiological
 Behavioural
Ecology
Ecology
Behavioural Ecology
Behavioural Ecology
 The
study of ecological and
evolutionary processes that explain the
occurrence and adaptive function of
behaviour
 Examples of potential questions:
– Why do birds migrate?
– Why do grazing animals condense into
herds?
Behaviour

Affects an individual’s ability to survive and
reproduce in a particular environment

Develops under the influence of both genetic
inheritance and environmental experience
(learning)
– the genetic component of behaviour is subject to
natural selection
Behaviour to maintain internal
conditions
Behaviour to get food or prevent
becoming food
Behaviour to reproduce
Plants manipulate behaviour
Outline
 Reproduction:
 Life
Why have sex?
histories and mate choice
 Predation:
– Optimal foraging
– Applications to fisheries management
Outline
 Reproduction:
Why have sex?
Life histories and mate choice
Predation:
– Optimal foraging
– Applications to fisheries management
Why have sex?
Ch. 7.1-7.2, Bush
Outline
 The
basics of sex
 The
evolution of sex
 Variations
in sexual systems
Outline
 The
basics of sex
 The
evolution of sex
 Variations
in sexual systems
Reproduction

The goal of reproduction, for any organism, is to
ensure the survival of its genetic lineage
Two ways to do this:

ASEXUAL: offspring are exact (almost) genetic
copies of a single parent

SEXUAL: chromosomes of two parents are
segregated and recombined so that no two offspring
are identical to each other or to either parent
Most organisms are sexual

Of the 1.8 million
known species only
2000 of them are
totally asexual
Asexuality is concentrated among
the basal organisms
Asexual reproduction

The cell divides to
produce two
daughter cells

This type of
reproduction can be
very rapid; several
generations can be
produced each hour
Sex = meiosis

Meiosis is the process
whereby gametes are
made with half the
number of
chromosomes

The original number of
chromosome is
reformed when two
gametes come together
Meiosis versus mitosis
Outline
 The
basics of sex
 The
evolution of sex
 Variations
in sexual systems
Why did sex evolve?

Life originated without sex
(as best we can tell) so
sexual reproduction is
something that had to evolve

There are a large number of
disadvantages to sexual
reproduction which makes
the evolution of sex a
conundrum
Sex is not necessary for all life
 Some
plants and animals have entirely
abandoned sex
Others have sex only when its
convenient and are asexual most of the
time (facultatively sexual)
Sex in the news…
Ancient asexuals: Bdelloid rotifers

bdelloid rotifers date
back ~100 million
years

Despite bdelloids'
asexuality, they've
diversified into 380
species
Facultative sexuality in animals

In some animals, such as
Hydra, asexual reproduction
can occur through budding

These animals are still
capable of reproducing
sexually as well

Sexual and asexual
processes are governed by
environmental conditions
Parthenogenesis – offspring from
unfertilized eggs
Cnemidophorus velox, a parthenogenic lizard
Aphids – asexual and sexual

Females give birth to
live females during the
summer months

As winter approaches,
both males and females
are produced, which
mate to produce eggs
The Cost of Sex
 The
cost of males
 The
cost of recombination
 The
cost of mating
The Cost of Males
Passing on genes is like tossing coins

Two copies exist for each
gene

Whether you pass on a
certain copy of a gene is an
independent event for each
child

If you have two children,
sometimes you will pass on
the same copy to both
children (leaving the second
copy passed on to neither
child)
Fitness
FITNESS:
– the number of offspring an individual
produces that survive to reproduce
themselves
 Fitness
= 1.0 means that individuals of
this phenotype are successfully passing
on 100% of their genes, on average
How is fitness calculated

Fitness = the number of genes passed on to the
next generation

Because diploid organisms (I.e., most organisms)
only pass on half of their genes to each child, they
must have two offspring living to reproductive age to
have Fitness = 1

Fitness = 1 does not exactly mean that you have
passed on 100% of your genes to the next
generation (Remember: sometimes you send two
copies of the same gene and zero copies of the
other)
Cost of recombination
Asexual
Sexual
F
F
F
Fitness
of females
2
F
F
F M
1
M
The Cost of Mating

Cost of sexual mechanisms
– Chemical attractants
– Sexual organs
– Flowers

Cost of mating behaviour
– Courtship is costly
– Potential exposure to predators
Injury
 Disease Transmission

Sexual Mechanisms
Mating Behaviour
Injury to females - unintentional

When males are much bigger than females,
the females can be injured by intercourse
Injury to females - intentional!
Callosobruchus maculatus
Male genitalia
Why hurt the female?

Reducing the fitness of your mate ought to reduce the fitness of
yourself as well

Copulation is not always a cooperative venture between the sexes.

In C. maculatus, females mate repeatedly

genital wounding could increase the fitness of male C. maculatus if:
– it causes females to postpone remating (less sperm competition)
– increase immediate oviposition (egg-laying) rates because females
perceive damage as a threat to survival and invest more in current
reproduction
Costs of mating are widespread

Female Drosophila melanogaster that mate more
often die more often

seminal fluid increases female death rate

Fluid is also responsible:
– in elevating the rate of female egg in elevating the rate of
female egg-laying,
– in reducing female receptivity to further matings
– in removing or destroying sperm of previous mates
Birds, bees, and STD’s

Most organisms are plagued
by a few sexuallytransmitted diseases

E.g., earwigs, frogs, koalas,
or humans

Ustilago violacea (smut
fungus) infects flowers of
Silene alba and is
transferred via pollinators
Sexuality must have its advantages
 Hardly
any asexual lineages seem old,
and fossil evidence has suggested that
asexuality is a dead end
 The
prevalence of sexuality amongst
species is caused not because asexual
species don't evolve, but because they
don't last
Sex increases variation
Sex increases variation

Genes from maternal
and paternal parent get
“shuffled up” when
gametes are made

Causes some gametes
to have “superfit”
genotypes and others
to have “superunfit”
genotypes
Sex leads to more variation in offspring
Sex and speed of evolution
 More
variation leads to natural selection
operating faster
 Most
selection, however, is stabilizing
selection, as individuals are welladapted for a given environment and try
to stay that way
Sex and speed of evolution

What aspect of the environment is so variable that
the production of variable offspring could offset the
cost of sex? - Parasites and pathogens

Hosts are constantly evolving to protect themselves
from parasites and parasites are constantly evolving
to overcome their host’s defenses

Parasites and hosts are locked in a host-parasite
arms race
Red Queen Hypothesis
"Well in our country," said Alice,
still panting a little. "you'd
generally get to somewhere
else-if you ran very fast for a
longtime as we’ve been
doing.”
"A slow sort of county!" said the
Queen. "Now, here, you see,
it takes all the running you
can do to keep in the same
place."
Evidence for Red Queen Hypothesis

In top minnows, sexual
and asexual lineages
coexist

Sexual lineages are the
least susceptible to
parasites

Genetic variation
needed to keep up with
evolution of parasites
Muller’s Ratchet

Vast majority of mutations
are detrimental

Mutation acquisition is a
one-way process in the
genomes of asexuals

In Salmonella typhimurium
444 lineages started from a
single colony

After 1700 generations, 1%
of lineages showed
decrease in fitness (growth
rate) but no lineages showed
increased fitness
Outline
 The
basics of sex
 The
evolution of sex
 Variations
in sexual systems
Variations in the sexual theme
 Are
 Do
 Do
there always two separate sexes?
females always have the offspring?
females control who fathers their
offspring?
Sexual systems
 Depends
on the sexual system of the
organism:
– Hermaphroditic
– Dioecious (Latin for “two houses”)
Flowering plants
Wide diversity of
sexual systems
ranging from strict
hermaphroditism to
dioecy
 Hermaphroditism is
the most common
(~90% of all
flowering plants)

Hermaphroditic animals
Snail copulation

Copulation involves a two- to
six-hour marathon that is
actually an exchange of
sperm between two
individuals, combined with
plenty of rubbing, biting and
"eye-stalk" waving

shoot centimetre-long darts
out of their bodies and into
the genital area of the other
(which happens to be just
behind the head on the right
side).
Helix aspersa
Why only two sexes at most?

If we describe the individuals that have the
offspring as females, then the other sex is
male

If we introduce another sex that also does not
have offspring (i.e., males) then we increase
the cost of males

a higher cost of males would be maladaptive
Seahorse anatomy
Mating and Males Giving Birth
Sperm storage in female insects
Many female insects have the ability to store sperm from
many males, only choosing the best to fertilize her eggs
when the reproductive season is over
Male Drosophila flies
Male Drosophila bifurca

Drosophila flies
have sperm cells
that are up to 6
centimetres long!

Their testes take up
11% of their body
mass
Summary

Considering the short-term advantages of asexuality,
it is not entirely clear why so many organisms are
sexual

Although the exact reason why sex is advantageous
has not been determined, the increase in variation
that sex brings is thought to play a large part

A wide variety of variations in sexuality have evolved
in terms of the separation of sexes, the placement of
parental care, and the timing of sexuality