2/19/2009
What is a gene?
• George Williams:
Notes on Evolution
– any portion of chromosomal material which
potentially lasts for enough generations to
serve as a unit of natural selection.
Differential reproduction
Altering the direction of selection…
• Of genetic variants is the principal driving
force of natural selection
• Alters the direction of change in organisms
– Such as shifting from global warming to global cooling
The causes of selection
The Causes of selection
• And the causes of mutation are
independent…
– When
Wh we introduce
i t d
M
McClintock’s
Cli t k’ work
k we
may find that the assumption of independence
may be partially incorrect
• Tend to remain consistently directional for
long periods of time (many generations).
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The Causes of selection
Mutations are caused by
• Darwin’s hostile forces of nature:
– Climate
– Weather
– Food shortages
g
– Predators
– Parasites
– Disease
– Competition for resources
– Competition for mates
Not all mutations
• Are equally likely to occur
• Mutations are random in respect to the direction
of selection
• The oldest genes would be expected to have the
lowest mutation rate. The rate of mutation must
be selected downward, or the gene would not
survive in the gene pool.
Heterozygote
• Humans have 23 pairs of chromosomes.
– For any gene on these chromosomes there
may be two different forms, or alleles (one
inherited from each parent) each designed to
produce
d
slightly
li htl diff
differentt enzymes.
– The instructions to produce different enzymes
pose a biological contradiction…should the
pigment in the iris be adjusted to build blue
eyes or brown?
•
•
•
•
Biohazards
Atmospheric
p
radiation
Chemicals
Disease vectors???
How do you explain genetic
variation in natural populations?
• New mutations
– New mutations are likely recessive, and a
gene can only be selected against when it is
p
p
phenotypically.
yp
y Hence,, some
expressed
recessive allele is unlikely to be reduced
below some base level.
• Selection varies across the range in
habitat of a species (latitude & body size)
• Heterozygote advantage
Heterozygote advantage
• Sometimes the heterozygote condition
yields the most fit phenotype.
• Sickle
Si kl cellll anemia:
i
– (Hn) allele codes for normal hemoglobin
– (Hs) allele codes for hemoglobin that is less
effective at transporting and releasing oxygen
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Sickle cell anemia:
• Heterozygous parents may produce the
following 4 types of offspring:
– Male (Hn-Hs)
Female (Hn-Hs)
• 1. (Hn-Hn)
Sickle cell anemia
– 2. (Hs-Hs)
• Anemic individual with poor oxygen transport,
frequently
q
y dies before maturity
y in ancestral
populations
– Normal hemoglobin, but readily parasitized by malaria
Sickle cell anemia
– 3. (Hn-Hs)
– 4. (Hs-Hn)
Heterozygote Advantage
• If the heterozygote phenotype is superior,
then selection will favor the preservation of
both alleles, and neither gene will be
supplanted by the other
other.
• Heterozygote individuals suffer from moderate
anemia, but are, in turn, more resistant to
parasitism by malaria
Why Reproduce Sexually?
• Gonochoristic organisms are biparental
and reproduce sexually.
– Sexual reproduction is the act of recombining
genetic materials in the same species yielding
novel arrays of genotypes in one’s offspring.
Why Reproduce Sexually?
• Parthenogenetic organisms are diploid
species where all members of the species
are female. The whiptailed lizard (center)
reproduces exclusively via
parthenogenenis.
• Not all species are biparental.
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Amazon Molly
Amazon Molly (top)
• The molly reproduces via gynogenesis. This
means that although females must mate with a
male, genetic material from the male is not
incorporated into the already diploid egg cells
that the mother is carrying (except in
extraordinary circumstances), resulting in
identical clones of the mother being produced en
mass. This unusual characteristic has led to the
Amazon molly becoming an all female species.
Parthenogenesis
• Both the Molly and whiptailed lizard
descended from gonochoristic ancestors
ancestors,
and evolved into an all female species
producing daughters which are a clone of
their mother. These species have rid
themselves of the need to reproduce
sexually.
HERMAPHRODITISM
• Some species produce both male and
female gametes within the same
individual.
– Many plants
– Earth worms
– Some insects
– Some reef fish
• Usually sequential
Found in some mammals
• Spontaneous parthenogenesis and
development of camel (Camelus
dromedarius) oocytes.
• Teratocarcinogenesis and spontaneous
parthenogenesis in mice.
HERMAPHRODITISM
• Hermaphrodites are divided into two main
categories: synchronous hermaphrodites,
and sequential hermaphrodites. In the
synchronous hermaphrodites, organisms
possess both active male and active
female reproductive organs at the same
time. In sequential hermaphrodites, both
male and female reproductive organs may
be present, but only one is active and
viable at any given time.
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HERMAPHRODITISM
• A few Serranids (sea basses, e.g. Serranus sp.)
and Hamlets are known synchronous
hermaphrodites. During mating, one individual
will lay eggs while another fertilizes the eggs,
after which both will reverse roles and perform
fertilization again. Synchronous hermaphrodites
do not fertilize themselves; Self-fertilization does
not promote genetic diversity, and can amplify
genetic defects from parent to offspring. The
interesting fact is most synchronous
hermaphrodites form monogamous pairs.
Black Sea Bass
Family Serranidae (sea basses)
• All sea basses and groupers belong to one
family (Serranidae). Many species, perhaps
most, are hermaphroditic (individuals may be
both sexes). Most species begin life as females
and eventually become males but the belted
sandfish (Serranus subligarius) is functionally
both male and female at the same time.
Heterogonic Species
• Some species may change back and forth
between sexual and asexual modes of
reproduction. Aphids are heterogonic.
Reproduction
Costs of Sexual Reproduction
• 1. Cost of recombination.
• There must be some costs and
advantages to reproduce in each of these
different ways.
– A very successful genotype (such as your
own) lasts for only 1 generation, and unless
you can be cloned, your very favorable
combination of genes will be loss forever
upon the day of your death.
• E.g. Heterozygote advantage
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Costs of Sexual Reproduction
Costs of Sexual Reproduction
• 2. Cost of meiosis:
– 50% reduction in the parent’s genes in their
offspring.
– The cost of omitting genetic material from a
zygote destined to require costly parental
investment, and replace it with genetic
material from a partner who may not invest in
this zygote.
These costs must be offset by
some advantages
• Heterogonic species may provide a clue:
– Invade a rich unoccupied habitat, then
reproduce asexually producing genetically
p g y
identical progeny.
– The habitat is nearly exhausted, and soon you
must disperse and seek a new beginning,
then reproduce sexually.
– Sexual reproduction is selected in the face
of uncertainty.
Bipartental Species
• Descended from a lineage where the absence of
predictability in the hostile forces of nature
favored a strategy where the genotype was
scrambled each generation.
• Hence,
Hence your competitors
competitors, parasites and
predators are less able to key in on the
exploitation of your progeny.
• The obstacles your progeny must face will be
unlike those ever encountered by you or your
ancestors!
• 3. Cost of mating:
– The bother and cost associated with finding
and securing a suitable mate.
– Hence, a portion of your parental or
reproductive effort is diverted into all the
business associated with mating, and this
may waste time and resources that could be
better spent in other pursuits.
Sexual Reproduction
– Your genotype worked thus far, but it may be
inadequate in the future.
– Don’t put all your eggs in one basket!
– Introduce diversity in your progeny.
– The optimal genotype cannot be predicted
with certainty.
– The only thing that is certain is change itself.
Predictions
• Selection for heightened diversity in
offspring would favor random or
promiscuous mating systems.
• Multiple paternity amplifies diversity within
a litter.
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Predictions
• Selection for moderate diversity in
offspring would favor monogamy and
inbreeding.
Predictions
• The extent of inbreeding and out-breeding
may relate to unpredictability in ancestral
environments.
• Identical offspring (armadillo).
Predictions
• Generation span may relate to
environmental predictability and rate of
change in the habitat.
Species
• What is a species??
– Members of a population that interbreed freely
and produce viable offspring.
– Short-life
Short life span organisms may be adopted to
perpetuate genes in highly unpredictable
(temporally disrupted) environments.
– Long-life span organisms may be specialized
to perpetuate genes in more predictable
environments.
– What about parthenogenesis?
– Recent evidence of spontaneous
parthenogenesis (some fish, birds, and
camels?).
Species
• Concept of a species suggests that there
are advantages in mating with organisms
similar to you.
• Sex evolved to amplify diversity in
offspring, but the species phenomenon
evolved to set limits upon that variability.
• Reproductive isolating mechanisms
– dialects
Geologic Time Scale
•
•
•
•
•
•
•
Earth: 4.5 billion years old.
Life: 4 billion years.
Vertebrates: 500 million
Mammals: 180 million
Man: 3 million
Fire: 500,000 years ?
Writing: 5,000 years
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Geologic Time Scale
• 12 hour clock:
– 2:40 AM life began
– 8:48 PM Cambrian explosion
– 9:20 PM vertebrates arise
– 11:02 PM mammals arise
– 11:59:02 PM man arise
– Last 10 seconds – fire
– Last 100 msec – writing
– Last nanosec – cell phones!
Geologic Time Scale
• Most of the history of life was dominated
by blue-green algae
• Then sexual reproduction arose as an out
come of the Cambrian Epoch
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