Download Evolution (the Cliff Notes version)

Evolution (the Cliff Notes version)
Millions of years
Natural selection in 5 easy steps
1. Individuals in a population vary in traits, such
as shape or size
Natural selection in 5 easy steps
2. Some of these trait differences are heritable –
meaning that they can be passed on to offspring
Natural selection in 5 easy steps
3. In each generation, more offspring are
produced than can survive.
• only some individuals will survive long
enough to produce offspring
• some of these individuals will produce
more offspring than others
The struggle for existence
Natural selection in 5 easy steps
4. Individuals whose inherited traits give them a
high probability of surviving and reproducing
in a given environment are likely to leave
more offspring.
Survival of the fittest!!
Natural selection in 5 easy steps
5. The unequal ability of individuals to survive
and reproduce will lead to gradual changes in
a population, with favorable characters
accumulating over generations.
Evolution!
Evolution (the Cliff Notes version)
Millions of years
Important points
• According to Darwin:
– At what level does natural selection occur?
– At what level does evolution occur?
Important points
• According to Darwin:
– At what level does natural selection occur?
– At what level does evolution occur?
– Natural selection acts on individuals
• Individuals vary in their ability to survive and produce
offspring
– Evolution can only be measured as changes in
relative proportions of heritable variation in a
population over many generations.
Natural selection
• Individuals do not evolve!!
• Natural selection DOES NOT involve organisms
“trying” to adapt.
• Natural selection DOES NOT give organisms
what they need to survive.
• Natural selection simply favors traits that
enhance survival and reproduction in a new
environment.
Survival of the fittest
• We often hear the process of natural selection
called “the survival of the fittest”:
– Define fitness
– Based on your knowledge of the process of
evolution by natural selection, why is this
statement an incomplete description?
Fitness
Sparrowhawks
• Of 693 female
offspring hatched,
only 194 survive to
maturity, 499 do not
• Of 194 females
surviving to breeding
age, 33 fail to produce
any offspring
– Thus: 532 of 693 or 85%
females leave no
descendents
• Australian red-bellied black snakes adapt to
the invasion of toxic cane toads
Red-bellied black snake,
Pseudechis porphyriacus
Morphology
• Larger snakes should be better able to withstand toxins
• Hypothesis: Snakes will evolve larger bodies in areas with
toads
• Analysis: Compared mean size of snakes (SVL: snout to vent
length) in areas that have had toads for different lengths of
time (statistically controlled for other variables that might
influence snake size like climate)
Physiology
• Hypothesis: Snake
populations with longterm exposure to toads
will show increased
tolerance of toad’s
cardiotoxin
• Measured swimming
speed of snakes before
and after non-lethal oral
dose of toxin calibrated to
body size (80 µg of toxin
per g body mass).
Behavior
• Hypothesis: Snakes in
populations with longterm avoidance of toads
will show an evolved,
innate avoidance of
toads.
• Methods: Offered cane
toads and non-toxic
frogs to snakes from
toad infected (40-60
yrs) and toad free areas.
Gave 24 h to consume.
For Red-bellied snakes
• How does the environment change for the
population? Is this a biotic or abiotic change?
• What are the different phenotypes or the variation
seen in the population?
• What is the specific selection agent (environmental
factor) acting on the population?
• As a result of this selective pressure, which
phenotypes had the highest fitness? Why?
• What is the result of the selective pressure?
Natural selection terminology
• With the coming of the Industrial Revolution and the
blackening of the environment, there was selection
for darker bodies in moths and other species.
• There is selection on body size in snakes.
• Because white flowers are visited by relatively few
pollinators and hence produce relatively few seeds,
white flowers are selected against.
Sexual Selection
Intrasexual competition: males
fight other males for mates
How should males differ from
females?
Giraffes
• Long neck in giraffes
– attributed to an evolutionary advantage of being able to
reach more food when competing with other browsers.
• May actually be a result of sexual selection.
Sexual Selection
Intersexual competition: females choose mates with specific traits
(phenotypes)
How should males differ from females?
Peafowl
• Do males with showier ornaments
have “better” genes?
– The larger a male’s peafowl’s tail,
the more females he is likely to
mate.
– If the “good genes” hypothesis is
correct, what relationship should
there be between fitness and
“showiness”?
Peafowl
Growth (measured by offspring
weight at 84 days of age)
Conclusion?
Survival (percent of released
offspring that survived for ≥2 years )
• Widowbirds
– Both sexes normally brown
– During breeding season, males molt and produce black
feathers
• Red and yellow wing patches
• Long tail (up to 0.5 m)
•
•
•
•
•
Tail Length Manipulations
Group 1—tails shortened
Group 2—tails uncut
Group 3—tails cut and re-glued onto the male, length unchanged
Group 4—tails elongated by gluing on the length cut from Group 1 males
Number of active nests before treatment
•
•
•
•
•
Tail Length Manipulations
Group 1—tails shortened
Group 2—tails uncut
Group 3—tails cut and re-glued onto the male, length unchanged
Group 4—tails elongated by gluing on the length cut from Group 1 males
Number of active nests AFTER treatment
Sexual selection can generate
different species
Wow, look at that blue fin! Sexy!
No thanks, I prefer mine orange. You can have him!
Let’s back up a bit
• Define evolution…
Let’s back up a bit
• Define evolution…
• Evolution is measured as changes in relative
proportions of heritable variation
(morphology, physiology and/or behavior) in a
population over many generations.
– Population shifts from a predominance of grey
snakes to a predominance of blue snakes.
– Proportion of long-billed hummingbirds in the
population increases
Another definition
• Geneticist’s definition of evolution: a change
in the relative frequencies of different alleles
in a population's gene pool over the course of
generations
– Gene pool: all the alleles that exist in the
population for a particular gene or set of genes.
Population genetics
Why is variation important?
How do you get variation?
• Mutations
– Imperfect copies during mitosis
– Environmental factors like UV and chemicals
How do you get variation?
• Mutation
– Source of new alleles
– Rare and random
– Harmful, beneficial or neutral
• However, since almost all conceivable beneficial
mutations of a population in a stable environment have
already been selected in the recent past, the
occurrence of new beneficial mutations are rare. (Ernst
Mayr, 2001. What Evolution Is).
How do you get variation?
Meiosis
Independent Assortment
Crossing over
Meiosis
Importance of variation
• In a population or species, if every organism
was genetically identical, what might happen?
How do you change the gene pool?
How do you change the gene pool?
• Through natural selection – the individuals
with alleles that increase fitness have more
offspring that carry those alleles, increasing
the proportion of those alleles in the gene
pool.
How do you change the gene pool?
• Through natural selection – the individuals
with alleles that increase fitness have more
offspring that carry those alleles, increasing
the proportion of those alleles in the gene
pool.
• Through sexual selection – the individuals
with alleles that allow them to attract more
mates have more offspring that carry those
alleles, increasing the proportion of those
alleles in the gene pool.
Changes in allele frequencies
• Natural selection
– Differential survival/fecundity of certain genotypes
• Non-random mating
– Many types of non-random mating. One type: sexual selection
• Mutation
– Mutation can cause (slight) changes in allele frequencies by:
• changing one allele to its alternative (e.g. A-->a or a -->A)
• creating new alleles for a gene locus (e.g., A ---> b )
• Gene “flow”
– Alleles entering/leaving population as a result of individuals
immigrating into, or emigrating out, of population.
• Genetic Drift
– More serious in small populations: bottleneck and founder effects
Mechanisms of Evolution
• Gene flow
– Individuals move between populations and bring
new alleles into the area.
Genetic drift
Founder effects
Bottleneck event
Proportion of
the gene loci
that are
polymorphic
Mean no. of gene loci
at which individuals
are heterozygous
Elephant seals
0%
0%
Values for >1000
other mammal
species w/ no
bottleneck event
15-50%
3-4%
Blue crabs – possible bottleneck?
Blue crabs – possible bottleneck?
• If you examined the genetics of blue crabs,
what would you expect to see if the population
had gone through a bottleneck?
Blue crabs – possible bottleneck?
• If you examined the genetics of blue crabs,
what would you expect to see if the population
had gone through a bottleneck?
– Low polymorphism and low heterozygosity
= low genetic diversity and evolutionary potential
Blue crabs – possible bottleneck?
• If you examined the genetics of blue crabs,
what would you expect to see if the population
had gone through a bottleneck?
– Low polymorphism and low heterozygosity
= low genetic diversity and evolutionary potential
– Baltimore crab populations: high haplotype
diversity and high microsatellite heterozygosity
= high diversity
• maintained by not overusing wild individuals in hatchery
Rats!
Coming to America!
Rats in Baltimore
• Rat populations in Baltimore were likely
introduced from Europe. What do you call this
phenomenon? How should this effect diversity
of the rat population in Baltimore?
Rats in Baltimore
• Rats in Baltimore = Founder event
– Low genetic diversity expected in Baltimore
Rats in Baltimore
• What aspects of the rat lifestyle may
counteract low diversity due to isolation and
founder events?
Rats in Baltimore
• Rats breed frequently and have large litters
– more opportunities for variation to enter
population.
• Rats can move long distances = gene flow
What about Baltimore rats?
Baltimore rats
• Most rats stick close to home – Baltimore rat
populations are geographically isolated and
genetically structured.
– The Jones Falls is a big barrier to rats
– Some rats to go much farther than others
– Neighborhood restoration causes large-scale
dispersal
If you were a rat…
• In terms of fitness –
– Why might it be advantageous to stay in the same
area as your relations?
– Why might it be advantageous to head out on
your own?
Speciation
• How do you get new species?
– Prezygotic mechanisms- prevent mating or fertilization
•
•
•
•
•
Mechanical isolation
Gametic isolation
Habitat isolation - Allopatry
Temporal isolation - Sympatry
Behavioral isolation - Sympatry
– Postzygotic mechanisms- prevent development of viable,
fertile young
• Hybrid inviability
• Hybrid sterility
• Hybrid breakdown
Lice/Human Evolution
Chimpanzee lice = Pediculus schaeffi
Head lice
Pediculus humanus capitis
“cooties”
Lice/Human Evolution
Human
Chimp
Gorilla
Orangutan
Gibbon
Old world monkey
30
25
20
15
10
million years ago
5
0
The human and chimpanzee lines diverged
approximately 6 million years ago
Lice/Human Evolution
The human and
chimpanzee lines diverged
approximately 6 million
years ago
Lice literally came
along for the ride.
What type of
speciation is this?
But there’s more…
Head lice
Pediculus humanus capitis
“cooties”
Body lice
Pediculus humanus corporis
Pubic lice
Phthirus pubis
“crabs”
Lice/Human Evolution
• How did we go from one louse species to three??
Phthirus – pubic lice
Closely related to gorilla lice
Pediculus – head and body – closely related to chimp lice
Lice/Human Evolution
• DNA comparison suggests the most recent
common ancestor of human head lice and
human pubic lice lived ~12 million years ago.
• The most recent common ancestor of gorillas
and humans lived ~ 12 million years ago
• Humans lose body hair 3-4 million years ago
• Head lice trapped in heads.
• DNA comparisons show that human pubic lice
and gorilla lice last shared a common ancestor
3-4 million years ago.
Lice/Human Evolution
• Conclusion:
– Common ancestor of humans, chimps and gorillas
had both Pediculus and Phthirus lice.
– When the human/chimp line split from gorillas,
Pediculus went extinct in gorillas and Phthirus
went extinct in the chimp/human ancestor.
– Early humans “caught” pubic lice from gorillas 3-4
million years ago.
• Sleeping in nests recently used by early gorillas.
• Preying on early gorillas.
– Lice are known to jump from prey to predator.
Lice/Human Evolution
• A DNA comparison suggests the most
recent common ancestor of human body
lice lived less than 72,000 years ago.
• Humans starting wearing tight fitting
clothes ~ 107,000 years ago.
• Very hotly contested (chicken and egg)
• Uncertain as to different species
– (intro to species concepts)
Cool websites
• http://www.pbs.org/wgbh/evolution/educato
rs/index.html
• http://evolution.berkeley.edu/
• http://www.sciencecases.org/three_lice/note
s.asp