Download What is a population?

Evolution of Populations:
The Origin of Species
How do species change and new
species come about?
Getting the terms right:
What is a population?
What is a species?
What is a gene pool?
• Population - same species, same place, same time
• Species - members of the same population that can
interbreed and produce fertile offspring..
Reproductive compatibility is key.
• Gene pool - the collection of all of the genes within a
population.
• Populations evolve. Individuals do not!
Natural selection works on individuals,
but populations/species evolve
•
•
•
•
Geospiza fortis (Galapagos
ground finch of Daphne
Major)
In 1977, 180 of 1200
individuals survived a
drought.
Larger harder-shelled seeds
survived the drought and so
did the bird with the deepest
and and larger beaks.
Evolution occurred - allele
frequencies change in a
population
Figure 23.1
How do Populations and
Species change?
2 Primary Sources of Genetic
Variation
• Mutations – creates variation
• Sex - shuffles the deck and spreads
mutations around…..
Three basic mechanisms of
evolutionary change
• 1. Natural Selection
• 2. Genetic Drift
• 3. Migration
(Gene Flow or Gene
Migration)
Evolution in Genetic Terms
In genetic terms,
evolution is any change in the relative
frequency of alleles in a population.
This can be measured.
+++++
As in albino otters …..
“Fitness”
Darwin called the ability of an
organism to survive and
reproduce in its specific
environment “fitness.”
Sexual Selection – the other half
of “fitness” is producing offspring!
• Darwin was stumped by some characteristics until he
understood the big role of sexual selection in natural
selection
• “The sight of a feather in a peacock’s tail…makes me
sick!”
• Handicap…
Reproductive Strategies
based on level of investment
• In vertebrates….
• Female makes larger investment in
offspring ! ….Choosy about mates
• Males make relatively small investment in
offspring. Larger investment not required
(no pregnancy, lactation…) and paternity
not always guaranteed, which discourages
investment. Wants as many mates as
possible, in evolutionary theory.
2 different types of sexual
selection
• Intrasexual selection
- competition for
mates (usually
between males)
• Intersexual selection
– mate choice
(usually females
choose the best
provider and
protector)
Choosing a mate is
complicated - female
sometimes has a tough choice
• High testosterone
creates bright
feathers and
aggressive behavior.
• Helpful for protection
– but may not make a
caring father.
Natural Selection Trends
Most traits are controlled by more than
one gene and are called polygenic.
When we graph their distribution in a
population , it looks like the classic Bell
Curve…
For example, human height is
controlled by over 220 genes
and distributes as a
Bell Curve:
3 Modes of Selection
that skew the Bell Curve:
Original
population
Original
populatio
n
Evolved
populatio
n
(a) Directional selection shifts the overall
makeup of the population by favoring
variants at one extreme of the
distribution. In this case, darker mice are
favored because they live among dark
rocks and a darker fur color conceals them
from predators.
Phenotypes (fur
color)
(b) Disruptive selection favors variants
at both ends of the distribution. These
mice have colonized a patchy habitat
made up of light and dark rocks, with the
result that mice of an intermediate color are
at a disadvantage.
(c) Stabilizing selection removes
extreme variants from the population
and preserves intermediate types. If
the environment consists of rocks of
an intermediate color, both light and
dark mice will be selected against.
There are other sources of
evolutionary change besides
natural selection….
Genetic Drift and Gene Flow
Genetic Drift
Random change in allele frequency.
Caused by a “freak” occurrence that
randomly (not selectively!) reduces the
population number and therefore the
genetic variation
• Bottleneck Effect – when a
disaster or other occurrence
severely reduces the
population in one generation
• Northern Elephant Seals –
hunted extensively
• -loss of variation
• -fixed harmful alleles
Genetic Drift – another cause
• Founder Effect
• Loss of genetic
variation that occurs
when a few
individuals from a
larger population
colonize an isolated
new area
• Trista da Cunha –
island colonized by
15 British in 1814 –
high rate of inherited
blindess
Gene flow
• Gene flow/ gene migration movement of fertile
individuals/gametes between
populations
• Reduces
differences/variation
between populations – think
of humans in the U.S.
today….
• Makes populations more
homogenous
• Cheetahs and pandas
• How does gene flow affect
speciation?
Which is more likely to have a
higher rate of gene flow – a
bird or a plant?
A human in the 21st
Century or a human in the
th
16 Century?
Interrupting Gene Flow to form a new Species:
Allopatric Speciation and Sympatric Speciation
• Allopatric - geographic
isolation/barrier
interrupts gene flow
between populations
(antelope squirrels and
the Grand Canyon)
• Over time, the separate
populations acquire
more and more
mutations until
interbreeding
impossible
Sympatric Speciation – Rare!
•
New species arises in
the midst of the parent
population
• Usually due to
disruptive selection
(Lake Victoria Cichlids –
caused by sexual
selection? )
• More common in
bacteria and plants than
animals – faster genetic
changes
MACROEVOLUTION:
the big picture over the looong term
Big Patterns and Ideas in Macroevolution:
• Extinctions – some individual species due to natural
selection and some massive due to big environmental
changes
• Adaptive Radiation – a single species into many
diverse forms and species (Darwin’s finches; also
dinosaurs then mammals….)
• Coevolution – interconnected species evolve in
response to each other (like the crabs and snails)
• Evolution of Development - “Evo-Devo” - a few
“master control genes” guide cell differentiation in
embryo. One gene can have huge impact.
• PACE OF
SPECIATION?
• Gradual or quick? No
consensus – still debated
• Gradualism vs. Punctuated
Equilibrium
• Although there exist some
debate over how long the
“punctuations” last,
supporters of punctuated
equilibrium generally place
the figure between 50,000
and 100,000 years. In other
words, its still pretty gradual!
How are differences between
species preserved?
• Reproductive
isolation –
individuals of
different species
cannot interbreed
• But why?
• What are the
barriers to gene
flow?
Prezygotic Barriers
• habitat isolation
(butterflies in canopies;
hair lice and pubic lice!)
• behavioral isolation (bird
song; firefly flash rhythms),
• temporal isolation (mating
times, seasons)
• mechanical isolation (great
danes and pugs; flower
anatomy)
• gamete isolation
(incompatibility)
Postzygotic barriers problems after mating
  Hybrid (offspring) viability –
offspring don’t survive
Frogs in a pond
  Hybrid fertility – offspring
sterile
Ex. Horses (Chr.# 64) and
Donkeys (Chr. # 62) produce
sterile mules with 63
chromosomes.
 Hybrid Breakdown – future
generations weak or sterile
Exaptations
• New use for a preexisting structure - Ex.
vestigial structures (penguin wings became
flippers), reptilian vs. mammalian jaw
• Mechanism for novel features to arise
gradually from a series of intermediates
Clines
• Gradual change in a trait along a
geographic axis – like Moose –