Transcript Charles Darwin and Natural Selection

Charles Darwin and
Natural Selection
Darwin journeyed on the
HMS Beagle as a naturalist
•
•
•
•
•
5 year journey
studied and collected many biological specimens
on Galapagos Islands, off coast of Ecuador, observed
animals such as finches, tortoises, and iguanas
Thirteen different but similar species of finches, each
with a distinctive bill that is specialized for a particular
food source.
Suggested that these birds migrated from Ecuador
and changed after they arrived.
Darwin’s ideas were influenced by:
• Jean Baptiste
Lamarck, who
hypothesized that
acquired traits
were passed onto
offspring
•Charles Lyell, a geologist, who
suggested that the Earth was much
older than 6000 yrs
•Thomas Malthus, who wrote that
human populations grow much faster
than their food supply
•Alfred Wallace, who suggested
natural selection after studying wildlife
in the Malay Archipelago.
Darwin observed differences
among island species.
Marine iguana
Land iguana
Thirteen different but similar
species of finches, each with a
distinctive bill that is specialized for
a particular food.
Suggested that these birds migrated from South
America and changed after they arrived
Key insights led to Darwin’s idea for natural
selection.
• Darwin noticed a lot of variation in
domesticated plants and animals.
• Artificial selection is the
process by which humans
select traits through
breeding.
• Heritability is the ability of a trait to be passed
down.
• There is a struggle for survival due to
overpopulation and limited resources.
• Darwin proposed that adaptations arose over
many generations.
• Natural selection is a mechanism by
which individuals that have inherited
beneficial adaptations produce more
offspring on average than do other
individuals.
Principles of Natural Selection
1. Variation. What can cause variation in a
population?
• Genetic differences and mutation
2. Overproduction. What are pros and cons
of overproduction?
• Having many offspring increases the
chance for survival, but also results in
competition for resources.
3. Adaptation. What determines whether
an adaptation is beneficial or not?
• A certain variation that allows an
individual to survive better than other
individuals it competes against.
4. Descent with Modification. How does
natural selection change a population
over time?
• Over time, more members of the species
will have adaptations that are well suited
for survival and reproduction in an
environment.
Elephants in Queen Elizabeth
National Park, Uganda, Africa
Normally, nearly all African
elephants, male and female,
have tusks. In 1930, only one
percent of the elephant population
in Queen Elizabeth Park was
tuskless because of a rare
genetic mutation. Food was
plentiful, and by 1963 there were
3,500 elephants in the park.
In the 1970’s, a civil war began in
Uganda. Much of the wildlife was killed for
food, and poachers killed elephants for
their ivory tusks. By 1992, the elephant
population had dropped to about 200. But
by 1998, the population had increased to
1,200. A survey revealed that as many as
30 percent of the adult elephants did not
have tusks. Ugandan wildlife officials also
noted a decline in poaching.
Natural selection acts on distributions of traits.
• A normal distribution graphs as a bell-shaped
curve.
• Populations have a
normal distribution
when they are not
undergoing natural
selection
• Microevolution is evolution within a
population.
– observable change in the allele frequencies
– can result from natural selection
Directional selection – favors one
of the extreme variations
• Woodpeckers with long beaks capture the
most insects, as they can reach the
insects deep in the tree trunk.
• Stabilizing selection –
favors the average
• Small spiders have a hard time capturing
prey
• Large spiders easily spotted by birds
• Medium sized spiders are best suited to
survive in their environment, reproduce
more often, leave more offspring.
Disruptive selection - favors both extremes
• On light colored rocks, the light limpets are
camouflaged and survive the best
• On dark rocks, the dark limpets are most
successful
• Tan (intermediate) limpets are visible on
both the light rocks and dark rocks, and
their numbers decline due to predation
Evidence of Evolution
A. Fossils
Fossil links found between
• fish and amphibians
• reptiles and birds
• reptiles and mammals
Whales from land mammals
Fossil linking fish and amphibians
• 365 million years old
• arm bone with fish fin
characteristics
• found in Pennsylvania
• thought to be from a
lobed-finned fish
Archaeopteryx – links reptiles and
birds
A fossil of Archaeopteryx was discovered at about the same time Darwin published On the Origin of
Species. This pigeon-size creature had a dinosaur like shape, complete with a long bony tail, heavy
jaws with serrated teeth, and three long fingers. It also had feathers like those of modern birds.
Hind leg bones in whales
An amphibious reptile
found in Texas, 2005
Diarthognathus, an animal with
reptile and mammal characteristics
Early mammals may have looked
like this
Evolution
of the
horse
B. Geography
– island species most closely resemble nearest mainland
species
– populations can show variation from one island to
another
C. Embryology
– Crabs and barnacles with similar larvae, different adult
body forms
Larva
Adult crab
Adult barnacle
• Early in development, vertebrate embryos
have similar characteristics such as a tail,
buds that become limbs, and pharyngeal
pouches that hold the gills of fish and
amphibians.
Vertebrate embryos
D. Anatomy
•Homologous structures – similar in
structure, with different functions
• Vestigial Structures
• Structures that are reduced in size and
either have no use or a less important use
than they do in other, related organisms.
• Examples: wings on flightless birds,
Human ear muscles, human wisdom teeth
human appendix , hind leg bones in
whales
The cassowary, a flightless bird
with wings
Wisdom teeth in human
Human appendix
– Analogous structures are not evidence of a
common ancestor.
– Analogous structures have a similar function.
Human hand
Fly wing
Mole foot
Bat wing
Evolutionary Biology Today
• DNA sequence analysis: two closely-related
organisms will have similar DNA
• Homeobox genes indicate a very distant
common ancestor.
– control the development of specific structures
– found in many organisms
Homeobox genes
Mechanisms of Evolution
1. Natural Selection: certain
traits might be an
advantage for survival
2. Mutation: creates new
genetic variation
3. Sexual selection: certain
traits may improve
mating success; alleles
for these traits increase
in frequency
Sexual selection occurs when certain
traits increase mating success.
• Sexual selection
occurs due to higher
cost of reproduction
for females.
– males produce many
sperm continuously
– females are more
limited in potential
offspring each cycle
Sexual dimorphism
Mating systems
Monogamy One male mates exclusively with one female
Polygamy Individuals mate with more than one partner
Polygyny Some males mate with more than one female
Polyandry Some females mate with more than one male
Promiscuity Males mate with more than 1 female and vice versa
Monogamy
Polygyny
Promiscuity
Harem polygyny
Elephant seals breeding females cluster together on beaches
- allows males to defend a harem of many females at once
Male elephant seals weigh up to 3x more than females!
Male reproductive success is highly variable:
8 individual males inseminated 348 females in one study!
Male elephant seals engage
in violent, bloody fights over
females – large size provides an
advantage in male combat
Alternative reproductive tactics
Remember…there may not be a “best” way to be a male (or female)
The Red Queen
Evolutionary arms-races
The Red Queen: “It takes all the running you can do, to
keep in the same place.”--Lewis Carroll, “Through the
Looking Glass”
The Red Queen Hypothesis: “For an evolutionary
system, continuing development is needed just in order to
maintain its fitness relative to the systems it is co-evolving
with.” --Leigh Van Valen (1973)
4. Gene flow: movement of individuals to
or from a population (also known as
migration). Immigrants add alleles,
emigrants take alleles away.
Example: troops of baboons in eastern
Africa. Females remain with the troop,
but younger or less dominant males
leave their birth troop, eventually joining
another troop. This ensures gene flow.
5.
Genetic drift is a change in allele
frequencies due to chance.
• Genetic drift causes a loss of genetic diversity.
• It is most common in small populations.
A) A population bottleneck can lead to genetic
drift.
– It occurs when an event
drastically reduces
population size.
– The bottleneck effect is
genetic drift that occurs
after a bottleneck event.
Example: In the 1800’s, northern elephant
seals were overhunted. The population
was reduced to about 20 individuals.
Hunting has ended, and there are now
about 100,000 seals. However, the
population has little genetic variation.
B) The founder effect is genetic drift that
occurs after start of new population
–It occurs when a few individuals start a new
population.
What is speciation?
The rise of two or more species
from one existing species
What causes speciation?
Types of reproductive isolation
• Geographic isolation
2. Temporal isolation
• timing of reproductive periods prevents
mating
3. Behavioral isolation
• includes differences in courtship or mating
behaviors
Example: female fireflies only respond to
light patterns of males of their own species
Satin bowerbird
and McGregor’s
bowerbird 
• What process keeps the number of total
species on Earth from growing
exponentially through speciation?
Draw a visual for divergent
evolution and one for
convergent evolution.
Give an example of each.
(convergent and divergent)
• Which type leads to analogous structures?
Convergent or divergent?
Convergent or divergent?
Polar bears and grizzly bears?
• Convergent
or divergent?
(Adaptive
radiation)
What is coevolution?
Evolutionary Arms Race
True or false:
When sexual selection is acting in a
species, all males tend to look alike.
• There are two groups of pine trees that
appear to be very similar phenotypically
and genotypically. However, one releases
pollen in January, when the female
structures of that group are receptive, and
one in March. What kind of reproductive
barrier is this?
Microevolution
• a change in gene frequency in a population —
such as all the individuals of one beetle species
living on a particular mountaintop.
Industrial Melanism
• Example is the peppered moth.
• Peppered Moth Simulation
Macroevolution
• generally refers to evolution above the
species level
• Macroevolution