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NATURAL SELECTION
7.11C IDENTIFY SOME CHANGES IN TRAITS THAT HAVE
OCCURRED OVER SEVERAL GENERATIONS THROUGH
NATURAL SELECTION AND SELECTIVE BREEDING
COULD YOU HAVE RIDDEN A HORSE
MILLIONS OF YEARS AGO?
• Horse evolution goes back more than
55 million years
EVOLUTION OF THE HORSE
THE ECOLOGICAL PRESSURES FACING THE SPECIES
DETERMINED THE TYPE OF DIFFERENCES, OR VARIATION
The development of larger limbs
and hooves allowed horses to
spread out and become more
successful in grassland habitats
as they developed.
HOW DO GENETICS CHANGE OVER
SEVERAL GENERATIONS?
• Changes occur over time from either natural
selection or selective breeding
• You now know that when DNA is copied,
mutations (mistakes) can occur
• Sometimes those mistakes are fatal, some
times they make no difference in survival,
and sometimes, every once in a great while,
that mutation becomes critical to survival
WHY CAN YOU RIDE ONE NOW?
• Natural selection caused some changes in
horses, and humans participated in selective
breeding which both domesticated the horse
and made specialized organisms.
WHAT EXACTLY IS NATURAL
SELECTION?
• The process by which individuals that are
better adapted to the environment survive
and reproduce more successfully than other
members of their species
• These are changes in genetics traits over
many generations
WHO FIGURED THIS OUT?
• CHARLES DARWIN – 1809-1882
• Most people in the world did not
become aware of the theory of
evolution until 1859, when Charles
Darwin published his book On the
Origin of Species by Means of
Natural Selection.
DARWIN’S OBSERVATIONS
• This book described the observations and evidence
that he collected over 20 years of intensive
research, beginning with a five-year voyage around
the world on a British research ship, the HMS Beagle.
• During this five-year voyage, Darwin was able to
make observations about plants and animals
spread around the world, and to collect specimens
to study when he returned to England.
DARWIN’S JOURNEY
• Each time the Beagle stopped at a port to do some
trading, Darwin went on land to explore and look
for the local plants, animals, and fossils. One of the
most important things Darwin did was to keep a
diary. He took extremely detailed notes and
drawings about everything he saw as well as his
thoughts.
• http://www.nhm.ac.uk/nature-online/science-ofnatural-history/expeditions-collecting/beaglevoyage/
THE GALAPAGOS ISLANDS
Darwin’s best known
discoveries were made on
the Galápagos Islands, a
group of 16 volcanic
islands near the equator
about 600 miles from the
west coast of South
America.
Allowed geographical
isolation to take place
GEOGRAPHIC ISOLATION
• Species evolved differently in on the islands than on
the “mainland”
• Darwin was amazed by the array of life he saw on
the Galápagos Islands. He saw animals unlike
anything he had ever seen before.
• For example, the iguanas (large lizards) differed
between islands. The members of one iguana
species spent much of their time swimming and
diving underwater for seaweed, while those of
another iguana species lived on land and ate
plants.
SIMILARITIES AND DIFFERENCES
Marine Iguanas
Marine Iguanas of the
Galapagos
Large claws for gripping slippery
rocks, eat seaweeds
Green iguanas in South
America
Smaller claws for climbing
trees, eat leaves
WHAT DOES THIS MEAN?
Key understanding: mainland species were
still related to the island species Curiously, they looked different, ate
different foods, and behaved differently,
but still had some similarities
So how did this happen?
Answer: gradual changes over many
generations
Darwin’s observations
• Of particular interest were the finches on
these islands
• They had varied sizes and shapes of the
beaks. This observation made him
wonder….
WHY?
FROM ONE FINCH…
• There are now at least 13 species of finches on the
Galapagos Islands, each filling a different niche on
different islands. All of them evolved from one
ancestral species, which colonized the islands only
a few million years ago.
WHAT DARWIN LEARNED FROM THESE FINCHES
• The one species of finch gradually evolved into separate
species because of food sources and other environmental
factors such as amount of water available
For example:
• Beaks highly adapted to food source
• Large beaks - meant there was little rainfall because
the seeds were large & tough
• Small beaks - Heavy rainfall – seeds – small & soft
• Other adaptations were sharp beaks for finding and
eating insects, wide beaks for cracking seeds
THE FINCHES ALSO EVOLVED TO EAT A
VARIETY OF FOODS -
MORE ADAPTATIONS
• Darwin studied hundreds more animal
and plant species. After nearly 20
years of research, he proposed
his theory of evolution by natural
selection.
• Another example of what he studied:
THE GALAPAGOS TORTOISE
The giant tortoises (Geochleone
nigra) in the Galapagos 2 m long
Saddle-shaped shell, long necks
more prone to standing upright to
forage on cactus
Nearest relative- Chacos Tortoise
(Geochelone chilensis) in
Argentina just 20cm long
Dome-shaped shell, short necks
graze on ground level vegetation
Theory of Natural Selection
(Darwin’s Conclusions)
• Species have changed over generations
through adaptations
• The changes allow them to survive and
reproduce in a new environment.
• Darwin proposed that evolution occurs by
means of natural selection
• New species may form when a group of
individuals is isolated from the rest of the
population
Darwin’s 5 key points in the
Theory of Natural Selection:
1. Populations have (genetic) variations.
2. Some variations are favorable.
3. More offspring are produced than
survive
4. Those offspring that survive have
favorable traits.
5. A population will change over time.
NATURAL SELECTION - QUICK EXPLANATION
How is this an example of natural selection?
Green bugs are more visible to its predator
Green bugs get eaten more easily
Brown bugs not as visible, they survive
Brown bugs reproduce, the helpful alleles for the
brown color are passed on to the next generation
After many generations, more members will be
brown
The environment has “selected” organisms with
the helpful traits, hence the term “natural
selection”
http://evolution.berkeley.edu/evolibrary/home.php
Life Sciences-HHMI Outreach. Copyright 2006 President and Fellows of Harvard College.
MYTHBUSTING!
• Survival of the “fittest” – what does that mean?
Fitness is the key to natural selection. We're not talking
about how many reps you can burn through at the gym -biological fitness is an organism's ability to successfully
survive long enough to reproduce.
Beyond that, it also reflects an organism's ability to
reproduce well. It isn't enough for a tree to create a bunch of
seeds. Those seeds need the ability to end up in fertile soil
with enough resources to sprout and grow.
DARWIN’S CONTEMPORARIES
Jean-Baptiste Lamarck,
1744-1829
Alfred Russell Wallace, 1823-1913
(with Darwin)
• Lamarck and Wallace had similar
findings as Darwin
• Wallace called the changes in species
over time “evolution”.
YOU MAY HAVE HEARD OF…
• Lamarckism (or Lamarckian inheritance) is
the idea that an organism can pass on
characteristics that it acquired during its
lifetime to its offspring.
FACTORS AFFECTING NATURAL SELECTION
1. Overproduction
2. Competition
3. Variations
OVERPRODUCTION
• What’s the point of so many offspring?
• Only the “fittest” survive, not all of your
offspring will survive long enough to
reproduce and pass on genetic heredity–
but some will!
COMPETITION
• Resources are limited! There is only so much
food, space, water and shelter
Members of the same species must compete
with each other, only the fittest survive
AND VARIATION ….
Any genetic difference
between individuals of the
same species
• very important in natural
selection
• the stronger individuals
survive and reproduce
• offspring with helpful traits
will survive and then pass
along those traits to their
offspring
What might those
traits be?
EXAMPLE 1 OF VARIETY OF TRAITS:
COUNTER-SHADING IN FISH
• Upper side of the animal is darker than the lower
side, blends in with sea floor
• Lower side of organism is lighter, organism blends in
with its surroundings – in this case the sky
Which do you think is better for the survival of these
species?
EXAMPLE 2 OF VARIETY OF TRAITS:
BEAKS IN BIRDS
EXAMPLE 3 OF VARIETY OF TRAITS:
CLAWS IN BIRDS
Variation in
talon size
and shape
due to
hunting
technique.
TO SUM IT UP…
• Watch this: Natural Selection
video
HW – NATURAL SELECTION WKSHT