Transcript Evolution for Beginners

Evolution for
Beginners
Evolution HistoryCarolous Linnaeus
• 1700’s
• Swedish Botanist
• Developed a system of classification
• Theorized that organism changed over time due to
hybridization just in plants.
Georges Louis Leclerc De
Buffon
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French Naturalist
1700’s
Proposed species shared ancestors
Suggested the Earth was much older than
6000 years.
Erasmus Darwin
• Charles Darwin’s own Grandfather
• English Doctor
• Proposed all living things were descended
from a common ancestor.
• More complex forms of life arose from less
complex ones
Jean-Baptiste Lamarck
• 1809
• Proposed all livings things evolved towards
perfection
• Believed species did not become extinct
only that they evolved into other forms.
• Believed that traits that were needed were
passed on to offspring. (inheritance of
acquired characteristics).
Geologic Change
• French Zoologist- Georges Cuvier
• Observed deeper layers of rock held quite
different fossils than the upper layers
• Fossils- are traces of living animals from
the past
• Developed the theory of Catastrophism –
states that natural disasters caused some
specied to become extinct and then other
animals moved in from elswhere.
Geologic Change Continued
• Scottish geologist James Hutton proposed
gradualism
• Gradualism- is slow changes that happened
over time in the past
• It is often used as term to explain the
gradual change in species though evolution.
Geologic Change Continued
• Charles Lyell an English Geologist
• Proposed the theory of uniformitarianismthis combined Hutton's theory of gradual
change over time with his idea that things
are occurring at constant rate and are
ongoing.
• This had a major impact on a young Carles
Darwin.
What is evolution?
A basic definition of evolution…
“…evolution can be precisely defined as any
change in the frequency of alleles within a
gene pool from one generation to the next."
- Helena Curtis and N. Sue Barnes, Biology, 5th ed. 1989 Worth Publishers, p.974
So what does the definition mean?
• Evolution is a change in the number of
times specific genes that code for specific
characteristics occur within an interbreeding
population
• Individuals don’t evolve, populations do
• There is no implied “improvement” in
evolution
Genetic Variation and
Evolution
• Evolution: changes through time
1. Species accumulate difference
2. Descendants differ from their ancestors
3. New species arise from existing ones
Charles Darwin
Charles Darwin was born on
February 12, 1809 in Shrewsbury,
England.
From 1831 to 1836 Darwin served as
naturalist aboard the H.M.S. Beagle on
a British science expedition around the
world.
He observed much variation in related
or similar species of plants and animals
that were geographically isolated from
each other.
These observations were the basis for
his ideas.
Darwin presumed that populations of individuals
changed over time, and, in 1844, he developed
the concept of the driving force for evolution. It
wasn’t until many years later that he published
his idea.
“I have called this principle, by which
each slight variation, if useful, is
preserved, by the term Natural Selection.”
—Charles Darwin from "The Origin of Species“,
1859
Darwin’s Observations
• He was struck by the many different
variations of individual species he observed.
• He observed tortoise shell differences
– Lots of low food no hump in shell.
– Food at a higher level tortoise has hump
• Finch beak differences.
– Soft food thin beak
– Hard nuts larger stronger beak
Darwin’s Observations
• Fossil evidence
• He found fossils of animals that look similar
to animals alive today, but are different
species.
• He saw marine fossils high on mountains
and concluded that Lyell’s theory that
geological change happens slowly over
time.
Darwin’s Thinking
• After the voyage he spent 20 years thinking
about what he saw.
• Local farmers helped him with his ideas
because he saw them use artificial selection
to specify what traits they wanted in captive
animals.
• Realized the traits had to be heritable or
passed from one generation to the next.
Evidence
• He theorized the animals on the island had
migrated from the mainland because they
very closely resembled those animals.
• This is what we now call biogeography.
• Embryology- Organism look very similar in
as they are developing, but develop
differently as adults.
Evidence Anatomy
• Homologous structures- structures that are
similar in structure but appear different in
different organisms and have different
functions.
• Example forelimbs of humans bats and
moles.
Evidence Anatomy
• Analogous Structures- Structures that have
similar function but are not similar in origin.
- Example wings of bats and flies.
• Vestigial Structures- organs that no longer
have any useful function, but probably did
in an ancestor.
– Example Human appendix
Paleontology
• Study of fossil remains of ancient animals.
• This was a very new science in Darwin’s
time.
• Fossil evidence shows how animals change
over time.
Genetic Evidence
• Today we can use genetic sequences and
analyze similarities between species.
• There are Pseudo-genes that are like
vestigial organs that no longer are used, but
may have been used in the past .
Natural selection: mechanism of
evolutionary change
Natural selection: proposed by Darwin as the
mechanism of evolution
• species have a variation of inherited characteristics
• they produce many offspring- overproduction
• the population includes more individuals with
specific characteristics that help them surviveadaptation
• the population evolves and is better adapted to its
present environment- Descent with modification
Natural Selection
Darwin knew nothing of
genes, but what he did have
were two observations and a
little inference that provided
the motive force for
evolution.
•Darwin: Evolution is descent with modification
Natural Selection
Observation 1: Organisms generally
have more offspring than can survive to
adulthood.
Observation 2: Offspring are not
identical. There is variation in their
appearance, size, and other
characteristics.
Natural Selection
Inference: Those organisms that
are better adapted to their
environment have a greater
likelihood of surviving to adulthood
and passing these characteristics
on to their offspring.
Survival of the “fittest.”
Survival of the “fittest.”
Darwin’s
theory for
how long
necks
evolved in
giraffes
The evolution of populations
Gene pool- the combined alleles of all the
members of a population.
1. Sources of variation
1. Sources of variation
Change can be rapid
• Ecologists Peter and Rosemary Grant
observed finches in the Galapagos.
• A drought in 1977 reduced soft seeds
finches preferred, but large tough seeds that
only the large beaked finches could eat were
still available
• The next year there was a drastic increase in
large beaded finches.
Selection
• Artificial selection: a breeder selects for
desired characteristics
Selection
• Natural selection: environmental conditions
determine which individuals in a population
produce the most offspring
• 3 conditions for natural selection to occur
– Variation must exist among individuals in a population
– Variation among individuals must result in differences
in the number of offspring surviving
– Variation must be genetically inherited
3. Maintenance of Variation
• Frequency-dependent selection: depends
on how frequently or infrequently a
phenotype occurs in a population
– Negative frequency-dependent selection: rare
phenotypes are favored by selection
– Positive frequency-dependent selection:
common phenotypes are favored; variation is
eliminated from the population
• Strength of selection changes through time
Maintenance of Variation
• Fitness of a phenotype does not
depend on its frequency
• Environmental changes lead to
oscillation in selection
Fitness and Its Measurement
• Fitness: A phenotype with greater
fitness usually increases in frequency
– Most fit is given a value of 1
• Fitness is a combination of:
– Survival: how long does an organism live
– Mating success: how often it mates
– Number of offspring per mating that survive
Maintenance of Variation
Directional selection: acts to eliminate one
extreme from an array of phenotypes
Stabilizing selection
• The intermediate
phenotype is favored
and becomes more
common in the
population
Disruptive Selection
• Both extreme
phenotypes are
favored in a
population.
Gene Flow
• The movement of alleles from one
population to another.
Genetic Drift
• A small population can produce changes in the
frequency of alleles by chance. (Example picking
flowers)
• Bottle neck affect- occurs after a population has
been greatly reduced the few members left do not
have the original diversity of the larger population.
• Founder affect- Occurs when a small number of
individuals colonize a new area.
• Evolution by drift is aimless, not adaptive. (may be
good or bad)
Sexual Selection
• This is referred to as non-random mating
• This occurs when certain traits increase
mating success.
– Intrasexual- males compete to mate. Example
bighorn sheep.
– Intersexual- males display traits that attract
females. Example birds of paradise
Genetic equilibrium is a
basic principle of
population genetics.
Hardy–Weinberg principle states that the genotype frequencies in a
population remain constant or are in equilibrium from generation to
generation unless specific disturbing influences are introduced.
Those disturbing influences include
1. Non-random mating- no sexual selection
2. New mutations- no new alleles introduced
3. large population-no random genetic drift
4. Gene flow- no immigration or emigration
5. All traits must equally aid survival- no natural selection
Five agents of evolutionary change
Speciation Through Isolation
• Speciation- the rise of two or more species from
one existing species
• Reproductive isolation- occurs when members of
two or more population can no longer mate
successfully. Last step in becoming new species.
• Behavioral isolation- differences in courtship or
mating behaviors
• Geographic isolation- physical barriers
• Temporal isolation- reproduction at different
times. Example - cicadas
Patterns of Evolution
• Natural selection is not random!
• Convergent evolution- when two unrelated
species develop similar characteristics.
• Divergent evolution- When closely related
species become increasingly different.
• Coevolution- Two or more species evolve
in response to one another.
– Examples Pg 328
Extinction
• Extinction- The elimination of a species
from the Earth.
– Background extinctions- continuous low rate of
extinction that are a part of the life cycle
– Mass Extinctions- caused by catastrophic event
Patterns of speciation
• Punctuated equilibrium- bursts of activity
followed by long periods of stability
• Adaptive radiation- the change of one
species into many different descendents