Transcript Patterns of Natural Selection

EVOLUTION
Evolution: genetic change in a
population of organisms over time

Macroevolution (large-scale)


Origins of new species and life forms as well as extinctions
Microevolution (small scale)

Changes in gene frequencies within a species
Jean-Baptiste Lamarck
Proposed that variation is created within a species based
on life experiences (acquired characteristics) which can
then be inherited by offspring
EXAMPLE:
Key: Change
is driven by
an inner
“need” or
desire! If you
work hard
enough, you
will have the
trait and pass
it on!
Charles Darwin

In 1831, Charles Darwin
took on the role of
naturalist of the ship
HMS Beagle

The Beagle set sail on a
five-year navigational trip
around the world
Charles Darwin (1809-1882)
Darwin’s Trip

Darwin studied a wide variety of plants and
animals across the globe, particularly on the
Galapagos Islands

In 1859, he published his book On the Origin
of Species

In it he proposed that evolution occurs through
natural selection
Darwin’s Evidences for Natural
Selection



Fossils of extinct species resembled living species
in the same area
Galapagos finches differed slightly in appearance
but resembled those on the S. American mainland
He believed it was “descent with modification” from
a common ancestor (AKA evolution!)
Darwin’s Book & Wallace

Darwin drafted a preliminary transcript of his book in
1842


However, he shelved it for 16 years, probably because of its
controversial nature
Alfred Russel Wallace (1823-1913) independently
developed a similar theory to Darwin’s

Correspondence between the two spurred Darwin to publish
his theory in 1859
Natural Selection
Natural Selection refers to the differential
reproduction of genotypes caused by
factors in the environment
More simply put… factors in the
environment, such as climate,
competition for food, or predators, affect
which organisms will survive and
therefore reproduce to pass on their
genes
Steps of Natural Selection





1. Gene variation exists among individuals in a population
(some are “fitter” than others)
2. This variation can be passed to offspring (variation is
due to differences in DNA)
3. All populations overproduce offspring (not all will live)
4. Individuals with traits that aid survival and reproduction
have a better chance of contributing to the next generation
(those that will live will have babies!)
5. Over time, the population changes such that the traits of
the more successful reproducers are more prevalent (those
that live pass on their traits that made them more successful
in the first place!)
Natural Selection
Adaptation: changes that increase the
likelihood of survival and reproduction of
particular genetic traits in a population
The Rate of Evolution

Different kinds of organisms evolve at different
rates
Ex: Bacteria evolve much faster than eukaryotes

The rate of evolution also differs within the same
group of species


In punctuated equilibrium, evolution occurs in
spurts
In gradualism, evolution occurs in a gradual, uniform
way
Scientists
still debate
as to
which
happens…
there
seems to
be
evidence
for both!
a) Punctuated equilibrium
b) Gradualism
Evidence for Evolution

Evidence for evolution comes from the
following

Fossil record

Molecular record

Anatomical record
Fossil Record

Fossils are the preserved remains, tracks, or traces of
once-living organisms


They form when organisms become buried in sediment and
calcium in hard surfaces mineralizes
Arraying fossils according to age often provides
evidence of successive evolutionary change
Fossils…how to put in order?

Relative Age: Looked at the fossils pulled
from the ground…Law of Superposition
states that the farther down it is, the older
it is

Absolute Age: Use radioactive isotopes to
find the precise age (fossil dating)

Fossils have been
found linking all the
major groups

The forms linking
mammals to reptiles
are particularly well
known
Embryology
All organisms in their early developmental patterns look
similar…links to a common ancestor?
Anatomical Record

Looking at the anatomy of organisms
shows similarities…

Homologous
structures
 Built of the
same basic
components,
but serve
different
functions
Anatomical Record

Analogous structures
 Different anatomical structures and ancestors,
but the structures serve the same purpose
Anatomical Record

Vestigial organs
 Structures that are no longer in use, but still
present
DNA Evidence


Evolutionary changes involve a continual accumulation
of genetic changes
 Distantly-related organisms accumulate a greater
number of evolutionary differences than closelyrelated ones
Compare the DNA sequences among organisms! The
most closely related organisms have more similar DNA
sequences!
**BEST EVIDENCE!!**
DNA Evidence
The greater the
evolutionary distance
The greater the number
of amino acid differences
Patterns of Evolution
Coevolution: long term, evolutionary adjustment of
organisms to each other
1.

Example: flowers and their pollinators
Patterns of Evolution
2.
Convergent Evolution: organisms that have
NO common ancestry look similar due to the
environment in which they live

3.
Example: sharks and dolphins (analogous
structures)
Divergent Evolution: organisms that HAVE
a common ancestry, but look different due to
the environment in which they lives

Example: finches!
Back to Natural Selection…


Fitness: an organism’s
fitness describes how
well that organism will
survive and reproduce
Artificial Selection: the
intentional reproduction
of specific individuals
in a population

Example: purebred dogs,
horses, crops, etc.
Patterns of Natural Selection

Directional Selection: One phenotype is more
favorable; allele frequency shifts in one direction
Example: Peppered Moth! Light colored was favored before Industrial
Revolution. After, there was a lot of soot in the air, darker moths became
favored. The gene frequency shifted in favor of dark moths!
Patterns of Natural Selection

Stabilizing Selection: population becomes less
varied and more normalized
Example: Human birth weights! Babies weighing between 7.5-8.5 pounds
at birth have the highest survival rate (98.5%). Anything below or above
this drops the survival rate. Birth weight is now stabilized around 8 lbs.
Patterns of Natural Selection

Disruptive Selection: extreme values for a trait
are favored over intermediates
Example: Think of fish! If you are small, you can hide. If you are large,
you can scare away predators or threats. If you are the intermediate
size…you’ll be eaten.
Hardy-Weinberg Equilibrium

Genetic variation in populations puzzled
scientists


Dominant alleles were believed to drive recessive
alleles out of populations
In 1908, G. Hardy and W. Weinberg pointed
out that in large populations with random
mating, allele frequencies remain constant

Dominant alleles do not, in fact, replace recessive
ones
Hardy-Weinberg Equilibrium


Hardy-Weinberg Equilibrium: A state in which genotype
frequencies and ratios remain constant from generation to
generation and in which genotype frequencies are a product of
allele frequencies
A population that is in Hardy-Weinberg equilibrium
is NOT evolving, meaning:
1. Large population size
2. Random mating
3. No mutation
4. No migration
5. No natural selection
Hardy-Weinberg Equilibrium

Let’s look at a gene for cat’s coat color. The cats can
either be black or white. There are 2 possible alleles:
B or b.
 The more common allele (B) is designated p
 The less common allele (b) is designated q
 p + q = 1 (Only 2 alleles, must add up to 100%!)
Hardy-Weinberg Equilibrium

The Hardy-Weinberg equilibrium can be written as
an equation
Individuals homozygous
2
2
for allele b
 1 = p + 2pq + q
Individuals homozygous
for allele B
Individuals heterozygous
for alleles B and b
The equation allows calculation of allele
frequencies
Hardy-Weinberg Equilibrium


Back to our example, let’s say that out of 100
cats, 16 are white and 84 are black. This gives
us some information…
Frequency of white cats (bb) = 0.16





q2 = 0.16
q = √0.16 = 0.4
p+q=1 Therefore… p=0.6
Frequency of homozygous dominant? p2
Frequency of heterozygous? 2pq
Hardy-Weinberg Example

If 9% of an African population is born with a
severe form of sickle-cell anemia (ss), what
percentage of the population will be
heterozygous(Ss) for the sickle-cell gene?
Why do allele frequencies change?

Five evolutionary forces can significantly alter the
allele frequencies of a population





1.
2.
3.
4.
5.
Mutation
Migration
Genetic drift
Nonrandom mating
Selection
Mutation

Errors in DNA
replication

The ultimate source of
new variation
However, mutations are
rare…they are only one
small factor

Migration

Movement of individuals from
one population to another
 Immigration: movement into
a population
 Emigration: movement out
of a population

A very potent agent of change
Genetic Drift

Random loss of alleles


Founder effect


More likely to occur in smaller
population
Small group of individuals
establishes a population in a new
location, those genes take over
Bottleneck effect

A sudden decrease in population size
to natural forces; whoever lives
passes their genes on
Nonrandom Mating



Mating that occurs more or less
frequently than expected by
chance
Inbreeding
 Mating with relatives
 Increases homozygosity
Outbreeding
 Mating with non-relatives
 Increases heterozygosity
Selection



Some individuals leave
behind more offspring than
others
Artificial selection
 Breeder selects for
desired characteristics
Natural selection
 Environment selects for
adapted characteristics