Download Natural selection






Individual organisms differ, and some of this variation is
heritable
Organisms produce more offspring than can survive, and many
that do survive do not reproduce
Because more organisms are produced than can survive, they
compete for limited resources
Each unique organism has different advantages and
disadvantages in the struggle for existence. Individuals best
suited to their environment survive and reproduce most
successfully. These organisms pass their heritable traits to their
offspring. Other individuals die or leave fewer offspring. This
process of natural selection causes species to change over time.
Species alive today are descended with modification from
ancestral species that lived in the distant past. This process, by
which diverse species evolved from common ancestors, unites all
organisms on Earth into a single tree of life.





1. Mutations
2. Genetic shuffling that results from sexual
reproduction
*The number of phenotypes produced for a
given trait depends on how many genes
control the trait
1. single-gene trait = leads to 2
phenotypes, ex: widow’s peak, tongue rolling
2. polygenic trait = many possible
genotypes and phenotypes, ex: height



Gene pool = consists of all genes, including
all the different alleles, that are present in a
population
Relative frequency = number of times that
the allele occurs in a gene pool compared
with the number of times other alleles for the
same gene occur. (%)
Evolution = any change in the relative
frequency of alleles in a population
◦ If the relative frequency of an allele changes over
time, that means the population is evolving

A. Fossil record
◦ Derived Traits = newly evolved features, such as
feathers, that do not appear in the fossils of
common ancestors
◦ Ancestral Traits = primitive features, such as teeth
and tails, that do appear in ancestral forms

B. Comparative Anatomy
◦ Homologous structures = anatomically similar
structures inherited from a common ancestor
 Arm, foreleg, fin, wing
◦ Vestigial structures = structures that are reduced
forms of functional structures in other organisms
 snake pelvis, kiwi wings, human appendix
◦ Analogous structures = can be used for the same
purpose and can be superficially similar in
construction but are not inherited from a common
ancestor
 Bird wings and insect wings





Adaptation = a trait shaped by natural selection
that increases an organism’s reproductive
success
Fitness = measure of the relative contribution an
individual trait makes to the next generation
 Types of Adaptation
Camouflage = morphological adaptations that
allow them to blend in with their envs.
Mimicry = one species evolves to resemble
another species (kingsnake and coral snake)
Antimicrobial resistance

Hardy-Weinberg principle = states that when allele
frequencies in a population remain constant a population is
in genetic equilibrium
◦ P2 + 2pq + q2 = 1
p = dominant, q = recessive
◦ Homozygous dom. + heterozygous + homozygous recessive = 1


Genetic equilibrium = the situation in which allele
frequencies remain constant
5 conditions are required to maintain genetic equilibrium
from generation to generation
◦
◦
◦
◦
◦
1.
2.
3.
4.
5.
Random mating
Large population
No movement into or out of the population (No interbreeding)
No mutations
No natural selection

A. Natural selection = process by which
favorable traits that are heritable become
more common in successive generations of a
population of reproducing organisms
◦ individuals with favorable phenotypes are more
likely to survive and reproduce than those with less
favorable phenotypes

1. Individuals at one end of the curve have
higher fitness than individuals in the middle
or at the other end
◦ Ex. Small and medium-sized seeds start to run low
and only large seeds  finches with large, thick
beaks survive and reproduce so average beak size
would increase

2. Individuals near the center of the
curve have higher fitness than
individuals at either end of the curve
◦ Ex. Babies that are too small or too large are less
likely to survive (average is selected for)

3. Individuals at the upper and lower ends of
the curve have higher fitness than individuals
near the middle
◦ Ex. Snakes live at edge of forest. Light and dark are
selected for. Medium color is disadvantage

Natural selection in which change in
frequency of a trait is based on the ability to
attract a mate
◦ ex. Brighter and bigger tails in peacocks
◦ Larger size and antlers in deer

Random change in allele frequency due to
chance
◦ More extreme in small populations
◦ May occur when a small group of individuals
colonizes a new habitat

A situation in which allele frequencies change as a result
of the migration of a small subgroup of a population

Occurs when a population declines to a very low number
and then rebounds

Speciation = formation of new species
◦ A population must diverge and then be
reproductively isolated

Reproductive isolation = when the members
of 2 pops. cannot interbreed and produce
fertile offspring

A physical
barrier divides
one population
into 2 or more
populations

A species evolves into a new species
without a physical barrier

Prezygotic isolating mechanisms =
operate before fertilization occurs
◦ Keep species separate

Postzygotic isolating mechanisms =
operate after fertilization
◦ hybrid remains infertile (liger, mule)

1. Behavioral isolation = occurs when 2 pops.
are capable of interbreeding but have
differences in courtship rituals or other
reproductive strategies that involve behavior
◦ Ex. Eastern and western meadowlarks have
overlapping ranges but do not mate with each other
because they have diff. mating songs

2. Geographic isolation = 2 pops. are
separated by geographic barriers such as
rivers, mountains, or bodies of water
◦ Ex. Colorado river split pop. of Abert squirrels and
2 separate gene pools were formed to form a new
species

3. Temporal isolation = 2 or more species
reproduce at different times
◦ Ex. 2 pops. of orchids release pollen on different
days so they cannot mate with each other,
eventually they may become separate species





Pg. 439
Explain
Explain
Explain
Explain
Adaptive radiation
Coevolution
Convergent evolution
Rate of Speciation
◦ Gradualism
◦ Punctuated equilibrium