Download Mechanisms for Evolution

Mechanisms for
Evolution
Populations and Evolution
A population is a group of organisms that interbreed
 Each population shares a gene pool (the different
alleles present in the population)
 Each population has a relative frequency of each
alleles, or a number of times the allele occurs in the
gene pool.
 The frequency of alleles in a population tend not to
change unless there is an outside force causing it.

(EQUILIBRIUM)
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There are FIVE mechanisms that can
change the allele frequencies in a
population:
mutation
migration
genetic drift
non-random mating
natural selection
Evolution occurs because of
changes to the equilibrium.
Mutations
Mutations are inheritable changes to
the genotype of an organism
 Mutations occur randomly and
spontaneously within a population
 Most mutations are harmful, but some are
useful

Mutations can affect allele frequency in
a population by adding new alleles for a
trait or changing the amount of each allele
present
 It can take a long time to eliminate a
mutation and a long time for a new
mutation to become prevalent

Migration
Movement into and out of a
population can change the allele
frequency in a population’s gene pool
 Immigration can add individuals with
variations to the population
 Emigration can remove individuals with
variations from a population
 Many species encourage migration which
can cause more gene flow which is the
process of transferring genes among
different populations
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Genetic drift is the random changes in
allele frequency due to population size.

In smaller populations the allele frequency
can change more rapidly than in larger
populations
Genetic Drift
Genetic
Drift
 The founder
effect cont’d
occurs when a few
individuals from a
larger population
colonize a new area.

The allele frequency
of this population
may differ from the
larger population
because of the
limited number of
individuals
Non-Random Mating
Having a limited number of individuals can
also impact mating.
 It’s a limitation due to lack of genetic
diversity within that population!!!
 Non random mating can influence allele
frequencies because:

◦ Mates can be limited by geography
◦ Mates can be chosen for their traits
◦ Mates can be more closely related to one
another
Natural Selection

process by which individuals more fit for
the environment survive and reproduce

The interaction of populations and the
environment results in changing allele
frequencies (ECOLOGY!!)

There are different types of natural
selection:
-stabilizing selection
-directional selection
-disruptive selection
-sexual selection
Stabilizing Selection

occurs when individuals with the
average form of the trait are most fit for
the environment and extreme traits are
eliminated

the most common form of selection
and works in all populations at all times
Example of stabilizing
selection

Lizard body size:
◦ Large lizards are easily seen by predators, but
smaller lizards cannot run as fast to escape the
predators
◦ Mid sized lizards are most fit in the
environment, so they survive and reproduce
more often, changing the allele frequencies in
the population
Directional Selection

occurs when individuals with one
extreme of variations are the most fit
in the environment

causes a gradual shift in allele
frequency to that extreme
Example of Directional Selection

Anteater tongue length:
◦ Anteaters with long tongues are most fit
because of the depth of the nests of the
termites they eat.
Disruptive Selection
occurs when both extremes of variations are
the most fit
 There is selection against the middle variations
 ie: Limpet shells
◦ dark limpets blend with bare rocks
◦ light limpets blend with barnacle covered rocks
◦ tan limpets are visible in both situations and
get preyed upon by birds

Sexual Selection

Sexual selection is the competition for
mates within a population

Mates tend to be chosen for their
phenotypes and females tend to
choose the males

The type of sexual selection can be
either stabilizing, directional OR
disruptive – this is determined by the
sexual preferences/practices of that
species.
Sexual Selection
◦ Male peacocks have
large tail feathers that
make it difficult to fly
and escape from
predators
◦ Female peacocks choose
males based on their tail
feather length and
fullness
◦ Over time males with
larger tail feathers
reproduce more causing
large tails to be selected
for reproduction