Download Speciation and Selection

Speciation and Selection
Mechanisms of Evolution
(don’t need to write down)
• Darwin’s theory of natural selection
explains how organisms adapt to
their environments and how
variations can give rise to
adaptations within species.
• Studies from population genetics
and molecular biology have led to
the development of evolutionary
theory.
What is Speciation?
It
is the evolutionary process by
which reproductively isolated
populations evolve to become
distinct species.
How does it happen?
 Populations
of the same species
become isolated
 Geographic
 Reproductive
Geographic Isolation
 Members
of a
population may
become
isolated from
one another by
geographic
boundaries like
mountains, rivers
or even oceans
Reproductive Isolation
When
the two populations
can no longer produce
viable (live), fertile offspring
(can produce own offspring
that can reproduce)
Reproductive Isolation
Allopatric Speciation
•
Occurs when a physical barrier
(mountains, islands, lava flows, rivers)
divides one population into two or
more populations
Sympatric Speciation
• Involves species who
evolve into a new species
without a physical barrier.
• The ancestor species and
the new species live side
by side during the
speciation process.
Rates of Speciation
• Gradualism: evolution proceeds in
small gradual steps.
• Theory of Punctuated Equilibrium:
Rapid spurts of genetic change
cause species to change quickly,
these periods punctuate much
longer periods when the species
exhibits little change.
Factors that lead to Evolution
Genetic
Drift
Gene Flow (movement of
genes)
Mutations (in DNA)
Sexual Selection
Artificial Selection
Genetic Drift
Genetic
drift is any change in
the allelic frequencies in a
population that is due to
chance
Causes a loss of genetic
diversity.
It is most common in small
populations.
Founder Effect
A
small sample of a population
settles in a location separated
from the rest of the population
Alleles that were uncommon in
the original population might
become common the new
population
Founder Effect
Leads
to:
 reduced
genetic variation
 a non-random sample of the
genes in the original population
Bottleneck
 It
occurs when an event
drastically reduces
population size and then
the population rebounds
 Reduced genetic
diversity, since less
diversity in gene pool
 The bottleneck effect is
genetic drift that occurs
after a bottleneck event
Gene Flow
A
population in genetic equilibrium
experiences no gene flow
 No
individuals leaving or entering
 Results
in low genetic variation
Nonrandom Mating
(don’t need to write down)
 Organisms
usually random mate
with individuals in a population
 Nonrandom mating promotes
inbreeding and could lead to a
change in allelic frequencies
favoring individuals that are
homozygous for particular traits
Mutation
 Though
many mutations cause
harm or are lethal, occasionally a
mutation provides an advantage to
an organism
 That
mutation may be passed on to
future generations
Natural Selection
 Natural
selection selects individuals that
are best adapted for survival and
reproduction
 Those organisms pass on their traits to
offspring
 Acts on an organism’s phenotype and
changes allelic frequencies
#1 Directional selection favors
phenotypes at one extreme.
Types of Selection
#2 Stabilizing selection favors
the intermediate phenotype.
Types of Selection
#3 Disruptive selection favors
both extreme phenotypes.
Types of Selection
Artificial Selection
The
process in which humans
select for or against particular
features in organisms
Example
Sexual Selection
Members
of one sex
choose mates with
particular features
Only
those with certain traits
succeed
Examples