Download Population Genetics and Speciation

Population Genetics
and Speciation
Chapter 16
Variation of Traits within a
Population
 Microevolution: is the evolution that occurs
within a population or a change in the gene
pool over a succession of generations
 Macroevolution is evolutionary change on a
grand scale, encompassing the origin of novel
designs, evolutionary trends and adaptive
radiation and mass extinction
Macroevolution
 Novel designs like feather & wings
 Trends like increasing brain size in
mammals
 Adaptive radiation is seen in flowering
plants
 Mass extinctions like the dinosaurs
Causes of Variation?
 Mutation
 Recombination
 Random pairing of gametes
The Gene Pool
 Definition: the total genetic information
available in a population
 Allele frequency: determined by dividing
the number of a certain allele by the total
number of alleles of all types in the
population
 Ex. Two alleles A, a. If in a set of 100
gametes, half are carrying allele A, then the
frequency of A is .5 or 50 %. The total of all
allele types must add up to 100.
Hardy-Weinberg Theorem
 Before we can look at microevolution we must
consider the H-W theorem. But first….. A few
definitions:
 Population-all the members of a single species
occupying a particular area at the same time.
 Species-organisms that share a common gene pool,
interbreed with one another
 Gene Pool- total of all the genes of all the
individuals in a population.
Hardy-Weinberg Genetic
Equilibrium
 A set of assumptions about an ideal,
hypothetical population that is NOT evolving.
 It states that the frequencies of alleles and
genotypes in a population’s gene pool remain
constant over the generation unless acted
upon by agents other than sexual
recombination.
 Sexual shuffling of alleles due to meiosis and
random fertilization have no effect on the
overall genetic structure of a population
5 conditions that must be
met to maintain equilibrium
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Population must be large
Population must be isolated from others
No mutations
Random mating must occur
No natural selection can occur
Getting the Hardy-Weinberg
Equilibrium Formula
 In a wildflower population there are two
alleles for color. A-pink and a-white.
 500 plants = 1000 alleles
 20 of those plants are white = 40 a alleles
 480 of those plants are pink
 320 are AA = 640 A alleles
 160 are Aa = 160 A alleles and 160 a alleles
 So the frequency of allele A is 800/1000
= .8 = 80%
 The frequency allele a is 200/1000 = .2 =
20%
 p = the frequency of allele A
 q = the frequency of allele a
 p + q = 1 (.8 + .2 = 1)
 If you consider genotypic frequencies
 AA = 320/500 = .64 = 64%
 Aa = 160/500 = .32 = 32%
 Aa = 20/500 = .04 = 4%
Hardy-Weinberg equation:
p2 +
2pq +
q2 = 1
(frequency of
AA genotype)
(frequency of
Aa genotype)
(frequency of
aa genotype)
For our example: .64 + .32 + .04 = 1
Disruption of Genetic
Equilibrium
 Evolution is the change in a populations’
genetic material over generations, that is,
a change of the population’s allele
frequencies or genotype frequencies.
 ANY exceptions to the five conditions
necessary for H-W equilibrium can result
in evolution.
Causes of Microevolution
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Genetic drift
Gene flow
Mutations
Nonrandom mating
Natural selection
 If any of these occur then equilibrium is NOT
present in the population!!
Gene Flow
 Populations may gain or lose alleles by
gene flow. This is genetic change due to
the migration of fertile individuals or
gametes between populations
 Ex. Human moving around the world.
Mutations
 A change in an organism’s DNA
Genetic Drift-changes in a gene
pool of a small population due to
chance
 Two situations that can lead to genetic drift:
 Bottleneck effect: disasters such as
earthquakes or floods reduce the pop.
drastically, killing victims unselectivelyreduces genetic variability.
 Founder effect: a small number of
individuals colonize an isolated island, lake
or other new habitat-reduced genetic
variability.
Nonrandom mating or
assortive mating
 Individual select
mates because of a
particular phenotype.
 Ex. Cardinal with the
brightest red feathers.
 Peacocks with the
most “eyes” in tail.
Natural Selection
 Differential success in reproduction
because an organism is more fit for their
environment.
 Which colored dot “mouse” became most
common in your Adaptation Activity?
Types of Selection
 Stabilizing: individual with the average
form of a trait have the highest fitness.
 Disruptive Selection: individual with either
extreme variation of a trait have a greater
fitness than individual with the average
form of the trait.
 Directional Selection:
individual that display
a more extreme form
of a trail have greater
fitness than
individuals with an
average form of a
trait.
Formation of Species
 Biological concept of Species:
 a population of organisms that can
successfully interbreed but cannot breed
with other groups.
It all begins with being
isolated.
 Geography: barriers formed by canyons,
mountains, water or deserts (cities and
highways) can divide or fragment and
isolate parts of populations from each
other.
 Natural selection and genetic drift cause
the two subpopulations to diverge,
eventually making them incompatible for
mating.
Allopatric Speciation
 New species arise because of
geographic isolation.
 More likely to happen in small populations
where gene pool will change quickly.
Reproductive Isolation
 May happen in the
absence of
geographic barriers.
 May be caused by
disruptive selection
 Temporal: different
breeding times
 Behavioral: different
mating calls.
Sympatric Speciation
 Two species develop reproductive
isolation within the same geographic area
by occupying different niches.
Rates of Speciation
 Gradualism: speciation occurs at a
regular, gradual rate. Change happens
slowly
 Punctuated equilibrium: sudden, rapid
change followed by long periods of
equilibrium or little change.