Download CHAPTER 23

The Evolution of Populations
and the Origin of Species
Chapters 23-24
• Evolution happens to populations/
species over time.
• Lamarckian evolution - evolution
result of change in individual in
response to environment (i.e.
giraffe stretching its neck to eat) incorrect hypothesis.
http://necsi.org/projects/evolution/lamarck/lamarck/giraffes.jpg
• Population composed of many
different genotypes, phenotypes
because of alleles carried in
population.
• Sum total of all alleles in population
- gene pool; variation in gene pool variations in individual phenotypes.
http://www.brooklyn.cuny.edu/bc/ahp/LAD/C21/graphics/C21_GenePool_2.GIF
• Measure of genetic variation in
population - allele frequency of
gene.
• # of copies of allele divided by
total # of copies of gene in
population.
http://anthro.palomar.edu/vary/images/map_of_A_blood_allele.gif
http://www.uni-kiel.de/medinfo/mitarbeiter/krawczak/folien/vorlesung2/img002.jpg
• If population does not change Hardy-Weinberg equilibrium – no
evolution.
• Frequencies change - evolution
occurring.
http://www.southtexascollege.edu/crj/human%20evolution.jpg
• Conditions for population to stay at
Hardy-Weinberg equilibrium:
• 1Random mating occurs.
• 2Population large enough to avoid
random statistical fluctuations in
frequencies.
• 3No mutation.
http://w3.dwm.ks.edu.tw/bio/activelearner/18/images/ch18c2.jpg
• 4No migration into/out of
population.
• 5No natural selection.
• Under conditions - free flow of
genes between members of same
species.
• Alleles shuffled up from 1
generation to next.
http://www.artgame.com/images3/migration.jpg
• In wildflower population of 500,
80% (0.8) of flower color alleles
are R and 20% (0.2) are r.
• Each gamete - 1 allele for flower
color; gamete drawn from gene pool
at random has 0.8 chance of
bearing R allele, 0.2 chance of
bearing r allele.
http://k43.pbase.com/u29/gaocus/upload/17965482.wildflower.jpg
• Rule of multiplication - frequencies
of 3 possible genotypes in next
generation.
• RR genotype - probability of picking
2 R alleles is 0.64 (0.8 x 0.8 = 0.64
or 64%).
• rr genotype - probability of picking
2 r alleles is 0.04 (0.2 x 0.2 = 0.04
or 4%).
• Heterozygous individuals - either
Rr or rR - R allele from sperm or
egg.
• Probability of ending up with both
alleles is 0.32 (0.8 x 0.2 = 0.16 for
Rr, 0.2 x 0.8 = 0.16 for rR, and 0.16
+ 0.16 = 0.32 or 32% for Rr + rR).
• p = gene frequency of dominant
allele, q = frequency of recessive
allele; p + q = 1.
• Equation for Hardy-Weinberg
principle is:
• p2 + 2pq + q2 = 1
• 2pq - # heterozygotes in
population.
http://bill.srnr.arizona.edu/classes/182/GeneFreqs/HardyWeinberg-lg.jpeg
• Hardy-Weinberg equilibrium,
frequency of dominant homozygous
curly hair (CC) is 64%. Percentage
with curly hair?
• p = frequency dominant allele (C) q
frequency recessive allele (c).
• CC frequency 64% so p2 = .64; p =
.8.
• (p + q = 1) q = 1 - .8 = 2.
• Individual with curly hair - either
CC or Cc.
• Percentage of population with curly
hair is p2 + 2pq = .64 + 2(.8*.2) = .96
or 96% of population.
Instabilities in populations
• Conditions can change HardyWeinberg equilibrium.
• Mutations cannot happen in
equilibrium; occurs in real world.
• Errors in DNA replication
accumulate over time as well as
mutagenic factors in environment.
• Mutations can lead to new alleles
not previously in gene pool.
http://www.sciencemuseum.org.uk/exhibitions/genes/images/1-3-5-1-2-2-2-2-2-0-0.jpg
• Mutations either neutral or harmful
on survival of individual.
• New phenotypes in population raw
material that natural selection acts
on to drive evolution; mutations only
source of new alleles.
• Migration affects equilibrium.
• Different populations have
different allelic frequencies in gene
pools.
http://www.geo.arizona.edu/Antevs/nats104/00lect26humigrout.jpg
• 1 population breeds with another
population, frequencies of alleles
change (gene flow).
• Small population more likely to have
random event than large population.
http://web.pdx.edu/~mfish/image019.jpg
• Genetic drift - changes in allele
frequencies in small population
caused by random events.
• Even in large population if small #
of individuals pass on traits can
decrease diversity.
• Individuals that do not pass traits
on may have harmful alleles - alters
gene pool of next generation (2
ways)
• 1Bottleneck - large population
reduced to small # by disease,
natural disaster, overhunting/fishing.
• Individuals left eventually
reproduce, generations not
representative of original gene pool.
• Inbreeding usually follows
bottleneck; individuals with same
recessive genes have more chance
of passing harmful gene on.
• Population more susceptible to
disease/infections that may not
have occurred with more diversity
in population.
http://www.brooklyn.cuny.edu/bc/ahp/LAD/C21/graphics/C21_Bottleneck_2.GIF
• 2Founder effect - small # of
individuals of species migrate into
new habitat.
• If only a few individuals colonize
new area, new population reflect
only their gene pool not larger gene
pool where they came from.
http://www.answersingenesis.org/creation/images/v18/i3/p13_step3.JPG
• Nonrandom mating - equilibrium
cannot occur.
• Individuals must choose mate
randomly without respect to
phenotype.
• If phenotype influences selection,
genotypes and phenotypes of
population will be changed.
http://bioweb.wku.edu/courses/Biol430/wsquirrelHR.jpg
• Self-fertilization in plants has this
effect.
• Reduces # of heterozygotes in
population; increases # of
homozygotes.
• Many species exhibit sexual
selection (form of nonrandom
mating)
http://www015.upp.so-net.ne.jp/shuri/shuri_study/what_is_si.GIF
• Natural selection - differential
production of offspring based on
inherited traits.
• Individuals with more favorable
phenotypes may survive, reproduce;
alters population frequencies.
• Fitness - key description of natural
selection.
http://www.telomere.org/images/Sloth.jpg
• Fitness - organism’s ability to
contribute alleles, traits to future
generations.
• Factors involved - ability to survive
to reproductive age, mate and
produce offspring, raise offspring
to maturity.
• Other factors - ability to escape
predation, gather food, attract
mates, or care provided to
offspring.
http://www.agpix.com/catalog/AGPix_CoTo11/large/AGPix_CoTo11_0092_Lg.jpg
• Individual with long life but few
offspring - poor fitness if other
individuals have more offspring.
• Animals that take care of offspring
- greater fitness than those that
do not.
• Balanced by having more offspring
that receive little care/fewer
offspring that receive more care.
http://www.facstaff.bucknell.edu/ddearbor/BFAL_feeds.jpg
• Three types of selective pressures
that affect natural selection over
time.
• Any given population, trait
distribution bell-shaped.
http://www.csulb.edu/~kmacd/346NotesI_files/normalCurve.gif
• 1Stabilizing selection - not change
average, tends to sharpen curve.
• Newborns can have problems if too
large or too small at birth –
stabilizing selection pushed average
to 8 pounds; perfect size for
newborn infant.
http://img.sparknotes.com/figures/A/a3aa6bb95c7d70781cc0089d17f9160f/stable.gif
• 2Disruptive selection - either side
of bell curve favored - leads to 2
different peaks in distribution of
population.
• 3Directional selection - change in
average for trait in population
occurs.
• Giraffes live in area where
vegetation high up - favor those
with longer necks - drives evolution
of longer necks.
• Kin selection - individuals with many
of same alleles live with other
members with same alleles.
• Lions live with many female
relatives, help raise young, even if
it is not his.
• Increases fitness of mother, even
if it doesn’t increase fitness of
male.
http://www.anth.ucsb.edu/faculty/gurven/images/orangatan.jpg
• Species - group of organisms able
to interbreed productively with
rest of group, not with other
organisms.
• Reproductive isolation – 2
populations of birds live in
different areas never mix - not
necessarily separate species.
http://www.lakelandwildlife.co.uk/images/species2.jpg
• If they breed and produce fertile
offspring when placed together same species.
• Separation of species can promote
evolution into 2 separate species no gene flow possible between 2
species.
• Result of separation of gene pools
over period of time.
• Cladogenesis - formation of 2
species from 1 ancestor species.
• Occurs when populations occupy
same area or when separated
geographically from each other.
• 1Allopatric speciation - populations
separated by geographic barrier
followed by reproductive isolation.
http://taxonomy.zoology.gla.ac.uk/~rdmp1c/teaching/L1/Evolution/l6/grandcanyon.gif
• Evolution of many diversely adapted
species from common ancestor adaptive radiation.
• 2Sympatric speciation - speciation
by populations that occupy same
region.
• New species arise within range of
parent populations.
• Can occur through sudden dramatic
genetic change - result of
polyploidy of genome.
• Can occur within species if
individual spontaneously (through
mistake in meiosis) produces
offspring with 2X normal
chromosomal number.
• Also result when cross between 2
related species produces hybrid
with chromosomal average of both
parents.
http://en.wikipedia.org/wiki/Hybrid_animals
A sheep goat
• Tetraploid cannot produce fertile
offspring if it mates with diploid,
could fertilize itself through selfpollination if plant.
• Polyploidy more common in plants has selective advantage over diploid
parents.
• Mechanisms of reproductive
isolation that cause speciation
divided into 2 groups.
• 1Prezygotic barriers prevent 2
species from mating.
• Occurs several ways.
http://www.sci.uidaho.edu/bionet/biol115/t9_species/images/L9_Mating-Barriers.jpg
• AHabitat isolation: species
separated because of where they
live.
• BBehavioral isolation: species have
different mating habits - never
interact.
• CTemporal isolation: breed at
different times of the year.
• DMechanical isolation: not
anatomically correct for each
other.
• 2Postzygotic barriers do not
prevent mating - prevent formation
of fertile offspring.
• Examples of this - reduced hybrid
viability (hybrid aborts
spontaneously), reduced hybrid
fertility (hybrids infertile), hybrid
breakdown (1st generation hybrids
viable and fertile, next generation
feeble or sterile).
http://www.kyhorsepark.com/imh/bw/images/dmconf.jpg
• Rate of speciation and relationship
between speciation and evolution
still controversy.
• Punctuated equilibrium - evolution
occurs rapidly in association with
speciation in small isolated
populations followed by long periods
where species changes little.