Microevolution - Population
Genetics
Evolutionary Biology
(BI 25) Lecture Notes
Microevolution
Definition - study of evolution
within species, evolution at the level of populations,
changes in gene frequencies in
populations, populations must be variable for evolution to occur,
variability among individuals must
have a genetic basis
Variation within populations
Variation occurs among individuals:
morphological, behavioral, physiological, must have a
genetic basis for evolution to
occur
Must be able to assess the nature
of the variation in populations
Problems with recognizing genetic
variation among populations - non-evolutionary variation
Age variation
3 year gulls from North America
Ring-billed Gulls
Adult
1
Adult
2
Immature
1
Herring Gulls
Immature
1
egg, larva, pupa adult stages
of invertebrates
deer - fawn versus adult
salmon - hooked mouth of adults
in some species prevents adults from feeding
Seasonal variation
plumage changes in birds: winter
versus breeding summer plumage
pelage changes in mammals -
winter versus summer coats
Sexual dimorphism
male and female differences
(Pinnipeds, whales, many species of birds, spiders, insects, etc...)
Environmental variation
Social variation (hymenopterous
insects - castes and occupations of different members of these
insect societies: workers,
soldiers)
Evolutionary variation
Individual variation - Pigeons,
other
Polymorphism - well recognized
morphs within species (mimetic butterflies, Screech Owl,
Snow Goose, snakes, Pepper
moths)
Balanced polymorphism (balance
or equilibrium achieved between 2 or more morphs)
Transitional polymorphism (one
morph in process of replacing another)
Geographic variation
Birds - Junco, Fox Sparrows,
Song Sparrow
Mammals - pocket gophers of
the western US
Tropical land snails
Other
Population genetics
Populations are composed of individuals
Each individual has two copies
of a gene or allele in their genotype, they are either:
AA, Aa or aa
Population is a gene pool with
gene and genotype frequencies
Calculations are relatively
simple
Find a specific phenotype and
genotype of interest: AA, Aa, aa
genotype frequency for AA
is the number of individuals with AA/total
number of individuals (population
size)
genotype frequency for Aa
is the number of individuals with Aa/total
number of individuals (population
size)
genotype frequency for aa
is the number of individuals with aa/total
number of individuals (population
size)
allele frequency is the number
of a alleles in the population/the total number
of alleles in the population
(twice the population size because every individual
has two alleles: AA or
Aa or aa)
allele frequency is the number
of A alleles in the population/the total number
of alleles in the population
(twice the population size because every individual
has two alleles: AA or
Aa or aa)
Hardy-Weinberg Equilibrium - model
of no change (Weinberg 1908)
No changes in gene frequencies
when there is no: assortative mating, genetic drift, mutation,
natural selection, gene flow (migration)
Derivation
Agents of Evolution - agents that
cause changes in populations
Assortative Mating - non-random mating
Types
+ Assortative Mating: Inbreeding - like x like individuals
- Assortative Mating: Outbreeding - like x unlike individuals
Consequences of Inbreeding
1) Harmful - increase the chances of 2 deleterious or lethal alleles coming together
2) Harmless - occurs in many species of plants that undergo selfing
Examples of Inbreeding from the literature
Stone (1963), Dobzhansky (1963),
Mettler et al. (1963) - inbreeding depression in Fruit Flies,
numbers are % survival rates
| Species |
Distant cousins |
Closely related |
Sibling |
| 1 |
90 |
75 |
65.5 |
| 2 |
90.2 |
83.2 |
80.5 |
| 3 |
84.3 |
|
63.5 |
Crow et al. (1956) - incidence of problems in human births
| Problem |
1st cousin |
2nd cousin |
unrelated |
| still birth |
.09 |
.06 |
.03 |
| infant death |
.14 |
.09 |
.07 |
Spuhler (1968) - human mating preferences (tall females preferred tall males)
Andersson (1982) - widow birds females showed preferences for males with longer tails
Moller (1989) - similar pattern in Barn Swallows
Jain (1976) - inbreeding with no problems: cleistogamous plants (flowers never open -
grasses and violets)
Faegri and Pijl (1971) - outbreeding, flower construction prevents selfing
Jones (1924) - corn, inbred lines were small and not productive, outbreeding promoted
genetic health
F coefficient - probability that two alleles in an individual are identical (derived from a single ancestral allele)
Ancestor has A1A2
, offspring in subsequent generations has A1A1 -
the probability of getting this kind
of offspring is F
High inbreeding means high
F, low inbreeding means low F
How F changes HW equilibrium, homozygotes increase and heterozygotes decrease each generation
f(AA) = p2 +Fpq
f(Aa) = 2pq(1-F)
f(aa) = q2 +Fpq
An extreme example of inbreeding - selfing, F=1/2
| Generation |
AA |
Aa |
aa |
| 0 |
1/4 |
2/4 |
1/4 |
| 1 |
3/8=1/4 +(1/2)(1/4) |
2/8=(1/2)(2/4) |
3/8=1/4 +(1/2)(1/4) |
| 2 |
7/16=3/8+(1/2)(1/8) |
2/16=(1/2)(2/8) |
7/16=3/8+(1/2)(1/8) |
| 3 |
15/32=7/16+(1/2)(1/16) |
2/32=(1/2)(2/16) |
15/32=7/16+(1/2)(1/16) |
Natural Selection - survival of the fittest
Conditions under which natural selection can occur
genetic variation among individuals
differential survival
differential reproductive success
What is the struggle in the environment against?
Abiotic factors - physical aspects of the environment
Climate - precipitation, wind, humidity, temperature, etc.
Other - altitude, daylength, water depth, water chemistry, tides, etc.
Biotic factors - biological aspects of the environment
parasitism, predation, competition, etc.
Types of selection
Directional Selection
Example: Galapagos Finches
Examples from the literature
Ricker (1981) - Pink Salmon decrease in size over time with fishing
Kettlewell (1973) - Pepper Moths
Wood (1981) - DDT resistance and insects
Antonovics et al. (1971) - plants and heavy metal resistance near mines
Hirschberg and McIntosh (1983) - weed resistance to herbicides (triazine)
Boag and Grant (1981) - Galapagos Finches (during drought, body size of birds increased)
Stabilizing Selection
Example: Baby weights
Examples from the literature
Bumpus (1899) - House Sparrows near Woods Hole, MA
Karn and Penrose (1951) - birth weight in human babies
Rendel (1953) - duck eggs parallel human birth weight results
Hecht (1952) - lizard size affected by predation versus territorial defense
Mason (1964) - milkweed butterflies (less variable males performed most of the matings)
Disruptive Selection
Examples from the literature
All cases of Batesian mimicry
Ford (1975) - multiple phenotypes of the Swallowtail Butterfly
Thoday and Gibson (1962) - laboratory example, selected for extreme Fruit Flies
Remington (1954) - field study of the Sulfur Butterfly (orange-winged versus white-winged morphs)
Other Types of Natural Selection
Frequency Dependent
All cases of Batesian mimicry - too many mimics spoils the program and predators find out
All cases of Mullerian mimicry - the more the merrier, more toxic mimics of each other - the faster the
predators find out
Clark (1962) - when morphs
reach a certain level in the population they are selected against by
fish predation
Ehrman (1967) - rare male Fruit Fly achieves the most matings
Sexual selection
first described by Darwin as a mechanism that leads to sexual dimorphism in a species
could be due to female mate choice
could be due to result of male versus male interactions
other (e.g., exploitation of different feeding niches)
Interesting products of Natural Selection
Heterosis - heterozygote advantage, sickle-cell anemia in the tropics (also example of balanced polymorphism)
Balanced Polymorphism - preservation
of genetic variation through natural selection, frequency of morphs is
stable over time
Transitional Polymorphism - one morph is being replaced by another over time due to selection
Example of multiple niche polymorphism
Clarke et al. (1963) - Papilo butterfly morphs feed on citrus plants versus umbelliferous plants
Tabachnik et al. (1979) - mosquito morphs lay eggs indoors versus outdoors
Jones (1980) - red-eyed Fruit Flies attracted to white light, white-eyed flies attracted to red light
Van Valen's Red Queen Hypothesis
- every species is constantly facing new challenges in the environment,
nature perpetuates cyclical
process that is a never ending arms race with the environment
Mathematical Models
Coefficients - used to compare
one or more genotypes with each other, coefficients are produced for one
genotype relative to another
s - selection coefficient (% that die each generation, s = 1-w)
w- fitness coefficient or adaptive value (% that survive, w = 1-s)
Example for genotypes
AA individuals produce 100 offspring and all offspring live to reproduce
s = 0 because all offspring survived
w = 1 that all offspring survived
aa individuals produce 100 offspring and only 90 live to reproduce
s = 10 that died/100 = .10
w = 90 that survived/100 = .90
Two models of selection
Allelic selection - against gametes or haploid organisms, more rapid because because both are exposed
(recessive allele can not hide dominant allele in a heterozygote)
Zygotic selection - against genotypes and diploid individuals
Allelic selection - against
gametes or haploid organisms, compute the relative frequency of alleles
after selection against a (recessive allele)
| Allele |
Frequency before selection |
Selection Coefficient |
Fitness Coefficient |
| A |
p |
0.0 |
1.0 |
| a |
q = 1-p |
s |
w=1-s |
| Frequency
of A before selection |
p |
| Frequency
of gametes before selection (total) |
p+q |
| Frequency
of gametes after selection (total) |
p+q(1-s)=
p+(1-p)(1-s)=
p+1-s-p+sp=
1+sp-s=
1-s(1-p)=
1-sq=
|
| Frequency
of A after selection = frequency of A before
selection/total after
selection
|
p/(1-s(1-p)=
p/1-sq=
|
| Rate of
change in the frequency of A=relative frequency
after selection minus
the, relative frequency before
selection, should be
positive because of selection
against a
|
(p/1-sq)-p=
(p/1-sq)-p[(1-sq)/(1-sq)=
p-(p-spq)/1-sq)=
p-p+spq/1-sq=
spq/1-sq =
|
Zygotic selection - against genotypes and diploid individuals
| Item |
AA |
Aa |
aa |
Total |
| Initial
frequency |
p2
|
2pq |
q2
|
1=p2+2pq+q2 |
| Adaptive
value (w) |
1 |
1 |
1-s |
|
| Frequency
after selection, does not equal one because of selection, must be
divided by the new relative population size after selection |
p2
|
2pq |
q2(1-s) |
p2+2pq+q2(1-s)
=
p2+2pq+q2-sq2
=
1-sq2
=
|
| Relative
frequency after selection, notice change in frequencies which is
different from the HW equilibrium |
p2/1-sq2 |
2pq/1-sq2 |
q2(1-s)/1-sq2 |
|
| Rate
of change in the recessive allele - a, note that it is negative
because it is being selected against |
Frequency
of a after selection |
Rate
of change in a during selection
qafter-qbefore=rate
|
|
(pq/1-sq2)+[q2(1-s)/1-sq2]=
pq+q2-sq2/1-sq2=
q(p+q)-sq2/1-sq2=
q-sq2/1-sq2=
q(1-sq)/1-sq2=
|
[q(1-sq)/1-sq2]-q=rate
[q(1-sq)/1-sq2]-[q(1-sq2/1-sq2)]=rate
q-sq2-q+sq3/1-sq2=rate
-sq2+sq3/1-sq2=rate
-sq2(1-q)/1-sq2=rate
|
Mutation
Occurs at 2 levels - genic level and chromosomal level
Chromosomal level mutations
Deletion - loss of a segment
Fusion - addition of a segment
Inversion - reversal of a segment
Aneuploidy - loss or addition of chromosomes due to non-disjunction during meiosis
Genic level mutations - exchanges or substitutions of nitrogenous bases (Purines - A, G; Pyrimidines - C, T)
Transition
substitution of purine for a purine - see bold text (G for A)
ATCG changes to GTCG
substitution of pyrimidine for a pyrimidine - see bold text (C for T)
ATCG changes to ACCG
Transversion
substitution of pyrimidine for a purine or vice-versa - see bold text
ATCG - TTCG
Frameshift mutation - loss or addition of a nitrogenous base in a gene sequence
addition - ATCG changes to AATCG
loss - ATCG changes to ACG, lost T
Consequences of mutations
Silent mutation - change does not affect amino acid sequence of protein production
Missense mutation - alters amino acid sequence
Nonsense mutation - turns off protein production
Mutation rates
rate = frequency of occurrence of new mutations
rate = number of mutations/gamete/generation
Examples from the literature
3 Types of Studies of Mutation in the Literature
1) Neutral mutations
2) Deleterious mutations
3) Adaptive mutations
Neutral Mutations
Prakash et al. (1969) - found evidence for at least 12 different alleles of the EST 5 locus in Fruit Flies,
none were lethal in homozygous condition
Margoliash (1972) - similar results with cytochrome C proteins
Deleterious Mutations
McKusick (1975) - cataloged over 2000 human genes that produce genetic disease
Suzuki et al. (1989) - temperature sensitive alleles in Fruit Flies (some flies had lethal genes
and they died due to paralysis when temperatures rose above 28oC
Ellergen and Fridolfsson (1997)
- mutation rates are higher in Collared Flycatcher males compared to females,
sperm production has the potential to produce more mutations than egg
production
Adaptive Mutations
Dobzhansky and Spassky (1947)
- examined 7 strains of D. pseudoobscura with low fitness (poor
egg viability) over 50 generations, 5 strains eventually recovered to
equivalent of wild populations due to mutations
Ayala (1966) - used D.
birchii, compared an irradiated population with a control population
that did not receive any treatment, both populations subjected to selective
pressures (limited food and space), irradiated population recovered
and at a much faster rate than the control group, due to new mutations
caused by radiation exposure
General Findings
Dobzhansky et al. (1977)
1) rate for human eye problems - 1.2 - 2.3 x 10-5 mutations/locus/gamete/generation
2) rate for dwarfism - 4.2 - 14.3 x 10-5 mutations/locus/gamete/generation
3) rate for sugar content of corn - 2.4 x 10-6 mutations/locus/gamete/generation
4) rate for coat color in mice - .97 - 7.1 x 10-5 mutations/locus/gamete/generation
Lande (1979)'s general conclusions
1) mutation rates differ among species
2) average rate = 1x10-3 to 1x10-8 mutations/locus/gamete/generation
Mathematical Model
Calculating gene frequencies in a population with a one-way mutation over time
A mutates into a at a rate of u (there will be a net loss of A over time), p = frequency of A allele,
Example for a one-way mutation
| Generation |
Frequency |
| 0 |
p |
| 1 |
po-upo=p1
po(1-u)=p1
|
| 2 |
p1-up1=p2
p1(1-u)=p2
po(1-u)(1-u)=p2
po(1-u)2=p2
|
| t |
po(1-u)t=pt |
Calculating equilibrium for two-way mutation, A mutates into a at a rate of u, a mutates into A at a rate of v
peq =v/(u+v)
qeq =u/(u+v)
Migration - gene flow among populations
Effects on populations
changes gene frequencies
homogenizes populations (makes different populations more similar to each other)
Models of Gene Flow
Continent - Island (one-way)
Island Model (genes move to and from all island)
Stepping Stone
Isolation by distance
Examples from the literature
Glass and Li (1953) - studied Rh alleles in humans (white and African-Americans)
Adams and Ward (1973) - studied MN blood groups (Africans, African Americans, White Americans)
Workman (1973) - South American populations
Mathematical Model
Q - frequency of a in the recipient population, q - frequency of a in the donor population
M - % of individuals in the recipient population that came from the donor population (frequency of migrants)
Genetic Drift - random fluctuations of genes in populations,
What causes drift
Factors
founder population - small sample of population leaves and colonizes new area
Example: Founder effect - plant seeds on seabirds
bottleneck effect - population size is reduced by external factors
Example: Bottleneck effect - Cheetah
Examples from the literature
Dobzhansky (1970) - compared
deviation in genotype frequencies between a group
of small versus large populations
Founder Examples
Maniatis et al. (1980) - Blood groups in the Dunkers (allele frequencies)
| Population |
IA |
IB |
IO |
| Dunker - US |
.38 |
.03 |
.59 |
| non-Dunker - US |
.26 |
.04 |
.70 |
| W. German |
.29 |
.07 |
.64 |
Afrikaner Population in South Africa (Dean 1972, Hayden 1981)
20 families are descendants of current population
high incidence of porphyria and Huntington's disease
Kidd and Cavalli-Sforza (1974) - cattle in Iceland versus mainland Europe, brought over
by the Vikings, exhibit different frequencies
Buri (1956)
brown alleles in Fruit Flies changed over a few generations, each new generation
was started by a founder of 8 males and 8 females, lead to elimination and fixation
in different lines of flies
Carson (1992)
Colonization patterns of Fruit Fly species in Hawaii, salivary chromosome banding patterns
reveal that founder populations were descendants of new species on many islands
Bottleneck Effect Examples
Bonnell and Selander (1974) - Northern Elephant Seal populations depleted by overhunting,
exacerbated by harem system of mating, leads to lack of genetic variation in today's
population
2 Effects of Genetic Drift
1) genetic divergence among populations
2) March towards homozygosity, fixation of alleles (p reaches 1 and q goes to 0 or vice versa)
Calculating Population Size
Ne = effective population size, population of breeders
Unequal sex ratio:
Fluctuations in population size: Harmonic Mean
Variability in fertility
among breeders, k - average number of gametes, variance in
number of gametes
Conservation Biology
Definition - identification of endangered populations and study of methods/solutions to protect them
Problems
Pollution, Development, Harvesting (hunting, commercial exploitation, poaching), Introductions
Solutions
In situ - conserve land/habitat/ecosystems, legislation and protection (Endangered Species Act, CITES Treaty)
Ex situ - zoo propagation of endangered species, captive breeding programs
Connection with evolution - How can Evolutionary Biologists play a role in conservation - 3 contributions
-
Assess biodiversity
-
Assess genetic health of a species
-
Study extinction and causes
Assess biodiversity
Identification of populations or conservation units - species, subspecies, local populations
Describe range of endangered populations, breeding and wintering ranges
Identify areas of endemism - region of origin and evolution
Techniques
Collect behavioral data and specimens from field expeditions
Examine morphological and genetic differences among populations
Examine behavioral data for isolating mechanisms
Examine existing specimens from museum collections
| Type
of Specimen |
Major
Groups |
Click
on Thumbnail to Enlarge |
| |
|
|
| Study
skin |
Birds |
|
| |
Mammals |
|
| Skeleton |
Birds
- whole skeleton |
|
| |
Mammal
skulls |
|
| |
|
|
| Alcoholic
or spirit specimen |
Fish |
|
| |
|
|
| Mounted
specimen |
Mammals |
|
| |
Birds |
|
| |
Beetles |
|
| |
Birdwing
Butterfly |
|
| |
Beetles
- pinned |
|
| |
|
|
| Fossil |
Imprint |
|
| |
Imprint |
|
| |
Fossil
teeth |
|
| |
Fossil
bones |
|
| |
Fossil
insect in amber |
|
| |
Stromatolite |
|
| |
|
|
| Birds
nest |
Black-capped
Chickadee cavity nest |
|
| |
Barn
Swallow mud+ nest |
|
| |
Indigo
Bunting cup nest |
|
| |
Killdeer
ground nest |
|
Some examples
Prosimians in Africa
- some new and endangered nocturnal species identified by behavior (vocalizations)
Key Deer - special population
of White-tailed Deer in Florida Keys, smaller than mainland populations
Dusky
Seaside Sparrow - special subspecies of Seaside Sparrow gone extinct
due to development
along Florida coast
Tigers in Asia - shrinking
breeding range and exploitation/poaching to make local delicacy
Mongolian
(from Harvard Museum of Natural History)
Bengal
(from Harvard Museum of Natural History)
Examples of loss of biodiversity
| Birds |
2000 species lost over
the past 2000 years
See Heywood for list
|
| Fish |
20% of freshwater fish
extinct or declining |
| Plants |
213-218/20,000 plant
species are extinct |
| Insects |
17%-60% of species lost
in different European countries |
Assess genetic health of a species - look for loss of genetic diversity
Use of biochemical genetics to identify populations with genetic drift (bottleneck)
Cheetah (from Harvard Museum of Natural History) - experienced genetic drift
2%-4% heterozygosity among 49 loci for allozymes
morphology - odontometric asymmetry due to developmental instability
skin graft test - little if any rejection after 78 days (1 week in most other cat species)
highly susceptible to the FIP virus
Consequences of loss of genetic diversity
increase in homozygosity - higher probability of expressing lethal and deleterious genes
susceptibility to disease and parasitism
negatively affects gamete production
Examples - see handout
Study extinction and causes
Examples of human/artificial disturbance to natural populations
Olson (1970s - 1980s) - Polynesians in the southwest Pacific
Birds of Hawaii - extinct
flightless birds, Drepanids, seabirds
Example of plants in Hawaii
90% of native flowering
plants from Hawaii are found only in Hawaii
Chart
- summary of status of flowering plants
plants of lowland dry
forest are most vulnerable
causes:
human disturbance, feral animals, alien plants - destroy habitat
Ethnobotany
of Hawaii - examples of endemic, indigenous and introduced plants
European explotiation of Madagascar
Dodo - link
Dodo - mount and skeleton (from Harvard Museum of Natural History)
European exploitation of North America
Paleoindians along with climate change
Extinction of Mammoths, Camels, Horses
subspecies of Bison
Alteration of environment due to fire hunting
replacement of forest with grassland
Piping Plover along the Atlantic Coast of the United States
Piping Plover warning signs and nest enclosures
Piping Plover wing feign display
Piping Plover nest with eggs
Piping Plover video - foraging at dusk
Great Auk off the coast of Newfoundland
Great Auk - link
Great Auk - mount and skeleton (from Harvard Museum of
Natural History)
Carolina Parakeet
Carolina Parakeet - link
Carolina Parakeet - mount (from Harvard Museum of Natural History)
Passenger Pigeon
Passenger Pigeon - link
Heath Hen
Heath Hen - link
Heath Hen - mount (from Harvard Museum of Natural History)
Ivory-billed Woodpecker
Ivory-billed Woodpecker - link
Ivory-billed Woodpecker - mount (from Harvard Museum of Natural History)
Labrador Duck
Labrador Duck - link
Labrador Duck - mount (male) (from Harvard Museum of Natural History)
Alaska - Arctic Fox farming on seabird islands
Prehistoric tribes - possible role in extinction of large mammals
Speciation - origin of new species
General Model
populations begin to diverge
from other similar populations (due to agents of evolution)
changes become so great, leads
to reproductive isolation
Allopatric speciation: differentiation of geographically isolated populations into species
Geographical or Continental Speciation - Examples
Other North American Species
Mengel's (1964) species groups of North American Warblers
Nashville Group
Connecticut Group
Black-throated Green Group
Yellow-rumped Group
Rising (1983)
Meadowlarks
Orioles
Flickers
| Yellow-shafted Flicker |
eastern North America |
| Red-shafted Flicker |
western North America |
Buntings
Grosbeaks
Remington (1964)
Butterflies
Deer
Explanation - Pleistocene Ice Age?? - see Klicka and Zink (1999)
Quantum Model - peripheral isolates become founder populations
Archipelago = Island Speciation
Bock (1970) - Hawaiian Honeycreepers
Darwin (1859) - Darwin's Finches
Mayr (1954) Papuan Kingfishers, mainland populations are very similar
but island populations have diverged
Carson (1975) - Hawaiian Drosophila
Current questions about allopatric speciation
which agents are responsible for changing gene frequencies
when did isolating mechanisms develop (during versus after isolation)
Parapatric speciation - species separated by hybrid zones and other ecological conditions
Mayr (1963) - Hooded and Carrion Crows in Europe
Nevo (1972) - mole rats in Asia
Sympatric speciation: splitting of populations in a common area into species
Instantaneous
Polyploidy
autopolyploidy - results from members of the same species
allopolyploidy - results from members of different species
Example: plant hybridization
Lewis and Lewis (1955) - plant genus Clarkia
primitive species, 2N are 7 and 8 chromosomes
advanced species, 2N are 18, 16, 15 chromosomes (probably 4N)
Gradual model - multiple niche polymorphism
Boiler and Bush (1973) - host
switching by parasites
Tauber and Tauber (1989) -
lacewing insects: C. carnea feeds on deciduous trees, C. downsei feeds
on conifers
Bush (1969) - Rhagoletis Fruit
Fly species - rapid host shift from Hawthorn trees to Apple, Cherry, Pear,
etc. occurred over last 100
years
Kettlewell's Peppered Moth
Isolating Mechanisms
Retention of Species Identity
Prezygotic mechanisms: prevent formation of zygotes
Postzygotic mechanisms: prevent functioning and development of zygotes
Prezygotic Isolating Mechanisms
Ecological isolation - Same area, but different habits and habitats
Nobs (1963) - Ceanothus bushes on different soils in California
C. jepsonii on serpentine soils
C. cuneatus and C. ramulosus on all other soil types
hybrids - rare
de Buck and others (1900s)
Anopheles mosquito species in different habitats
different species in brackish, fresh and stagnant waters
Temporal isolation - Breeding periods at different times
Stebbins (1950) - species of the genus Pinus in California
P. radiata and P. muricata have different pollination periods (Feb and Apr respectively)
Blair (1941) - species of the genus Bufo breeding at different times
B. americanus breeds April - June
B. fowleri breeds February - May
Behavioral or Ethological isolation - Species specific mating rituals
Littlejohn (1965) - frog and toad vocalizations and mating
Stein (1960's) - Empidonax flycatchers
Trail's Flycatcher divided into Willow and Alder Flycatchers based on song differences
also includes Acadian, Least, Yellow-bellied Flycatchers
Fish Crow and American Crow
Lanyon (1957, 1962) - Eastern and Western Meadowlarks in the United States
Pitocchelli (1990) - Mourning and MacGillivray's Warblers
Eastern and Western Wood Pewees in the United States
Sex Pheromones in insects and mammals
Hawaiian Drosophila
Fireflies - different signal patterns for different species
Cricket song and female choice
Mechanical isolation - General structural differences in genitalia or other structures prevent interbreeding
Dufour (1844) - lock and key relationship of male and female genitalia in some insects
Grant and Grant (1964) - flower structure in Salvia flowers restrict pollinators
S. mellifera - small to medium size bees
S. apiana - large bumble bees
Dodson (1967) - orchids
flowers of some species mimic female insects of certain bees and wasps, encourages pseudo-copulation
and pollination
Gametic Isolation - Prevention of gamete fusion, Sperm not attracted to eggs of other species, Sperm
incapable of penetrating eggs,
Growth of pollen tubes impeded
Dobzhansky et al. (1977) - marine invertebrates release millions of gametes into the ocean
Smith (1970) - flowers inhibit the fertilization of foreign pollen
Dobzhansky et al. (1977) - Drosophila and insemination reaction - vaginal swelling incapacitates foreign sperm
Postzygotic Isolating Mechanisms
3 types
Hybrid inviability - embryos die early
Hybrid sterility - adults are somatically vigorous but can not reproduce
Hybrid breakdown - adults are somatically vigorous and can reproduce but future offspring fail to reproduce
Hybrid inviability
Dobzhansky et al. (1977) - Sheep and goat - hybrid embryo dies soon after fertilization
Moore (1949) - Rana frogs and varying degrees of inviability (die at different times after fertilization)
Sonneborn (1900's) - Paramecia hybrids dies soon after conjugation
Hybrid sterility
Dobzhansky et al. (1977) - horse x donkey - mule is sterile, problems occur before meiosis
Karpechenko (1927) - radish x
cabbage - sterile offspring, problems during meiosis or tetraploid individuals
that can not back cross
Dobzhansky et al. (1977) - Drosophila
D. pseudoobscura x D. persimilis - hybrid males unable to produce sperm but females are fertile
Hybrid Breakdown
Dobzhansky et al. (1977) - Drosophila
D. pseudoobscura x
D. persimilis - hybrid females are fertile but offspring of the F1 generation
experience problems
Stephens (1950) - different species of Cotton (Gossypium) experience breakdown
Origin of isolating mechanisms
Accidental by-product of
genetic divergence, could occur during allopatry - could be due to pleiotropy,
epistasis; selection for sexual traits which tend to be more variable than other
phenotypic traits (Civetta and Singh 1998's study of testis length in Drosophila
species, Buckley et al.'s study of pheromone differences among Drosophila species,
etc.)
Examples
Dobzhansky et al. (1977) - Crepis tectorum x C. capillaris - F1 hybrids die because of lethal gene
Mayr (1954) - founder populations,
genetic revolution, populations at the periphery of the range are more divergent
Kilias and Alahiotis (1982) - separated lab populations of D. melanogaster for 6 years under different conditions
which produced sterility and isolation
Carson (1986) - origin of 26 species of Hawaiian picture-winged Drosophila, founder populations responding to
microhabitat changes
Increased divergence among allopatric populations compared to sympatric
Patterson and Stone (1952) - European species (D. littoralis) more isolated from American populations of
D. americana, texana and novamexicana than any of the American populations are from each other
Natural selection against hybrids and hybridizing parents - occurs when when species get back together after allopatry
Examples
Koopman (1950) - selection against hybrids between D. pseudoobscura and D. persimilis
selected against hybrids and parents of heterozygotes, reduced frequency of hybrids from 50% - 5%
Paterniani (1969) - against hybrids between yellow sweet and white flint strains of corn
selected against parents of heterozygotes, reduced frequency of hybrids from 40% - 5%
Increased divergence among sympatric populations compared to allopatric
Wasserman and Koepfer (1977) - found that sympatric strains of D. arizonensis and D mojavensis
hybridized less than allopatric strains
Noor (1995) - similar results with D. pseudoobscura and D. persimilis
Species Concepts
Why so many concepts?
Theoretical implications
problems finding a universal concept that applies to all species
problems on agreeing upon criteria to delineate species
reproductive isolation
amount of divergence
other
Practical applications
Morphological divergence and ability to interbreed
Examples
Subspecies of Song Sparrow in the United States
Subspecies of Fox Sparrow in the United States
Subspecies of Dark-eyed Juncos in the United States
Subspecies of Horned Larks in the United States
Yellow-rumped Warblers
No morphological divergence and reproductive isolation - sibling or cryptic species
Examples
Empidonax Flycatchers in the eastern United States
Acadian, Least, Yellow-bellied, Willow, Alder Flycatchers
Fish Crow and American Crow
Eastern and Western Meadowlarks in the United States
Mourning and MacGillivray's Warblers
Eastern and Western Wood Pewees in the United States
sibling species of frogs
Spatial separation and no test of reproductive continuity
Nashville Warbler
Temporal isolation - paleontology dilemma
Hybridization among populations from different species
Species of North American Birds
Blue-winged and Golden-winged Warblers of the United States
Lazuli and Indigo Buntings of the United States
Black-headed and Rose-breasted Grosbeaks of the United States
Uniparental species
2 Broad Categories
Process Species Concepts - how speciation is achieved is incorporated into the concept
Process-free Species Concepts - process is not important
Process Species Concepts
Biological Species Concept: Species are groups of interbreeding natural populations that are reproductively isolated
from other such populations.
emphasis: reproductive isolation
problems: universal application - unisexual species, paleospecies and the temporal dimension
Recognition Concept: Species are inclusive groups of individual biparental organisms which share a common
fertilization system. One of the important components of the fertilization system us a subset of
adaptations which are involved in signaling between mating partners which constitute the Specific-
Mate-Recognition System (SMRS).
emphasis: reproductive isolation
problems: universal application - unisexual species, paleospecies and the temporal dimension
Ecological Spcecies Concept: A species is a lineage (or closely related set of lineages) which occupies an adaptive zone
minimally different from that of any other lineage in its range and which evolves separately from all other
lineages outside its range.
emphasis: natural selection, co-adapted genes and ecological niche
problems: ignores other agents responsible for evolution
Process-free Species Concepts
Phenetic Species Concept: Species are those groups of organisms that have the most overall phenotypic similarity.
Overall phenotypic similarity is determined by statistical methods which analyze phenotypic characters
(usually external measurements) and provide measures of similarity and dissimilarity that are used to
define species.
emphasis: mathematical similarity
problems: consistency of application among species, misses cryptic/sibling species
Evolutionary Species Concept
A species is a single lineage of ancestral descendant populations of organisms which maintains
its identity from other such lineages and which has its own evolutionary tendencies and
historical fate
emphasis: evolutionary tendencies and roles, temporal continuity
problems: defining evolutionary roles and tendencies, dealing with gradually changing roles
Phylogenetic Species Concept: A species is the smallest diagnosable cluster of individual organisms within which
there is a parental pattern of ancestry and descent.
emphasis: diagnostic characters
problems: elevating too many subspecies to species level
Literature Cited
Andersson, M. 1982. Female choice
selects for extreme tail length in a widowbird. Nature199: 818-820.
Crow et al. (1956)
Dobzhansky, T. et al. 1963. __________.
Faegri, K. and L. van der Pijl. 1971.
The principles of pollination ecology. New York, Pergamon Press.
Jain, S. K. 1976. Evolution of inbreeding
in plants. Ann. Rev. Ecol. Syst. 7: 468-495.
Jones, D. F. 1924. The attainment
of homozygosity in inbred strains of maize. Genetic 9: 405-418.
Mettler et al. (1963)
Moller, A. P. 1989. Viability costs
of male tail ornaments in a swallow. Nature 339: 132-134.
Spuhler, J. N., 1968 Assortative
mating with respect fo physical characteristics. Eugen. Quart. 15: 128-140.
Stone, W. S. et al. 1963.
BI
25 Homepage
Saint
Anselm College Homepage
Blackboard
at Saint Anselm College
Dr.
Jay's Homepage
Trademark and Disclaimers
Copyright © 2001
Jay Pitocchelli. All rights reserved. The contents of this page are the intellectual
property of Dr. Jay Pitocchelli for
distribution to students enrolled in Evolutionary Biology BI 25 at Saint Anselm
College. These pages may not be copied, photocopied, reproduced, translated,
or published in any electronic or machine-readable form in whole or in part
without prior written approval of Jay Pitocchelli. Students enrolled in Evolutionary
Biology BI 25 at Saint Anselm College have permission to print this material
for their lecture notes. All formulae from Strickberger, M. W. 2000. Evolution.
(3rd ed.) Massachusetts, Jones and Bartlett Publishers; Ridley, M. 1996. Evolution.
(2nd ed.). Massachusetts, Blackwell Science, Inc.
