Chapter 17 SPECIES

Summary of Chapter 17: EVOLUTION, D. Futuyma, 2

n
E., 2!!"
The diversity of organisms is the consequence of cladogenesis, the branching or multiplication of
lineages, each of which the evolved by anagenesis along it own path.
Each branching in the phylogenetic tree marks a speciation event the origin of two species from one.
WHAT ARE SPECIES?
The word species in !atin means "kind#.
There are many definitions of species.
PHYLOGENETIC SPECIES CONCEPTS - PSC
$%n irreducible &basal' cluster of organisms diagnosably different from other such cluster, and within
which there is a parental pattern of ancestry and descent.# Cracraft, 1()('
*hylogenetic species concepts emphasi+e the phylogenetic history of organisms common ancestry.
% species is the smallest lineage that can be united by synapomorphic characters.
,ynapomorphies are characters shared by two or more ta-a that are derived from a common
ancestor.
The members of the species should share characteristics that other groups lack. these characteristics are
"diagnostic.#
This definition makes no reference to reproductive boundaries.
BIOLOGICAL SPECIES CONCEPT- BSC
/t was defined by Ernest 0ayr in 1(12 ",pecies are groups of actually or potentially interbreeding
populations, which are reproductively isolated from other such groups.#
*opulations of similar organisms that interbreed in the wild and produce viable and fertile offspring.
3ased on 3uffon4s ideas.
5eproductive isolation &no genetic e-change' is a key element of this definition.
3ased on se-ual reproduction.
5eproduction without human interference.
0orphological similarities and differences do no not suffice to define species.
Vaia!ion "i!#in pop$la!ions characteristics vary among the members of a single population of
interbreeding individuals.
Geogap#ic %aia!ion populations of a species differ. there e-ists a spectrum from slight to great
difference, e.g. human populations.
Si&ling species these are reproductively isolated populations that are difficult or impossible to
distinguish by morphological features, but which are often recogni+ed by differences in ecology,
behavior, chromosomes and other characters.
'O(AIN AN' APPLICATION O) THE BIOLOGICAL SPECIES CONCEPT
%ll concepts have limitations
6ifficulties in7
1. Testing allopatric populations.
2. Evaluating differences in fertility of offspring.
8. 9ossils cannot be tested.
1. %se-ual reproducing organisms are genetically isolated because they cannot reproduce
se-ually.
:. ,elf;pollinated plants.
<. *lasmid;mediated hori+ontal gene e-change between different species.
6=0%/>
The domain of the 3,C is restricted to se-ual, outcrossing organisms, and to short intervals of time.
E. g. 9ossils cannot be tested. ase-ual reproducing organisms are genetically isolated because they cannot
reproduce se-ually. self;pollinated plants
% second meaning of "species# is a ta-onomic category, ?ust like genus or family.
3=56E5!/>E C%,E,
/nterbreeding versus reproductive isolation is not an either@or, all or none situation.
>arrow hybrid +ones e-ist where genetically distinct populations meet and interbreed to a limited e-tent,
but in which there e-ist partial barriers to gene e-change.
The hybridi+ing entities are often recogni+ed as species but may be called se*ispecies.
% collection of semispecies is a s$pespecies.
The biological species concept is sometimes difficult to apply in botany.
Aeographic variation in status occurs when genetically different populations appear to be conspecific in
certain geographic regions, but to be different species elsewhere.
*5%CT/C%! 6/99/CB!T/E,
E. g. Testing allopatric populations. evaluating differences in fertility of offspring.
The greatest practical limitation of the 3,C lies in determining whether or not geographically segregated
&allopatric' populations belong to the same species.
*opulations with intrinsic barriers to gene e-change can undergo independent evolutionary change, even
if they should become sympatric.
5ange e-tension or coloni+ation could well bring presently isolated populations in contact, so the
evolutionary future of the populations depends on whether or not they have evolved reproductive
isolation.
%llopatric populations have been classified as species if their differences in phenotype or in 6>%
sequence are as great as those usually displayed by sympatric species in the same group.
WHEN SPECIES CONCEPTS CON)LICT
%llopatric populations that can be distinguished by fi-ed characters are species according to the *,C, but
if the diagnostic differences are slight, advocates of the 3,C may recogni+e the populations as geographic
variants of a single species.
/n some cases, a local population of widespread species evolves reproductive isolation from other
populations, which remain reproductively compatible with one another. Bnder the 3,C, two species
would be recogni+ed. under the *,C, the various distinguishable populations of the paraphyletic group
might be named as distinct species.
BARRIERS TO GENE )LOW
Aene flow between biological species is largely or entirely prevented by biological differences that have
often been called isola!ing *ec#anis*s+
=ther terms are isolating barriers or barriers to gene flow.
Bnder 3,C, speciation consists of the evolution of biological barriers to gene flow.
Pe*a!ing &aies
%. Ecological isolation potential mates do not meet.
1. Te*poal isola!ion reproductive period occurs at different time of the year.
2. Ha&i!a! isola!ion live in the same locality but in different habitats, e.g. primarily aquatic while
the other mostly terrestrial.
3. *otential mates meet but do not mate.
8. Be#a%ioal isola!ion differences in courtship or life style.
1. Pollina!o isola!ion different pollinators respond to different colors, scent or form of flowers.
Pos!*a!ing, pe-.go!ic &aies/ (ating between species occurs but the fertili+ation of ova does not
occur.
:. (ec#anical isola!ion copulation occurs, but no transfer of male gametes takes place because of
failure of mechanical fit of reproductive structures.
<. Cop$la!o. isola!ion failure of fertili+ation because of behavior during copulation or because
genitalia fail to stimulate properly.
7. Ga*e!ic isola!ion gamete recognition is based on the presence of specific molecules on the
coats around the egg, which adhere only to complementary molecules in the sperm.
Pos!-.go!ic &aies prevent the hybrid +ygote to develop into a viable, fertile adult.
E0!insic hybrid fitness depends on conte-t.
1. Ecological in%ia&ili!. hybrids do not have ecological niche in which they are competitively
equal to parent species.
2. Be#a%ioal s!eili!. hybrids are less successful than parent species in obtaining mates.
In!insic hybrid fitness is low because of problems that are relatively independent of environmental
conte-t.
1. Red$ced #.&id %ia&ili!. hybrid +ygote dies in the early stages of development or fails to
reach se-ual maturity.
2. Red$ced #.&id 1e!ili!. hybrid does not produce functional gametes.
8. H.&id &ea2do"n offspring of hybrids fail to produce functional gametes or do not reach
se-ual maturity.
*ossible causes of hybrid sterility
1. 5educed fertility of hybrids can be caused by structural differences between the chromosomes
that cause segregation of some aneuploid gametes during meiosis unbalanced number of
chromosomes.
2. 6ifferences between genes from the two parents interact disharmoniously.
Haldane3s R$le
Cybrid sterility and inviability is often reduced to the #e!eoga*e!ic se0 male in mammals and most
insects. female in birds and butterflies.
#hen $n the off%pr$n& of t'o $fferent an$ma( ra)e% one %e* $% a+%ent, rare, or %ter$(e, that %e* $% the
hetero,y&ou% -hetero&amet$). %e*.
Caldane, D. 3. ,. &1(22' ,e- ratio and unise-ual sterility in hybrid animals. /. 0enet. 45 1E1;1E(.
H.&id &ea2do"n occurs in the 92 generation and backcross offspring, between species and among
different geographic populations of the same species
The common interpretation of this phenomenon is that the 91 generation produced various combinations
of alleles that were disharmonious.
%lleles at different loci within the same population have presumably been selected to from harmonious
combinations. They are coadap!ed and the population is said to have a coadap!ed gene pool.
HOW SPECIES ARE 'IAGNOSE'
0orphological and other phenotypic characters, serve as markers for reproductive isolation.
/f a sample of sympatric organisms falls into two discrete clusters that differ in two or more character, it
is likely to represent two species.
'I))ERENCES A(ONG SPECIES
5eproductive isolation.
%daptive differences to different ecological factors.
>eutral differences that have arisen by genetic drift and mutation.
Character differences may have evolved d$ing the process of speciation, and partly a1!e the
reproductive barriers evolve.
1. The strength of both pre+ygotic and post +ygotic isolation increases gradually with the time since
the separation of the populations.
2. The time required for full reproductive isolation to evolve is very variable, but both average, it is
achieved when the genetic distance &6' is about E.8E ; E.:8, based on a molecular clock for
6rosophila.
Gene!ic dis!ance is a measure of the dissimilarity of genetic material between different
species or individuals of the same species.
Aenetic distance between humans and chimps is 1.<F and it suggests that the last
common ancestor e-isted about : million years ago.
8. %mong recently diverge populations or species, premating isolation is, overall, a stronger barrier
to gene e-change that post+ygotic isolation &hybrid sterility or inviability'.
1. /n the early stages of speciation, hybrid sterility or inviability is almost always seen in males
only. female sterility or inviability appears only when ta-a are older. *ost+ygotic isolation
evolves more rapidly in males than in females.
3ecause differences continue to accumulate long after two species achieve complete reproductive
isolation.
,ome of the genes and traits that now cause reproductive isolation may not have been the ones that were
instrumental in forming the species in the first place.
THE GENETIC BASIS O) REPRO'6CTIVE BARRIERS
GENES A))ECTING REPRO'6CTIVE ISOLATION
The most e-tensive information on genetic reproductive barriers has been obtained from studies
conducted on certain Dro%oph$(a species.
Gu and his coworkers &1(()' have suggested that as many as 1E gene differences on the H chromosome
and 12E in the genome as a whole might cause hybrid male sterility among closely related species
studied.
% similar study conducted by *resgraves &2EE8' suggested that about 2EE genes can contribute to hybrid
inviability.
=rr and /rving &2EE1' found that male hybrids between two populations of Dro%oph$(a p%euoo+%)ura,
one from the B, and the other form near 3ogota, Colombia, appears to be based on differences in about
five gene regions of which four are required for sterility.
/t seems that early hybrid sterility requires few gene differences.
These genes do not affect non;hybrid individuals.
,terility and inviability must stem from interactions between genes in the two different species.
Epis!a!ic in!eac!ions contribute to post+ygotic isolation.
Evidence of co*ple0 epis!a!ic in!eac!ions supports the idea that species consist of distinct coadapted
gene pools, or systems of genes that interact harmoniously within species but interact disharmoniously if
mi-ed together.
E-periments have shown that the H chromosome has greater effect on causing sterility than autosomes.
,terility must be caused by the epistatic interaction of the H chromosome of one species with the
autosomal genes of another species.
/t has been suggested that favorable H;linked genes evolve faster than autosomal genes because they are
sub?ect to greater natural selection due to the males carrying only one.
%utosomal genes affecting male sterility have diverged faster than those affecting female sterility,
possibly because of se-ual selection.
*remating isolation is frequently based on polygenic traits, although in some cases a few traits are
involved.
0ale and female components of communication that result in se-ual isolation are usual genetically
independent.
)6NCTIONS O) GENES THAT CA6SE REPRO'6CTIVE ISOLATION
There is little understanding of the function of the genes that cause inviability and sterility.
*resgraves and collaborators &2EE8' have shown that selection drove the non;synonymous substitutions in
one of the 8E or so proteins making the nuclear pore comple- that controls the passage of 5>% and
proteins between the nucleus and the cytoplasm.
This selection renders the hybrids between 6rosophila melanogaster and 6. simulans that inherit the
Nup"1 gene inviable.
CHRO(OSO(E 'I))ERENCES AN' POST7YGOTIC ISOLATION
Chromosome differences among species include alterations of chromosome structure and differences in
the number o chromosome sets &polyploidy'.
5eciprocal translocations can align the arms of a metacentric chromosome of a hybrid individual with two
arms of two different chromosomes that came from the other parent causing aneuploid segregation.
Chromosome fusion causes differences in the number of chromosome pairs in the burrowing mole rat
Spa(a* ehren+e&er$ in two populations in Egypt, Turkey, ,yria, /srael and !ebanon.
The species consists of four parapatric groups found in /srael with the following number of
chromosomes 2n I :2, 2n I :1, 2n I

:) and 2n I <E.
This is called a s$pespecies.
Cybrids are found in a very narrow +one 2.) to .8 km wide.
%neuploidy in hybrids may reduce fertility.
Corses have <1 chromosomes and donkeys <2. Their hybrid, the mule, is viable and has <8 chromosomes.
9urthermore, the chromosomes are not homologous. The chromosomes of the parents fail to pair up
during meiosis thus causing the sterility of the mule.
Evidence that chromosome hetero+ygotes have reduced fertility because of meiotic irregularities is
convincing for some plants and mammals, but not for other organisms.
CYTOPLAS(IC INCO(PATIBILITY
% possible cause of or contributor to speciation in insects is c.!oplas*ic inco*pa!i&ili!. caused by the
endosymbiotic bacteria of the genus #o(+a)h$a.
=ffspring of infected or uninfected males and an infected female develop normally.
=ffspring of an infected male and an un;infected female are unviable because the paternal chromosomes
are destroyed very early in development.
#o(+a)h$a modifies the chromosomes of the infected male in such a way that they have to be fi-ed by the
#o(+a)h$a in the eggJs cytoplasm.
THE SIGNI)ICANCE O) GENETIC ST6'IES O) REPRO'6CTIVE ISOLATION
The 'o&-#ans2.-($lle incompatibility is the result of epis!a!ic inco*pa!i&le gene in!eac!ions

between diverging populations@species and is recogni+ed as the

basis of post;+ygotic reproductive
isolation.
9unctional mismatch between genes gives rise to hybrid sterility or inviability.
5eproductive isolation requires that populations diverge by at least two allele substitutions.
The number of gene differences that suffice for post+ygotic isolation may be rather small but more
differences are incorporated over time.
5eproductive isolation eventually becomes irreversible and the evolutionary lineages undergo
independent genetic change thereafter.
%bstract
Ne' %pe)$e% ar$%e a% reprou)t$2e $%o(at$on e2o(2e% +et'een $2er&$n& popu(at$on%. 3ere 'e re2$e' re)ent 'or4 $n the &enet$)%
of po%t,y&ot$) reprou)t$2e $%o(at$on 5 the %ter$($ty an $n2$a+$($ty of %pe)$e% hy+r$%. O2er the (a%t fe' year%, re%ear)h ha% ta4en
t'o ne' $re)t$on%. F$r%t, 'e ha2e +e&un to (earn a &oo ea( a+out the popu(at$on &enet$) for)e% r$2$n& the e2o(ut$on of
po%t,y&ot$) $%o(at$on. It ha%, for $n%tan)e, +e)ome $n)rea%$n&(y )(ear that )onf($)t5r$2en pro)e%%e%, ($4e %e*ua( %e(e)t$on an
me$ot$) r$2e, may )ontr$+ute to the e2o(ut$on of hy+r$ %ter$($ty. Se)on, 'e ha2e +e&un to (earn %ometh$n& a+out the $ent$ty
an mo(e)u(ar )hara)ter$%t$)% of the a)tua( &ene% )au%$n& hy+r$ pro+(em%. 6(thou&h mo(e)u(ar &enet$) ata are ($m$te, ear(y
f$n$n&% %u&&e%t that %pe)$at$on &ene% )orre%pon to (o)$ ha2$n& norma( fun)t$on% '$th$n %pe)$e% an that the%e (o)$ %omet$me%
$2er&e a% a )on%e7uen)e of e2o(ut$on $n &ene re&u(at$on. 3ioEssays 221E):;1E(1, 2EEE. K 2EEE Dohn Giley L ,ons, /nc.
,peciation by post+ygotic isolation forces, genes and molecules. C. %llen =rr
M
, 6aven C. *resgraves
(OLEC6LAR 'IVERGENCE A(ONG SPECIES
6ifferences between species in allo+ymes and 6>% sequences are presumably selectively neutral or
nearly so.
>ot specific level of allo+ymes or 6>% divergence can tell that two populations have become separate
species.
,ome reproductively isolated populations display little or no divergence in molecular markers,
presumably because reproductive isolation has evolved very recently.
This isolation may be the result of genetic changes in one or a few characters.
Two populations that become isolated from each other at first share many of the same gene
lineages inherited from their polymorphic common ancestor.
Gith respect to some loci, individuals in each population are genealogically less closely related to
one another than they are to some individuals in the other population.
Aenetic drift or directional selection for a favorable mutation in each population eventually
results in the loss of all the ancestral lineages of 6>% sequence variants e-cept one. N
Coalescen! T#eo..
Aene lineages are lost by genetic drift at a rate inversely proportional to the effective population
si+e.
%t one point one population will become monophyletic for a single gene lineage, while the other
population, if it is larger, retains both this and other gene lineages.
%t this time, the more genetically diverse population will be paraphyletic with respect to this
gene, and some gene copies sample from population 2 will be more closely related to some gene
copies in population 1.
Eventually both populations will become monophyletic for gene lineages and the relation ship
among genes will reflect the relationship among populations.
This process of sorting of gene lineages into species is called lineage so!ing.
,hared polymorphism can be maintained for a long time if natural selection maintains the variation in
both species.
/ncomplete lineage sorting between closely related species if possible.
HYBRI'I7ATION
H.&idi-a!ion occurs when offspring are produced by interbreeding between genetically distinct
populations.
/n some cases hybridi+ation may be the source of new adaptations or even of new species.
PRI(ARY AN' SECON'ARY HYBRI' 7ONES
% #.&id -one is a region where genetically distinct populations meet and mate, resulting in at leas t
some offspring of mi-ed ancestry.
% character or locus that changes across a hybrid +one e-hibits a cline that may be quite steep. This is
called a cline.
Pi*a. #.&id -ones originate in situ as natural selection alters allele frequencies in a series of more or
less continuously distributed populations.
The position of the +one is likely to correspond to a sharp change in one or more environmental
factors.
>atural selection on different loci or characters would result in clines with different geographic
positions, and that selectively neutral variation would not display a clinal pattern.
Seconda. #.&id -ones are formed when two formerly allopatric populations that have become
genetically differentiated e-pand so that they meet and interbreed.
Cybrids between populations that meet at secondary hybrid +ones often have low intrinsic fitness
due to hetero+ygote disadvantage or breakdown of coadapted gene comple-es N !ension -ones.
The clines in characters that differentiate the populations need not match changes in the
environment and are e-pected to be coincident.
Clines in selectively neutral markers should be coincident with the others.
GENETIC 'YNA(ICS IN A HYBRI' 7ONE
6ispersal, selection and linkage all affect the distribution of alleles and phenotypic characters in hybrid
+ones.
The geographic position of a tension +one is not determined by ecological factors.
6ispersal of semispecies into the range of the other, followed by random mating, constitutes gene flow
that tends to make the cline in allele frequency broader and shallower.
The alleles of one population cannot increase in frequency within the other population because of
hetero+ygote disadvantage.
Cetero+ygotes, hybrids, act as a barrier to gene flow if they have low fitness.
In!ogession, in genetics &particularly plant genetics', is the movement of a gene from one species into
the gene pool of another by backcrossing an interspecific hybrid with one of its parents.
%n e-ample of introgression is that of a transgene from a transgenic plant to a wild relative as the
result of a successful hybridi+ation.
Bnlinked alleles of one population will diffuse into the other population creating the introgression of
those genes. These loci will have shallow cline.
% lowered fitness of hybrids at one locus reduces the flow of neutral or advantageous alleles between
populations, but the reduction is greater for alleles at closely linked loci that a t loosely liked or unlinked
loci.
The cline of a gene in the hybrid +one will be steep if the hybrids have reduced fitness or are linked to a
locus that is selected against.
The steepness of a cline depends on the rate of dispersal, the strength of selection and linkage to selected
loci.
THE )ATE O) HYBRI' 7ONES
Cybrid +ones may have several fates
1. % hybrid +one may persist indefinitely with selection maintaining steep clines at some loci even while
the clines in neutral alleles dissipate due to introgression.
2. >atural selection may favor alleles that enhance pre+ygotic isolation, resulting ultimately in full
reproductive isolation.
8. %lleles that improve the fitness of hybrids may increase in frequency. /n the e-treme case, the
post+ygotic barrier to gene e-change may break down, and the semispecies may merge into on
species.
1. ,ome hybrids may become reproductively isolated from the parent forms and become a third species.