Species versus gene selection

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Submitted on 1 Jan 1989

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Species versus gene selection

S.C. Tsakas

To cite this version:

Letter

to

the Editor

Species

versus

_gene

selection

S.C.

Tsakas

Agricultural

University of Athens, Department of Genetics, l!otanikos,

118.55 Athens

Greece;

and

_{University of California}

at

_{Irvine, Department of Ecology}

and

_Evolutionary

1Jiology,

Irvine,

CA

_92717,

USA

(received

17

_{January 1989, accepted}

17

_{April 1989)}

Species

selection has been

_{recently promoted}

_(Gould

&

_Eldredge,

_1988a,

_1988b)

as

the

_driving

force in

_{macroevolution,}

and viewed as an

_explanation

for the

_variability

in rates

_observed,

both

_temporally

and

_spatially,

at the

_phenotypic

level. This has rekindled the contention between microevolutionists

_(Maynard

_Smith,

_1987,

₁₉₈₈₎

and

_{macroevolutionists,}

which has existed since Darwin’s era.

One of the main differences between micro

_(molecular)

and macro

(phenotypic)

evolution,

is the stasis

_stage

_possible

for the

_latter,

while the molecular evolution

proceeds

on as far as the

_lineage

continues.

_Stabilizing

selection

_{(Charlesworth}

et

_al.,

1982),

proposed

by

microevolutionists to

_explain

_this,

is countered

_by

the

_proposal

of

_species

selection

_by

macroevolutionists. Further

_complicating

the

_matter,

are the

_existing

_exceptions

which weaken both

concepts.

As Gould

&

_Eldredge

so

imaginatively

put

it,

&dquo;Can we

_{really hope}

to

_explain

_why

the world holds more than a million

_species

of insects and

_only

a dozen or so of

_priapulids

_by

relative

adaptive

success of their

_{equally complex}

_morphology

alone?&dquo;

It is a

pleasant

_surprise

to find that we are able to propose an answer to this and other difficult

_evolutionary

_questions,

such as:

Why,

for

example,

do

limpets,

lingula, depnoi fishes,

horse shoe

crabs,

dragonflies,

orthoptera, tortoises,

crocodiles,

platypus,

and bats remain in

_comparative

_evolutionary

_stasis,

even

_{though they}

live under variable

_{environments,}

_compete

with other

_species

and have such

_long

existences? Is this the result of

_sampling

or does a common denominator exist? The answer can be found in the

_speciation

burst

_hypothesis

_(Tsakas

&

_David,

_1986),

and the common characteristic is that their most sensitive

_stages

are

_protected

or

less

_exposed

to ultraviolet

_light

_and/or

_{cosmic rays} and

_consequently

_experience

low mutation

_rates,

_especially

chromosomal. Chromosomal aberrations have

_long

been

proposed

as

_being

_responsible

for

_speciation

events

_{(White, 1978),}

and then for

macroevolution.

_However,

it is _{necessary to}

_clarify

what less

_exposed

means here:

the

limpets

are less

_{exposed by}

_having

a short larval

_stage

and

_by

the

_shielding

of their

_shell;

the

_{depnoi fishes, by being}

_aquatic

and

_burrowing

into the

_dry

lake

bed;

the

_{dragonflies, by having}

_aquatic

and covered larval

forms;

the

_{orthoptera, by}

developing

in the earth

_and/or

_living

_there;

the

_crocodiles,

tortoises and

_platypus,

external

_protection;

and bats are less

_{exposed by being}

nocturnal or

inhabiting

caves.

Thus

_species,

with lower exposure

during

their sensitive

stages,

and/or

less

exposure to their

_gametic

material

_finally,

will be

_expected

to show lower

_diversity

and

_{macroevolutionary}

rates. Evidence of the same kind is found in the

_plant

world.

Such a case occurs between

_phanerogamic

and

_cryptogamic

_plants,

wherein the latter whose

_gametic

material is much less

_exposed,

shows a

_{significantly}

lower

phenotypic diversity

than the

_{phanerogamic.}

Ultraviolet

_light

is

_{experimentally}

used

as a

_mutagenic

factor on

pollen

for

_artificially

_produced

mutations.

Microevolution,

therefore, through

its

_variability

in mutation _{rate, appears} to affect

macroevolution,

in addition to the

_already

known influences

_through

_genetic

drift and natural selection. It is

_important

to notice that

_higher

mutation _{rates may}

&dquo;trigger&dquo;

natural selection

_{by providing}

more

_variability

and

_genetic

drift there

_by

increasing

genetic

isolation. Another

_outcome,

is the increased

_genetic

load

_(genetic

deaths),

especially

in

_species

with small effective

_population

size

_(Kimura

et

al.,

1963),

and

_thus,

an increased risk of extinction

_(Tsakas

&

_David,

_1987).

From this

point

of

_view,

the

_advantage

of the insect

_kingdom

is evident. The sustained

_high

level of

_diversity

in

_{insects, comparative}

to the

_priapulids,

can be due to their

generally

greater

exposure, while

they

are able to bear the

_consequent

_genetic

load

due to their enormous

_population

size.

It seems

_apparent

that macroevolution is

probably

more related to

microevolu-tionary

processes than ever before

considered,

and

_{population genetics}

may

provide

surprisingly

useful answers if the correct

_questions

are addressed and there is a

will-ingness

to listen. REFERENCES

Charlesworth

_B.,

Lande R.

&

Slatkin M.

₍₁₉₈₂₎

A neo-Darwinian

_commentary

on macro-evolution. Evolution

36,

474-498

Gould S.J. &

_Eldredge

N.

_(1988a)

_Species

selection: _{its range} and _power. Nature

334,

19 9

’

Gould S.J.

&

_Eldredge

N.

_(1988b)

Punctuated

_{equilibrium prevails.}

Nature

_332,

211-212

Kimura

M.,

Maruyama

T. & Crow J.F.

₍₁₉₆₃₎

Mutation load in small

populations.

Genetics

_48,

1303-1310

Maynard

Smith J.

₍₁₉₈₇₎

Darwinism

_stays

_unpunctured.

Nature

_330,

516

Maynard

Smith J.

₍₁₉₈₈₎

Punctuation in

_perspective.

Nature

_{332, 311-312}

Tsakas S.C.

&

David J.R.

₍₁₉₈₆₎

_Speciation

burst

_hypothesis:

an

_explanation

for

the variation in rates of

_phenotypic

evolution. Genet. Sel. Evol.

18,

351-358

Tsakas S.C. & David J.R.

₍₁₉₈₇₎

_{Population genetics}

and the Cretaceous extinction.

Genet. Sel. Evol.

_19,

487-496

White M.J.D.

₍₁₉₇₈₎

Chromosomal models

_{of speciation.}

In: Modes

_{of Speciation,}