MORPHOLOGICAL DIFFERENCES BETWEEN DROSOPHZLA
PSEUDOOBSCURA POPULATIONS SELECTED FOR
OPPOSITE GEOTAXES AND PHOTOTAXES1
GEORGES PASTEUR2
The Rockefeller Uniuersity, N e w York, N.Y.
Received September 27, 1968
J
. HIRSCHand his students have constructed classification mazes, with the aid
of which it has proved possible to select populations of Drosophila melunogaster
for positive and for negative geotactic or phototactic behavior. The experiments
performed with such mazes have been highly successful: within less than ten
generations, two populations originated from the same stock and selected for
opposite taxes show a strong divergence (HIRSCH
1962; DOBZHANSKY
and SPASSKY 1962; HADLER
1964; DOBZHANSKY
and SPASSKY
1967).
When genetically distinct populations are hybridized and the hybrid populations obtained are used for selection studies, the polygenic combinations selected
for during similar experiments are unlikely to be identical. If a hybrid population is divided into two lines by selection for opposite behaviors, some structural
divergence may be found between them. If the structure involved has an influence on the behavior under selection, the divergence is expected to appear
again in sister hybrid populations similarly treated. On the other hand, if the
structural divergence does not reappear, or appear to be reversed, in a sister population, then the structure involved is likely to be physiologically irrelevant to the
behavior selected; the difference between the outcomes is accounted for by differences in the sets of polygenes picked out by the behavioral selection, through
linkage or pleiotropy.
The present study is an attempt to detect morphological dissimilarities between lines selected for opposite phototaxis or geotaxis, and investigate whether
the characters involved have been directly selected or not, using such comparative
tests.
MATERIALS A N D METHODS
All the populations studied were originally derived from hybrids between twelve strains of
D. pseudoobscura homozygous for the CH gene arrangement in the third chromosome, crossed
to twelve strains homozygous for the AR gene sequence in the same chromosome. All these strains
are descended from flies collected at Pinon Flats, Mount San Jacinto, California. The techniques
and the results of the selections made in the populations before my own experiments were started
are described in detail by DOBZHANSKY
and SPASSKY
(1967) and DOBZHANSKY,
SPASSKY,
and SVED
(1969). A summary of the origins of my material can be seen in the following tabulation:
Research supported under Coneact AT(30-1)-3096, U.S. Atomic Energy Commission.
Present address: The University of Texas Southwestern Medical School, Dallas, Texas 75235.
Genetics 62: 837-847 August 1969.
838
GEORGES P A S T E U R
Population
No.
17
19
18
20
25
27
29
31
Behavior
selected
positive geotaxis
negative geotaxis
negative phototaxis
positive phototaxis
positive phototaxis
negative phototaxis
positive geotaxis
negative geotaxis
Selected for
generations
Relaxed for
generations
30
30
20
20
16
15
16
15
None
None
10
10
None
None
None
None
These eight populations represent four pairs of twin populations selected in opposite directions.
The investigations presented here were started with samples taken from cups withdrawn from
the population cages in which these populations were maintained. An equal number of at least
5 “positive” and 5 “negative” replicate culture bottles were prepared from every one of the populations 17 through 20, by counting out 10 males and 10 females for each bottle. SPASSKY’S
(194.3)
cream of wheat-molasses medium was used throughout, and the same amount of a live yeast
suspension was placed in each culture. The bottles of each set of replicates from two twin populations were placed on the same shelf in an incubator maintained at a constant temperature;
bottles from the positively selected population and from the negatively selected one were alternated. When the females had deposited eggs, the flies were transferred to a similar set of bottles.
Further similar experiments were performed with populations 23 and 31, using one set of 5+5
replicates or more, and with populations 25 and 27, using one set of 5+5, or two successive sets
of 4f4 replicates.
From the progenies thus developed in environments as uniform as possible, flies were taken at
random for measurements (usually 10 males and 10 females from each replicate). Right wings,
right hind legs, and antennae were prepared in permanent slides for later measurement; in the
populations selected for phototaxis, the greatest diameter of the eye had been measured beforehand on the same specimens, just after etherization. As a precaution, only cultures in which no
fewer than 30 and no more than about 150 flies had emerged were used, although there is no
indication of any density-dependence of the present dimensions even when higher or lower
numbers of flies are produced (AND-RSON
1966, and personal observations). The length of the
wing was measured as the distance from the anterior crossvein, where it joins the third longitudinal vein, to the tip of the wing. The length of the hind tibia was measured as the distance
between the distal end and the minute postero-proximal bump. These measurements are reported
in the following tables in units of an eyepiece micrometer, 1 unit being equal to 25.3 p (wing)
or 19.05 p (eye and tibia), depending on the objective used. On the antennae, the larger branches
and the small hairs were counted separately.
RESULTS
Body size: ROBERTSONand REEVE (1952), and many others after them (review
1966) have used the wing length as a measure of the body size of
in ANDERSON
Drosophila. Tibia length may be used as a complementary estimate. Tables 1
and 2 show that flies selected for positive geotaxis are larger than those negatively
selected; this is true in every one of the 16 relevant comparisons, and was clear
enough to make the divergence visible to the naked eye. An additional set of
replicates was raised from populations 17 and 19 when they had been released
from the selection for seven generations. A careful examination with the dissecting microscope revealed that some difference in body size between the ‘Lpositive”
GEOTAXIS AND PHOTOTAXIS IN
D.pseudoobscura
839
TABLE 1
Wing length in populations selected for geotaxis and phototaxis
Population
Geotaxis
Nos. 17 & 19 ( I )
Nos. 17 & 19 (11)
Nos. 29 & 31
Nos. 29 & 3 1
Phototaxis
Nos. 20 & 18 ( I )
Nos. 20 & 18 (11)
Nos. 25 & 27
Nos. 25 & 27 ( I )
Nos. 25 & 27 (11)
Temperature
Positive
Males
15"
15"
15"
20"
67.03 f
66.86 t
65.12 f
61.41. t
20"
20"
20"
19"
19"
58.26 rfr .W
58.25 f .51
60.0+ t .60
60.63 rfr 1.29
64.75 f .73
.66
.45
.49
.44
Negative
Positive
Females
62.2) f .27
61.90 t .56
58.34 t 1.10
55.72 f 1.31
73.53
72.21
69.63
66.15
59.67 t
60.46 f
59.74 f
62.00 f
62.18 f
61.63 t .61
63.01 t .76
64.08 f 1.00
64.95 rfr 1.35
68.76 f. .66
.09
.41
.44
.78
.80
Negative
t .97 68.71 t .79
f. .72 67.35 f 5 7
f 1.13 63.62 f .46
k .4Q 58.86 f. 1.23
64.20 f .25
65.43 t .27
64.36 t .27
66.76 f. .97
66.39 t 1.32
Means from five replicates (four in the last two lines).
and the "negative" progenies remained, but it was no longer distinguishable to
the naked eye.
Quite different is the situation with the populations selected for phototaxis.
When I began m y experiments, the twin populations 20 and 18 had been maintained for 10 generations with relaxed selection. Nevertheless, h e positively
phototactic flies had definitely shorter wings and tibiae than the photonegative
ones. No significant differences were found, however, when populations 25
and 27 were tested; these had been under selection for 16 and 15 generations,
respectively. The results are shown in Table 1 for wing length. To be sure, this
table shows that the flies of the "positive" population 25 were significantly larger
in the second (11) replication at 19°C than in the first (I). This discrepancy is
probably due to some environmental accident (such as fortuitous changes in
food-making) to which, interestingly enough, the negatively selected population
was conspicuously insensitive.
Tibia: wing ratio: Some significant divergences were found in this character,
but they were consistent neither in photoselected nor in geoselected populations.
For example, in flies selected for geotaxis, one of the two tests on populations
TABLE 2
Tibia length in populations selected for geotaxis
Population
Nos. 17 & 19 ( I )
Nos. 17 & 19 (11)
Nos. 29 & 31
Nos. 29 & 31
Temperature
15"
15"
15"
20"
Means from five replicates.
Positive
38.15
37.33
35.66
35.24
Males
Negative
t .39 35.01
f .39 33.85
rfr .47 33.04
t .22 31.96
f
t
f
f
Positive
.21
.20
.79
.90
Females
39.05 f .47
37.81 31 .40
36.96 t 1.08
36.16 f .20
Negative
36.46
35.26
34.38
32.00
t .49
t .57
fk .37
f 1.01
840
GEORGES PASTEUR
TABLE 3
Eye diameter in populations selected for phototaxis
Population
Temperature
Positive
Absolute size
Nos. 20& 18 (I)
20"
24.51 f
20"
24.35 t
Nos. 20 & 18 (11)
20"
24.80 t
Nos. 25 & 27
19"
24.84 f
Nos. 25 & 27 (I)
19"
25.64 f
Nos. 25 & 27 (11)
Eye diameter/wing length (percent)
Nos. 20& 18 (I)
20"
31.67 i
Nos. 2085 18 (11)
20"
31.36 t
30.98 t
Nos. 25 & 27
20"
Nos. 25 &27 (I)
19"
30.72 t
Nos.25&27 (11)
19"
29.94 t
Males
Negative
*
Females
Positive
Negative
.19
.I5
.26
.49
.33
24.38
25.06 t
25.20 i
25.68 f
25.93 ?
.I5
.24
.27
.44
.23
25.10 f
25.44 t
25.59 f
25.66 f
26.35 f
.24
.27
.46
.68
.23
25.72 f
26.59 t
26.47 f
26.80 f
26.99 t
.41
.19
.16
.48
.51
.32
.28
.21
.18
.36
30.66 i
31.09 t
31.63 f
31.05 t
31.30 f
.I6
.23
.13
.40
.I2
30.55 i
30.28 f
29.95 t
29.62 t
28.74 f
.26
.17
.16
.22
.I6
30.01. f
30.39 f
30.84 t
30.10 t
30.49 t
.I9
.21
.I8
.20
.18
Means from five replicates raised at 20°C or four raised at 19°C.
17 and 19, the index was definitely higher in "negative" than in "positive"
males; but the reverse was true in one of the two tests on populations 29 and 31.
(Respective probabilities were about .01 and .005 for the twin populations being
homogenous in the male sex.) Remembering that geopositively selected flies were
always much smaller than geonegatively selected ones, this can at least allow us
to affirm that a size allometry of the limbs is not involved in the strong divergences shown by Table 2 in tibia length.
Eye diameter: Negatively phototactic flies have slightly but fairly consistently
larger eyes than the photopositive ones (Table 3 ) . Although most of the differences are not statistically significant, the means for the photonegative series are
greater than those for the photopositive one in 9 out of 10 comparisons. The relative eye size, as measured in terms of the eye: wing length ratios, is greater in the
photonegative population No. 27 than in the photopositive No. 25. This difference is not found between the populations 18 and 20; if anything, the ratios tend
to be higher in the positive population.
Aristae: The data reported in Table 4 show indications of higher numbers of
TABLE 4
Numbers
Population
Nos. 17&19 (I)
Nos. 1 7 & 19 (11)
Nos. 29 & 31 (I)
Nos.'L9&31 (11)
of
branches on the aristae in populations selected for geotaxis
Temperature
15"
15"
15"
20"
Means from six replicates.
Positive
7.21
7.13
7.07
7.14
Males
f .07
f .04
t .02
f .04
Negative
6.73 i
6.66 t
6.98 f
7.04 t
.07
.02
.OS
.I6
Positive
7.13 -C
7.08 f
7.02 -C
7.08 i
Females
.05
.03
.05
.02
Negative
6.57 f
6.58 -C
7.02 i
6.74
*
.03
.04
.04
.21
GEOTAXIS AND PHOTOTAXIS
IN
D.pseudoobscuru
841
branches in the aristae of geopositive than in geonegative flies, although this
difference is much more pronounced
- - between populations 17 and 19 than between
populations 29 and 31. (A sampling from the males of populations 17 and 19
obtained at their eighth generation of relaxed selection showed that they had
retained a significant divergence in this character.) The numbers of small hairs,
however, showed reversed differences between the two pairs of populations at
15"C, as well as between the two pairs of populations selected for phototaxis. No
consistent differencesappear in the latter for the numbers of branches.
Other sense organs: Unsuccessful attempts have been made to find differences
in the numbers of certain macrochaetae on the second antennal segment of photoand geoselected flies, and in sensilla campaniformia on the halteres and wings of
flies selected for geotaxis. It is, of course, possible that a histological study may
reveal significant differences. Such organs serve perception of gravity in a num1964).
ber of insects (SCHNEIDER
Abnormal wing venation: DRUGER(1963) found, in experimental D.pseudoobscuru populations of exactly the same origin as those which are studied here
and bred at 16"C, 3.8% of the individuals with some portion of an extra distal
crossvein between the second and third longitudinal veins. He noted also a small
abnormal longitudinal vein, between the third and fourth ones, that tended to be
associated with the extra crossvein. These additional veins did not appear at 25 O C.
Among flies selected for negative geotaxis and bred at 15" or 20°C, traces of
extra veins appeared only twice in about 1,500 individuals examined. As shown
in Table 5, the extra crossvein was on the contrary quite unusually frequent in
the samples of populations No. 17 (11.8:h at 15") and No. 29 (20.5% at 15",
13.0% at 20°), selected for positive geotaxis. The lower frequency in population
17 is counter-balanced by very high frequency of the extra longitudinal vein
(32.8%), which is virtually lacking in population 29 (0.4%). Table 6 shows the
association between the abnormalities in venation in the form of a contingency
table. The total number of flies showing any venation abnormality is 433 4- 47 =
~
TABLE 5
Extra crossveins on one or on both wings in populations selected for positive geotaxis
Population Temperature
Sex
Complete
Both
One
None
13
0
1.0
24
5
2.2
37
5
3.2
62
9
5.4
531
631
88.2
14
1
1.9
56
10
8.4
66
10
9.7
267
355
79.5
51
2
6.8
45
3
6.1
321
361
87.0
No. 17
15"
Fem a1es
Males
Total percent
No. 29
15"
Females
Males
Total percent
4
0
0.5
1
20"
Females
Males
Total percent
0
No. 29
0
0.0
0
0.1
* In absence of complete extra crossvein.
Partial'
One
Both
842
GEORGES P A S T E U R
TABLE 6
Association of wing venation abnormalities in the population N o . 17, selected for positive geotaxis
Extra longitudinal vein
Present
Absent
Extra crossvein only
Extra crossvein plus other abnormalities
Abnormalities other than the crossvein
Total with both kinds
91
20
6
35
9
3
126
29
9
117
47
164
837
1153
__
1317
316
Without either
~
~
433
Totals
Totals
884
480 out of 1,317, or 36.5%-a high percentage indeed. However, eight generations later, without selection, the frequency had turnel back to a frequency similar to that observed by DRUGER.
Color of testes: Normal D.pseudoobscura males have testes of a bright red
color. They remain so in populations selected f o r negative geotaxis. However, in
the positively selected population No. 29, testes in young males vaned, as seen
through the abdominal wall, from pale yellow to bright orange. When samples
of geopositive and geonegative males were compared side by side, the average
difference was striking. In old males, the testis color darkens, making comparisons
more difficult. Populations 17 and 19, also selected for geotaxis, were examined
for testis color eight generations after the selection was relaxed. A similar but
extremely faint color difference was found only by comparison of several pairs
of testes after dissection of young males; though this difference was seen by two
observers (TH.DOBZHANSKY
and the writer), it was so weak in fact that a conclusive statement is impossible. However, a firmer divergence may have been
overlooked when populations 17 and 19 were freshly selected, since I sought it in
these populations only after having discovered it in populations 29 and 31.
TABLE 7
Male/female ratios for metric characters in populations selected for phototaxis
Trait
Populations
Temperature
Tibia length
Tibia length
Tibia length
Tibia length
Tibia length
Eye/wing ratio
Eye/wing ratio
Eye/wing ratio
Eye/wing ratio
Eye/wing ratio
Nos. 20 and 18 (I)
Nos. 20 and 18 (11)
Nos. 25 and 27
Nos. 25 and 27 (I)
Nos. 25 and 27 (11)
Nos. 20 and 18 (I)
Nos. 20 and 18 (11)
Nos. 25 and 27
Nos. 25 and 27 (I)
Nos. 25 and 27 (11)
20"
20
20"
19"
19"
20
20"
20"
19"
19"
Positive
.986 f .018
.967 t .010
.994 f .014
.986 f .012
,991 t .023
1.036 f .007
1.036 t .007
1.037 t .005
1.038 & ,009
1.042 f .012
Means from five replicates raised at 20°C or four raised at 19°C.
Negative
.960 & .013
.943 t .006
.969 f .009
.971 t .O&
,986 t .012
1.021 f .007
1.021 f . O M
1.025 t .006
1.032 f .013
1.027 f .008
GEOTAXIS AND PHOTOTAXIS IN
D.pseudoobscura
843
Sexual dimorphism: Most unexpectedly, not only have divergences been found
in sexual dimorphism, but they were quite consistent, for tibia and eye dimensions, in the populations selected for phototaxis (Table 7 ) . The differences in
tibia size and eye size between males and females of negatively selected lines
were larger than those from positively selected lines; in other words, dimorphism
for these traits was less pronounced in “positive” than in “negative” flies selected
for phototaxis. Changing sexual dimorphism was also observed in the numbers of
arista ramifications in response to both photoselection and geoselection, but not in
a consistent way.
N. B.-Tables illustrating inconsistent changes are deposited in the GENETICS
Editorial Office.
DISCUSSION
These experiments and observations reveal that morphological response to
selection f o r a behavioral trait may be tremendous. Morphological differences had
not been detected previously in Drosophila selected for opposite geotaxes or phototaxes (DOBZHANSKY
and SPASSKY
1967, and personal communication). In the
present study, most of the relatively few characters investigated showed some
divergence in lines selected in opposite directions. Among these characters were
qualitative (wing venation, testis color) as well as quantitative traits. There is no
reason to think that other, uninvestigated characters do not respond in similar
proportion. Populations selected for opposite taxes may become, in ten generations
or so, more distinct than drosophilid subspecies, by usual morphological standards.
stressing the enormous power of this selection along with the enormous variability
stored in the secularly selected genutype.
The so-called correlated response of many characters to artificial selection
exercised for only one of them has been known very long (DARWIN
1859, chap. I;
HASKELL
1954). In the present case, the new question at issue is: since the one
character selected is behavioral, which of the morphological traits affected simdtaneously were relevant to this behavior? As a working hypothesis, four possibilities, which explain the designed choice of metric and meristic measurements,
have been considered, namely: (1) Selection for geotaxis might affect body size
simply because heavier flies may have a tendency for positive geotaxis. The same
hypothesis occurred independently to S. KESSLER(personal communication) as a
result of observations in D.melanogaster selected in a similar maze; (2) selection
for geotaxis might affect the relative limb size, since flies with stronger and possibly larger limbs could conceivably climb upwards more easily. Observations
made at the Rockefeller University and elsewhere (film by LOUISLEVINE)show
that flies walk, and do not fly, in the experimental mazes; (3) selection for phototaxis might affect the size of the eye, for obvious reasons; (4)selection for phototaxis or geotaxis might affect other sense organs easily accessible to observation,
i.e. (in the case of photoselection) the antennae, the use of which might be
increased in a dark environment, or (in the case of geoselection) both the halteres
and antennae, since these organs could bear some responsibility in the perception
of gravitational forces.
844
GEORGES PASTEUR
A strikingly consistent morphological response is the strong divergence in body
size in the two pairs of populations selected for geotaxis. The positive geotactic
tendencies of larger flies seem clear. (According to data kindly supplied by B.
SPASSKY,
the divergence in geotactic scores for the first pair of populations was
already noticeably reduced five generations after selection had been relaxed, due
to genetic homeostasis. That their divergence in body size was no longer perceptible to the naked eye three generations later can be attributed to the same
cause.)
Another character that might affect geotactic behavior is the number of
branches on the aristae. This number is larger in positively selected flies from both
pairs of populations tested, although more significantly so in the first pair than
in the second.
The results obtained from eye measurements, in populations selected for phototaxis, may seem less conclusive. In both absolute and relative sizes, the “negative”
flies of both sexes proved to have a constantly larger eye in the second pair of
populations tested, while the body size difference was fluctuating. On the contrary,
it was the eye size differences which were fluctuating in the first pair of populations; while the “negative” flies were very significantly larger, their eyes were
much less definitely so in absolute size, which fitted in with their tendency to be
smaller in relative size.
The other characters tested do not fit the working hypothesis at all. There was
either no response to selection (numbers of certain sensoria) or the responses
observed did not follow a consistent pattern. For example, the divergence in
numbers of arista ramifications was distinctly reversed between the two pairs of
populations selected for phototaxis, although one of the two pairs had not been
selected for ten generations. Similarly the tibiawing ratio (a measure of relative
limb size) evolved differently in populations identically selected for geotaxis, and
moreover it did respond also to selection for phototaxis.
Beside the characters tested in relation to the hypotheses, others were noticed
to diverge between the twin populations oppositely selected. Again, among these
traits some did not behave the same way in identically selected pairs of populations (extra longitudinal vein in geotaxis, sexual dimorphism of arista ramifications), and others showed a consistent response throughout (sexual dimorphisms of eye and tibia in phototaxis; testis color in geotaxis).
On the whole, there has been a considerable indeterminacy in responses of the
morphological characters investigated to selection for phototaxis and geotaxis.
Strongly dissimilar responses have also been observed in chromosomal frequencies
(DOBZHANSKY
and SPASSKY
1967, and unpublished data). This indeterminacy
must not be confused with such instability in selection outcomes as is obtained
when the selection is exerted on populations of geographically mixed origin, as,
for instance, in MARIEN’S
experiments (1958) on development time and SOLIMA
SIMMONS’( 1 966 and references therein) on chromosomal frequencies. I n the
present study, populations of geographically common origin are dealt with; as
such, and as all the sister populations identically se!ected by DOBZHANSKY
and
SPASSKY,
they have developsd similar tactic behaviors. However, it is well known
that similar phenotypes can be arrived at by different genotypic evolutionary
GEOTAXIS AND PHOTOTAXIS IN
D. pseudoobscura
845
changes, as exemplified in nature by sibling species. The findings reported here
are a further example of this phenomenon: the same behavioral phenotype has
been arrived at by populations the morphology of which discloses clearly that
they are genotypically different. Correlated responses to artificial selection for
one trait are largely due to the linkage of polygenes controlling different characters (MATHER
and HARRISON
1949). Owing to the randomness of recombination,
it is logical that such responses show unpredictability and unrepeatability. Different morphological outcomes were observed in 14 strains of Drosophila melanogaster selected for a non behavioral trait ( SOKAL1959).
In this study the character under artificial selection is a behavioral trait. Therefore, some associated morphological changes may be naturally selected, if they
improve the behavioral efficiency of the flies, and, by so doing, their fitness. Such
changes are unlikely to lie in those traits that have evolved in contrary directions,
nor are they in testis color, f o r which it is hard to imagine any function in geotaxis,
nor extra wing venation, which does not develop at the selection temperature
(25"C), nor sexual dimorphism. As an adaptive change, evolution of body size in
geotaxis is not the serious candidate it may seem to be from Tables 1 and 2: 1 )
Unselected flies of the same population, but raised at 16" or 25"C, give similar
tactic responses in geotactic mazes, although the former are twice as big as the
latter; 2) ERLENMEYER-KIMLING,
HIRSCH
and HEISS(1962) have observed that
selected females give more extreme responses than males; however, divergence
in size is not higher in females than in males, in the present observations; 3 )
through selection for phototaxis, a divergence appeared in two sexual dimorphisms
that was as consistent as the difference in body size obtained through geoselection;
no matter how difficult the exact explanation of this puzzling evolution may be,
it is obviously irrelevant to phototactic behavior and can only depend on genetic
connection. Consistency of the response of a character in the two pairs of populations tested does not prove that this character is adaptively significant.
The best evidence for a change having adaptive significance lies eventually in
the definite tendency of the eye to be larger in flies selected for negative phototaxis, especially in the second pair of populations tested, where it was independent
of the fluctuations of body size (compare Table 3 with Table 1) .The slight opposite tendency in the first pair of populations may be ascribed to interfering linkage. In the same way, development of arista branches, as reflected by their numbers, could be relevant in geotaxis, the weaker response in the second pair of
populations tested being ascribed to interfering linkage as well (Table 4).Different experiments are in progress or in project to attempt to ascertain these points
and locate phototactic and geotactic receptors.
The author wishes to thank warmly Dr. FRANCISCO
J. AYALA,Mrs. OLGAPAVLOVSKY
and Mr.
BORISSPASSKY
for their technical help or advice, as well as Dr. JOHN SVED and Mrs. NICOLE
PASTEUR
for valuable discussion. Most of all, I am thankful to Prof. THEODOSIUS
DOBZHANSKY
for
his guidance, as well as Mr. ROLLIN
C. RICHMOND
and Dr. COSTASD. KASTRITSIS
for their help in
preparing the manuscript.
SUMMARY
A search has been made for morphological differences between populations of
846
GEORGES P A S T E U R
Drosophila pseudoobscura selected for positive and negative phototaxis and geotaxis. Progenies from sister populations selected in opposite (plus and minus)
directions were raised simultaneously under uniform environmental conditions,
and samples were taken for measurements and other observations.-By repeating
similar tests on distinct but identically selected populations and comparing the
results, two kinds of morphological changes have been observed: 1) changes which
were duplicated in the populations; 2) changes which appeared only in one population or were even reversed from one population to another. The changes
that fall into the second category are considered to be due mainly to linkage
and presumably irrelevant to geotactic or phototactic behavior. They exemplify
the fact that the same behavioral phenotype may be arrived at through different
genotypic evolutionary changes, and the indeterminacy of correlated responses
to artificial selection for the same trait in populations of sympatric origin.
Into the first category fall changes involving body size in geotaxis (populations positively selected produced much larger flies) , eye size in phototaxis
(larger in populations negatively selected), arista branches in geotaxis (more
numerous in positively selected flies), along with extra wing venation in geotaxis,
sexual dimorphism in phototaxis, and possibly testis color in geotaxis. It is suggested that the second, and, to a lesser degree, the third changes are more likely
to have resulted from direct selection than the first. Further experiments are
underway to clarify these matters.
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