180:
malaria exists. Maintenance of the HgbS allele through positive selection is supported by significant evidence that heterozygotes have decreased fitness in regions where malaria is not prevalent. In
Surinam, for example, the allele is maintained in the gene pools of descendants of African slaves, as the Surinam suffers from perennial malaria outbreaks. Curacao, however, which also has a significant population of individuals descending from African slaves, lacks the presence of widespread malaria, and therefore also lacks the selective pressure to maintain the HgbS allele. In Curacao, the HgbS allele has decreased in frequency over the past 300 years, and will eventually be lost from the gene pool due to
108:
296:
210:, rare morphs of prey are actually fitter due to predators concentrating on the more frequent morphs. As predation drives the demographic frequencies of the common morph of prey down, the once rare morph of prey becomes the more common morph. Thus, the morph of advantage now is the morph of disadvantage. This may lead to boom and bust cycles of prey morphs. Host-parasite interactions may also drive negative frequency-dependent selection, in alignment with the Red Queen hypothesis. For example, parasitism of freshwater New Zealand snail (
366:
270:
318:
In this species predation by birds appears to be the main (but not the only) selective force driving the polymorphism. The snails live on heterogeneous backgrounds, and thrush are adept at detecting poor matches. The inheritance of physiological and cryptic diversity is preserved also by heterozygous
419:
passing from 1,000 feet (300 m) to 4,000 feet. Also, the same areas sampled at different times of year yielded significant differences in the proportions of forms. This indicates a regular cycle of changes which adjust the population to the seasonal conditions. For these results selection is by
397:
as a possible explanation of the results. Stocks containing inversions at a known initial frequency can be maintained in controlled conditions. It was found that the various chromosome types do not fluctuate at random, as they would if selectively neutral, but adjust to certain frequencies at which
179:
which kills a large number of people each year. This is an example of balancing selection between the fierce selection against homozygous sickle-cell sufferers, and the selection against the standard HgbA homozygotes by malaria. The heterozygote has a permanent advantage (a higher fitness) wherever
163:
from both parents. In such individuals, the hemoglobin in red blood cells is extremely sensitive to oxygen deprivation, which results in shorter life expectancy. A person who inherits the sickle cell gene from one parent and a normal hemoglobin allele (HgbA) from the other, has a normal life
222:
it became. Note that in these examples that no one phenotypic morph, nor one genotype is entirely extinguished from a population, nor is one phenotypic morph nor genotype selected for fixation. Thus, polymorphism is maintained by negative frequency-dependent selection.
231:
The fitness of a genotype may vary greatly between larval and adult stages, or between parts of a habitat range. Variation over time, unlike variation over space, is not in itself enough to maintain multiple types, because in general the type with the highest
307:
is likely, with the birds preferentially taking the most common morph. This is the 'search pattern' effect, where a predominantly visual predator persists in targeting the morph which gave a good result, even though other morphs are available.
81:
There are several mechanisms (which are not exclusive within any given population) by which balancing selection works to maintain polymorphism. The two major and most studied are heterozygote advantage and frequency-dependent selection.
73:
Evidence for balancing selection can be found in the number of alleles in a population which are maintained above mutation rate frequencies. All modern research has shown that this significant genetic variation is ubiquitous in
377:
Values for heterozygote inversions of the third chromosome were often much higher than they should be under the null assumption: if no advantage for any form the number of heterozygotes should conform to
617:
Koskella, B. and Lively, C. M. (2009), EVIDENCE FOR NEGATIVE FREQUENCY-DEPENDENT SELECTION DURING EXPERIMENTAL COEVOLUTION OF A FRESHWATER SNAIL AND A STERILIZING TREMATODE. Evolution, 63: 2213–2221.
218:
results in decreasing frequencies of the most commonly hosted genotypes across several generations. The more common a genotype became in a generation, the more vulnerable to parasitism by
288:(large stones). Here fragments accumulate, permitting researchers to analyse the snails taken. The thrushes hunt by sight, and capture selectively those forms which match the habitat
141:. Due to unexpected high frequencies of heterozygotes, and an elevated level of heterozygote fitness, heterozygotic advantage may also be called "overdominance" in some literature.
427:
By 1951 Dobzhansky was persuaded that the chromosome morphs were being maintained in the population by the selective advantage of the heterozygotes, as with most polymorphisms.
362:. All the flies look alike whatever inversions they carry, so this is an example of a cryptic polymorphism. Evidence accumulated to show that natural selection was responsible:
319:
advantage in the supergene. Recent work has included the effect of shell colour on thermoregulation, and a wider selection of possible genetic influences is also considered.
99:
Sickle-shaped red blood cells. This non-lethal condition in heterozygotes is maintained by balancing selection in humans of Africa and India due to its resistance to the
647:
Bertram, Jason; Masel, Joanna (20 March 2019). "Different mechanisms drive the maintenance of polymorphism at loci subject to strong versus weak fluctuating selection".
929:
292:. Snail colonies are found in woodland, hedgerows and grassland, and the predation determines the proportion of phenotypes (morphs) found in each colony.
198:
Frequency-dependent selection occurs when the fitness of a phenotype is dependent on its frequency relative to other phenotypes in a given population. In
311:
The polymorphism survives in almost all habitats, though the proportions of morphs varies considerably. The alleles controlling the polymorphism form a
423:
Lastly, morphs cannot be maintained at the high levels found simply by mutation, nor is drift a possible explanation when population numbers are high.
303:
A second kind of selection also operates on the snail, whereby certain heterozygotes have a physiological advantage over the homozygotes. Thirdly,
969:
315:
with linkage so close as to be nearly absolute. This control saves the population from a high proportion of undesirable recombinants.
1000:
96:
922:
54:
alone. Balancing selection is rare compared to purifying selection. It can occur by various mechanisms, in particular, when the
193:
383:
107:
915:
1056:
401:
Different proportions of chromosome morphs were found in different areas. There is, for example, a polymorph-ratio
835:
Painter T.S. 1933. "A new method for the study of chromosome rearrangements and the plotting of chromosome maps".
295:
336:
393:, which enabled feeding, breeding and sampling whilst preventing escape. This had the benefit of eliminating
1051:
995:
819:
727:
705:
359:
122:
116:
67:
974:
959:
451:
446:
436:
412:
342:
331:
365:
964:
441:
262:. Unbanded is the top dominant trait, and the forms of banding are controlled by modifier genes (see
236:
fitness will take over, but there are a number of mechanisms that make stable coexistence possible.
938:
355:
304:
145:
1077:
682:
508:
407:
351:
244:
Species in their natural habitat are often far more complex than the typical textbook examples.
1020:
946:
674:
535:
500:
492:
176:
169:
134:
100:
59:
31:
1041:
1005:
664:
656:
618:
527:
484:
394:
259:
258:, is famous for the rich polymorphism of its shell. The system is controlled by a series of
254:
130:
608:
David Wool. 2006. The
Driving Forces of Evolution: Genetic Processes in Populations. 80-82.
1046:
1036:
979:
402:
153:
583:
Allison A.C. 1956. The sickle-cell and
Haemoglobin C genes in some African populations.
233:
207:
181:
1071:
1015:
888:. eds Lewontin RC, Moore JA, Provine WB and Wallace B. Columbia University Press N.Y.
622:
553:
528:
55:
51:
17:
686:
389:
Using a method invented by L'Heretier and
Teissier, Dobzhansky bred populations in
512:
1010:
278:
206:
the fitness of a phenotype decreases as it becomes more common. For example, in
549:
347:
157:
63:
47:
496:
156:. Sickle cell anemia is caused by the inheritance of an allele (HgbS) of the
137:
than a homozygous individual. Polymorphisms maintained by this mechanism are
416:
312:
269:
263:
43:
721:
Cain A.J. and Currey J.D. 1968. Climate and selection of banding morphs in
678:
504:
382:(number in sample) = p+2pq+q where 2pq is the number of heterozygotes (see
743:
Cain A.J. and
Sheppard P.M. 1950. Selection in the polymorphic land snail
472:
149:
75:
669:
95:
907:
660:
35:
488:
202:
the fitness of a phenotype increases as it becomes more common. In
364:
358:
and discovered that all the wild populations were polymorphic for
294:
268:
106:
160:
39:
911:
793:
Jones J.S., Leith B.N. & Rawlings P. 1977. Polymorphism in
851:
Stalker H.D and Carson H.L. 1948. "An altitudinal transect of
164:
expectancy. However, these heterozygote individuals, known as
599:
Sickle cell anemia. 2009. Encyclopædia
Britannica. Chicago.
534:(7th ed.). Oxford: Oxford University Press. p.
129:, an individual who is heterozygous at a particular gene
471:
Charlesworth, Deborah; Willis, John H. (November 2009).
763:
Cain A.J. and
Sheppard P.M. 1954. Natural selection in
526:
King, R.C.; Stansfield, W.D.; Mulligan, P.K. (2006).
277:
In
England the snail is regularly preyed upon by the
1029:
988:
945:
58:for the alleles under consideration have a higher
273:Grove snail, dark yellow shell with single band
637:, p26, Heterozygous advantage. MIT Press 1965.
111:Malaria versus sickle-cell trait distributions
923:
725:from the climate optimum to the present day.
8:
886:Dobzhansky's genetics of natural populations
560:. Oxford: Clarendon Press. pp. 493–513.
411:along an 18-mile (29 km) transect near
930:
916:
908:
699:Cain A.J. and Currey J.D. Area effects in
668:
172:, may suffer problems from time to time.
50:at frequencies larger than expected from
799:Annual Review of Ecology and Systematics
552:(1940). "Polymorphism and taxonomy". In
94:
473:"The genetics of inbreeding depression"
463:
813:Cook L.M. 1998. A two-stage model for
204:negative frequency-dependent selection
200:positive frequency-dependent selection
901:. 4th ed. Chapman & Hall, London.
797:: a problem with too many solutions.
175:The heterozygote is resistant to the
7:
873:Genetics of the evolutionary process
784:, 4th ed. Chapman & Hall, London
25:
1001:Models of nucleotide substitution
42:) are actively maintained in the
875:. Columbia University Press N.Y.
623:10.1111/j.1558-5646.2009.00711.x
420:far the most likely explanation.
227:Fitness varies in time and space
350:and neighbouring states. Using
144:A well-studied case is that of
1:
334:and his co-workers collected
194:Frequency-dependent selection
188:Frequency-dependent selection
127:heterotic balancing selection
354:technique, they studied the
284:, which breaks them open on
323:Chromosome polymorphism in
1094:
1057:Nonsynonymous substitution
384:Hardy–Weinberg equilibrium
191:
114:
346:from wild populations in
182:heterozygote disadvantage
38:(different versions of a
530:A dictionary of genetics
337:Drosophila pseudoobscura
212:Potamopyrgus antipodarum
1052:Synonymous substitution
996:Models of DNA evolution
572:Encyclopædia Britannica
477:Nature Reviews Genetics
398:they become stabilised.
299:Two active grove snails
820:Phil. Trans. R. Soc. B
728:Phil. Trans. R. Soc. B
706:Phil. Trans. R. Soc. B
373:
360:chromosomal inversions
300:
274:
139:balanced polymorphisms
123:heterozygote advantage
117:Heterozygote advantage
112:
104:
91:Heterozygote advantage
30:refers to a number of
975:Stabilizing selection
960:Directional selection
452:Fluctuating selection
447:Stabilizing selection
437:Directional selection
368:
332:Theodosius Dobzhansky
298:
272:
240:More complex examples
152:disease that damages
110:
98:
18:Balanced polymorphism
965:Disruptive selection
871:Dobzhansky T. 1970.
635:Genetic polymorphism
442:Disruptive selection
356:polytene chromosomes
68:genetic polymorphism
1030:Molecular processes
955:Balancing selection
939:Molecular evolution
899:Ecological genetics
782:Ecological genetics
558:The New Systematics
372:polytene chromosome
305:apostatic selection
214:) by the trematode
32:selective processes
28:Balancing selection
970:Negative selection
853:Drosophila robusta
374:
301:
275:
146:sickle cell anemia
113:
105:
34:by which multiple
1065:
1064:
947:Natural selection
661:10.1111/evo.13719
585:Ann. Human Genet.
282:Turdus philomelos
252:The grove snail,
177:malarial parasite
170:sickle cell trait
101:malarial parasite
16:(Redirected from
1085:
1042:Gene duplication
1006:Allele frequency
932:
925:
918:
909:
902:
897:Ford E.B. 1975.
895:
889:
882:
876:
869:
863:
849:
843:
833:
827:
811:
805:
791:
785:
780:Ford E.B. 1975.
778:
772:
761:
755:
745:Cepaea nemoralis
741:
735:
719:
713:
697:
691:
690:
672:
644:
638:
633:Ford E.B. 1965.
631:
625:
615:
609:
606:
600:
597:
591:
581:
575:
570:Heredity. 2009.
568:
562:
561:
546:
540:
539:
533:
523:
517:
516:
468:
391:population cages
260:multiple alleles
255:Cepaea nemoralis
220:Microphallus sp.
216:Microphallus sp.
21:
1093:
1092:
1088:
1087:
1086:
1084:
1083:
1082:
1068:
1067:
1066:
1061:
1047:Silent mutation
1037:Gene conversion
1025:
984:
980:Selective sweep
941:
936:
906:
905:
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582:
578:
569:
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543:
525:
524:
520:
489:10.1038/nrg2664
483:(11): 783–796.
470:
469:
465:
460:
433:
381:
328:
250:
242:
229:
196:
190:
154:red blood cells
119:
93:
88:
23:
22:
15:
12:
11:
5:
1091:
1089:
1081:
1080:
1070:
1069:
1063:
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1059:
1054:
1049:
1044:
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1027:
1026:
1024:
1023:
1021:Fay and Wu's H
1018:
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1008:
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998:
992:
990:
986:
985:
983:
982:
977:
972:
967:
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951:
949:
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937:
935:
934:
927:
920:
912:
904:
903:
890:
877:
864:
844:
828:
817:polymorphism.
806:
786:
773:
756:
736:
714:
692:
655:(5): 883–896.
639:
626:
610:
601:
592:
576:
563:
541:
518:
462:
461:
459:
456:
455:
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432:
429:
425:
424:
421:
399:
387:
379:
327:
321:
249:
246:
241:
238:
234:geometric mean
228:
225:
208:prey switching
192:Main article:
189:
186:
133:has a greater
115:Main article:
92:
89:
87:
84:
70:is conserved.
66:. In this way
24:
14:
13:
10:
9:
6:
4:
3:
2:
1090:
1079:
1076:
1075:
1073:
1058:
1055:
1053:
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1043:
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1038:
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1034:
1032:
1028:
1022:
1019:
1017:
1014:
1012:
1009:
1007:
1004:
1002:
999:
997:
994:
993:
991:
987:
981:
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388:
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376:
375:
371:
367:
363:
361:
357:
353:
349:
345:
344:
343:D. persimilis
339:
338:
333:
330:In the 1930s
326:
322:
320:
316:
314:
309:
306:
297:
293:
291:
287:
286:thrush anvils
283:
280:
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247:
245:
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237:
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226:
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205:
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195:
187:
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173:
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167:
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159:
155:
151:
148:in humans, a
147:
142:
140:
136:
132:
128:
124:
118:
109:
102:
97:
90:
85:
83:
79:
78:populations.
77:
71:
69:
65:
61:
57:
56:heterozygotes
53:
52:genetic drift
49:
45:
41:
37:
33:
29:
19:
954:
898:
893:
885:
880:
872:
867:
859:
856:
852:
847:
839:
836:
831:
826:, 1577-1593.
823:
818:
814:
809:
801:
798:
794:
789:
781:
776:
768:
764:
759:
751:
748:
744:
739:
731:
726:
722:
717:
709:
704:
700:
695:
670:10150/632441
652:
648:
642:
634:
629:
613:
604:
595:
587:
584:
579:
571:
566:
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406:
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369:
341:
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329:
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251:
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219:
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211:
203:
199:
197:
174:
165:
143:
138:
126:
120:
80:
72:
27:
26:
1011:Ka/Ks ratio
771:39: 89-116.
279:song thrush
248:Grove snail
1016:Tajima's D
842:: 585–586.
804:, 109-143.
574:. Chicago.
550:Ford, E.B.
458:References
413:Gatlinburg
408:D. robusta
370:Drosophila
348:California
325:Drosophila
290:least well
158:hemoglobin
150:hereditary
86:Mechanisms
64:homozygote
48:population
1078:Selection
862:, 237–48.
857:Evolution
734:: 483-98.
649:Evolution
554:J. Huxley
497:1471-0064
395:migration
352:Painter's
313:supergene
264:epistasis
76:panmictic
62:than the
44:gene pool
1072:Category
769:Genetics
754::275-94.
749:Heredity
687:83461372
679:30883731
590:, 67-89.
505:19834483
431:See also
166:carriers
837:Science
712:: 1-81.
556:(ed.).
168:of the
135:fitness
60:fitness
36:alleles
989:Models
884:1981.
815:Cepaea
795:Cepaea
765:Cepaea
723:Cepaea
701:Cepaea
685:
677:
513:771357
511:
503:
495:
747:(L).
683:S2CID
509:S2CID
403:cline
131:locus
125:, or
46:of a
675:PMID
501:PMID
493:ISSN
340:and
161:gene
40:gene
855:".
824:353
732:253
710:246
665:hdl
657:doi
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485:doi
405:in
266:).
121:In
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673:.
663:.
653:73
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667::
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621::
538:.
515:.
487::
380:s
378:N
103:.
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