Rich KingDepartment of Biological Sciences

Detecting Natural Selection in Real Time: Detecting Natural Selection in Real Time: Examples from Lake Erie Snake PopulationsExamples from Lake Erie Snake Populations

What is Natural Selection?

As many more individuals of each species are born than can possibly survive; and as, consequently, there is a frequently recurring struggle for existence, it follows that any being, if it vary however slightly in any manner profitable to itself, under the complex and sometimes varying conditions of life, will have a better chance of surviving, and thus be naturally selected. From the strong principle of inheritance, any selected variety will tend to propagate its new and modified form.

What is Natural Selection?

As many more individuals of each species are born than can possibly survive; and as, consequently, there is a frequently recurring struggle for existence, it follows that any being, if it vary however slightly in any manner profitable to itself, under the complex and sometimes varying conditions of life, will have a better chance of surviving, and thus be naturally selected. From the strong principle of inheritance, any selected variety will tend to propagate its new and modified form.

Variation

What is Natural Selection?

As many more individuals of each species are born than can possibly survive; and as, consequently, there is a frequently recurring struggle for existence, it follows that any being, if it vary however slightly in any manner profitable to itself, under the complex and sometimes varying conditions of life, will have a better chance of surviving, and thus be naturally selected. From the strong principle of inheritance, any selected variety will tend to propagate its new and modified form.

Variation

Fitness Differences

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What is Natural Selection?

As many more individuals of each species are born than can possibly survive; and as, consequently, there is a frequently recurring struggle for existence, it follows that any being, if it vary however slightly in any manner profitable to itself, under the complex and sometimes varying conditions of life, will have a better chance of surviving, and thus be naturally selected. From the strong principle of inheritance, any selected variety will tend to propagate its new and modified form.

Variation

Fitness Differences

Inheritance

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What is Natural Selection?

As many more individuals of each species are born than can possibly survive; and as, consequently, there is a frequently recurring struggle for existence, it follows that any being, if it vary however slightly in any manner profitable to itself, under the complex and sometimes varying conditions of life, will have a better chance of surviving, and thus be naturally selected. From the strong principle of inheritance, any selected variety will tend to propagate its new and modified form.

Variation

Fitness Differences

Inheritance

Evolution

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How can natural selection be detected? Darwin’s view:

“. . . natural selection will always act with extreme slowness . . .” “Its action depends on . . . physical changes, which are generally very slow . . .” “. . . variation itself is apparently always a very slow process.” “The process will often be greatly retarded by free intercrossing.” “. . . I do believe that natural selection will always act very slowly, often only at long intervals of time.”

By extension: Natural selection should be difficult to detect

How can natural selection be detected? A more contemporary view (J. A. Endler, 1986, Natural Selection

in the Wild): “Natural selection is ubiquitous enough to be found in a wide variety of organisms . . .” “. . . strong selection is by no means uncommon in natural populations.”

By extension: Natural selection should be readily detected by any of a variety of methods.

Western Lake Erie Islands

• 18 islands• 1 – 4000 hectares• 1 – 30 km from mainland• shorelines of limestone and dolomite that resisted glaciation• isolated ca. 4000 years ago• islands = natural laboratories of evolutionary biology• renowned for abundance of snakes: “Les Isles aux Serpentes”

• Medium-sized non-venomous live-bearing colubrid• Active April – October• Courtship May – June• 20+ offspring in September• Maturity in 2 – 4 years• Longevity 10 + years • Maximum size > 1 m• Aquatic prey• Terrestrial retreats/hibernation

Lake Erie Watersnakes

Variation in Lake Erie Watersnake Color Pattern

Mainland populations

Island populations

Variation in Lake Erie Watersnake Color Pattern

Mainland populations

Island populations

Inheritance of Lake Erie Watersnake Color

Pattern• Pattern elements remain fixed in shape and position• Resemblance among relatives suggests significant heritability (h2)• Captive matings suggest a major locus with regular dominant to reduced pattern

h2 = 0.20 - 0.34

h2 = 0.40 - 0.62

Evidence for Natural Selection Method I: Correlation with environmental factors

Environmental differences impose different selective regimes, resulting in differences in phenotype among populations

Island shoreline Mainland marsh

Evidence for Natural Selection Method I: Correlation with environmental factors

Environmental differences impose different selective regimes, resulting in differences in phenotype among populations

Island shoreline Mainland marsh

Suggests color pattern is target & predation is agent of selection

• Other targets & agents? • Current vs. past selection? • Strength of selection?

Method IX: Nonequilibrium prediction of fitness differences or their consequences

Phenotype Performance Selection

Do snakes differing in color pattern differ in crypsis (the degree to which they are visible to predators)?

Island shoreline

Mainland marsh

Island shoreline

Mainland marsh

Quantitatification of crypsis: compare size of pattern elements between snakes and

backgrounds

Method IX: Nonequilibrium prediction of fitness differences or their consequences

Phenotype Performance Selection

Performance functions predict that on islands, visual predators should impose selection favoring less patterned morphs among neonates and more patterned morphs among adults.

• Current vs. past selection? • Strength of selection?

Method VII: Genetic demography or cohort analysis (longitudinal analysis)Do snakes differing in color pattern differ in survival?

• 317 newborns marked & released in fall, 1983• 54 survivors recaptured in spring & summer, 1984• color-pattern of survivors was compared to snakes that were not recaptured

Method VII: Genetic demography or cohort analysis (longitudinal analysis)Do snakes differing in color pattern differ in survival?

Yes, among newborns, survival was highest for snakes with reduced patterns.

• Color pattern is target of selection• Survival (via differential predation?) is agent of selection• Selection is occurring currently• Selection differential () = -0.30 for number of dorsal blotches• Relative fitness of regularly patterned snakes is 78-90% that of reduced pattern snakes (s = 0.1 – 0.22).

Method VIII: Comparisons among age classes or life-history stages (cross-

sectional analysis)Selection should result in differences in color pattern frequencies between different age classes.

• Prediction from crypsis analysis (Method IX) is that frequency of regularly patterned snakes should be greater among adults than subadults. • Compared color pattern frequencies between subadults and adults at 5 island sites.

Method VIII: Comparisons among age classes or life-history stages (cross-

sectional analysis)Selection should result in differences in color pattern frequencies between different age classes.

No consistent pattern of selection from sub-adults to adults; selection differentials () = -0.05 – 0.01.

•Adult snakes have outgrown most significant predators?

Method V: Long-term studies of trait frequency distributions

Natural selection should result in trait frequencies that remain constant over time (stabilizing selection, directional selection balanced by gene flow) or change monotonically (directional selection).

Morph frequencies have remained relatively constant from 1980 – present

Method V: Long-term studies of trait frequency distributions

Natural selection should result in trait frequencies that remain constant over time (stabilizing selection, directional selection balanced by gene flow) or change monotonically (directional selection).

Morph frequencies have changed markedly between historic (<1961) and recent (1980-2003) samples

• Change in selection?• Change in gene flow?

Method IV: Deviation from formal null

modelsIn the absence of selection, variation in color pattern frequencies among populations should equal that resulting from the combined effects of random genetic drift and gene flow.

• Assessed allozyme (protein) variation at 5 island and 2 mainland sites (presumably neutral)• Compared to variation in color pattern allele frequency

Method IV: Deviation from formal null models

• FST provides a measure of genetic differentiation among populations• Variation in putative color pattern allele is similar to that of allozymes among 5 islands• Variation in putative color pattern allele is markedly greater than that of allozymes when mainland populations are included

• Pattern is consistent with selection favoring different color pattern alleles in island vs. mainland populations

5 Islands

5 Island + 2 Mainland sites

Allozymes 0.016 0.074

Color pattern 0.019 0.493

Method X: Equilibrium prediction of the outcome of natural selection

Color pattern variation in Lake Erie watersnakes appears to represents a balance between

1) selection favoring a reduction in color pattern in island populations and

2) gene flow from the mainland where regularly patterned snakes predominate.

Information on inheritance, strength of selection, and rate of gene flow can be used to predict equilibrium conditions.

Method X: Equilibrium prediction of the outcome of natural selection

Under an island-continent model with selection favoring reduced pattern snakes on islands and gene flow from mainland populations consisting solely of patterned snakes, change in allele frequency,

q – hq2

q = - q - mq 1 – hq2

Whereq = frequency of reduced pattern alleleh = the strength of selection = 1-(1/(1-s))m = rate of gene flow.

Using allozyme-based estimates of FST and Nm and mark-recapture estimates of N, m ≈ 0.0008From cohort analysis (Method VII), s ≈ 0.10

Predicted equilibrium allele

frequency:q = 0

Observed allele frequencies are lower than predicted • Gene flow underestimated?• Strength of selection overestimated?

Predicted equilibrium allele

frequency:q = 0

Observed allele

frequencies

ConclusionsMultiple methods demonstrate convincingly that

Lake Erie watersnakes are subject to natural selection 1)Color pattern is the target of selection2)Survival (via differential predation) is the agent of

selection3)Selection occurs in present-day populations4)Strength of selection is moderately high

Some unexpected results and directions for future research1)Predicted pattern of selection on older snakes (from

analysis of crypsis) was not evident 2)Historic and recent samples exhibit unexplained

differences in morph frequency3)Observed color pattern allele frequencies differ from

predicted equilibrium allele frequency4)Selection in mainland habitats has not received

detailed study

More generally: watersnakes are not unique

Endler’s 1986 compilation documents selection in 141 species involving 314 traits and includes 566 estimates of selection coefficients & gradients (Methods VI, VII, VIII)

Kingsolver et al. summarize >2,500 estimates of selection coefficients and gradients from 1984-1997(Kingsolver et. al. 2001, American Naturalist 157:245-2261; Hoekstra et al. 2001, PNAS 98:9157-9160)

Examples include morphological, physiological, biochemical, and life history traits and mortality, fecundity, and sexual selection

Examples of directional selection outnumber stabilizing selection

Shortcomings (there is more work to be done!)Lack of replication, small sample sizeMorphological traits and directional selection

predominateViability, fecundity, and sexual selection need to be

studied on similar timescalesMore sophisticated analyses (fitness surfaces,

population genomics) are warranted