Introduction to Ecological Genetics

Darwin’s Four Postulates

Individual variation (phenotypes vary, ecology and genetics)

In every generation, more progeny produced than can survive (population dynamics, ecology)

Survival and reproduction of individuals not random

Within the current conditions, individuals with some phenotypes produce more offspring, find more mates, survive better (i.e. are more fit)

These individuals are selected by environment (ecology)

Some phenotypic variants passed on to offspring (heritable information, genetics)

Thus, more fit phenotypes are better represented in the next generation (evolution)

Published in:  Darwin, C.  1859.  On the Origin of Species by Means of Natural Selection, 1st edition

See online facsimile:

http://www.esp.org/books/darwin/origin/facsimile/

Individual variation (phenotypes vary)

Height histogram

Calculate mean and variance

Mean

Variance

Compare with runners’ height histogram

Runners from research unit at University of Indiana in Bloomington, IN

2.  In every generation, more progeny produced than can survive

Darwin read Malthus’ treatise on population growth

See online HTML version

http://www.ac.wwu.edu/~stephan/malthus/malthus.0.html

Population growth models

Exponential growth

To read more about exponential growth

http://www.ento.vt.edu/~sharov/PopEcol/lec5/exp.html

Not all progeny can survive

Carrying capacity

Density-dependent growth

To read more about logistic growth

http://www.ento.vt.edu/~sharov/PopEcol/lec5/logist.html

Examples of using r to measure fitness

Bennett, A.F., Lenski, R.E., & Mittler, J.E. (1992). Evolutionary adaptation to temperature I. Fitness responses of Escherichia coli to changes in its thermal environment. Evolution, 46:16-30.

//links.jstor.org/sici?sici=0014-3820%28199202%2946%3A1%3C16%3AEATTIF%3E2.0.CO%3B2-O

Back to Darwin

Survival and reproduction of individuals not random

Within a particular environment, individuals with certain phenotypes survive and reproduce better than others

These are the selected individuals

Fitness

Metric to measure selection

Note:  Fitness itself is a phenotype

Like all phenotypes, it varies with the environment

Measure fitness as population growth rate

Early example: Gause, G. F.  1934.  The Struggle for Existence.  Hafner Press, NY

Online HTML with facsimile figures

http://www.ggause.com/Contgau.htm

Yeast Monocultures

Found logistic growth with carrying capacity of 8.70 units

Note early exponential phase of growth, which could be used to estimate r

Yeast Mixed cultures

Result

Still logistic growth
Looking at r during exponential phase, Saccharomyces has higher fitness than does Schizosaccharomyces

Modern use of same method for E. coli

Bennett, A.F., Lenski, R.E., & Mittler, J.E. (1992). Evolutionary adaptation to temperature I. Fitness responses of Escherichia coli to changes in its thermal environment. Evolution, 46:16-30.

//links.jstor.org/sici?sici=0014-3820%28199202%2946%3A1%3C16%3AEATTIF%3E2.0.CO%3B2-O

Studied adaptation to High and Low Temperatures by E. coli.

Used doubling time to estimate fitness of a bacterial lineage

Calculating doubling time

Can use exponential growth model to calculate doubling time

Integrate exponential equation to obtain a population projection equation
Predicts population growth at any time, assuming constant exponential growth
Mathematically describe doubling time
Substitute into the population projection equation
Divide both sides by N0
Take natural log of both sides
Rearrange to solve for doubling time

ln(2) = 0.6931

Measuring fitness components

Not possible to measure population growth rate of a single genotype for most organisms

Use fitness components:  e.g. fecundity, survival

For species with complex life histories, fecundity and survival vary with age

Use age-structured models (life table analysis)

Estimate replacement rate of a phenotype as if growing in a pure population, R0
If survivorship = probability of survival to age x+1 from age x
And L = life expectancy,
l(x) = prob. of survival from birth to age x  (cumulative),
and m(x) = fecundity (# offspring) at age x
Then,  (in stable population)
At low density, replacement rate: 
This equation is a discrete solution to the continuous logistic equation

Other surrogates for fitness

Survival and reproduction of individuals not random (continued)

Selection gradient

Plot fitness value versus phenotypic value

Calculate slope of regression

Where w = relative fitness, z = phenotypic value, a = intercept, b = slope
Directional selection gradient

Selection differential

Change in population mean value of a trait within a generation

 = population mean value before selection
 = population mean value after selection
 = selection differential

Example:  goldenrod ball-galler – Eurosta solidaginis

Eurosta

For more information, see:  http://www.facstaff.bucknell.edu/abrahmsn/solidago/main.html

phenotype = gall diameter

See data from student projects, Williams College and Culver College
http://www.williams.edu/Biology/studentprojects/Biol015/ballgall/index.html
 
Gall Contents
Average Diameter
Diameter Range
Standard Deviation
Eurosta solidaginis
21.96 mm
14.20 - 29.40 mm
+/-3.31 mm
Eurytoma gigantea
17.58 mm
15.35 - 20.25 mm
+/-2.47 mm
All galls
21.4
12.5 – 29.45 mm
+/-3.56 mm
See data from another student project, http://homepages.culver.edu/faculty/jcoelho/biol206/eurosta.htm
Gall Contents
Average Diameter
Diameter Range
Standard Deviation
Eurosta solidaginis
19.16 mm
9.00 – 24.00 mm
+/-3.37 mm
Eurytoma gigantea
15.94 mm
6.00 – 24.00 mm
+/-4.23 mm
All galls
18.41 mm
6.00 – 24.00 mm
+/-3.81 mm

Also interesting research on freeze tolerance of the gallers

http://www.naturenorth.com/winter/gallfly/Fgallfly.html

What’s the selection differential for each population?

What happens in next Eurosta generation?

Some phenotypic variants passed on to offspring (heritable information)

Darwin was unclear on how this happened, but knew that it did in some way

If a trait is heritable or phenotypic variants are “genetically controlled” more fit phenotypes will be better represented in subsequent generation

Adaptation:  evolution by natural selection

Evolutionary response modeled by Breeder’s Equation

R = s h2

Evolution versus natural selection

Evolution does not equal natural selection

Evolution is descent with modification

Due to any of several mechanisms

Natural selection is a mechanism by which evolution results in adaptation

Adaptation

Process by which populations become more fit in current environment

Any trait that allows an individual, population, or lineage to be more fit in current environment

Evolution can happen by processes other than natural selection

These processes modeled by Hardy-Weinberg Equation

Ideal population will not evolve

It is at evolutionary equilibrium

Where allele frequencies will stay at: 

 

Ecology

Explicitly assumed to have no effect

No differential fitness (no selection)

Single population (no gene flow)

No population dynamics (infinite population size)

No behavior (random mating)

Genetics

Explicitly assumed to be “perfect”

Perfect information transfer across generations

No mutation

Perfect translation of genotype to phenotype

No environmental contribution to phenotype
Single gene controls phenotype