Evolution Biol 4802

Lecture 18 - Chapter 12

 

Topics for today

    1. What is the difference between natural selection and evolution?
    2. Modes of natural selection
    3. Genetical theory of natural selection

 

Scenario 1 – traits are genetically based

Natural selection occurs

·         Interaction between phenotypes and the environment resulting in fitness differences

Evolutionary response to natural selection occurs

·         Genetic change in genotypic frequency of offspring

Scenario 2 – traits variation is environmentally induced

Natural selection occurs

·         Interaction between phenotypes and the environment resulting in fitness differences

      No evolutionary response to natural selection

·         No genetic change in genotypic frequency of offspring because the phenotype is related to environmental variation rather than genetic variation

 

Key points

Polymorphism at many alleles leads to “quantitative variation”

 

Environmental factors can have an:

The environmental context produces a different phenotypic outcome

 

Example

·         Brown frog Rana pirica

·         Tadpole body type is altered in the presence of the gape-limited predatory salamander making it impossible to swallow. Kishida, et al. 2005.Evolutionary Ecology Research, 7(4): 619-631

·         Different genes are expressed in the presence of the predatory salamander. Mori et al. 2005. Biochem.  & Biophys. Research. Comm. 330:1138–1145

Example

·         Populus euramericana  (clone I-214) exposed to elevated and ambient CO2 in a FACE (free-air CO2 enrichment) experiment.

·         Global analysis using cDNA microarrays was conducted following 6-years of exposure

·         Does gene expression differ in different environments?

o   Same genotype

o   Different tissue ages

·         Gene expression differs between CO2 environments and development. Taylor et al. 2005 New Phytologist 167: 143–154

 

How can we understand polygenic traits?

 

Three modes of selection

Fig. 12.1

Directional selection

Stabilizing selection (balancing)

Diversifying selection (disruptive)

 

Episodes of selection in life

Fig. 12.3

“Components of fitness are combined (usually by multiplying them)…” page 273. Bogus method.  New method forthcoming – keep tuned.

 

Unifying life history analyses for inference of fitness and population growth

R. G. Shaw, S. Wagenius, H. Hangelbroek, J.R. Etterson, C.J. Geyer

 

Genetical theory of natural selection

            Box 12.A

           

 

 

 

 

 

Key points from simulations

Selection on beneficial alleles

         An allele with even a miniscule advantage will eventually be fixed by selection

         An advantageous allele moves to fixation more slowly if it is recessive, especially if it starts at low frequency

Selection on deleterious alleles

         An allele with even miniscule disadvantage will eventually be eliminated by selection (purging selection)

         A deleterious allele is purged more slowly if it is recessive, especially if it starts at low frequency

Selection on heterozygotes

         Heterozygote advantage leads to maintenance of alleles

         Heterozygote disadvantage leads to multiple fixation points depending upon initial allele frequencies