EVOLUTIONARY ECOLOGY SOME USEFUL DEFINITIONS Lecture #2

1. “Nothing in biology makes sense except in the light of evolution” Theodosius Dobzhansky The book is one written by G.E. Hutchinson 1965 “ The ecological theater and the evolutionary play”

The quote is self-explanatory and the intent of the books title is to state that the process of evolution takes place in an ecological context - evolutionary events occur because of the relationships between an organismic unit and their environment

To develop the tie between ecology and evolution we will begin with a discussion of Natural Selection 1. The theory of natural selection is a truly fundamental unifying theory of life 2. A thorough appreciation of it is is essential background for understanding evolutionary ecology 3. Natural Selection comes as close to being a “fact” as anything in biology

Many think that Natural Selection is synonomous with Evolution; however, it’s not! - Evolution refers to temporal changes, whereas natural selection specifies one particular way in which these changes are brought about - Evolution more specifically refers to a change in gene frequencies or descent with modification

Darwins’ Definition of Natural Selection 1. In any population, more offspring tend to be produced than can survive to reproductive age. 2. Members of the population vary in form and behavior. Much of the variation is heritable. 3. Some varieties of heritable traits are more adaptive than others

Darwins’ definition 4. Because bearers of adaptive traits have a greater chance of reproducing, their offspring tend to make up an increasingly greater proportion of the reproductive base for each new generation. This tendency is called differential reproduction. 5. “Differential reproduction” is natural selection. Adaptive forms of traits show up (are selected for) with increased frequency in a population, because their bearers contribute proportionately more offspring to succeeding generations

Endler’s (1986) definition of Natural Selection a. variation in some attribute or trait b. a consistent relationship between that trait and mating ability, fertilizing ability, fecundity and/or survivorship: fitness differences c. a consistent relationship, for that trait, between parents and their offspring, which is at least partially independent of common environmental effects: i.e., there is a heritable component

Then: 1. There will be a within-generation effect; individuals of a given age will differ predictably from the individuals which do not survive to that age 2. There will also be a between-generation effect; the offspring generation will differ predictably from their parental generation

The modern statement of the theory of natural selection is in terms of genes; i.e., what changes during evolution is the relative frequency of genes

Modern restatement in genetic terms 1. All organisms have genes which code for protein synthesis. These proteins regulate the development of the nervous system, muscles and structure of the individual. 2. Within a population many genes are present in two or more alternative forms, or alleles, which code for slightly different forms of the same protein. These will cause differences in development and so there will be variation within a population.

Modern restatement 3. Any allele that can make more surviving copies of itself than its alternative will eventually replace the alternative form in the population. Therefore, natural selection is the differential survival of alternative alleles

Modern restatement - genes will be selected through a complex interaction with their environment, including their genetic environment (other genes) and ecological environment (climate, competitors, herbivores, etc.) - the individual can be regarded as a temporary vehicle by which genes survive and are replicated - the most successful genes will be those which promote an individuals survival and reproductive success

Selection The three most common include –Stabilizing –Directional –Disruptive

Stabilizing Selection - reducing the variance of extremes Fitness Freq Before Freq After Phenotype

Disruptive Selection - can increase the variance in a trait Fitness Freq Before Freq After Phenotype or Phenotype

Directional Selection - affects the mean value of the trait; the variance may also change Fitness Freq Before Freq After Phenotype

Other types of selection encountered: Other types of selection encountered: - Frequency-dependent - Density-dependent and density-independent - Hard and soft - Artificial - Sexual - Mortality - Phenotypic - Correlational

Frequency Dependent Selection - occurs when the fitness of a phenotype depends upon the frequency of that phenotype in the population. Selection of this sort could favor either rare or common phenotypes - leads to oscillations in the mean value of a trait

Density-dependent & Density- independent Selection - density-dependent selection, selection may only emerge when competition between individuals becomes pronounced, i.e., when density is high - by contrast, the effects of extreme weather may be density-independent, so the survival or death of an organism occurs regardless of density

Hard & Soft Selection Parallels density-dependent & density- independent - Soft selection occurs when individuals are in competition - only a certain proportion will be able to survive - Hard selection - the survival or death of one individual is not conditional on the suvival or death of another individual

Artificial Selection Selective breeding; fitness differences imposed by humans - goal oriented By contrast, natural selection is blind and operates without a goal; humans can certainly impose natural selection

Sexual Selection Traits that seem more a hinderance than a help Selection that assisted individuals in gaining mates –competition for mates –preference for particular traits in the other sex

Mortality Selection Consistent phenotypic-specific mortality A single component of fitness variation and therefore an incomplete definition

Phenotypic Selection Phenotypic selection requires conditions a and b of Endler’s definition - trait variation & fitness differences Does not include genetic change

Correlational Selection - certain combinations of traits or alleles are favored - this will result in patterns of gametic phase (“linkage”) disequilibrium for suites of traits

Example of Correlational Selection Brodie, E.D Genetic corelations between morphology and antipredator behavior in natural populations of the garter snake Thamnophis ordinoides. Nature.

Examples Gibbs and Grant Oscillating selection on Darwin’s finches. Nature

Directional Selection - Darwin’s Finches

Hori, M Frequency-dependent natural selection in the handedness of scale-eating cichlid fish. Science

Frequency-Dependent Selection in Cichlid Fish

Disruptive Selection in Salmon

Fitness (Dawkins 1982) 1. Historical 2. Genotypic 3. Individual (Classic) 4. Inclusive 5. Personal

Historical Fitness Definition used by Wallace, Spencer and Darwin - The fittest individuals would be those with the keenest eyes, the strongest muscles, the sharpest ears, the swiftest reflexes - a definition close to everyday usage, physical fitness

Genotypic Fitness Used by population geneticists - May be regarded as a measure of the number of offspring that a typical individual of genotype Aa is expected to bring up to reproductive age

Individual Fitness (Classic Fitness) Individual reproductive success - Fitness is a property of an individual organism, often expressed as the product of survival and fecundity

Inclusive Fitness Calculated from an individuals own reproductive success plus his effects on the reproductive success of his relatives, each one weighed by the appropriate coefficient of relatedness - IF = W ind + (W rel ) X (r) i Inclusive Fitness = Direct Component + Indirect Component

Personal Fitness Backwards way of looking at inclusive fitness - Personal fitness focuses on the effects that the individual’s relatives have on his or her fitness

Natural Selection as a Tautology Natural selection is defined as “survival of the fittest” therefore the fittest survive is circular or an inherently true statement

There are at least two reasons why this is misleading 1. Fitness can be directly related to biological properties and the design of organisms. 2. It is heritable variation in the biological properties of organisms which gives rise to fitness differences and the process of natural selection - From heritable variation in some attribute or trait and a correlation between that trait and fitness we can deduce the effects - evolutionary change

Therefore, natural selection is a syllogism (2 premises leading to a conclusion) rather than a tautology (or circular reasoning) New insights can be gained from an understanding of the process

How did we get into this tautology mess? Herbert Spencer & Alfred Russel Wallace were responsible for the phrase “Survival of the fittest”