T. Dobzhansky (geneticist)

T. Dobzhansky (geneticist)

“Nothing in biology makes sense

except in the light of evolution”

Adaptation

A genetically determined characteristic that influences an organism's ability to survive and reproduce in a particular environment.

Characteristics can be morphological, behavioral, or physiological.

Evolution

• Is a genetic change in a population (not an individual) over time

• Ecologists look at phenotypic (physical changes), in most cases, because that is how we recognize populations.

• It is, in fact, changes in the genotype, or more specifically, the gene pool.

Allele Frequencies

The frequency of occurrence of alleles in a population.

If we use the simple one dominant and one recessive allele model, this can be demonstrated by:

p = frequency of the dominant allele

q = frequency of the recessive allele

Example

AA - 30 individuals

Aa - 20 individuals

aa - 50 individuals

p = 2(# individuals AA) + # individuals Aa 2(Total # individuals in population)

p + q = 1; therefore q = 1 - p

Example

p = 2(30) + 20 200= 0.4

p + q = 1; therefore q = 1 - 0.4 = 0.6

With these values, we can calculate the probability of what genotypes would be present in the next generation if this population were to mate randomly

Genotypic Frequencies

p2 = probability of AA

q2 = probability of aa

2pq = probability of Aa

p2 + 2pq + q2 = 1

Mechanisms for Evolutionary Change

Mutation

Genetic Drift (small population size)

Gene Flow (immigration and emigration)

Non-Random Mating

Natural Selection

Basic Tenet of Natural Selection

The most fit organisms (survivors) will reproduce and pass their genes

on to the next generation.

Hardy-Weinberg Equilibrium

In diploid, sexually reproducing organisms, phenotypes, genotypes and genes all tend to come to equilibrium in

populations in certain conditions are met

Hardy-Weinberg Equilibrium No Mutation

Large Population Size

No immigration or emigration

Random Mating

No Selection for Traits

Genetic Drift

• Random or chance events lead to a change in the genetic makeup of a population

• Limited to small populations

• “Bottleneck”

p = 0.7

Random event leads to loss of individuals

p = 0.33

Population will come to reflect surviving population

Effective Population Size (Ne)

The number of individuals actually participating in random mating.

This number is always smaller than the actual population size

- number of old- number too young- small number of one sex

Gene Flow

• Change in gene pool of a population from immigration or emigration.

• Founder Effect

Identification of Human Migration fromMitochondrial DNA

Steps for Natural Selection

• Variation occurs in every group of living organisms. Individuals are not identical in any population.

• Every population produces an excess of offspring.

• Competition will occur among these offspring for the resources they need to live.

Steps for Natural Selection

• The most fit offspring will survive.

• If the characteristics of the most fit organisms are inherited, these favored traits will be passed on to the next generation.

Common Types of Individual Selection

• Stabilizing selection

• Direction selection

• Disruptive selection

Figure 21.12

Figure 21.12

figure 21-12.jpg

Figure 21.13

Figure 21.13

Figure 21.14

Similarities in Adaptive Strategies

• Convergent Evolution – similar responses to similar environmental situations BUT not related evolutionarily

• Example – Fig. 2.9

!!!!!!!!!!VARIATION!!!!!!!!!!!

Properties of Fitness

• Fitness is a property of a genotype, not an individual or population.

• Fitness is specific to a particular environment. As the environment changes, so does the fitness of genotypes.

• Fitness is measured over one generation or more.

Outcomes of Selection

• Changes in Genetic Make-up of a Population

• Rise of new species – IF certain conditions met

Distribution of a Species

Allopatric Speciation

Geographic Barrier Splits Distribution

No longer interbreeding; therefore, no exchangeof genes and could be undergoing different selectionpressures

Over time, the gene pool of each group can become quit different

If two groups are brought back together anddo not interbreed, they are now two separate species

Distribution of a Species

Parapatric Speciation

Individuals move into a new habitat

If no interbreeding occurs between individualsin new habitat and those in the old, reproductiveisolating mechanisms can develop.

Isolating mechanism develops within the existing distribution of a species

Sympatric Speciation

Isolating mechanism develops within the existing distribution of a species

Sympatric Speciation

Reproductive Isolating Mechanisms

• Prezygotic mechanisms prevent fertilization or zygote formation. Temporal shifts - do not become reproductively

active at the same time. Behavioral shifts - do not recognize courtship

behaviors (female bird doesn't recognize the dance of a male).

Mechanical shift - change in reproductive structure making it physically impossible to mate.

Habitat shifts – populations live in the same regions but occupy different habitats/microhabitats.

Reproductive Isolating Mechanisms

• Postzygotic mechanisms zygote forms but

Does not complete development

Develops into a weak, unhealthy individual

Is sterile in either the F1 or F2 generation

Evolution of Interactions Among Species

Mimicry

• Batesian mimicry - a benign species resembles a noxious or dangerous species.

• Müllerian mimicry - both the mimics and the model are noxious or dangerous.

Coevolution

• Evolutionary change in one species results in a reciprocal response of another species

• Many examples – excellent one is diversification of flowering plants and insect pollinators

• Parasitism• Predator-Prey Interactions (including

herbivory)• Competition

• Other topics of note– Sexual selection– Kin Selection