Ecology Chapter 6

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Evolution

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-change in allele frequency(proportion) over time
-defined more broadly as descent with modification

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defined more broadly as descent with modification

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change in organisms over time.

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genotype

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genetic makeup, and is represented by letters, one for each allele.

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Four key processes influence evolution

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•Mutation
•Natural selection
•Genetic drift
•Gene flow

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natural selection

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-proposed by Charles Darwin
-Individuals with certain heritable characteristics survive and reproduce more successfully than individuals with other heritable characteristics
-Natural selection acts as a sorting process.
-Individuals with favored traits contribute more offspring to the next generation, and their alleles will increase in frequency in the population.
-The population will evolve, but individuals do not evolve.

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Phenotype

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the observable characteristics of individuals that are determined by the genotype.

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recombination

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production of offspring that have combinations of alleles that differ from those of their parents

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at what rate do mutations occur?

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10^–4to 10^–6new mutations per gene per generation. In each generation, one mutation would occur in every 10,000 to 1,000,000 copies of a gene.

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when does natural selection occur?

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when individuals with particular heritable traits tend to leave more offspring than individuals with other heritable traits.

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3 General models of natural selection

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•Directional selection
•Stabilizing selection
•Disruptive selection

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Directional selection

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Individuals with one extreme of a heritable phenotypic trait (for example, large size) are favored.
-ex: Drought favored large beak size in medium ground finches.

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Stabilizing selection:

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-Individuals with an intermediate phenotype are favored
-ex:Parasites and predators of Eurostaflies result in stabilizing selection. Parasitic wasps select for small gall size; birds select for large gall size.

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Disruptive selection

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-Individuals at both phenotypic extremes are favored.
-Example: African seedcrackers (birds) have two food sources—hard seeds that large beaks are needed to crack, and smaller, softer seeds that smaller beaks are more suited to.

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Genetic drift

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-occurs when chance EVENTS determine which alleles are passed to the next generation.
-Genetic drift is significant only for small populations.
-ex: If the wildflower population had 10,000 individuals, the chance of the moose killing 40% of the population without killing any aaplants is essentially zero.
-It is more likely that individuals of all genotypes would be killed, and allele frequency would stay the same.

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four effects of genetic drift on small populations

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1. Because it acts by chance alone, it causes allele frequencies to fluctuate at random. Some alleles may disappear, other may reach 100% frequency (fixation).
2. Because some alleles are lost, genetic drift reduces genetic variation of the population.
3. Frequency of harmful alleles can increase. If the allele has only mildly deleterious effects, genetic drift can ―overrule‖ natural selection.
4. Differences between populations can increase.

-The second and third effects can have dire consequences.
=>Loss of genetic variation reduces the ability of the population to respond to changing environmental conditions.
=>Increase of harmful alleles can reduce survival and reproduction.
=>These effects are important for species that are near extinction.

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Gene flow

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-occurs when alleles are transferred from one population to another via movement of individuals or gametes.

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what are the 2 effects of gene flow?

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-1. Populations become more similar genetically (though not necessarily phenotypically).
2. New alleles can be introduced into a population.

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adaptive evolution

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-By consistently favoring individuals with certain alleles, natural selection causes adaptive evolution, in which traits that confer advantages tend to increase in frequency over time.
-There are many examples of rapid adaptive evolution, including increased antibiotic resistance in bacteria; increased insecticide resistance in insects; drab coloration in guppies, which makes them harder for predators to see; and increased beak size in Geospizafinches.

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Clines

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-are gradual changes in a characteristic over a geographic region.
-Example: In the fruit fly Drosophila, the alcohol dehydrogenase (Adh) gene exhibits a cline in which the AdhSallele decreases in frequency as latitude increases.
=>AdhS allele is less effective in cooler temperatures

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what large-scale processes shape the long-term patterns on evolution

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speciation, mass extinction, and adaptive radiation

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Species

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-are groups of organisms whose members have similar characteristics and can interbreed.
-Ecological interactions influence the number of species alive today.

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Speciation

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the process by which one species splits into two or more species.
-Most commonly occurs when a barrier prevents gene flow between two or more populations of a species.
-Barriers can be geographic or ecological.
-The populations then diverge genetically over time.

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reproductive isolating barriers

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-key step of speciation
–biological mechanisms that prevent successful reproduction between two populations or incipient species.

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mass extinctionevents.

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-The fossil record documents five mass extinctionevents.
-Large proportions of Earth’s species were driven to extinction worldwide in a relatively short time—a few million years or less.
-was followed by great increases in the diversity of some of the surviving groups.
-remove competitor groups, allowing survivors to expand into new habitats or new ways of life.

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adaptive radiations.

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-increases in diversity over a short time period
-Great increases in diversity can also occur when a group of organisms evolves major new adaptations.
-ex: Stems and waxy cuticles provided early terrestrial plants with support against gravity and protection from desiccation.