Ch. 6 Evolution And Ecology

Front 1

evolution

Back 1

change over time in allele frequencies of different alleles in a population

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descent with modification

Back 2

• populations accumulate differences over time and hence, when a new species forms, it differs from its ancestors
• however, although a new species differs from its ancestors in some ways, it also resembles its ancestors and continues to share many characteristics with them

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mutations

Back 3

• if there were a species with no mutations, each gene would have only one allele and all members of a population would be genetically identical...evolution could not occur
• weak agent of allele frequency change but because it provides new alleles on which natural selection can occur, mutation is central to evolutionary process

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natural selection can be divided into 3 categories...

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1. direct

2. stabilizing

3. disruptive

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direct selection

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occurs when individuals with one extreme of a heritable phenotypic trait are favored over other individuals

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stabilizing selection

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individuals with intermediate phenotype are favored

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disruptive selection

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individuals with phenotype at either extreme are favored

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gene flow

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• occurs when alleles are transferred from one population to another via the movement of individuals or gametes
• has two effects: (1) populations become more similar and (2) new alleles can be introduced into a population

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genetic drift

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occurs when chance events affect which alleles are passed from one generation to the next

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four effects of genetic drift on evolution in small populations

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1. can cause allele frequencies to fluctuate randomly over time...eventually some alleles disappear from population while other reach fixation
2. reduces genetic variation of population
3. can increase frequency of harmful allele
4. can increase genetic difference between populations because chance events may cause an allele to reach fixation in one population, yet be lost from another

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adaptive evolution

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a process of change in which traits that confer survival or reproductive advantages tend to increase in frequency over time...enhances fitness

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clines

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patterns of change in a characteristic of an organism over a geographic region

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constraints on adaptive evolution

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1. lack of genetic variation

2. evolutionary history

3. ecological trade-offs

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lack of genetic variation

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if none of the individuals in a population has a beneficial allele of a particular gene that influences survival and reproduction, adaptive evolution cannot occur at that gene

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evolutionary history

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natural selection brings about changes by modifying traits that are already present in an organism, not by creating advantageous traits from scratch

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ecological trade-offs

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trade-offs occur in all organisms and they ensure that adaptation will never be perfect…instead, adaptations represent comporomises in the abilities of organisms to perform many different and sometimes conflicting functions

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speciation

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• 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

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how do new species evolve?

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• genetic divergence: when a barrier to gene flow is established between populations, they diverge genetically over time
• hybridization: members of two different species produce fertile hybrid offspring
• key step is always the evolution of barriers that prevent its members from breeding freely with members of the parental species
• natural selection and genetic drift can both promote accumulation of genetic differences between populations, but gene flow typically acts to slow down speciation

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allopatric speciation

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geographic barrier present

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sympatric speciation

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• new species evolve from single ancestral species while inhabiting same geographic area
• isolated and relatively homogenous habitats, host plants

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parapatric speciation

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occurs when populations are separated by an extreme change in habitat

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adaptive radiation

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• mass extinction followed by great increases in diversity of some of the surviving groups
• a group of organisms gives rise to many new species that expand into new habitats or new ecological roles in a relatively short time