Microevolution:
The Forces of Evolutionary Change
Part 2
Lecture 23
Outline
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Conditions that cause evolutionary change
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Natural vs artificial selection
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Nonrandom mating and sexual selection
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The role of chance events
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Genetic drift
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Bottlenecks
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Founder effects
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Gene flow
Conditions that Cause Evolutionary Change in Natural Populations
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Natural selection is an important, but not the only, force that results in biological evolution
Micro­evolution occurs when the frequency of an allele in a population changes
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Natural selection
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Artificial selection
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Sexual selection, mate choice, and nonrandom mating
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Mutation
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Genetic drift
Review of Natural Selection
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The differential survival and reproduction of organisms whose genetic traits better adapt them to a particular environment
Change in the number of individuals in a population that carry copies of a specific allele
Determined by an individual's phenotype
Evolution and Mutation
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Mutation is the source of new alleles
Evolution is the change in allele frequency over time Some other evolutionary mechanism required for new alleles to become more common
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The genetic makeup of populations, and ultimately species, changes as natural selection permits differential survival of genetic variants (mutants) that are better adapted to a particular environment.
Disease Evolution
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All organism evolve
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One is not “more evolved” than another
In most examples of disease evolution we are looking at the evolution of the disease rather than of the host: flu, HIV, antibiotic resistant bacteria, cholera, tuberculosis, etc.
(exception: balanced polymorphism)
Our immune system and the drugs we take are part of the disease's environment
natural =variation differential heredity
selection
reproduction
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Change in allele frequency in a population is called microevolution.
Can take place over relatively short time period
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Some phenotypes are better adapted to particular environment.
Natural selection removes some alleles from the population
Allele frequencies will change in response to environmental change Role of the Environment
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Two fundamental forces are operating: genetic variation and environmental change. Those with more adaptive traits survive in a new or changing environment.
Environment includes
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Climate
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Abiotic conditions/resources: water, soils, habitat
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Parasites, predators, and competitors
Parasites, predators, and competitors also evolve
Role of the Environment
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Natural selection reflects adaptation to a prevailing environmental condition. The direction of natural selection can change. A phenotype that is adaptive in one set of circumstances may be a liability in another.
Broad­ranging species often show local adaption among sub­populations
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Affects management, conservation, and climate change
Reproductive Isolation
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Over time, an isolated sub­population could change so that it can no longer breed with the original group. Eventually, a new species would arise.
Macro­evolution represents accumulated changes in allele frequency in two populations that preclude their interbreeding, leading to the formation of new species (or their extinction).
Macro­evolutionary changes tend to span many generations, but in some instances can occur in only a few generations.
Drivers of Microevolution
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Mutation
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Natural selection
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Artificial selection
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Sexual selection, mate choice, and nonrandom mating
Genetic drift
Artificial Selection:
Proof of the Power of Selection
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Also called selective breeding or domestication
Process of intentional or unintentional modification of a species through human actions which encourage the breeding of certain inherited traits over others
The breeding potential of individuals who possessed desirable characteristics is intentionally encouraged, whereas the breeding of individuals with less desirable characteristics is discouraged.
Artificial Selection
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“Selection was the keystone of man's success in making useful races of animals and plants”
­Darwin, 1837
No real difference in genetic processes underlying artificial and natural selection
Underscores the power of selection in generating evolutionary change
Non­random Mating
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A variation of Natural Selection
Completely random matings (where each individual has as equal chance of mating with every other member of the population) are nearly impossible to achieve
Sexual Selection & Mate Choice
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Sexual selection: the "natural selection of traits that increase an individual's reproductive success." These traits contribute to attraction, courtship, or mating.
Most species exhibit some sort of preferences in mate choice
The alleles for these desired traits will become more common in future generations.
Peacock
Peahen
Bower Bird
Sexual selection:
male­male competition
Genetic Drift
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Changes in allele frequency in a population that result from RANDOM survival or reproduction of individuals with certain characteristics.
Survival or reproduction of an organism is sometimes just a matter of LUCK or CHANCE, not because of their phenotype or genotype.
Example: if a Florida Panther is killed by a truck on a highway, that is bad luck. The panther did not get hit because of some allele it carried.
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Genetic Drift contrasts with natural selection.
In selection the environmental events that affect a population may be random, but the survival or reproduction of the individuals depends on their phenotypes.
Example of selection: If the panther population is infected with FIV (feline AIDS), individuals with alleles that give them resistance to the disease will survive.
Types of random effects
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Gene flow/Migration
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Founder Effects
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Population Bottlenecks
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Genetic drift
Gene Flow / Migration
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Gene Flow is the movement of alleles from population to population.
Immigrating individuals introduce new alleles by mating with members of the population they join.
Any advantage given to individuals with new alleles will change the population due to subsequent natural selection.
Because geographic barriers greatly influence migration patterns, allele frequencies may differ between adjacent but separated geographic regions.
Founder Effect
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Establishment of a new population (geographically isolated) from a single or very few individuals.
The gene pool of a founding population only contains a subset of the total genetic diversity of the original population
Different from gene flow because the migrating individuals are establishing a new population where none existed before. Population Bottleneck
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Occurs when population size is drastically reduced
The new population has lost much of the genetic diversity that was present in the larger ancestral population as a result of inbreeding.
Bottlenecks and Conservation
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Common problem among endangered species
Florida Panther
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Hybrids with Texas pop'n have greater lifespan and fewer genetic problems.
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Pop'n has tripled
Greater Prairie Chicken [Dr. Paige, Animal Bio] Genetic Drift
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Chance events affect which individuals survive and reproduce in a population.
Small, random changes in allele frequency can "build up" over generations and result in a significant change in allele frequency.
Population Size
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Chance changes in allele frequency tend to have small effects on large populations
Small populations (e.g. endangered species) are much more susceptible Tend to be inbred and have little genetic variation anyway.
Micro­evolution Recap
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Microevolution (a change in allele frequency in a population) can occur as a result of several factors that fall into two broad categories.
Factors in which an individual's chance of survival and reproduction depend on the individual's phenotype.
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Natural Selection
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Artificial Selection
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Mate Selection, Sexual Selection, Nonrandom Mating
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Mutation
Micro­evolution Recap
2. Factors in which an individual's chance of survival and reproduction, are just that, a matter of CHANCE. An individual's phenotype and genotype has nothing to do with it.
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Gene Flow
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Founder Effect
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Population bottle neck
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Genetic Drift