Class Notes (807,647)
Canada (492,772)
Biology (2,161)
BIOL 239 (260)


4 Pages
Unlock Document

University of Waterloo
BIOL 239
Diana Parry

22.2- What Are the Mechanisms of Evolutionary Change? • Hardy-Weinberg equilibrium is a null hypothesis that assumes evolutionary forces are absent. • Known evolutionary mechanisms: • Mutation • Nonrandom mating • Gene flow • Natural selection • Genetic drift Mutations Generate Genetic Variation • Origin of genetic variation is mutation; mutation is any change in an organism’s DNA • Most mutations are harmful to their bearers or are neutral, but if environmental conditions change, previously harmful or neutral alleles may become advantageous • Mutations can restore to populations alleles that other evolutionary processes have removed • Most mutations appear to be random and are harmful or neutral to their bearers. • Some mutations can be advantageous. • Mutation rates are low; one out of a million loci is typical. • Although mutation rates are low, they are sufficient to create considerable genetic variation. • Rates as high as one mutation per locus in a thousand zygotes per generation are rare; one in a million is more typical • One condition for Hardy–Weinberg equilibrium is that there is no mutation. • Although this condition is never met, the rate at which mutations arise at single loci is usually so low that mutations result in only very small deviations from Hardy–Weinberg expectations. • If large deviations (from H-W expectations) are found, it is appropriate to dismiss mutation as the cause and look for evidence of other evolutionary agents. Gene flow may change allele frequencies • Gene flow results when individuals migrate to another population and breed in new locations.  Immigrants • No immigration is allowed for a population to be in Hardy–Weinberg equilibrium. Genetic drift may cause large changes in small populations • Genetic drift is the random loss of individuals (and their alleles)-may produce large changes in allele frequencies from one generation to the next • In very small populations, genetic drift may be strong enough to influence the direction of change of allele frequencies even when other evolutionary agents are pushing the frequencies in a different direction. • Organisms that normally have large populations may pass through occasional periods when only a small number of individuals survive (a population bottleneck). Genetic variation can be reduced by genetic drift. o Population bottlenecks occur when only a few individual survive a random event, resulting in a shift in allele frequencies within the population • Founder effect- random changes in allele frequencies resulting from establishment of a population by a very small number of individuals o When a few pioneering individuals colonize a new region, the resulting population will not have all the alleles found among members of the source population. Nonrandom Mating Changes Genotype Frequencies • Nonrandom mating occurs when individuals mate either more often with individuals of the same genotype or more often with individuals of a different genotype. • The resulting proportions of genotypes in the following generation differ from Hardy– Weinberg expectations. • If individuals mate preferentially with other individuals of the same genotype, homozygous genotypes are overrepresented and heterozygous genotypes are underrepresented in the next generation. • Conversely, individuals may mate preferentially with individuals of a different genotype • Self-fertilization (selfing) is another form of nonrandom mating that is common in many organisms, especially plants. • Selfing reduces the frequencies of heterozygous individuals below Hardy–Weinberg expectations and increases the frequencies of homozygotes, without changing allele frequencies, and thus not result in adaptation • Sexual selection- is a particularly important form of nonrandom mating that does change allele frequencies and often results in adaptations 22.3- What Evolutionary Mechanisms Result in Adaptation? • Recall: for adaptation (and evolution) to occur, individuals that differ in heritable traits must survive & reproduce with different degrees of success. • When some individuals contribute more offspring to the next generation than others, allele frequencies in the population change in a way that adapts individuals to the environments that influenced their success: natural selection. • The reproductive contribution of a phenotype to subsequent generations, relative to the contributions of other phenotypes, is called its fitness. • The fitness of a phenotype is determined by the average rates of survival and reproduction of individuals with that phenotype. Natural Selection Produces Variable Results • Most characters (traits) are influenced by alleles at more than one locus and are more likely to show quantitative rather then qualitative variation. • For example, the body size of individuals in a population is influenced by genes at many loci, and distribution of body sizes is likely to be a bell-shaped curve. Quantitative variation: avg body size of a population may increase or decrease as a result of selection. • Natural selection can act on characters with quantitative variation in three ways:  Stabilizing selection-preserves the average characteristics of a population by favoring average individuals  Directional selection-changes the characteristics of a population by favoring individuals that vary in one direction from the mean of the population  Disruptive selection- changes the characteristics of a population by favoring individuals that vary in opposite directions from the mean of the population • Stabilizing selection favors average individuals.  the extremes of a population con
More Less

Related notes for BIOL 239

Log In


Don't have an account?

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.