chapter 23 Microevolution

What mechanisms cause evolution

natural selection, gentic drift, and genetic flow

they alter the allele frequency of a population over time

microevolution

a change in allele frequencies in a population over generations.

genetic drift

chance fluctuations in allele frequencies over generations tend to reduce genetic variation.

gene flow

the transfer of alleles between populations, tends to reduce genetic differences between populations over time.

natural selection

the process where organisms with helpful traits survive and reproduce more than others, passing those traits to the next generation

In genetic terms, selection results in alleles being passed to the next generation in proportions that differ from the present generation.

genetic variation

differences among individuals in the composition of their genes or other DNA sequences.

a heterozygous individual

two different alleles for a given gene

a homozygous individual

two identical alleles for that gene

nucleotide variability

genetic variation at the molecular level of DNA, but doe snot often result in phenotypic variations. the ones that do

why does nucleotide variability not often lead to phenotypic varaitions

Many nucleotide variations occur within introns

introns

noncoding segments of DNA lying between exons

exons

the regions retained in mRNA after RNA processing

Phenotype

the product of an inherited genotype and many environmental influences

only the genetically determined part of phenotypic variation can have evolutionary consequences

Why do many nucleotide variations within exons also have no effect on phenotype?

Most do not change the amino acid sequence of the protein.

mutation

a change in the nucleotide sequence of an organism’s DNA

how are mutations caused

errors in DNA replication, exposure to UV light and other high-energy forms of radiation, and exposure to certain chemicals

point mutation

onse base in a gene, which is enough to have a significant imapct on a phenotype

persistence of harmful alleles

harmful alleles that are recessive can be hidden from selection, in diploid organisms, thus can persist for generations by propa- gation in heterozygous individuals. this is because tehir harmful effects cna be masked by more dominant alleles

heterozygote protection

the genetic phenomenon where a normal, dominant allele masks or compensates for a harmful recessive allele in a heterozygous individual, preventing the disease or defect from manifesting

neutral variations

differences in DNA sequence that do not confer a selective advantage or disadvantage.

eg, redundancy in our genetic code

importance of amino acid for mutations

Even a point mutation in a gene that encodes a protein will have no effect on the protein’s function if the amino acid doesn’t change.

however, a change in the amino acid may not affect the pro- tein’s shape and function.

mutations in multi-cellar organisms

only muta- tions in cell lines that produce gametes can be passed to offspring.

Chromosomal changes

these changes which delete, disrupt, or rearrange many loci are usually harmful

however, if the gene stays in tact it will not effect the phenotypic

Chromosomal changes, duplication

Duplications of large chromosome segments are often harmful, but the duplication of smaller pieces of DNA may not be.

these duplications taht do not have severe effects persis over generations, thus mutations accumulate causing an expanded genome with new functions

sexual reproduction

in such a society, most genetic variation is from the unique combination of alleles that each individual receives from its parents.

teh differences from tehse alleles result from previous mutations

3 mechanims involved with genetic variation from sexual reproduction

crossing over, independent assortment of chromosomes, and fer- tilization

crossing over

During meiosis, homologous chromosomes, one inherited from each parent, trade some of their alleles by crossing over.

meiosis

a specialized type of cell division that reduces the chromosome number by half

independent assortment of chromosomes

after cross over, the resulting recombinant chromosomes are then distributed at random into gametes

fertilisation

after crossover and assortment of chormosones: because myriad possible mating combinations exist in a population, fertilization typically brings together gametes with different genetic backgrounds.

population

a group of individuals of the same species that live in the same area and interbreed, producing fertile offspring.

gene pool

consists of all copies of every type of allele at every locus in all members of the population.

fixed in the gene pool

If only one allele exists for a particular locus in a population thus all individuals are homozygous for that allele

calculating allele frequency

dividing the count of a specific allele by the total number of all allele copies for that gene in the population

calculating allele frequency, dominant vs recssive alleles

Hardy-Weinberg equilibrium

In a population that is not evolving, allele and genotype frequencies will remain constant from generation to generation, provided that only Mendelian segregation and recombination of alleles are at work.

• a population is in Hardy-Weinberg equilibrium only if the observed genotype frequency of one homozygote is p2, the observed frequency of the other homozygote is q2, and the observed frequency of heterozygotes is 2pq.

you have to consider the combination of alleles in all of the genetic crosses in a population. We do thsi y finding tehf reqnecies for each alle and multiplying tehm togtehr to see teh probabikty for that combination

Mendelian segregation

allele pairs separate randomly during the formation of reproductive cells so that each gamete receives only one allele for each trait

conditions for the Hardy-Weinburg equilibrium and consueqences

1. no mutations: The gene pool is modified if mutations occur or if entire genes are deleted or duplicated.

2. Random mating: If individuals mate within a subset of the population, such as near neighbors or close relatives (inbreeding), random mixing of gametes does not occur and genotype frequencies change.

3. No natural selection: Allele frequencies change when individuals with different genotypes show consistent differences in their survival or reproductive success.

4. Extremely large population size: In small populations, allele frequencies fluctuate by chance over time (genetic drift).

5. No gene flow: By moving alleles into or out of populations, gene flow can alter allele frequencies.

when is Hardy-Weinburg equation used

as an initial test of whether evolution is occurring in a population, also has medical applications, such as estimating the percentage of a population carrying the allele for an inherited disease.

adaptive evolution

By consistently favoring some alleles over others, caused by natural selection

the founder effect

When a few individuals become isolated from a larger popula- tion, this smaller group may establish a new population whose gene pool differs from the source population

the bottleneck effect

A sudden change in the environment, such as a fire or flood, may drastically reduce the size of a population. A severe drop in population size can cause the bottleneck effect

By chance alone, certain alleles may be overrepresented among the survivors, others may be underrepresented, and some may be absent altogether.

effects of genetic drift

1. Genetic drift is significant in small populations.

2. Genetic drift can cause allele frequencies to change at random.

3. Genetic drift can lead to a loss of genetic variation within populations.

4. Genetic drift can cause harmful alleles to become fixed.

effects of genetic drift, 1

Genetic drift is significant in small populations.

Chance events can cause an allele to be disproportionately over- or underrepresented in the next generation. Although chance events occur in populations of all sizes, they tend to alter allele frequencies substantially only in small populations.

effects of genetic drift, 2

2. Genetic drift can cause allele frequencies to change at random.

Because of genetic drift, an allele may increase in frequency one year, then decrease the next; the change from year to year is not predictable. Thus, unlike natural selection, which in a given environment consistently favors some alleles over others, genetic drift causes allele frequencies to change at random over time.

effects of genetic drift, 3

2. Genetic drift can lead to a loss of genetic variation within populations.

By causing allele frequencies to fluctuate randomly over time, genetic drift can eliminate alleles from a population. Because evolution depends on genetic variation, such losses can influence how effectively a population can adapt to a change in the environment.

effects of genetic drift, 4

2. Genetic drift can cause harmful alleles to become fixed.

Alleles that are neither harmful nor beneficial can be lost or become fixed (reach a frequency of 100%) by chance through genetic drift. In very small populations, genetic drift can also cause alleles that are slightly harmful to become fixed. When this occurs, the population’s survival can be threatened (as in greater prairie chickens).

relative fitness

the contribution an individual makes to the gene pool of the next generation relative to the contributions of other individuals.

how does natural selection alter frequency of distrubution of heritable traits

depending on which phenotypes in a population are favored: through directional selection, disruptive selection, and stabilizing selection.

directional selection

occurs when conditions favor individuals exhibiting one extreme of a phenotypic range, thereby shifting a population’s frequency curve for the phenotypic character in one direction or the other

often when environment chnages

Disruptive selection

occurs when conditions favor individuals at both extremes of a phenotypic range over individuals with intermediate phenotypes.

Stabilizing selection

acts against both extreme phenotypes and favors intermediate variants. This mode of selection reduces variation and tends to maintain the status quo for a particular phenotypic character.

sexual selection

a process in which individuals with certain inherited characteristics are more likely than other individuals of the same sex to obtain mates.

can result from sexual dimorphism

sexual dimorphism

a difference in secondary sexual characteristics between males and females of the same species

These distinctions include differences in size, color, ornamentation, and behavior.

intrasexual selection

selection within the same sex, individuals of one sex compete directly for mates of the opposite sex.

intersexual selection

individuals of one sex (usually the females) are choosy in selecting their mates from the other sex.

balancing selection

a type of natural selection that maintaining two or more phenotypic forms in a population

includes frequency-dependent selection and heterozygote advantage.

frequency-dependent selection

the fitness of a phenotype depends on how common it is in the population.

heterozygote advantage

occurs when an individual with two different alleles (heterozygote) has a higher survival and reproduction rate than individuals with two identical alleles (homozygotes)

defined for genotype and not phenotype

Why Natural Selection doesnt lead to Perfect Organisms

1. Selection can act only on existing variations.

   1. Natural selection favors only the fittest phenotypes, which may not be the ideal traits.

2. Evolution is limited by historical constraints.

   1. Each species has a legacy of descent with modification from ancestral forms. Evolution does not scrap the an- cestral anatomy and build each new complex structure from scratch

3. Adaptations are often compromises

   1. Organisms face many such trade-offs in which the ability to perform one function may reduce the abil- ity to perform another

4. Chance, natural selection, and the environment interact.

   1. Chance events can affect the subsequent evo- lutionary history of populations. the environment at a particular location may change unpredictably limiting the extent to which adaptive evolution results in organisms being well suited for current environmental conditions.