21: Microevolution-Genetic Changes w/in populations

microevolution

- describes small-scale genetic changes in population

- responds to shifting environmental circumstances

- ex : antibiotic resistance in bacteria

- study via analyzing variations within natural population and how and why these variations are inherited

macroevolution

- describes larger-scale evolutionary changes in a species and more inclusive groups

- occurs at or above the level of the species

- can result in speciation or the emergence of new species

- results from gradual accumulation of microevolutionary changes

population

all individuals of a single species that live together in the same place and time

phenotypic variations

- all difference in appearance or function that are passed from gen to gen

- caused by : genetic differences, differences in environmental factors, and interactions bt genetics and environment

only _________ based variation is subject to evolutionary change

genetic

quantitive traits

- a measurable phenotype that depends on the cumulative actions of many genes and the environment

- able to be described on a bar graph or curve

qualitative traits

- trait that fits into discrete categories

- polymorphism : exiting in >= 2 discrete states

- described by percentage/frequency of each trait

- intermediate forms are often absent

- frequently controlled by one or just a few genes -> simply-inherited traits

processes that generate genetic variations

1) production of new alleles -> small-scale mutations

2) rearrangement of existing alleles into new combos -> larger scale changes in chromosome structure/# and from crossing over, independent assortment, and random fertilizations

how biologist detect genetic variations?

1) gel electrophoresis : inferring the genetic variation in gene coding for a protein -> separates >= 2 forms of a protein that differs significantly in shape, mass, or net electrical charge

2) DNA sequencing tech : allows direct survey of genetic variation; can help accumulate astounding knowledge of genome -> study of DNA polymorphisms

population genetics

- understanding the causes of genetic variation within populations

- describe genetic structure of population

- create hypotheses formalized in mathematical models to describe evolutionary proceses

- test the predications of these models to evaluate the idea about evolution

gene pool

- sum of gene copies (alleles) at all gene loci in all individuals in the population

allele

1 of 2 or more versions of DNA sequence at a given genomic location (locus)

genotype frequencies

percentages of individuals in a population possessing each genotype

- each diploid organism has two copies of each gene

- 2 alleles may be same or different

- 3 types, pp, pq, and qq (p is dom, q res)

- p^2 + 2pq + q^2 = 1

allele frequencies

- relative abundances of the different alleles

- can be calculated

- two types (p and q)

- if genotype has a unique phenotype -> incomplete dominance -> allele frequencies can be counted directly

- p + q = 1

null models

- theoretical reference points against which observations can be evaluated

- predict what would happen if a particular factors had no effect

hardy-weinberg principle

- mathematical model that described genotype frequencies for sexually reproducing organisms

- the population's allele and genotype frequencies remain constant unless there is some type of evolutionary force acting upon them

- genetic equilibrium only if all are met : (1) no mutations are occurring; (2) population is closed to migration; (3) population is infinite in size; (4) all genotypes survive and reproduce equally well; (5) individual mate randomly wrt genotypes

- if these conditions are not met -> change of allele frequencies -> microevolution

factors of microevolution

mutation, gene flow, genetic drift, natural selective, nonrandom mating

what is the major source of heritable variations

mutations

- note : little to no immediate effect but creates new genetic variations over time

- most animals -> only mutations in germ line are heritable

deleterious mutations

- alter an individual's structure, function, or behavior in HARMFUL ways

- ex : ehlers-danlos syndrome -> mutation in coding for collagen -> loose skin, weak joints

- dermatosparaxis ehlers-danlos syndrome -> connective tissue disorder -> fragile skin, bruising and scarring, hernias, puffy eyelids, etc

lethal mutations

- may kill all carriers (if dominant) or only homozygous carriers (if recessive)

- any mutation that causes an individual to die before it reaches its reproductive age

- ex huntington's disease (dominant); cystic fibrosis (res); sickle cell anemia (res)

neutral mutations

- neither harmful or helpful

- mostly arise due to change in 3rd position of a codon

- degeneracy of the genetic code -> still produce the same amino acid

- can be beneficial later if environment changes

advantageous mutations

- has benefit to the individual that carries it

- natural selection may preserve the new allele and increase its frequency over time

gene flow

- movement of organisms or their gametes (eg pollen), or genetic variation from one pop. to another (migration)

- immigrants reproduce -> introduce new alleles -> shift alleles and genotype frequencies away from hardy-weinberg model

- there must be reproduce of immigrants into the gene pool, not just the act of migrating

- evolutionary importance depends on : (1) degree of genetic differentiation bt populations -> can be beneficial or harmful; (2) rate of gene flow bt the populations

genetic drift

- chance events that cause allele frequencies in a population to change unpredictable

- founder effect : extreme ex; small group splits from main pop. and find new colony -> ex amish in PA and ellis-van creveld syndrome; alleles may be missing or rare alleles occur more highly

- population bottleneck : population is sharply reduced in size by natural disaster or animal poaching; ex elephant seals have no variation in 24 proteins; lack of more rare/low frequency alleles

- more common in small populations

- generally leads to loss of alleles and reduce genetic variability

relative fitness

- number of surviving offspring that an individual produces compared w/others in the population

- differences is the main source for natural selection -> natural selection has little to do w/genes affecting an individuals post-reproductive life

directional selection

- extremely common, used in artificial selection

- individuals near one end of the phenotype spectrum have the highest relative fitness

- shift the mean phenotype towards the end the distribution favored

- body size in guppies

stabilizing selection

- when individuals expressing intermediate phenotypes have the highest relative fitness

- eliminated the phenotype extremes -> reduces phenotypic variation

- ex birthweights in human -> mean of 7-8lbs

disruptive selection

- when extreme phenotypes have higher relative fitness than intermediate phenotypes

- promotes polymorphism

- least common

- ex bill size in medium ground finches

sexual selection

- special process that fosters evolution towards showy structures

- "special case" of natural selection

- acts on an individual's ability to obtain or successfully copulate w/a mate -> bright colors, elaborate courtship behaviors

- probable cause of sexual dimorphism (differences bt sexes in the same species)

intersexual selection

- based on interactions bt male and females

- males produce useless structures to make females attracted

- ex african male widowbird -> longer tail length

intrasexual selection

- based on interactions bt male and male

- males produce structures to intimidate, injure, or kill rival males

inbreeding

- form of nonrandom mating

- genetically related individuals mate w/each other

- self-fertilization is an extreme example

- recessive phenotypes are often expressed

balanced polymorphisms

- when 2 or more phenotypes or maintained in fairly stable proportions over gens.

- when heterozygotes have higher relative fitness, different alleles favored in different environments, or when rarity of phenotype provides an advantage (sickle cell and malaria)

frequency dependent selection

- genetic variability is maintained in a pop. bc a rare phenotype has a higher relative fitness -> becomes the more common phenotype -> rare phenotype loses its advantage

- ex bees pollinating elderflowers w/ color polymorphism -> rarer the color, the higher the reproductive success or higher relative fitness

neutral variations

- some genetic variation at loci coding for enzymes and other soluble proteins is selectively neutral (when different forms of the proteins function equally well)

- directly proportional to population size and length of time over which variations have accumulated

adaptive trait

- any product of natural selection that increases the relative fitness of an organism in its environment

- crucial for species survival and evolution of new species

- not all traits are adaptive

- adaptive changes in a organism's morphology must be based on SMALL modifications of existing structures

adaptation

- accumulation of adaptive traits over time