Lecture 7 Flashcards
Most important causes for evolutionary change in populations
genetic drift and natural selection
genetic drift definition
random fluctuations in allele frequencys, no selective pressure so its a random non adaptive change
Genetic drift occurs because natural populations are
finite in size
Random fluctuations in allele frequencies can
result in non adaptive evolution
natural selection results in
adaptive evolution
in genetic drift
both conclusions of hardy weinberg equasions have been violated
conclusions of hardy Weinberg violated in genetic drift are
allele frequencys changed from generation to generation and genotype frequency were not equal to expect frequency of 0.25 AA+0.5 Aa +0.25 aa=1
Evolution occurs through
genetic drift
For genetic drift the alleles included in any generation are
a sample of the alleles of the previous generation
all samples are
subject to sampling error or random variation
proportions of alleles are likely to differ by
change alone, from the proportions they are drawn from
genetic drift is
analogous to the null hypothesis in statistics
genetic drift can cause
speiciation, without natural selection
way to portray genetic drift
a random walk, you start with p=0.5, each generation there’s a equal probability that p will move higher or lower, no stabilizing force so p will wanted to 1 or 0, is fixed 0 is lost
in any given generation alleles are more likely to get lost or fixed in
smaller populations
we use the loss of — as a measure of the rate of genetic drift within a population
heterozygosity, H=2p(1-p)
As the frequency of one allele approches 1
the frequency of heterozygotes declines, rate of inbreeding increases, can lead to interbreeding depression
the probability of an allele becoming fixed equals
the alleles frequency in the population
the allele is more liekly to become fixed in
small populations
genetic drift proceeds faster in
small populations
if an allele has just arisen by mutation and is represented by only one among the 2N gene copies in the population its frequency is
pt=1/(2N)
Census size
number of adults we count
in actual populations the census size is
greater than the number that actually contributes alleles to the next generation
in effective population size the
rate of genetic drift is greater than would be expected using a populations census size
effective population size (Ne) definition
the number of individuals in an ideal population in which the rate of genetic drift would be the same as in the actual population
bottleneck
A severe temporary reduction in population size, sometimes occurs when a new population is established by a small number of colonists
founder effect
genetic drift in a small colonizing population, if population grows large fast it will not be greatly altered, if it remains small genetic drift will decrease genetic variation and alter allele frequency, new mutations may restore genetic variation
consequences of population bottlenecks
fixation of deletreious alleles which reduces survival and reproduction, increasing the extinction risk, and interbreeding depression
interbreeding increases
homozygosity and decreases heterozygosity
many lethal alleles are
recessive and rare
inbreeding increases
homozygosity in individuals that are more liekly to share the same recessive deleterious alleles
inbreeding depression
is the reuslting decline in fitness and fecundity that occurs when closely related individuals reproduce
inbreeding depression is well known in
small captive populations on endangered species, we have special breeding programs to minimize inbreeding
inbreeding also increases risk of extinction of
small populations in nature
Neutral theory of molecular evolution definition
the great majority of mutations that are fixed are effectively neutral with respect to fitness and are fixed by genetic drift
most genetic variation at the molecular level is
selectively neutral and lacks adaptive significance
small minority of mutations in DNA are
advantageous and are fixed by natural selection, many are disadvantoeous and are eliminated by natural selection
The neutral theory futher holds that
evolutionary substitutions at the molecular level proceed at a constant rate, so the degree of sequence difference between species can serve as a molecular clock
molecular clock
enables us to determine divergence times between species
in order to assign concrete dates
the clock must be calibrated by using the fossil record
