BIO LECTURE OCT 6

BIO152: genetic drift : 

BIO152: genetic drift Lecture 9 October 6,2008

Lecture 11 Outline : 

10/5/2008 2 Lecture 11 Outline Neutral evolution Pattern Process Genetic drift = sampling error Genetic drift in small populations Bottleneck effect Founder effect

Facilitated Study Groups-week of Oct 6MC & Critical reading skills : 

10/5/2008 3 Facilitated Study Groups-week of Oct 6MC & Critical reading skills Critical reading skills & continued practice with how to understand multiple choice questions

Neutral evolution : 

10/5/2008 4 Neutral evolution Frequency of traits (alleles) in a population may change without selection When an allele is not under selection, the frequency will drift (change) randomly IF there is no correlation between trait and reproductive success, then change in the frequency of that trait is due to genetic drift not natural selection.

Contribution of natural selection & drift to evolution : 

10/5/2008 5 Contribution of natural selection & drift to evolution Predications a) If genetic variety is inconsequential to survival & reproduction (= neutral) ►genetic drift is main agent for evolution OR b) If genetic variety leads to phenotypic differences which affect fitness, ►natural selection is the main driving force.

Sampling error & genetic drift : 

10/5/2008 6 Sampling error & genetic drift Genetic drift — one of the basic mechanisms of evolution — is simply the evolutionary equivalent of a sampling error. (Freeman 25.3)

Beads and sampling error : 

10/5/2008 7 Beads and sampling error

Genetic drift : 

10/5/2008 8 Genetic drift Drift is example of when evolution is due to chance, not selection (selection is NOT due to chance: Remember that selection is due to some characteristic providing an advantage or disadvantage for surviving and having more viable offspring)

Slide 9: 

10/5/2008 9 "If a population is finite in size (as all populations are) and if a given pair of parents have only a small number of offspring, then even in the absence of all selective forces, the frequency of a gene will not be exactly reproduced in the next generation because of sampling error. “ Griffiths, Intro to Genetic Analysis

Coin toss until 2 heads or 2 tails[2 coins] : 

10/5/2008 10 Coin toss until 2 heads or 2 tails[2 coins] Generation 1 2 3 4 e.g., Heads: 1 0 Tails: 1 2

Slide 11: 

10/5/2008 11

Aside—terms again : 

10/5/2008 12 Aside—terms again Allele = version of a particular gene e.g., A or a are 2 versions of the gene “genotype” = AA, Aa, and aa.

Simulated drift—4 populations starting with 8 copies of alleles A & a (like heads&tails on coin) #copies of A1 : 

10/5/2008 13 Simulated drift—4 populations starting with 8 copies of alleles A & a (like heads&tails on coin) #copies of A1

Clicker: what does it mean that an allele has reached fixation? The fixed allele : 

10/5/2008 14 Clicker: what does it mean that an allele has reached fixation? The fixed allele has been eliminated from the population. has a frequency of 1.0 (100%) Is dominant to all other alleles Is adaptively advantageous

Clicker: what does it mean that an allele has reached fixation? The fixed allele : 

10/5/2008 15 Clicker: what does it mean that an allele has reached fixation? The fixed allele has been eliminated from the population. has a frequency of 1.0 (100%) Is dominant to all other alleles Is adaptively advantageous #4 p 536 Freeman—incorrect answer given in text (typo!!)

random genetic drift : 

10/5/2008 16 random genetic drift E.g., 1. pair of diploid sexually reproducing parents (such as humans) have only a small number of offspring; not all of the parent's alleles will be passed on to their progeny due to chance assortment of chromosomes at meiosis.

Random genetic drift e.g., 2 : 

10/5/2008 17 Random genetic drift e.g., 2 2. probability of a given seed falling on fertile ground

Also accidental random events that influence allele frequency : 

10/5/2008 18 Also accidental random events that influence allele frequency Disasters such as earthquakes, floods, or fires may reduce the size of a population drastically, killing victims unselectively. What does unselectively mean?

Slide 19: 

10/5/2008 19 Natural selection is driven by the environment ►may result in adaptation Genetic drift is non-directed; drift is the result of chance events.

Genetic drift effects evolution : 

10/5/2008 20 Genetic drift effects evolution Drift reduces genetic variation in populations, potentially reducing a population’s ability to evolve in response to new selective pressures. Genetic drift acts faster and has more drastic results in smaller populations. This effect is particularly important in rare and endangered species. Genetic drift can contribute to speciation. E.g., a small isolated population may diverge from the larger population through genetic drift.

Slide 21: 

10/5/2008 21 Genetic drift can cause big losses of genetic variation from small populations very quickly

Bottlenecks—type of genetic drift : 

10/5/2008 22 Bottlenecks—type of genetic drift

Slide 23: 

10/5/2008 23 Population bottlenecks occur when a population's size is drastically reduced for at least one generation. Why does genetic drift act more quickly to reduce genetic variation in small populations?

Bottleneck examples : 

10/5/2008 24 Bottleneck examples Northern elephant seal Hunting reduced population size to ~ 20 individuals at the end of the 19th century. Now population over 30,000 << less genetic variation compared to a population of southern elephant seals

2. Cheetah : 

10/5/2008 25 2. Cheetah Cheetahs have unusually low genetic variability VERY low variability: even skin grafts between non-related cheetahs show no rejection of the donor skin. Hypothesis: Prolonged period of inbreeding following a genetic bottleneck at end the last ice age (10,000 years ago)

3. American Bison : 

10/5/2008 26 3. American Bison

Bottleneck : 

10/5/2008 27 Bottleneck Inbreeding during recovery can lead to large population with low levels of variation

Founder effect—type of genetic drift : 

10/5/2008 28 Founder effect—type of genetic drift New colony is started by a few members of the original population. reduced genetic variation from the original population. a non-representative sample of the genes in the original population.

Slide 29: 

10/5/2008 29 The founder effect ►the new population may be distinctively different, both genetically and phenotypically, than the parent population from which it is derived.

Slide 30: 

10/5/2008 30 Founder effects are common in island ecology, but the isolation need not be geographical. E.g., the Amish populations in Pennsylvania & Ontario --grown from a very few founders, have not recruited newcomers, and tend to marry within the community, exhibit founder effects. phenomena such as polydactyly (extra fingers and toes) more common in Amish communities than in the US or Canadian population in general.

e.g., Founder effect : 

10/5/2008 31 e.g., Founder effect biocolonization of Surtsey, Iceland, a new volcanic island that erupted offshore between 1963 and 1967. (pictures from Wikipedia)

Genetic drift: compare bottleneck and founder effect : 

10/5/2008 32 Genetic drift: compare bottleneck and founder effect How are they DIFFERENT? How are they SIMILAR?