Mechanisms of Evolutionary Change - Topic 4

What is the evolutionary process triangle - Where does drift fit in

Evolution by random change = WHAT

Evolution by random change = GENETIC DRIFT

Evolution by random change = GENETIC DRIFT

• Individual organisms can get WHAT or WHAT

• Good luck and bad luck are unrelated to the individual’s WHAT

• The net result is random changes in WHAT, just due to chance that is WHAT

Evolution by random change = GENETIC DRIFT

• Individual organisms can get LUCKY or UNLUCKY

• Good luck and bad luck are unrelated to the individual’s PHENOTYPES

• The net result is random changes in ALLELE FREQUENCIES, just due to chance that is GENETIC DRIFT

Genetic drift is WHAT, not WHAT

Genetic drift is STATISTICS, not BIOLOGY

Genetic drift results from WHAT

Genetic drift results from RANDOM SAMPLING ERROR

• More sampling error in WHAT samples

• Analogously: more genetic drift in WHAT populations

• More sampling error in SMALLER samples

• Analogously: more genetic drift in SMALLER populations

• Drift tends to WHAT genetic variation WHAT populations

• Drift tends to WHAT genetic variation WHAT populations

• Drift is stronger in WHAT populations

• Drift tends to REDUCE genetic variation WITHIN populations

• Drift tends to INCREASES genetic variation BETWEEN populations

• Drift is stronger in SMALLER populations

Genetic drift example: Northern elephant seals

• underwent a WHAT in the late 1800’s due to commercial hunting

• Numbers reduced to WHAT in 1892, all at one breeding ground in mexico

Genetic drift example: Northern elephant seals

• underwent a BOTTLENECK in the late 1800’s due to commercial hunting

• Numbers reduced to 20-100 in 1892, all at one breeding ground in mexico

Bottleneck effect

Temporary reduction in population size, leading to genetic drift

Genetic drift example: Northern elephant seals

Potential conservation implications, because drift isn’t the only evolutionary force at work

• Post-bottleneck seals are more likely to be WHAT at any given locus (drift: we know this from sequencing DNA from thousand-year-old seal bones)

• More homozygous seals have WHAT (inbreeding depression, a form of natural selection;)

Genetic drift example: Northern elephant seals

Potential conservation implications, because drift isn’t the only evolutionary force at work

• Post-bottleneck seals are more likely to be HOMOZYGOUS at any given locus (drift: we know this from sequencing DNA from thousand-year-old seal bones)

• More homozygous seals have LOWER FITNESS (inbreeding depression, a form of natural selection;)

Northern elephant seal recovery:

• Commercial hunting WHAT in the late 1800’s (too few seals)

• Seal abundance recovered to >WHAT today

• Reduced WHAT didn’t prevent recovery, at least in this case → lost WHAT

Northern elephant seal recovery:

• Commercial hunting STOPPED in the late 1800’s (too few seals)

• Seal abundance recovered to >200,000 today

• Reduced GENETIC VARIATION didn’t prevent recovery, at least in this case → lost more GENETIC DIVERSITY

Founder effect

A special case of bottlenecks and genetic drift

Founder events in human history:

• WHAT: human populations closely related to neighbours

• All non-African descend from a small population of humans who lived in WHAT

• WHAT in genetic diversity reflects the founder effect

Founder events in human history:

• PHYLOGENY: human populations closely related to neighbours

• All non-African descend from a small population of humans who lived in ETHIOPIA

• DECLINE in genetic diversity reflects the founder effect

• Ellis-van Creveld syndrome in the Old Order Amish in Pennsylvania, USA

• Rare WHAT allele, carried by one of the WHAT founders of the community in the 1700’s

• Community members mostly married one another

• Symptoms can include WHAT, other skeletal anomalies

• 1/5000 Amish births vs 7/1000000 births in the general population

• Ellis-van Creveld syndrome in the Old Order Amish in Pennsylvania, USA

• Rare RECESSIVE allele, carried by one of the 200 founders of the community in the 1700’s

• Community members mostly married one another

• Symptoms can include POLYDACTYLY, other skeletal anomalies

• 1/5000 Amish births vs 7/1000000 births in the general population

Other historically isolated human populations elsewhere have high incidence of other genetically-based WHAT

Other historically isolated human populations elsewhere have high incidence of other genetically-based PHENOTYPES (eg 10% colour blind on Pingalep Island in Micronesia)

Genetic drift → increases genetic variation WHAT population

Genetic drift → increases genetic variation WHAT population

Genetic drift causes allele frequencies to WHAT in populations - WHAT

Genetic drift causes allele frequencies to WHAT in populations - WHAT

Genetic drift causes a reduction in WHAT within a population - variation is lost most rapidly in small populations (WHAT and WHAT events)

Genetic drift causes a reduction in WHAT within a population - variation is lost most rapidly in small populations (BOTTLENECK and FOUNDER events)

Genetic drift causes WHAT

Genetic drift causes WHAT

Breeding sockeye salmon in body size and shape

• Males are WHAT and WHAT-bodied than females

• Variation in WHAT and WHAT within sexes too

Breeding sockeye salmon in body size and shape

• Males are LARGER and DEEPER-bodied than females

• Variation in WHAT and WHAT within sexes too

Salmon are bigger and deeper-bodied in bigger streams, where there’s less bear WHAT

Salmon are bigger and deeper-bodied in bigger streams, where there’s less bear WHAT

Bears foraging at breeding sites WHAT bigger, deeper-bodied salmon to prey on

Bears foraging at breeding sites WHAT bigger, deeper-bodied salmon to prey on

Heritable variation for adult body size in salmon