Lecture 10 Flashcards
Hurricanes Irma & Maria
lizards that survived the hurricane had larger toepads and shorter femurs
- help them hand on
- hurricanes could help drive natural selection and if the adaption stayed it could be a driver for natural selection
Hurricane-induced selection for bigger toepads
the graph showed hurricane induced selection for bigger toe pads
- used the specific x and y axis bc they wanted to correct for body size
- comparing the how the average toe pad length change before and after the hurricane to view natural selection
- selection for bigger toe pads after the storm
Toe pads: So far, we have seen evidence of natural selection (fitness differences among phenotypes), but have we seen evidence of evolution?
no this is not evidence of evolution because it is missing heritable genetic differences to prove there is evolution we could see if the bigger toe size was passed on to the next generation or if we had info of the heritability
so we know there has been selection but don’t know if there was evolution
a paper in 2020 came out and said what about hurricanes?
hurricane effects on neotropical lizards span geographic and phylogenetic scales
from an another study (2020) same info (toe pads and hurricanes) but went back a year later and measured again
before the hurricane the toe sizes were the smallest
after the hurricane, a year later the toe size was the same (bigger)
Populations of Anolis sagrei that experienced more hurricanes in the last 70 y (red) had larger toepads than those that were hit less often (blue).
noted that different islands are more or less likely to experience hurricanes
recored which islands had more hurricanes
on graph:
- more hurricanes= red
- less hurricanes= blue
islands with higher hurricane activity will have higher toe pad areas
same species of lizards in different environment
Anolis species (n=188) that experienced more hurricanes have larger or smaller toepads than those that were hit less often
larger, instead of looking at one species they looked at a bunch
when looking across many species, the ones that live in high hurricane have different toe pads evidence that there is evolution
Chance: Random Genetic Drift
Due to random sampling, allele frequencies fluctuate
all sorts of genetic variation just by chance
why do allele frequencies fluctuate (due to random sampling) especially key in small populations?
because fluctuations can result in loss of alleles
more likely to lose alleles just by chance, once lost it cannot come back
Direction of genetic drift is…
unpredictable
Genetic drift increases or reduces variation within a population
reduces, (causes loss of alleles/ increases homozygosity/ decreases heterozygosity)
Looking at the effect of genetic drift on allele frequencies in PopG
make population smaller
results: lots of genetic drift
after 100 generations there was a lot of changes compared to before when its was a line no population have gone extinct but a lot have lost genetic variation (become fixed)
Population size:
1000
Fitness of genotype AA:
1
Fitness of genotype Aa:
1
Fitness of genotype aa:
1
Mutation from A to a:
0
Mutation from a to A:
0
Migration rate:
0
Initial frequency of allele A:
0.5
# Generations:
100
# Populations:
10
Genetic drift causes populations to diverge from one another
the blue line represents no drift
populations look different but no natural selection, just by chance
above the blue line is A got fixed
below the blue line A is lost (only have a)
Bowling as a metaphor for genetic drift
Population size and allele frequencies
Population size = width of the bowling alley
Allele frequency = how close you are to the edge (population doesn’t go extinct but allele is lost and theres no getting it back )
Genetic drift is pervasive in biology
very common and important
genetic bottlenecks
reduction to small numbers
subject to genetic drift & founder effects (genetic variation is dependent on whats in the founding individuals)
Effects can last a long time, even if population rebounds / grows rapidly
Bottlenecks can be due to
– population crashes
– establishment of a new population from a small number of individuals (founders)
Northern elephant seal
~100,000 today, but all descended from ~40 in 1850!
example of how effects can last a long time, even if population rebounds / grows rapidly
Gene frequency in small populations of mutant drosophila
A classic experiment demonstrating genetic drift
Followed the dynamics of 2 alleles in 107 populations that were started from only 16 flies each generation
looking at the frequencies of A and a
- Initial populations: frequencies are 50/50
- Generation 1: seeing some spread
- Generation 5: a lot more scatter, some populations losing their alleles
- Generation 19: 30 of the population are fixed for one alleles and the other 30 is fixed for the other allele aka lose one or the other
Peter Buri’s experiment REAL-LIFE
lots lose the a allele
lots lose the A allele
more have become fixed
real life experiment would look the same if there where a smaller amount of individuals
difference in speed
Wright-Fisher model of genetic drift in a population of 16 individuals THEORETICAL
in wright fisher we are assuming that all contribute to reproduction but this is not true
so the actual number is smaller than 16 (effective population size)
How do Peter Buri’s experiment (REAL-LIFE) and Wright-Fisher model of genetic drift in a population of 16 individuals (THEORETICAL) compare?
How and why are they different?
not exactly the same, there are some important differences
Peter Buri’s experiment (REAL-LIFE) populations lost more!
