Allele Frequency Changes: Migration and Drift

Amy Hauver Author

07/20/2022 Added

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This video describes how gene flow and genetic drift impact allele frequencies for some pesticide resistant populations.

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[00:00:09.750]In this video,
[00:00:10.740]we will discuss the final two of the forces
[00:00:13.260]that affect the gene pool: migration and drift.
[00:00:16.950]There are many forces that affect
[00:00:18.480]and cause changes in the allele frequencies in a gene pool,
[00:00:22.020]but we only discuss four of them.
[00:00:24.290]In the first video, we discussed mutation and selection,
[00:00:28.320]and now in the second video,
[00:00:29.700]we will talk about migration and drift.
[00:00:33.750]An observable example
[00:00:35.340]that many people have witnessed is the migration
[00:00:37.650]of the monarch butterfly.
[00:00:40.680]They migrate each year
[00:00:42.930]during the summer and autumn,
[00:00:44.430]from overwintering sites in North America,
[00:00:47.100]to the mountainous sites in Mexico.
[00:00:49.410]As these butterflies migrate,
[00:00:50.820]they encounter different population of monarchs,
[00:00:53.190]and they may enter interbreed.
[00:00:54.660]This transfer of genetic information
[00:00:57.000]from one population to another introduces new alleles
[00:01:00.570]or altering alleles and new allele frequencies.
[00:01:05.640]A smaller scale migration example is gene flow.
[00:01:08.760]You would not typically think of a plant like horseweed
[00:01:11.550]as being able to migrate since it's immobile
[00:01:13.950]as soon as it's rooted as a seedling.
[00:01:16.620]Many plants have accessories or attractants
[00:01:18.900]that aid them in dispersing their seed and pollen
[00:01:21.690]and their pollen is mobile via wind or pollinators.
[00:01:25.890]Horseweed has accessories that promote dispersal by wind.
[00:01:30.450]Plants utilize different dispersal methods
[00:01:32.610]to enhance gene flow in a multitude of ways.
[00:01:35.790]Many plants have accessories or attractants
[00:01:37.860]that aid them in dispersing their seed
[00:01:39.960]and pollen is mobile via wind or pollinators.
[00:01:42.690]Some seeds are attracted for consumption by birds or mammals
[00:01:45.690]and will be transported by them,
[00:01:47.340]and others have accessories like horseweed or dandelion
[00:01:49.980]that promote dispersal by wind.
[00:01:52.290]This table shows some of the methods
[00:01:53.850]by which weed seeds can be dispersed,
[00:01:56.190]increasing gene flow to new populations.
[00:01:59.490]The different methods are compared on a distance basis
[00:02:02.310]from within field distance to between field,
[00:02:05.520]to between fields and across regions.
[00:02:09.360]Within field movement is between zero and 1,000 feet.
[00:02:13.410]Within field movement of weed seeds can be through rain,
[00:02:16.950]insect movement, like ants, wind, birds,
[00:02:21.150]soil runoff, small animals that carry seeds short distances
[00:02:26.160]and mechanical movement,
[00:02:27.930]like a combine moving across the field
[00:02:30.480]and also a contaminated seed that's being planted.
[00:02:34.800]Between field movement is a greater distance
[00:02:37.260]and therefore more limited.
[00:02:39.000]Between field dispersal of weed seed
[00:02:41.700]is around 1,000 feet to 10,000 feet.
[00:02:44.940]This can be through windblown dispersal, birds, soil runoff,
[00:02:49.410]small animals carrying the seed short distances,
[00:02:52.110]mechanical movement, like a combine moving to a new field,
[00:02:56.190]contaminated seed, and animals moving longer distances,
[00:02:59.580]like the burs of sticking to the fur of a deer.
[00:03:06.120]Longer distances causing resistance to spread
[00:03:08.820]between regions are commonly through contaminated seed,
[00:03:12.690]movement of equipment to new areas,
[00:03:14.790]and animals moving long distances,
[00:03:16.980]especially migratory birds.
[00:03:19.470]These are all examples of how nature
[00:03:21.390]and humans can influence gene flow of resistance trait
[00:03:24.510]from one field population to a new population,
[00:03:27.900]increasing the spread of resistance.
[00:03:32.160]The last force that affects the gene pool
[00:03:34.260]of a population that we will cover is drift.
[00:03:37.500]Genetic drift is changes in the allele frequencies
[00:03:40.260]of an existing gene variant
[00:03:42.120]in a population due to random sampling of individuals.
[00:03:47.130]A simple example to visualize
[00:03:49.110]of drift is the flower colors in peas.
[00:03:52.320]The phenotypes, what we observe,
[00:03:54.930]are the white or purple colors of the petals.
[00:03:57.990]The genotypes are WW,
[00:04:00.780]or big WW, and big W, little w
[00:04:03.750]for the white flowers.
[00:04:05.130]So the white color is dominant
[00:04:07.050]and the genotype for purple flowers is little w, little w,
[00:04:10.920]making purple recessive.
[00:04:13.080]This image shows two populations
[00:04:15.060]and represents the first generation
[00:04:17.520]that is affected by drift in population one and two.
[00:04:21.210]Both populations start with the same number
[00:04:23.220]of individuals with the same number of purple
[00:04:25.680]to white flowers and the same genotypes
[00:04:29.250]for population one and two.
[00:04:34.710]Through a random sampling of individuals,
[00:04:38.130]these five plants were able to survive
[00:04:39.990]and reproduce in population one.
[00:04:42.630]They were all white flowers.
[00:04:44.490]Two of the individuals carry
[00:04:45.810]the dominant homozygous genotype,
[00:04:48.150]and three have the heterozygous white genotype.
[00:04:52.500]Think about what you would expect the next generation
[00:04:54.870]to look like for population one
[00:04:56.700]after these five individuals reproduce.
[00:05:00.630]Population two is also affected by drift
[00:05:03.180]through a random sampling of five individuals
[00:05:05.520]where these five flowers reproduce.
[00:05:08.220]There are three white and two purple flowers
[00:05:10.320]with three white having a heterozygous genotype
[00:05:13.290]and the two purple having the recessive genotype.
[00:05:16.530]Think about what you would expect this next generation
[00:05:18.750]to look like for population two
[00:05:20.550]after these five individuals reproduce.
[00:05:26.940]The second generation begins
[00:05:28.560]to show the effects of drift
[00:05:30.150]on the two different populations' gene pools.
[00:05:33.120]The second generation of population one has a higher ratio
[00:05:36.120]of white flowers with only purple,
[00:05:38.160]while the second generation
[00:05:40.110]of population two has a higher ratio of purple flowers.
[00:05:45.210]Through another random sampling of individuals,
[00:05:50.100]these five plants were able to survive
[00:05:52.200]and reproduce in population one.
[00:05:54.630]There are five white flowers,
[00:05:56.160]where all of the individuals carry
[00:05:57.540]the dominant homozygous genotype.
[00:06:00.060]Think about what you would expect the next generation
[00:06:02.460]to look like for population one.
[00:06:05.520]Population two is also affected again
[00:06:07.950]by drift through another random sampling
[00:06:10.230]of individuals where these five plants,
[00:06:12.870]two white and three purple flowers, reproduce.
[00:06:16.590]Think about what you would expect
[00:06:17.880]the next generation to look like.
[00:06:21.420]The third and final generation
[00:06:23.130]of population one has ended
[00:06:24.870]with a gene pool of all white flowers.
[00:06:27.420]For population two, the gene pool has drifted
[00:06:30.240]to be majorly purple,
[00:06:31.620]nearly opposite of what the starting population was
[00:06:34.080]for both of the populations.
[00:06:37.170]Here is the simplified version of what's occurred.
[00:06:39.990]The starting populations are the same
[00:06:41.940]for population one and population two
[00:06:44.220]with mostly white and a few purple flowers.
[00:06:46.890]But after multiple generations of genetic drift,
[00:06:49.680]the populations have shifted to be all white,
[00:06:52.410]population one, or mostly purple, population two.
[00:06:59.940]Here is the previous example
[00:07:01.620]but including the concept of insecticide resistance
[00:07:04.500]with the idea of genetic drift.
[00:07:07.050]Even with no additional selection pressure,
[00:07:09.240]like pesticide application,
[00:07:11.070]a population can still become resistant
[00:07:13.170]due to standing variation in drift.
[00:07:15.870]If there are two individuals
[00:07:17.220]in each starting population
[00:07:18.810]that carry the resistance trait
[00:07:20.640]and there are no pesticide selection pressures,
[00:07:23.490]the population can still become resistant
[00:07:25.560]due to random sampling.

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Allele Frequency Changes: Migration and Drift

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