explain how genetic drift can cause large changes in small populations

1. explain how genetic drift can cause large changes in small populations

2. Genetic Drift • a change in the frequency of an allele in a population due to random sampling • the alleles in the offspring are a sample of those in the parents, and... • ... there is an element of chance in determining whether any given individual survives and reproduces. • Genetic drift may cause alleles to disappear completely and thereby reduce genetic variation

3. Modelling Genetic Drift – The Jar of Marbles Analogy Imagine a jar containing 20 marbles – 10 blue and 10 red - representing 20 organisms Different colours of marble represent different alleles The random reproduction of the organisms can be modelled as follows: • Select one marble from the first jar, at random • Place a marble of the same colour in a second jar • Replace the first marble in the original jar and repeat until the second jar holds 20 marbles

4. Random sampling error means that the next generation is unlikely to have exactly the same proportions of red and blue marbles With each subsequent generation, the red allele is less likely to be picked for reproduction Eventually, the red allele may go ‘extinct’

5. Where there are few copies of an allele in a population, the effect of genetic drift is larger • Where there are many copies of an allele in a population, the effect of genetic drift is reduced Kimura proposed the neutral theory of molecular evolution in 1968 Most evolutionary changes and most of the variation within and between species is not caused by natural selection but by random drift of mutant alleles that are neutral NOTE: this theory explains variation – NOT adaptation! Motoo Kimura 木村 資生 (1924 – 1994)

6. Welcome to the tropical island paradise of New Basildon! Main Island Squawkiraptorragulii East Thurrock St Clere’s New Basildon is home to the world’s largest population of Squawky Birds

7. The birds on Main Island originally showed a lot of variation Yellow eyes Brown Red legs Blue Blue eyes Red Black Yellow legs

8. During tropical storms, a few birds can be blown off Main Island and out to sea • The small island of St Clere’s has a population of squawky birds which are descended from a small number of birds who were blown onto the island from Main Island • All squawky birds on St Clere’s are brown or black with yellow legs and eyes St Clere’s Main Island

9. The Founder Effect • the loss of genetic variation that occurs when a new population is established by a very small number of individuals from a larger population

10. Squawky birds are considered a pest by plantation owners because they eat coconuts In 1903, a new predator was introduced to Main Island to control their numbers... ... the Fluffy Vampire Bat

11. By 1910 the fluffy vampire bats had reduced the population of squawky birds to one breeding pair

12. Removal of the Fluffy Vampire Bats by the New Basildon SPB allowed the Squawky Bird population to recover, but... ... all the Squawky Birds on Main Island are blue or red, with blue eyes and red legs

13. Population Bottlenecks • a sharp reduction in the size of a population due to: - environmental events (e.g. earthquakes, floods, fires, or droughts) - human activities (hunting, habitat destruction) • Population bottlenecks produce a smaller population with reduced genetic diversity. • In subsequent generations, genetic diversity remains lower, only slowly increasing with time as random mutations occur.

14. Effects of a Bottleneck Event Recovery Population Size Bottleneck event Extinction Time

15. Population Bottlenecks and Conservation • The reduction in population size and genetic variation reduces the robustness of the population • The population is less able to survive selecting environmental changes such as: - climate change - changes in availability of resources (food, shelter, etc.) - introduction of new diseases

16. The island of East Thurrock has few trees and no mammals. • There are plentiful peanut plants which provide food for the birds. • The birds do not need to fly to obtain food or to escape predators The Squawky Birds of East Thurrock have small wings and are incapable of flight. East Thurrock

17. Relaxation of Selective Pressure • The random nature of mutations ensures that all genes will be subject to mutations • When natural selection is relaxed, there is no mechanism for weeding out mutations that disrupt the function of that gene • Without selection, disruptive mutations will accumulate in the population • As these mutations accumulate, the proportion of individuals carrying the functional gene drops, so... • ... with each subsequent generation, it becomes less likely that the functional gene will be passed on.

18. Fumarase deficiency or "Polygamist Down's" • An autosomal recessive allele affecting the enzyme fumarase which converts fumarate to malate in the mitochondria • Fumarase deficiency causes causes encephalopathy, severe mental retardation, unusual facial features, brain malformation, and epileptic seizures • Fumarase deficiency is prevalent amongst members of the Fundamentalist Church of Jesus Christ of Latter-Day Saints • Members of the FLDS Church practise edogamy and polygyny • 75 – 80% FLDS Church members are descendents of the Church’s founders - John Y. Barlow and Joseph Smith Jessop

19. The Wisent or European Bison • The wisent was hunted to extinction in the wild – the last wild wisent was killed by poachers in 1927 • At the time, fewer than 50 wisent were held in zoos • Current wild populations have been reintroduced using animals bred from 12 zoo animals • Current wisent populations have very low genetic variation... • ... which may be affecting the reproductive ability of bulls.

20. The Golden Hamster • Most Golden Hamsters are descended from a mother hamster and her litter found in the Syrian desert in 1930 by zoologist Israel Aharoni • Aharoni took the animals back to Jerusalem to breed as laboratory animals • Some escaped from the cage through a hole in the floor • Most of the wild golden hamsters in Israel today are believed to be descended from this litter The Cheetah • Cheetahs are so closely related that skin can be grafted from one cheetah to another without rejection • This suggests an extreme bottleneck event in the past

21. Astyanax mexicanus • Fish living in rivers on the surface have eyes and are silver coloured • Populations living in caves are eyeless albinos • In the dark there is no selective advantage to maintaining eyes or having any particular colour

22. Champsocephalus gunnari The Mackarel Icefish • Icefish do not produce haemoglobin or red blood cells • Sequencing of the icefish genome reveals that they have fossil haemoglobin genes... • ... genes which have been disrupted by so many mutations that they are no longer functional • The solubility of oxygen in water increases as the temperature of the water decreases • In the Antarctic temperatures, the water in the fish’s plasma will carry enough to supply the muscles’ needs