Molecular Evolution Course #27615

Transcription

1 Molecular Evolution Course #27615 Anders Gorm Pedersen Molecular Evolution Group Center for Biological Sequence Analysis Technical University of Denmark (DTU)

2 Neutral Theory of Molecular Evolution Evolution is a two-step process: 1. Mutation (random) 2. Selection (non-random) Detrimental mutation =>! negative selection =>! Mutation not seen Beneficial mutation =>! positive selection =>! Mutation seen

3 Selectionist Views of What Drives Molecular Evolution Majority of all mutations are detrimental and not seen Most observed substitutions have adaptive value Classical school: Single predominant version of gene ( wild type ) present in population Natural selection rapidly fixates new, advantageous mutations Balance school: Appreciable amount of polymorphism in gene pool Polymorphism maintained actively by natural selection (e.g., sickle cell anemia)

4 Neutralist Views of What Drives Molecular Evolution Electrophoretic studies in 1960ʼs showed much higher polymorphism than anticipated by either classical or balance school selectionists Kimura and others proposed the Neutral Theory of Molecular Evolution. Detrimental mutation => negative selection => Mutation not seen Neutral mutation => no selection => Mutation may be seen (genetic drift) Beneficial mutation => positive selection => Mutation seen

5 Difference Between Selectionist and Neutralist Views of Evolution Fraction of random mutations assumed to be deleterious, neutral, and advantageous Selectionist view: Most observed mutations represent functional innovation Neutralist view: Most observed mutations represent conservative changes, changes in unimportant regions

6 All Agree that Adaptations are Caused by Natural Selection

7 All Agree that Adaptations are Caused by Natural Selection

8 All Agree that Adaptations are Caused by Natural Selection

9 All Agree that Adaptations are Caused by Natural Selection

10 The molecular clock

11 Genetic drift Gen. 1

12 Genetic drift Gen. 1

13 Genetic drift Gen. 1

14 Genetic drift Gen. 1

15 Genetic drift Gen. 1 Gen. 2

16 Genetic drift Gen. 1 Gen. 2

17 Genetic drift Gen. 1 Gen. 2

18 Genetic drift Gen. 1 Gen. 2 Gen. 3

19 Genetic drift Gen. 1 Gen. 2 Gen. 3

20 Genetic drift Gen. 1 Gen. 2 Gen. 3

21 Genetic drift Gen. 1 Gen. 2 Gen. 3 Gen. 4

22 Genetic drift Alleles will eventually reach a frequency of 0 or 1 Genetic diversity decreases Effect is more strongly felt in small populations

23 Time to fixation and time between fixations

24 Drift and mutation

25 Genetic Drift: The bottleneck effect Alleles in original population Alleles remaining after bottleneck

26 Bottleneck effect Cheetah

27 Northern Elephant Seal Bottleneck Effect Reduced to 20 individuals in 1896 Now 30,000 individuals, with no detectable genetic diversity

28 Genetic Drift: The founder effect Change in allele frequencies when a new population arises from only a few individuals. e.g., only a few fish are introduced into a lake. e.g., only a few birds make it to an island.

29 New Atlantic population, maybe from only 10 individuals Scorpaenidae Lionfish Pterois volitans Founder Effect

30 Exercise: Genetic drift simulation Starting point: population with N individuals, fraction p has genotype A, fraction (1-p) has genotype a All individuals produce 200 offspring of same genotype as parent. (offspring also has fraction p genotype A) Survival rate = 1/200 => Constant population size N Death strikes randomly: each generation N random individuals survive Investigate drift of allele A frequency:. Find proportion of populations where A is fixed (p=0.4; N=20, N=80) Find average time to fixation of A (p=0.4, N=10, 20,...130)