1 1 Population Genetics

2 2 The Gene Pool Members of a species can interbreed & produce fertile offspring Species have a shared gene pool Gene pool – all of the alleles of all individuals in a population

3 3 The Gene Pool Different species do NOT exchange genes by interbreeding Different species that interbreed often produce sterile or less viable offspring e.g. Mule

4 4 Populations A group of the same species living in an area No two individuals are exactly alike (variations) More Fit individuals survive & pass on their traits

5 5Speciation Formation of new species One species may split into 2 or more species A species may evolve into a new species Requires very long periods of time

66 Modern Evolutionary Thought

7 7 Modern Synthesis Theory Combines Darwinian selection and Mendelian inheritance Combines Darwinian selection and Mendelian inheritance Population genetics - study of genetic variation within a population Population genetics - study of genetic variation within a population Emphasis on quantitative characters (height, size …) Emphasis on quantitative characters (height, size …)

8 8 Modern Synthesis Theory 1940s – comprehensive theory of evolution (Modern Synthesis Theory) 1940s – comprehensive theory of evolution (Modern Synthesis Theory) Introduced by Fisher & Wright Introduced by Fisher & Wright Until then, many did not accept that Darwin’s theory of natural selection could drive evolution Until then, many did not accept that Darwin’s theory of natural selection could drive evolution S. Wright A. Fisher

9 9 Modern Synthesis Theory TODAY’S theory on evolution Recognizes that GENES are responsible for the inheritance of characteristics Recognizes that POPULATIONS, not individuals, evolve due to natural selection & genetic drift Recognizes that SPECIATION usually is due to the gradual accumulation of small genetic changes

10 Modern Synthesis Theory Darwin & Wallace (Natural Selection) + Mendel(Genetics) + Wright & Fisher(Population Genetics) + Mayr & others(Biogeography, Paleontology) ____________________________________ = Modern Evolutionary Synthesis a more complete understanding of how evolution proceeds in the wild

11 Macro- vs. Micro- Evolution Macroevolution - change that results in the formation of new species, new taxonomic groups, evolutionary trends, adaptive radiation, and mass extinction. Microevolution: the generation-to- generation change in the frequency of alleles in a population – Evolution at its smallest scale – The core definition of evolution  Microevolutionary changes give rise to macroevolution

12 Microevolution Changes occur in gene pools due to mutation, natural selection, genetic drift, etc. Gene pool changes cause more variation in individuals in the population Example: Bacteria becoming unaffected by antibiotics (resistant)

13 Microevolution of Species

14 Causes of Microevolution Genetic Drift - the change in the gene pool of a small population due to chance Natural Selection - success in reproduction based on heritable traits results in selected alleles being passed to relatively more offspring (Darwinian inheritance) - Cause ADAPTATION of Populations 14

15 Causes of Microevolution Gene Flow -is genetic exchange due to the migration of fertile individuals or gametes between populations Mutation - a change in an organism’s DNA - Mutations can be transmitted in gametes to offspring Non-random mating - Mate selection may be influenced by geographic proximity - Individuals may select a mate with traits similar to their own 15

16 Genetic Drift

17 Genetic Drift Changes in a population’s allele frequencies due to chance Drift can cause rapid change in small populations A problem when populations get too small 17

18 Factors that Cause Genetic Drift Bottleneck Effect - a drastic reduction in population - (volcanoes, earthquakes, landslides …) - Reduced genetic variation - Smaller population may not be able to adapt to new selection pressures 18

19

20 Factors that Cause Genetic Drift Founder Effect - occurs when a new colony is started by a few members of the original population - Reduced genetic variation - May lead to speciation 20

21 Founder’s Effect 21

22 Loss of Genetic Variation Cheetahs have little genetic variation in their gene pool This can probably be attributed to a population bottleneck they experienced around 10,000 years ago, barely avoiding extinction at the end of the last ice age 22

23 Natural Selection

24 Natural Selection Natural selection – differential success in survival and reproduction N.S. leads to populations becoming better adapted to their environments (adaptation) N.S. works most powerfully in large populations (effect of drift is small, and changes due to N.S. are preserved) 24

25 It IS possible to measure Natural Selection in the wild.

26 Modes of Natural Selection 26

27 Modes of Natural Selection Directional Selection - Favors individuals at one end of the phenotypic range - Most common during times of environmental change or when moving to new habitats 27

28 Modes of Natural Selection Disruptive selection - Favors extreme over intermediate phenotypes - Occurs when environmental change favors an extreme phenotype 28

29 Modes of Natural Selection Stabilizing Selection - Favors intermediate over extreme phenotypes - Reduces variation and maintains the current average - Example: - Human birth weight 29

30 Modes of Natural Selection Sexual Selection - Traits that enable an organism to reproduce, without necessarily helping them to survive are selected for - Any inherited trait that improves the mating success of certain individuals will become more pronounced in succeeding generations. Example: Bright plumage 30

31 Gene Flow

32 Gene Flow Migration of individuals can change the allele frequencies in a population Immigration – movement into a population Emigration – movement out of a population 32

33 Mutation

34 Mutations Mutations are the origin of all differences between alleles But mutations are rare, so they must still spread by drift or selection if they are going to impact allele frequencies in a population 34 Sickle cell anemia is the result of a single point mutation

35 Speciation

36Speciation Speciation is the formation of a new species A Species is a population of organisms whose members can interbreed and produce fertile offspring. 36 So, how does one species become two?

37 Reproductive Isolation Reproductive Isolation occurs when a population splits into two groups and the two groups no longer interbreed. When populations become reproductively isolated, they can evolve into two separate species 37

38 Mechanisms of Isolation Behavioral Isolation occurs when two populations that are capable of interbreeding develop differences in courtship rituals or other behaviors Ex: Eastern meadowlarks will not respond to western meadowlark songs, and vice versa. 38

39 Mechanisms of Isolation Geographic Isolation occurs when two populations are separated by geographic barriers such as rivers, mountains or bodies of water Ex: The Kaibab squirrel is a subspecies of the Abert’s squirrel that formed when a small population became isolated on the north rim of the Grand Canyon. 39

40 Mechanisms of Isolation Temporal Isolation occurs when two or more species reproduce at different times. Ex: Two species of frogs inhabit the same area, but one reproduces from January to March, whereas the other reproduces from March to May 40