Evolution of Populations Microevolution Chapter 23

Micro- Evolution Natural selection – types Sexual selection Microevolution Hardy Weinberg Conditions Genetic drift –Bottle neck –Founder effect

Charles Darwin

Microevolution Slight changes in gene frequencies between generations Populations change, not individuals Example: –Antibiotic resistance

less than 1 in 1,600 1 in 400-1,600 1 in in in more than 1 in 64 Distribution of malaria cases in Africa, Asia, and the Middle East in the 1920s Frequency of people with the sickle-cell trait

Hardy Weinberg Under these conditions, populations do not change – No Evolution No mutations Random Mating No Natural Selection No Gene Flow Large Population Size

G.H. Hardy

Wilhelm Weinberg

Hardy Weinberg Equation looks at individual traits, one at a time – p & q are alleles Probably couldn’t meet the conditions for all traits at once for long. Evolution probably always working at some level. Shows us the factors that alter a populations genepool- evolution.

Population All the individuals of the same species in a given location at a given time The potentially interbreeding group The basic unit of evolution Populations evolve, not individuals

Fig Porcupine herd Porcupine herd range Beaufort Sea NORTHWEST TERRITORIES MAP AREA ALASKA CANADA Fortymile herd range Fortymile herd ALASKA YUKON

Gene flow Allows gene to move between populations –immigration Any new trait arising in one population can move to others Keeps species together as a interbreeding unit. Blocking gene flow helps form new species.

Fig XX XX

Fig NON- MINE SOIL MINE SOIL NON- MINE SOIL Prevailing wind direction Index of copper tolerance Distance from mine edge (meters)

Microevolution in humans: Populations became isolated for several thousands of years Slight morphological changes came about by natural selection by climate: –Skin tone and sunlight (uv,vitamin D, Folic acid) –Eye shape and winds, and ice etc. –Height in some populations.

Gene flow and human micro- evolution Isolated populations now coming back together sharing traits

Fig

Sexual Selection

Fig SC male gray tree frog Female gray tree frog LC male gray tree frog EXPERIMENT SC sperm  Eggs  LC sperm Offspring of LC father Offspring of SC father Fitness of these half-sibling offspring compared RESULTS 1995Fitness Measure1996 Larval growth Larval survival Time to metamorphosis LC better NSD LC better (shorter) LC better (shorter) NSD LC better NSD = no significant difference; LC better = offspring of LC males superior to offspring of SC males.

Sexual Dimorphism Sexual selection results in the males and females having different morphology, at least in breeding season. –Size – elephant seals, primates –Color- bird plumage

Genetic Drift Random events in a small population can alter the genepool. Does not increase fitness.

Fig Generation 1 C W C R C R C W C R C R C W p (frequency of C R ) = 0.7 q (frequency of C W ) = 0.3 Generation 2 C R C W C W C R p = 0.5 q = 0.5 Generation 3 p = 1.0 q = 0.0 C R

AA in five populations allele A lost from four populations Generation (25 stoneflies at the start of each) In small populations, random deaths influence outcome, by fixing or eliminating alleles.

allele A neither lost nor fixed in large population Generation (500 stoneflies at the start of each)

Special cases of genetic drift: –Bottleneck – a large population reduced by disaster. A few survivors re-grow the population, but with much less diversity. –Founder effect a small population colonizes a new area. Who is in the small population affects the genepool of the new population.

phenotypes of original population phenotype of island population A seabird carries a few seeds, stuck to its feathers, from the mainland to a remote oceanic island.

Fig Number of alleles per locus Range of greater prairie chicken Pre-bottleneck (Illinois, 1820) Post-bottleneck (Illinois, 1993) Minnesota, 1998 (no bottleneck) Nebraska, 1998 (no bottleneck) Kansas, 1998 (no bottleneck) Illinois 1930–1960s 1993 Location Population size Percentage of eggs hatched 1,000–25,000 <50 750,000 75,000– 200,000 4, < (a) (b)

Types of Natural Selection “weeds out” less fit traits. Reduces genetic diversity in population. Adaptive evolution Directional Selection favors one extreme trait Stabilizing Selection favors the most common form of a trait Disruptive Selection favors the extremes, often forming disjunct populations.

Fig (a) Color-changing ability in cuttlefish (b) Movable jaw bones in snakes Movable bones

Fig Original population (c) Stabilizing selection (b) Disruptive selection (a) Directional selection Phenotypes (fur color) Frequency of individuals Original population Evolved population

Range of values at time 3 Number of individuals Range of values at time 2 Number of individuals Directional selection Range of values at time 1 Number of individuals

Directional Selection modifies Beak depth during drought periods

Range of values at time 1 Number of individuals Range of values at time 2 Number of individuals Stabilizing Selection Range of values at time 3 Number of individuals

percent of population birth weight (pounds) percent mortality Stabilizing selection

Range of values at time 1 Number of individuals Disruptive Selection Range of values at time 3 Number of individuals Range of values at time 2 Number of individuals

Galapagos Finches Specialization to different feeding sources may have diversified the species.

Diversifying selection lead to two beak depths in Cameroon finches

Number of individuals Widest part of lower bill (millimeters) nestlings drought survivors

Frequency Dependent Selection “Right-mouthed” 1981 “Left-mouthed” Frequency of “left-mouthed” individuals Sample year ’82 ’83 ’84 ’85 ’86 ’87 ’88 ’89’90

Ecotypes Locally adapted populations. Local weather or other conditions selects for adaptations. Still one species, but distinguishable from other ecotypes When distributed along a gradient (elevation, north to south) form a cline.

Fig Georgia Warm (21°C) Latitude (°N) Maine Cold (6°C) Ldh-B b allele frequency

A cline:

All made by Artificial Selection from wild mustard Artificial Selection: human designed breeding of plants and animals for desired traits by selecting which individuals get to reproduce.

Polymorphism

Don’t confuse: Polymorphism Sexual Dimorphism Ecotypes - Cline

Fig –2.5% Distribution of malaria caused by Plasmodium falciparum (a parasitic unicellular eukaryote) Frequencies of the sickle-cell allele 2.5–5.0% 7.5–10.0% 5.0–7.5% >12.5% 10.0–12.5%

Fig. 23-UN2 Sampling sites (1–8 represent pairs of sites) Salinity increases toward the open ocean N Long Island Sound Allele frequencies Atlantic Ocean Other lap alleles lap 94 alleles Data from R.K. Koehn and T.J. Hilbish, The adaptive importance of genetic variation, American Scientist 75:134–141 (1987). E S W