Transcript Populations
Studying organisms in their environment organism population community ecosystem Bio biosphere biosphere Populations Occupy niches in ecosystems Grow exponentially or logistically (carrying capacity) A group of the same kind of organisms living in the same place at the same time. EVOLVE! Allele frequencies can be described by the Hardy-Weinberg Equilibrium: p2 + 2pq + q2 = 1 Will remain the same if no migration, no selection, random mating. No evolution Measured by random sampling Not realistic but, offers a standard to compare rates of evolution to – Null Hypothesis Life takes place in populations • Population – group of individuals of same species in same area at same time rely on same resources interact interbreed Population Ecology: What factors affect a population? Population Size • Changes to population size – adding & removing individuals from a population • • • • birth death immigration emigration Population growth rates • Factors affecting population growth rate – sex ratio • how many females vs. males? – generation time • at what age do females reproduce? – age structure • how females at reproductive age in cohort? Population growth change in population = births – deaths Exponential model (ideal conditions) dN = riN growth increasing at constant rate dt N r ri t d = # of individuals = rate of growth = intrinsic rate = time = rate of change intrinsic rate = maximum rate of growth every pair has 4 offspring every pair has 3 offspring Exponential growth rate • Characteristic of populations without limiting factors – introduced to a new environment or rebounding from a catastrophe Whooping crane coming back from near extinction African elephant protected from hunting Introduced species • Non-native species – transplanted populations grow exponentially in new area – out-compete native species • loss of natural controls • lack of predators, parasites, competitors – reduce diversity – examples • • • • African honeybee gypsy moth zebra mussel purple loosestrife gypsy moth kudzu Zebra musselssel ~2 months ecological & economic damage reduces diversity loss of food & nesting sites for animals economic damage Purple loosestrife 1968 1978 reduces diversity loss of food & nesting sites for animals Regulation of population size marking territory = competition • Limiting factors – density dependent • competition: food, mates, nesting sites • predators, parasites, pathogens – density independent • abiotic factors – sunlight (energy) – temperature – rainfall competition for nesting sites swarming locusts Logistic rate of growth • Can populations continue to grow exponentially? Of course not! no natural controls K= carrying capacity What happens as N approaches K? effect of natural controls – varies with changes in resources What’s going on with the plankton? 10 8 6 4 2 0 1915 1925 1935 1945 Time (years) 500 Number of cladocerans (per 200 ml) • Maximum population size that environment can support with no degradation of habitat Number of breeding male fur seals (thousands) Carrying capacity 400 300 200 100 0 0 10 20 30 40 Time (days) 50 60 Population of… China: 1.3 billion India: 1.1 billion Human population growth Doubling times 250m 500m = y () 500m 1b = y () 1b 2b = 80y (1850–1930) 2b 4b = 75y (1930–1975) What factors have contributed to this exponential growth pattern? Is the human population reaching carrying capacity? adding 82 million/year ~ 200,000 per day! 20056 billion Significant advances in medicine through science and technology Industrial Revolution Bubonic plague "Black Death" 1650500 million Populations evolve • Natural selection acts on individuals – differential survival • “survival of the fittest” – differential reproductive success • who bears more offspring • Populations evolve – genetic makeup of population changes over time – favorable traits (greater fitness) become more common Presence of lactate dehydrogenase Mummichog 5 Agents of evolutionary change Mutation Gene Flow Genetic Drift Non-random mating Selection Variation & natural selection • Variation is the raw material for natural selection – there have to be differences within population – some individuals must be more fit than others Where does Variation come from? – random changes to DNA Wet year Beak depth • Mutation • errors in mitosis & meiosis • environmental damage – mixing of alleles • recombination of alleles – new arrangements in every offspring • new combinations = new phenotypes – spreads variation • offspring inherit traits from parent Dry year 1977 Dry year 1980 1982 1984 11 Beak depth of offspring (mm) • Sex Dry year 10 9 8 Medium ground finch 8 9 10 11 Mean beak depth of parents (mm) 1. Mutation & Variation • Mutation creates variation – new mutations are constantly appearing • Mutation changes DNA sequence – changes amino acid sequence? – changes protein? • changes structure? • changes function? – changes in protein may change phenotype & therefore change fitness 2. Gene Flow • Movement of individuals & alleles in & out of populations – seed & pollen distribution by wind & insect – migration of animals • sub-populations may have different allele frequencies • causes genetic mixing across regions • reduce differences between populations