Unit 4: Principles of Ecology Section 1 – How Populations Grow Section 2 – How Populations Evolve

What is a Population? Population: Growth is based on: All individuals of a species that live in the same location at the same time Growth is based on: Fecundity – ability to give birth Resources Adaptability Demography: study of all populations in one area of an environment

Three Features of a Population Population Size Ability to survive depends on the number of individuals in a population Smaller population are more vulnerable to becoming extinct due to random events The smaller pops. have more inbreeding that can decrease the fitness of the population Decreased fitness: more individuals with recessive and/or harmful traits

Three Features of a Population Population Density The amount of individuals that live in a given area Smaller populations – more space = less availability to reproduce Larger population – more choice in mate selection

Three Features of a Population Dispersion The way the organisms are arranged in their space Random Even Clumped Arranged by self-determination or chance Arrangement consists of a regular pattern The individuals are spaced by clusters or chunks

Population Modeling  

Population Modeling  

Population Modeling Resources & Population Size Logistic Growth A graph of population size is restricted by density-dependent factors Density-Dependent Factors Factors that account for the amount of individuals in the population that are depleting resources and reduce the capability of population growth Density Independent – weather and climate

Rapid Growing Populations R-strategists: bacteria, some plants and insects Environmental conditions allow these species to flourish and reproduce quickly When environment conditions worsen: The population depletes quickly The organisms have a short-life either way Small and quick maturing offspring allowing for no parental care necessary

Slow Growing Populations K-strategists: whales, mammals, etc. Population density is usual near carrying capacity Longer life spans and slower maturing processes Parental care is required and intense

Hardy-Weinberg Principle The frequencies of alleles in a population do not change unless evolutionary forces act upon the population Only true if: The population is large Members do not mate with relatives No evolutionary forces ↓ Shifted by: Mutation, Gene Flow, Genetic Drift, Nonrandom mating & Natural Selection

Mutation A source of genetic variation Change of allele frequencies due to mutation are very slow Not all mutations affect the phenotype The mutations appear in the genetic code

Gene Flow Movement of alleles to or from a population Caused by migration The movement of individuals Immigrants: add alleles Emigrants: remove alleles Make a drawing

Nonrandom Mating Preferred mating with individuals nearby or with similar phenotypes Inbreeding (mating with relatives) causes a lower frequency of heterozygotes – predicted by H-W In actuality, inbreeding only increases the amount of homozygotes Also caused by selective mating choosing by females – they prefer stronger and attractive traits No change to frequencies Stronger traits – ability to gather food and handsome individuals

Genetic Drift Random changes in allele frequencies due to an event Isolated and smaller population are affected greater by drift Cheetahs Uniform DNA to prevent extinction Reduced disease resistance

Natural Selection Direct changing of allele frequencies by deviating from the H-W proportions Selection against = reduces the favor of that trait Ultimately helps raise the favor of the other trait or individual (homozygote or heterozygote) Not always the most beneficial trait Example: a male bird has strong abilities to gather food, but is also a carrier of a fatal disease, the phenotype of gathering food can be passed on with the carrier allele as only alleles expressed are acted upon

How Selection Acts Only traits expressed are acted upon – sometimes causing rare recessive alleles to passed on Once heterozygous individuals produces homozygous offspring – natural selection can be activated The traits need to become common enough to be selected against Hemophilia – recessive disorder

Persistence of Genes Homozygous recessive = 1/100 Heterozygous = 18/100 So 18 individuals carry the allele without expression These individuals maintain that gene without even knowing it The conditions cannot be eliminated as they are not common enough

Distribution of Traits Populations are shaped by polygenic traits in reference to natural selection Genes that contribute the most to the distributed are influenced the greatest These traits and genes are measured by measuring each individual then providing an average Normal distribution: The average having the highest amount of individuals – creating a hill-shaped diagram Ask what polygenic means

Directional Selection The frequency of one trait moves in one direction towards the average to become closer normal distribution Purpose: trying to eliminate one extreme from the range to balance out the average Need an example – more spines – more protection

Stabilizing Selection Distribution becomes narrower and selection favors similar individuals opposed to the extremes In other terms: favoring intermediate phenotypes