Section 15.2 Summary– pages 404-413 Population Genetics and Evolution The principles of evolution are rooted in population genetics Section 15.2 Summary– pages 404-413

Population Genetics and Evolution A population consists of all the individuals of a species that live together in one place at one time and can successfully breed. 2/16/2019

Population characteristics Population size~ number of individuals in a population Density ~ number of individuals per unit of area Dispersion~ pattern of spacing •random ~ unpredictable, patternless spacing (a) •clumped ~ patchy spacing (b) •uniform ~ even spacing (c) 2/16/2019

Factors that affect Populations Birthrate (natality)~ number of offspring produced 2/16/2019

Factors that affect Populations Death rate (mortality)-how many people die 2/16/2019

Factors that affect Populations Age structure~ relative number of individuals of each age 2/16/2019

Factors that affect Populations Survivorship curve~ plot of numbers still alive at each age 2/16/2019

Carrying capacity is maximum number of individuals a habitat can support 2/16/2019

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Population limiting factors Density-dependent factors Competition Predation stress/crowding waste accumulation Density-independent factors •weather/climate •periodic disturbances 2/16/2019

Population “strategies” r-selected opportunistic Short lifespan Many small offspring reproduces once no parental care High death rate 2/16/2019

Population “strategies” K-selected Long lifespan Few large offspring several reproductions late in life extensive parental care Low death rate 2/16/2019

Population Growth Models Exponential model (blue) (J-curve) r-selected Logistic model (red) (S-curve) K-selected species 2/16/2019

Section 15.2 Summary– pages 404-413 Changes in genetic equilibrium Another mechanism that disrupts a population’s genetic equilibrium is genetic drift— the alteration of allelic frequencies by chance events. Section 15.2 Summary– pages 404-413

Section 15.2 Summary– pages 404-413 Changes in genetic equilibrium The movement of genes by migration is called gene flow. When an individual leaves a population, its genes are lost from the gene pool. When individuals enter a population, their genes are added to the pool. Section 15.2 Summary– pages 404-413

Section 15.2 Summary– pages 404-413 Natural selection acts on variations Some variations increase or decrease an organism’s chance of survival in an environment. There are three different types of natural selection that act on variation: stabilizing selection directional selection disruptive selection Section 15.2 Summary– pages 404-413

Section 15.2 Summary– pages 404-413 Natural selection acts on variations Stabilizing selection is a natural selection that favors average individuals in a population. Selection for average size spiders Normal variation Section 15.2 Summary– pages 404-413

Section 15.2 Summary– pages 404-413 Natural selection acts on variations Directional selection occurs when natural selection favors one of the extreme variations of a trait. Selection for longer beaks Normal variation Section 15.2 Summary– pages 404-413

Section 15.2 Summary– pages 404-413 Natural selection acts on variations In disruptive selection, individuals with either extreme of a trait’s variation are selected for. Selection for light limpets Normal variation Selection for dark limpets Section 15.2 Summary– pages 404-413

Section 15.2 Summary– pages 404-413 Natural selection acts on variations Natural selection can alter the genetic equilibrium of a population’s gene pool over time. Significant changes in the gene pool could lead to the evolution of a new species over time. Section 15.2 Summary– pages 404-413

Section 15.2 Summary– pages 404-413 In nature, physical barriers can break large populations into smaller ones. Geographic isolation occurs whenever a physical barrier divides a population. A new species can evolve when a population has been geographically isolated. Section 15.2 Summary– pages 404-413

Section 15.2 Summary– pages 404-413 The Evolution of Species When geographic isolation divides a population of tree frogs, the individuals no longer mate across populations. Tree frogs are a single population. Section 15.2 Summary– pages 404-413

Section 15.2 Summary– pages 404-413 The Evolution of Species The formation of a river may divide the frogs into two populations. Section 15.2 Summary– pages 404-413

Section 15.2 Summary– pages 404-413 The Evolution of Species Over time, the divided populations may become two species that may no longer interbreed, even if reunited. Section 15.2 Summary– pages 404-413

Section 15.2 Summary– pages 404-413 As populations become increasingly distinct, reproductive isolation can arise. Reproductive isolation occurs when formerly interbreeding organisms can no longer mate and produce fertile offspring. Section 15.2 Summary– pages 404-413

Section 15.2 Summary– pages 404-413 Reproductive isolation can result in speciation Reproductive Isolation can occur when there are changes in the species: Genetic material (DNA) Behavior Section 15.2 Summary– pages 404-413

The Western meadowlark (left) and the Eastern meadowlark (right) appear to be identical, and their ranges overlap, but their distinct songs prevent interbreeding 2/16/2019

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