The Evolution of Populations Chapter 11

Genetic Variation in Populations Microevolution - the observable change in the allele frequencies of a population over time. Occurs on a small scale In populations there are a wide range of phenotypes Ex: short/fat penguins and taller/slim penguins

Genetic Variation in Populations In order to have a wide range of phenotypes, there must be genetic variation The greater the variation in phenotypes, the more likely it is that at least some individuals can survive in a changing environment

Genetic Variation in Populations For example, a short/fat penguin might be better able to stay warm in an especially cold winter. A tall/slim penguin might be able to dive better, thus catching more fish in a period of food shortage

Genetic Variation in Populations Gene pool - All of the alleles of every individual in a population

Genetic Variation in Populations The allele frequency is a measure of how common a certain allele was in the population

Genetic Variation in Populations To calculate allele frequency for “b”: 7 b 7 B Total alleles = 14 7b/14 TOTAL = 50%

Genetic Variation - 2 Main Sources Mutations A random change in DNA Can form a new allele Mutations that occur in sex cells can be passed on to offspring Recombination Occurs during meiosis Parents’ alleles can be arranged in new ways during crossing over

How do we analyze genetic variation in a population? With graphs!

Natural Selection in Populations “mean” = average “distribution” = range of data; a set of data and their frequency of occurrence

Natural Selection in Populations Normal distribution/bell shaped curve frequency is highest near the mean and decreases toward each extreme end of the range

Natural Selection in Populations Phenotypes in the middle of the range are most common Ex: people of average height (5-6ft) would fall in the shaded region Anyone shorter or taller (the extremes) would fall outside the shaded region

Directional Selection Directional selection - Favors a phenotype at one extreme of a trait’s range. Causes a shift in the population’s phenotypic distribution Extreme/rare phenotype becomes more common

Directional Selection Ex: Peppered moth example After the tree trunks became dark again, the trait for white bodies was selected against because it was no longer beneficial. This caused a shift in the distribution. The mean range now included mostly black moths.

Stabilizing Selection Stabilizing Selection- The intermediate phenotype is favored and becomes most common in the population. Distribution becomes stable in the middle range/decreases genetic variation

Stabilizing Selection Ex: Human birth weight Babies with either a very low or very high weight at birth are more likely to experience complications than babies of average weight

Disruptive Selection Disruptive selection- Both extreme phenotypes are favored Intermediate is selected against

Disruptive Selection Ex: The African swallowtail butterfly has traits to be either orange, black (the extremes), or tan (the intermediate) Predators don’t eat the orange or black ones because orange and black butterflies of different species are poisonous. Thus, the extreme trait is favored.

Other Mechanisms of Natural Selection Gene flow: Organisms move to new population & reproduce --> its alleles are added to new population’s gene pool and removed from original one. Increases genetic variation of receiving population Lack of gene flow can lead to speciation Speciation - the rise of two or more new species from a previously existing species

Other Mechanisms of Natural Selection Genetic Drift Small populations = more affected by chance Elimination/sudden increase of alleles in a small population = drift Drift = loss of genetic diversity

Other Mechanisms of Natural Selection Bottleneck Effect Drift that occurs after an event reduces population size Reduced population size = reduced genetic variation

Other Mechanisms of Natural Selection The Founder Effect Small number of individuals colonize a new area Gene pool of new colony is usually very different (less diverse) from original population

Other Mechanisms of Natural Selection Effects of Drift Loss of genetic variation Less likely to have a large amount of individuals that can survive a changing environment Higher chance of lethal genes being passed on instead of eliminated

Other Mechanisms of Natural Selection Sexual Selection Males continually produce sperm, small investment, can have many mates Females have limited number of eggs, long gestational periods, invest more time/resources Makes females choosy about mates, don’t want to have loser babies

Other Mechanisms of Natural Selection Sexual Selection Certain traits increase mating success Intra- : competition between males Inter- : showy males to attract mates Some showy traits can put animals in danger, but suggest good health/fertility/ability to care/defend to potential mates. GOOD QUALITY.

Video Bell Ringer! What kind of sexual selection is feautured in this video? What traits might the beetle have to make him successful? Sexual Selection of Darwin Beetles What kind of sexual selection is featured in this video? Why are females so choosy about mates? Sexual Selection Birds of Paradise

Hardy-Weinberg Equilibrium Describes populations that are not evolving “Equilibrium” - Genotype frequencies in a population stay the SAME over time as long as 5 conditions are met.

5 Conditions Very large population - no drift No emigration/immigration - no gene flow No mutations - no new alleles can be added to gene pool Random mating - no sexual selection No natural selection - all traits aid in survival equally

Hardy-Weinberg Equilibrium Equilibrium is only met if ALL 5 conditions are met!!!! Real populations rarely meet all 5

Why study Hardy-Weinberg? Biologists can compare real data to data predicted by the model. Can learn more about the ways populations evolve and about how to test factors that can lead to evolution.

Review on what leads to evolution Genetic drift Gene flow Mutation Sexual selection Natural selection

Speciation Through Isolation Isolated populations - when gene flow stops Isolated populations adapt to their environments differently, which can lead to differences in the gene pools

Speciation Through Isolation Reproductive isolation - members of different populations can not mate successfully Reproductive organs not compatible Producing barren offspring Final step of speciation

What causes isolation? Behavioral isolation - differences in courtship or mating behaviors Bird dances Frog songs Firefly flash patterns

What causes isolation? Geographic isolation - Physical barriers that can separate populations Mountain ranges Rivers Valleys

What causes isolation? Geographic Isthmus of Panama separated a large population of wrasse fish Gene pools changed over time Now 2 separate species

What causes isolation? Temporal isolation - mating seasons do not line up High competition for mates at one time

Evolution Through Natural Selection is Not Random “Chance” and “random” relate to how easily an outcome can be predicted Genetic drift Mutations

Evolution Through Natural Selection is Not Random The only “random” thing is a change in the environment Natural selection pushes a population’s traits in an advantageous direction The response of species to environmental challenges is not random

Patterns In Evolution Convergent Evolution - evolution toward similar characteristics in unrelated species Adaptations to similar environments

Patterns In Evolution Shark and dolphin tails Separated by 300 million years of evolution

Patterns In Evolution Divergent Evolution - Closely related species evolve in different directions Different appearances due to adapting to different environments

Patterns In Evolution Red fox and Kit fox Red coat, temperate region vs. Sandy coat/big ears, desert regions

Patterns In Evolution Species interactions can lead to connected evolutionary pathways Coevolution - process in which two or more species evolve in response to changes in each other

Patterns In Evolution Acacia plant and stinging ant Ant protects plant from predators and plant provides nectar for the ant

Patterns In Evolution “Evolutionary Arms Race” - coevolution in competitive relationships

Patterns In Evolution Garter snake and rough skinned newt Newt produces a neurotoxin that concentrates on their skin Garter snakes have evolved a resistance to the toxin Driven newts to have extreme levels of toxins

Patterns In Evolution Extinction - The elimination of a species from Earth Most often occurs when a species cannot adapt to an environmental change

Patterns In Evolution Background Extinction - occur continuously at a low rate Usually affect one or a few species in a small area

Patterns In Evolution Mass Extinction - Very rare, but very intense, often on a global level Can destroy many species There have been at least 5 mass extinctions in the last 600 million years

Patterns In Evolution Punctuated Equilibrium - episodes of speciation occur suddenly in geologic time and are followed by long periods of stability

Patterns In Evolution Adaptive Radiation - Diversification of one ancestral species into many descendant species Descendant species usually adapted to a wide variety of environments