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Chapter 11 - The Evolution of Populations

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Robert Mottershead

on 4 October 2016

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Transcript of Chapter 11 - The Evolution of Populations

Chapter 11 - The Evolution of Populations
A population shares a common gene pool.

Genetic variation leads to phenotypic variation.
Phenotypic variation is necessary for natural selection.
Genetic variation is stored in a population’s gene pool.
made up of all alleles in a population
allele combinations form when organisms have offspring

Genetic variation in a population increases the chance that some individuals will survive.

measures how common allele is in population
can be calculated for each allele in gene pool

Allele frequencies measure genetic variation.

usually occurs during meiosis
parents’ alleles arranged in new ways in gametes

can form new allele
can be passed on to offspring if in reproductive cells

Recombination forms new combinations of alleles.

Mutation is a random change in the DNA of a gene.

Genetic variation comes from several sources.

Hybridization is the crossing of two different species.
occurs when individuals can’t find mate of own species
topic of current scientific research

Genetic variation comes from several sources.

Populations, not individuals, evolve.

highest frequency near mean value
frequencies decrease toward each extreme value

Traits not undergoing natural selection have a normal distribution.

A normal distribution graphs as a bell-shaped curve.

Natural selection acts on distributions of traits.

Microevolution is evolution within a population.
observable change in the allele frequencies
can result from natural selection

Natural selection can change the distribution of a trait in one of three ways.

Directional selection favors phenotypes at one extreme.

Natural selection can take one of three paths.

Natural selection can take one of three paths.

Stabilizing selection favors the intermediate phenotype.

Disruptive selection favors both extreme phenotypes.

Natural selection can take one of three paths.

Natural selection is not the only mechanism through which populations evolve.

bald eagle migration

Gene flow occurs when individuals join new populations and reproduce.
Gene flow keeps neighboring populations similar.
Low gene flow increases the chance that two populations will evolve into different species.

Gene flow is the movement of alleles between populations.

Genetic drift causes a loss of genetic diversity.
It is most common in small populations.
A population bottleneck can lead to genetic drift.
It occurs when an event drastically reduces population size.
The bottleneck effect is genetic drift that occurs after a bottleneck event.

Genetic drift is a change in allele frequencies due to chance.

It occurs when a few individuals start a new population.
The founder effect is genetic drift that occurs after start of new population.

The founding of a small population can lead to genetic drift.

less likely to have some individuals that can adapt
harmful alleles can become more common due to chance

Genetic drift has negative effects on a population.

Sexual selection occurs due to higher cost of reproduction for females.
males produce many sperm continuously
females are more limited in potential offspring each cycle

Sexual selection occurs when certain traits increase mating success.

intrasexual selection: competition among males
intersexual selection: males display certain traits to females

There are two types of sexual selection.

The next slides review the five factors that can lead to evolution.

Mutations produce the genetic variation needed for evolution.

Genetic drift changes allele frequencies due to chance alone.

Real populations rarely meet all five conditions.
Real population data is compared to a model.
Models are used to studying how populations evolve.

Hardy-Weinberg equilibrium describes populations that are not evolving.

Hardy-Weinberg equilibrium provides a framework for understanding how populations evolve.

Genotype frequencies stay the same if five conditions are met.
very large population: no genetic drift
no emigration or immigration: no gene flow
no mutations: no new alleles added to gene pool
random mating: no sexual selection
no natural selection: all traits aid equally in survival

Hardy-Weinberg equilibrium describes populations that are not evolving.

Biologists use models to study populations.
Hardy-Weinberg equilibrium is a type of model.

Hardy-Weinberg equilibrium describes populations that are not evolving.

expected in all populations most of the time
respond to changing environments

In nature, populations evolve.

Natural selection selects for traits advantageous for survival.

Sexual selection selects for traits that improve mating success.

Gene flow moves alleles from one population to another.

p2 + 2pq + q2 = 1

must know frequency of recessive homozygotes

"The Hardy-Weinberg equation is based on Mendelian genetics. It is derived from a simple Punnett square in which p is the frequency of the dominant allele
and q is the frequency of the recessive allele."

Predicted genotype frequencies are compared with actual frequencies.
used for traits in simple dominant-recessive systems

The Hardy-Weinberg equation is used to predict genotype frequencies in a population.

Behavioral barriers can cause isolation.
called behavioral isolation
includes differences in courtship or mating behaviors

Populations can become isolated in several ways.

Populations become isolated when there is no gene flow.
Isolated populations adapt to their own environments.
Genetic differences can add up over generations.

The isolation of populations can lead to speciation.

New species can arise when populations are isolated.

Temporal barriers can cause isolation.
called temporal isolation
timing of reproductive periods prevents mating

called geographic isolation
physical barriers divide population

Geographic barriers can cause isolation.

Speciation is the rise of two or more species from one existing species.

members of different populations cannot mate successfully
final step to becoming separate species

Reproductive isolation can occur between isolated populations.

Two or more species can evolve together through coevolution.
evolutionary paths become connected
species evolve in response to changes in each other

Species can shape each other over time.

Evolution occurs in patterns.

A pattern of punctuated equilibrium exists in the fossil record.
theory proposed by Eldredge and Gould in 1972
episodes of speciation occur suddenly in geologic time
followed by long periods of little evolutionary change
revised Darwin’s idea that species arose through gradual transformations

Speciation often occurs in patterns.

destroy many species at global level
thought to be caused by catastrophic events
at least five mass extinctions in last 600 million years

Mass extinctions are rare but much more intense.

occur at roughly the same rate as speciation
usually affects a few species in a small area
caused by local changes in environment

Background extinctions occur continuously at a very low rate.

Coevolution can occur in competitive relationships, sometimes called evolutionary.

Coevolution can occur in beneficial relationships.

Convergent evolution describes evolution toward similar traits in unrelated species.

ancestral species diversifies into many descendent species
descendent species usually adapted to wide range of environments

Many species evolve from one species during adaptive radiation.

occur at roughly the same rate as speciation
usually affects a few species in a small area
caused by local changes in environment

Extinction is the elimination of a species from Earth.
Background extinctions occur continuously at a very low rate.

Species can become extinct.

Natural selection can have direction.
The effects of natural selection add up over time.

Evolution through natural selection is not random.

kit fox

red fox


How do convergent and divergent evolution illustrate the directional nature of natural selection?

Divergent evolution describes evolution toward different traits in closely related species.

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