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Biology Chapter 17

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Franchesca Ramirez

on 12 June 2014

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Transcript of Biology Chapter 17

Biology Chapter 17
Evolution of Population
Lesson 1: Genes and Variation
Genetics Joins Evolutionary Theory
When Darwin developed his theory of evolution, he did not know the sources of variation in how heredity worked. Also, didn't know the source of variation in a population.
Mendel's studies were published while Darwin was alive. But, no one understood the importance of Mendel's work.
Around 1900, Mendel's work was rediscovered.
Scientists combined Mendel's work with Darwin's theory, in understanding how traits were inherited.
Today, evolution can be described in terms of genetics.
Genotype and Phenotype in Evolution
Most animals and plants have two sets of genes, one set from each parent.
These genes come in different forms, called alleles.
The set of allele found in an organism's is called its genotype.
An organism's genotype and its environment determine its phenotype.
Phenotype is an organism's appearance and other characteristics, or traits.

Natural selection act on phenotypes; it not act directly on genes.
Some individuals in any population have phenotypes(or traits) are better fit for their environment.
Those individuals have higher fitness; meaning that they will have more offspring and pass on more of their genes to the next generation
Gene Pools
A population is a group of individuals that mate and produce offspring.
Gene pool

contains all the allele of all the genes in a population.
In gene pool, some alleles are common while, others are rare.
Allele frequency
is the number of time an allele occurs in a gene pool, compared to the total number of alleles for the same gene in that pool.
Allele frequency is a percentage.
If the frequency of an allele change over time, the population is evolving.
Only populations evolve, not individuals.
Sources of Genetic Variation
The members of a population differ each other.
Genetics explains the source of these variations.
Lesson 2: Evolution as Genetic Change in Populations
Lesson 3: The Process of Speciation
Lesson 4: Molecular Evolution
A mutation is a genetic change.
Most mutations are neutral and have little effect on fitness.
Some mutations lower fitness by making survival and reproduction more difficult.
Other increase fitness by making survival and reproduction easier.
A mutation must be passed from one generation to the next; and only one are carried in egg or sperm cell.
Sexual Reproduction
The genes from two parents combine in new ways, during the sexual reproduction.
This process can produce millions different gene combinations.
Crossing-over also mixes genes and it happens during meiosis when chromosome pairs trade pieces of DNA.
Sexual reproduction creates new genotype, but it does not change the frequency of alleles in the whole population

Lateral Gene Transfer
Some single-celled organisms can pass genes from one individual to another.
The passing of genes to an organism that is not an offspring is called called lateral gene transfer.
Lateral gene transfer is important in the evolution of single-celled organisms.
Single-Gene and Polygenic Traits
Genes control in different ways.
In some cases, a single gene controls a trait; other cases. several genes work together to control a trait.
Polygenic Traits
Polygenic trait
that is controlled by two or more genes.
Can have many many possible genotypes and phenotypes.
Single Gene Traits
trait is a trait controlled by just one gene.
May have just two or three phenotypes.
How Natural Selection Works
Fitness describes individuals that have trait that help them survive and reproduce.
Their trait are favored by natural selection.
Individuals with high fitness have more offspring and pass on more of their genes.
The alleles that produce these traits become more common in the population.

This is the process of evolution.
Natural Selection on Single-Gene Traits
In single-gene traits, natural selection can lead to change in allele frequencies.
When natural selection favors one trait over another, the allele for the favored trait become more common over time.
Natural Selection on Polygenic Traits
Traits that are controlled by more than one gene produce a range of phenotypes.
This range can often be shown by a bell curve.
Natural selection can act on polygenic traits in one of three: directional selection, stabilizing selection, or disruptive selection.
Directional Selection
Sometime natural selection favors organisms at one end of the bell curve.
When individuals at one end of curve have higher fitness than the other, directional selection take place.
Stabilizing Selection
Sometime natural selection favors the average individuals.
When individuals near the center of the bell curve have the highest fitness, stabilizing selection take place.
It shifts the end of the curve closer to the middle.
Disruptive Selection
Sometime the most extreme traits are most likely to survive to survive and reproduce, while the averge types have a harder time.
Disruptive Selection
can eventually create two distinct phenoypes.
These phenotpes are shown by a curve with two peaks.
Genetic Drift
In small populations, change events can cause evolution.
Genetic drift
is a random change in the frequency of the alleles in agene pool.
It may be caused by the bottleneck effect or the founder effect.
Genetic Bottlenecks
Sometime, a disaster can kill off most of a population.
Example, a flood or disease may leave only few alleles that is very different from the lost population.
A change in allele freuqency following a dramatic loss of population is called the
bottleneck effect.
The bottleneck effect can greatly reduce the gentic diversity in a population.
The Founder Effect
*Genetic drift can also happen when a few individuals move into a new haitat
*The small group may have a set of alleles that is different from the main population.
*The founder effect is a change in allele frequency that results from a small group starting a new population.
Evolution Versus Genetic Equilibrium
*If a population is not evolving, the allele frequencies in the gene pool do not change.
*This condition is called genetic equilibrium.
Sexual Reproduction and Allele Frequency
In sexual reproduction, genes are shuffled.
Meiosis and fertilization do not change the allele frequencies in the total population.
It alone does not affect genetic equilibrium.
The Hardy-Weinberg Principle
The Hardy-Weinberg principle states that allele frequences in a population will stay in genetic equilibrium unless something causes them to change.
This principle predicts that five conditions can upset genetic equilibrium and causes a population to evole.
Five conditions: Nonrandom Mating, Small Population, Movement into or out of the population, Mutations, and Natural Selection.
Nonrandom Mating
Individuals choose mate that have particular traits, such as size or color.
Sexual selection is when indididuals pick mates with certain heritable traits.
If mates are selected for a particular trait, the frequency of that trait will increase.
Small Population
Genetic drift can affect small population greatly.
Evolution take place more easily in a small population.
Movement Into or Out of the Population
Individuals that join a population may add new allelesto the gene pool.
Individuals that leave can lower the frequency of certain alleles.
It can create new alleles.
These new alleles ca change the allele frequencies of the population.
Natural Selection
If different genotype have different fitness, individuals with those genotype will be more likelyto survive.
Genetic equilibrium will be upset and the population will evolve.
In real population, one or more of these conditions is usally.
Most of the time, most species are evolving.
Isolating Mechanisms
Biologists define a
as a population whose members can breed and produce fertile offspring.
If a members of a population can no longer mate, a new species may evolve.
The formation of a new species is called
Breeding connects the gene pool of species.If a species is split into two part, genetic change in one part of the gene pool cannot spread to other.
When two poulations can no longer mate and produce offspring,
reproductive isolation
has occurred.
Geography, behavior, and time can separate population from each other.
Geograhic Isolation
When population are separated by a barrier, such as river, mountain, or ocean,
geograhic isolation
can occur.
Behavioral Isolation
If two interbreeding populations develop different behaviors, such as different mating dance, then
behavioral isolation
can happen.
Temporal Isolation
A third isolating mechanism is temporal isolation.
Temporal isolation
happen when population that live in teh same habitat reproduce at different times.
Speciation in Darwin's Finches
Since Darwin, many scientists hve studied the Galapagos finches. The Grants showed that under the right conditions, natural selection can happen quickly.
That it make the Galapagos finches a good choice for describing how speciation might work in real life.

Speciation probably began when a small number of finches founded a new population.
Over time, the gene pool changed.
According to the currently accepted hypothesis, isolation, competition, and natural selection formed new species that chould no longer interbreed.
Founders Arrive
Many years ago, a few finches from South America arrived on one of the islands.
The small group survived and reproduced. Because of the founder effect, the allele frequencies of this new population were different from the original population.
Geographic Isolation
These finches do not usually fly over open water, so the population was geographically isolated.
The founder effect, geographic isolation, and natural selection led to the evolution of new species of finch.
Eventually some of these finches moved to a new island, where the process was repeated.
Change in Gene Pools
The environment on the second island may have different plants with larger seeds.
Directional selection would have favored birds with larger beaks.
Over time, a population with larger beaks evolved.
Behavioral Isolation
A few birds from the second island cross back to the first island.
Will the two population interbreed? Probably not.
Finches choose mate carefully. They like mates that look like themselves.
Even though they share the same habitat, the two population are now isolated by behavior.
They have become two different species.
Competition and More Evolution
The two new species live together and compete for seeds.
Individuals with very small beaks have greater success than birds with average-size beaks.
Also can specialize in eating very small seeds that birds with average-size beaks cannot eat.
Over time the differences lead to reproductive isolation
A third species evolves.
Over many years, this process could have repeated itself again on different islands.
The combination of geographic isolation, behavioral isolation, and natural selection could have produce the 13 different finch species found in the Galapagos today.
Molecular Clocks
A genome is the complete set of genes found in an organism. Ex. Your body has about 25,000 working genes!
Scientists often study genomes to learn how organisms have evolved.
Scientists can test different hypotheses.
One way to test these hypotheses is b6y using DNA as a molecular clock.
A molecular cock uses mutation rate in DNA to estimate how long ago two organisms shared a common ancestor.
Ticking Mutations
Some kinds of mutations happen at a steady rate, like seconds ticking on a clock.
Some mutations help or hurt the survival of an organism. Because these mutations are affected by natural selection, they cannot be used as "molecular ticks."
But many do not affect an organism's fitness.
Such mutation are said to be neutral.
Neutral mutation change DNA without affecting an organism's fitness.
Neutral mutations can collect in DNA of different species at about the same rate.
Scientists can compare the number of neutral mutations in specific sequences of DNA from different organism.
By counting mutations, researchers can estimate how long it has been since the two organisms shared a common ancestor.
Rate of Mutation
Not all genes mutate at the same rate.
These gene are useful for measuring evolution that happens over a short time.
Other genes mutate more slowly and are used to measure evolution that take place over a much longer time.
Gene Duplication
Humans have about 25,00 working genes.
Where it came form? It probably evolved from a much smaller number of genes in the earliest forms of life.
Copying Genes
One way the number of genes grew so large was by duplication, or copying, of existing genes.
It can happen during meiosis as chromosome pairs exchange DNA.
Sometime one chromosome end up with extra DNA during exchange.
Duplicate Genes Evolve
Sometime duplicated gene mutates.
The mutation change the way the gene works.
The mutate copy of the gene perform a new role, while the old continues its old role.
Gene Families
Several copies of gene can form a gene family.
They usually make slightly different proteins that carry out similar roles.
This gene family evolved from just one original gene.
Developmental Genes and Body Plans
A new way of study evolution looks at gene activity during development.
Small change in the way gene work in an embryo can lead to big change in the adult body.
Hox Genes and Evolution
Hox genes are group of genes that play an important role in animals development.
It determine which part of the embryo develop arms, legs, or wings.
Also can control the size and shape of the adult body part.
A change in one Hox gene can cause a major difference between two group of animals.
Timing Is Everything
The growth of an embryo is carefully controlled by genes.
Growth starts and stop at exact times.
Small change in starting and stopping time can make a big difference in final organism.
Molecular clock
- a method that uses mutation rates in DNA to estimate the length of time that two species have been evolving interdependently.
Behavioral isolation
- a form of reproductive isolation in which two population develop differences in courtship rituals or other behaviors that prevent them from breeding.
Temporal isolation
- a form of reproductive isolation in which two or more species reproduce at different times.
- a population whose members can breed and produce fertile offspring.
Reproductive isolation
- the separation of a species or population so that members can no longer interbreed.
Geographic isolation
- a form of reproductive isolation in which two population are separated by geographic barriers such as rivers, mountains, or bodies of water.
Genetic equilibrium
- a situation in which the allele frequencies in a population remain the same.
Hardy-Weinberg principle
- the principle that state that allele frequencies in a population remain constant unless one or more factors cause those frequencies to change .
Sexual selection
- the process by which individuals select mates on the basis of heritable traits.
Directional Selection
- a form of natural selection in which individuals at one end of a distribution curve have higher fitness than individuals in the middle or at the other end of the curve.
Stabilizing selection
- a form of natural selection in which individuals near the center of a distribution curve have higher fitness than individuals at either end of the curve.
Disruptive selection
- a form of natural selection in which individuals at the upper and lower ends of a distribution curve have higher fitness than individuals near the middle of the curve.
Genetic drift
- a random change in allele frequency caused by a series of change occurrences that cause an allele to become more or less common.
Bottleneck effect
- a change in allele frequency following a dramatic reduction in the size of a population.
Founder effect
- a change in allele frequency as a result of the migration of a small group of a population.
Gene pools-
all the genes, including all the different alleles for each gene, that are present in a population at any one time.
Allele frequency
- the number of times that an allele occurs in a gene pool, compared to the total number of alleles in that pool for the same gene.
Single-gene trait
- a trait controlled by one gene that has two alleles.
Polygenic trait
- a trait controlled by two or more genes.
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