Kirby Ellis and Janie Salmon p.2
Chapter 17 outline
17.1 Genes and Variation
Genetics Joins Evolutionary Theory
● Heritable traits are controlled by genes carried on chromosomes
● Changes in this creates genetic variation
● Variation is raw material for natural selection
Genotype and Phenotype in Evolution
● Forms of a gene- alleles
● Genotype is the combination of alleles it carries
● Genotype combined with environment creates phenotype
● Phenotype is all physical and behavioral characteristics
● Natural selection acts on phenotype, not genotype
Populations and Gene pools
● Population- group of individuals of the same species
● A gene pool is all the genes of a whole population
● Allele frequency is how often a certain gene is present in a gene pool
● Evolution involves a change in the frequency of alleles in a gene pool over time
Sources of Genetic Variation
Mutations
● A mutation is a change in the genetic material of a cell
● There are such things as neutral mutations which do not change the organisms phenotype
● Mutations may affect overall fitness of an organism
○ Fitness is an individuals ability to survive
● Most mutations or mutual
● Mutations are only passed from generation to the next generation
Genetic Recombination in Sexual Reproduction
● Genetic recombination happens from sexual reproduction- this is how you look different from
your parents
● Crossing over is another way this happens
● This is when paired chromosomes swap lengths of DNA at random
Lateral Gene Transfer
● This is when organisms swap genes
● This can increase genetic variation
● This happens in single celled organisms mostly
Single Gene and Polygenic Traits
● The number of phenotypes produced for a trait depends on how many genes control the trait
Single gene trait
● A trait that is controlled by only one gene
Polygenic Traits
● These are traits controlled by two or more genes
● Each gene controlled by this has at least two alleles
● This means there are a lot of phenotypes and genotypes
○ For example height in humans is a polygenic trait
17.2 Evolution as Genetic Change in Populations
How Natural Selection Works
● When organisms reproduce they pass down copies of its genes
● This means fitness is passing genes to the next generation
● This means an evolutionary adaptation is a trait that increases an organism’s ability to pass down
it’s alleles
Natural Selection on Single-Gene traits
● Natural selection on single gene traits can change allele frequency, and phenotype
Natural Selection on Polygenic traits
● This can change fitness of phenotypes and produce directional selection, stabilizing selection, or
disruptive selection
Directional Selection
● When individuals on one end of the curve have higher fitness than those on the other end, or the
middle do
● Phenotypes change because some individuals are able to survive better
Stabilizing Selection
● When individuals in the middle of the curve have higher fitness than those at either ends
● This keeps the curve at its center position
Disruptive Selection
● When individuals at both ends of the curve have higher fitness than those in the middle
● If this lasts long enough the single curve can develop into two curves
○ This means it creates two different phenotypes
Genetic Drift
● In a small population some individuals may leave more descendants with their alleles
● This causes there to be more or less of a certain allele in a population
● This random change in allele frequency is called genetic drift
Genetic Bottlenecks
● The bottleneck effect is a change in allele frequency following a dramatic reduction of size in a
population
The Founder Effect
● Allele frequency changes because of a migration of a small subgroup of a population
Evolution vs. Genetic Equilibrium
●
If the allele frequencies in a gene pool do not change it means the population is in genetic
equilibrium
Sexual Reproduction and Allele Frequency
● Sexual reproduction may not result in a change in genetic equilibrium
The Hardy-Weinberg Principle
● States that allele frequency in a population should remain constant unless one or more factors
cause those frequencies to change
● This helps with making predictions about outcome of sexual reproduction
● These next five conditions disrupt this principle
○ Usually one of these five happens in a normal population, which causes evolution to
occur
Non-random Mating
● Sexual selection based on traits of the mate
Small Population Size
● Genetic drift is more likely to occur which would disrupt the constant allele frequency
Immigration or Emigration
● Individuals who join a new population and add new alleles to the gene pool
● Individuals who leave and remove alleles from the gene pool
Mutations
● Can introduce new alleles into a gene pool
Natural Selection
● Different genotypes have different fitness and genetic equilibrium will be disrupted
17.3 The Process of Speciation
Isolating Mechanisms
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Species: population or group of population’s whose members can interbreed and produce fertile
offspring.
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Speciation: formation of new species.
Reproductive isolation: when two species stop breeding with eachothers causing the gene pool to split
Splits the population into two or more groups and the two species no longer interbreed
When a population becomes reproductively isolated they evolve into two different species.
It can develop through behavioral isolation, geographic isolation, and temporal isolation.
Behavioral Isolation
Two populations develop different ritual or behaviors and no longer inter breed even though they
potentially could
Ex. Two bird species that are the same but use different mating songs so therefore they do not mate.
Ex. Eastern and Western meadowlarks
Geographic Isolation
Two populations are separated by rivers, mountains, bodies of water etc. (ß geographic barriers)
Eventually separate gene pools form
Ex. Abert’s squirrel lived in the Southwest 10,000 years ago a population of squirrels became
isolated in the north of the Grand Canyon. Natural selection and genetic drift worked separately on both
species and now there are two species of squirrels the Kaibab and Abert’s, which are similar but have
many differences like fur color etc.
Does not always produce isolation ex. Birds not isolated by rivers but rodents are.
Temporal Isolation
Two or more species reproduce at different times
Ex. Three species of orchids live in the same rain forest and each species blooms on a different day
so therefore they cannot interbreed or pollinate with each other.
Darwin’s Finches
Galapagos islands finches
The finches varied by beak length and shape, and color of feathers
Geographic isolation, founding a new population, changes the new populations gene pool,
behavioral isolation, and ecological competition caused the speciation in the finches
Many years ago, finches from South America arrived on the Galapagos
They survived on the island and reproduced.
The environment on the island was different from the South American environment.
They eventually evolved into a new species through geographic isolation and natural selection
Later some birds moved to another island and were therefore geographically isolated and they
evolved differently
The population on each island adapted to the different islands, plants with small thin-shelled seeds
vs. large thick-shelled seeds made beaks different. ß Eventually two different populations with different
phenotypes came about .
They also eventually became behaviorally isolated because of their beaks because they are now
behaviorally isolated they can also become reproductively isolated.
Competition resulted in the birds with the highest fitness to reproduce.
Geographic isolation, genetic change, behavioral isolation, and competition provided variation and
speciation with the Galapagos finches to produce the different species of finches on the islands.
17.4 Molecular Evolution
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Molecular clock: uses mutation rates in DNA to estimate the time that two species have been
evolving independently.
Neutral Mutations as “Ticks”
Mutations occur all the time
These mutations cause changes in the sequence of DNA
Mutations have positive and negative effects on phenotypes and are under pressure from natural
selection
Some mutations have no effect on phenotype and these accumulate in the DNA of different species
at the same rate
DNA sequences in two species are compared to show how many mutations have occurred in each
group, and the more differences there are the more time has gone by because they share a common
ancestor.
Calibrating the clock
There are many different molecular clocks in a genome due to the rate of accumulating mutations
Clocks allow researchers to time different evolutionary events
Estimating how often mutations occur can check the accuracy of these molecular clocks this is done
by comparing the number of mutations in a gene whose age has already been determined or is already
known.
Gene Duplication
Over 25,000 working genes in the human genome
Genes evolve through the duplication and modification of existing genes.
Copying Genes
During meiosis sometimes when crossing-over there is an unequal swapping of DNA and the extra
DNA carries the part of the gene a full gene or a longer length of chromosome.
Sometimes the entire genome can be duplicated
Duplicate genes may undergo mutations that change their function, but the gene which they copied
is still there and functioning
The new genes can evolve without affecting the original gene function or product
Multiple copies of a duplicated gene can turn into a group of related genes called a gene family
Gene families and their members produce similar proteins
EX. The globin gene family evolved from a single ancestral globin gene
Developmental Genes and Body Plans
Darwin once thought that in the growth of embryos the change could cause transformation of adult
body shape and size
Hox genes and Evolution
Hox genes determine which parts of an embryo develop arms, legs, or wings
Groups of these genes control size and shape of these structures
Homologous Hox genes shape the bodies of animals
Small changes in Hox gene activity during embryological development can produce large changes in
adult animals.
A change in one Hox gene accounts for a major evolutionary difference between two animal groups
Small changes in starting and stopping times for embryo growth can make a difference in organisms.
Small timing changes can make the difference between long, slender fingers and short, stubby toes