Chapter 17 Evolution of Populations

17.1 Genes and Variation

Genetics Joins Evolutionary Theory Genotype and Phenotype in Evolution

Typical plants and animals contain two sets of genes, one contributed by each parent Specific forms of genes, called alleles, vary from person to person

Genotype – particular combination of alleles an organism carries

Phenotype – all physical, physiological, and behavioral characteristics of an organism.

Natural selection acts directly on phenotype, not genotype – acts on an organism’s characteristics, not directly on its alleles

Some individuals have phenotypes that are better suited to their environment than are the phenotypes of other individuals – better suited reproduce and pass on more offspring

Genetics Joins Evolutionary Theory

Populations and Gene Pools Population – a group of individuals of the same species

that mate and produce offspring Because members of a population interbreed, they share a

common group of genes called a gene pool Gene pool – consists of all genes, including the different

alleles for each gene, that are present in a population Researchers study gene pools by examining the number of

different alleles they contain Allele Frequency – the number of times an allele occurs in a

gene pool, compared to the total number of alleles in that pool for the same gene.

Evolution, in genetic terms, involves a change in the frequency of alleles in a population over time Natural selection operates on individual organisms, but the

changes it causes in allele frequency show up in the population as a whole.

Sources of Genetic Variation

Three sources of genetic variation are mutation, genetic recombination during sexual reproduction, and lateral gene transfer

1. Mutations – change in the genetic material of a cell Some involve changes within individual genes others involve

changes in larger pieces of chromosomes Some mutations – called neutral mutations – do not change an

organism’s phenotype May be lethal – cause genetic diseases May lower fitness by decreasing an individual’s ability to survive

and reproduce Each of us born with roughly 300 mutations that make parts of

our DNA different from our parents – most of those are neutral – one or two are potentially harmful – a few may be beneficial

Mutations matter in evolution only if they can be passed from generation to generation Mutation must occur in germ cells that produce sperm or eggs

Sources of Genetic Variation2. Genetic Recombination in Sexual

Reproduction In humans, who have 23 pairs of chromosomes, this

process can produce 8.4 million gene combinations http://

www.dnalc.org/view/16931-2D-Animation-of-Genes-and-Inheritance.html

Crossing over is another way genes are recombined https://

highered.mcgraw-hill.com/sites/dl/free/0072835125/126997/animation5.html

Occurs during meiosis – paired chromosomes swap lengths of DNA at random – further increases new genotypes created in each generation.

This is why – in species that reproduce sexually – no two siblings (except identical twins) ever look exactly alike

With independent assortment and crossing over , you can easily end up with your mom’s eyes, your dad’s nose, and hair that combines qualities from both parents

Sources of Genetic Variation3. Lateral Gene Transfer – some organisms pass genes from

one individual to another, or even from one species to another Most of the time, in eukaryotic organisms, genes are passed only

from parents to offspring Bacteria swap genes on plasmids as though the genes were

trading cards Can occur between organisms of the same species or organisms

of different species Can create genetic variation in any species that picks up the

“new” genes Important in the evolution of antibiotic resistance in bacteria

Single-Gene and Polygenic Traits The number of phenotypes produced for a trait

depends on how many genes control the trait. Some snails have dark bands on their shells, others

do not – presence or absence of dark bands is a single-gene trait Single-gene trait – trait controlled by only one gene Gene controlling shell banding has two alleles – allele

for a shell without bands (W) is dominant over an allele for shell with dark bands (w)

All genotypes for this trait have one of two phenotypes – shell with bands or shell without bands

Graph shows presence of dark bands may be more common in populations – true even though it is the recessive

Phenotypic ratios determined by the frequency of alleles in the population as well as by whether the alleles are dominant or recessive

Single-Gene and Polygenic Traits Polygenic traits – many traits are controlled by

two or more genes Each gene of a polygenic trait often has many

possible genotypes and even more different phenotypes

Human height – varies from very short to very tall

17.2 Evolution as Genetic Change in Populations

How Natural Selection Works

Each time an organism reproduces, it passes copies of its genes on to its offspring Evolutionary fitness – success passing genes to the next

generation Natural selection on single-gene traits

Can lead to changes in allele frequencies and, thus, to changes in phenotype frequencies Ex: A lizard experiences mutations in one gene that

determines body color. Normal color is brown, the mutations produce red and black lizards. The red lizards are more visible to predators, eaten, less likely to

survive and reproduce. Allele for red color becomes less common Black lizards – might absorb sunlight and warm up faster on cold

days – high body temperature may allow the lizards to move faster to feed and avoid predators – may allow them to produce more offspring Allele for black color may increase in frequency which would cause the

black phenotype to increase

How Natural Selection Works Natural Selection on Polygenic Traits – more complex

Can affect the relative fitness of phenotypes and thereby produce one of three types of selection: directional selection, stabilizing selection, or disruptive selection

1. Directional Selection – when individuals at one end of the curve have higher fitness than individuals in the middle or at the other end

Range of phenotypes shifts because some individuals are more successful at surviving and reproducing than are others

Limited resources – food – affect individuals’ fitness Darwin’s fitness – birds with bigger, thicker beaks can feed

more easily on larger, harder, thicker shelled seeds If supply of small and medium-sized seeds runs low, leaving only

larger seeds, birds with larger beaks would have an easier time feeding – big-beaked birds would be more successful in surviving and passing genes to the next generation

How Natural Selection Works2. Stabilizing Selection – when individuals near

the center of the curve have higher fitness than individuals at either end

Ex: Mass of human infants at birth – very small babies are likely to be less healthy and thus, less likely to survive. Babies who are much larger than average – difficulty being born.

How Natural Selection Works

Disruptive Selection – when individuals at the outer ends of the curve have higher fitness than individuals near the middle of the curve. Acts against individuals of intermediate type If pressure of natural selection is strong and lasts

long enough, the situation can cause the single curve to split into two. Disruptive selection creates 2 distinct phenotypes

Bird population living in an area where medium-sized seeds become less common and large and small seeds become more common. Population might split into two groups – one with smaller beaks and one with larger beaks

Genetic Drift In small populations, individuals that carry a

particular allele may leave more descendants than other individuals leave, just by chance. Over time, a series of chance occurrences can

cause an allele to become more or less common in a population

This kind of random change in allele frequency is called genetic drift

Genetic Drift

Genetic Bottlenecks Sometimes a disaster, such as disease, can kill

many individuals in a population By chance, the smaller population’s gene pool may

have allele frequencies that are different from those of the original gene pool. If the reduced population later grows, its alleles will be

different than the original population’s Bottleneck effect – a change in the allele

frequency following a dramatic reduction in the size of a population Severe bottleneck can sharply reduce genetic diversity

Genetic Drift The Founder Effect – when allele frequencies

change as a result of he migration of a small subgroup of a population When a few individuals colonize a new habitat Founding individuals may carry alleles that differ

from those in the main population, just by chance Ex: evolution of several hundred species of fruit flies on

different Hawaiian Island – they all descended from the same mainland fruit fly population

http://www.youtube.com/watch?v=Q6JEA2olNts

17.3 The Process of Speciation

Isolating Mechanisms Species – a population or group of populations

whose members can interbreed and produce fertile offspring Speciation – the formation of a new species

What happens if members of a species stop breeding with other members? The gene pool can split – once split into two groups,

changes in one of the gene pools cannot spread to the other

They can no longer interbreed – reproductive isolation occurs

Reproductive Isolation – When populations become reproductively isolated, they can evolve into two separate species. Can develop in a variety of ways, including behavioral isolation,

geographic isolation, and temporal isolation

Isolating Mechanisms Reproductive Isolation

1. Behavioral Isolation – when two populations that are capable of interbreeding develop differences in courtship rituals or other behaviors

Eastern and western meadowlarks are similar birds whose habitats overlap – will not mate with each other, partly because they use different songs to attract mates

2. Geographic Isolation – when two populations are separated by geographic barriers such a rivers, mountains, or bodies of water

Abert’s squirrel – lives in Southwest – about 10,000 years ago, a small population became isolated on the north rim of the Grand Canyon – separate gene pools formed – genetic changes that appeared in one group were not passed on the other – natural selection and genetic drift worked separately on each group – led to the formation of a subspecies, the Kaibab squirrel – Abert’s and Kaibab very similar, indicating they are closely related – however, they differ in significant ways, such as fur coloring

Isolating Mechanisms Reproductive Isolation

3. Temporal Isolation – when two or more species reproduce at different times

Ex: suppose three similar species of orchids live in the same rainforest – each species has flowers that last only one day and must be pollinated on that day to produce seeds – because the species bloom on different days, they cannot pollinate one another

Speciation in Darwin’s Finches How might the founder effect and natural

selection have produced reproductive isolation that could have led to speciation among Galapagos finches? Speciation in Galapagos finches occurred by

founding of a new population, geographic isolation, changes in the new population’s gene pool, behavioral isolation, and ecological competition

Founders Arrive – few finches from South America – species M – arrived on the Galapagos Islands – may have gotten lost or blown off course – once there, they survived and reproduced Founder Effect – allele frequencies could have

differed from original South American population

Speciation in Darwin’s Finches Geographic Isolation – Island’s environment

was different from the South American environment Combination of founder effect, geographic

isolation, and natural selection enabled the island finch population to evolve into anew species – species A

Later a few from Species A crossed to another island – these birds do not usually fly over open water so they move from island to island very rarely – became geographically isolated and no longer shared a common gene pool

Changes in Gene Pools – over time, natural selection would have caused a population to evolve larger beaks (second island produced larger, thick-shelled seeds), forming a distinct population, B, characterized by a new phenotype

Speciation in Darwin’s Finches

Behavioral Isolation – if a few birds from the second island cross back to the first island – they will probably not breed with population-B birds Finches prefer to breed with birds that have the same sized beak as they do Because the populations on the two islands have evolved differently sized

beaks, they probably will not mate with each other Remain reproductively isolated – even when the individuals live in the same

place – populations have now become two distinct species Competition and Continued Evolution – as these two species live

together, they compete for seeds During dry season, most different birds have highest fitness More specialized birds have less competition for certain seeds and other

foods Over time, they may evolve in a way that increases the differences between

them Species-B birds may evolve into new Species-C

Over many generations, the process could have produced the 13 different finch species found there today.

17.4 Molecular Evolution

Timing Lineage Splits: Molecular Clocks

Molecular Clock – uses mutation rates in DNA to estimate the time that two species have been evolving independently Neutral Mutations as “Ticks”

Simple mutations occur all the time, causing slight changes in the sequence of DNA – may have positive or negative effect on organism’s phenotype

Many mutations have no effect on phenotype Tend to accumulate in the DNA of different species at about

the same rate – researchers can compare such DNA sequences in two species – comparison can reveal how many mutations have occurred independently in each group

The more differences between the DNA sequences, the more time has elapsed since the species shared a common ancestor

Timing Lineage Splits: Molecular Clocks Molecular Clock

Calibrating the Clock – there is not just one clock in a genome There are many clocks that “tick” at different rates

This is because some genes accumulate mutations faster than others

These different clocks allow researchers to time different evolutionary events – researchers choose a different molecular clock to compare great apes than to estimate when mammals and fishes share a common ancestor

Researchers check accuracy by estimating how often mutations occur – compare the number of mutations in a particular gene in species whose age has been determined by other methods

Developmental Genes and Body Plans Hox Genes and Evolution – Hox genes determine

which parts of an embryo develop arms, legs, or wings – also control the size and shape of those structures Small changes in Hox gene activity during

embryological development can produce large changes in adult animals Insects such as fruit flies and crustaceans such as brine

shrimp descended from a common ancestor that had many legs

Mutation in a single Hox gene, known as Ubx, turns off the growth of legs in the abdominal region of insects