1. Chapter 17 Evolution of Populations

2. 17.1 Genes and Variation

3. 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

4. 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.

5. Sources of Genetic Variation
Three sources of genetic variation are mutation, genetic recombination during sexual reproduction, and lateral gene transfer
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

7. Sources of Genetic Variation
Genetic Recombination in Sexual Reproduction
In humans, who have 23 pairs of chromosomes, this process can produce 8.4 million gene combinations
Crossing over is another way genes are recombined
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

8. Sources of Genetic Variation
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

9. 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

10. 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

11. 17.2 Evolution as Genetic Change in Populations

12. 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

13. 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
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

14. How Natural Selection Works
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.

15. 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

16. 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

17. 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

18. 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

19. 17.3 The Process of Speciation

20. 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

21. Isolating Mechanisms Reproductive Isolation
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
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

22. Isolating Mechanisms Reproductive Isolation
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

23. 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

24. 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

25. 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.

26. 17.4 Molecular Evolution

27. 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

28. Timing Lineage Splits: Molecular Clocks
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

29. 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