How PopulationsEvolve
CHARACTERISTICS OF LIFE
GROUPS OF ORGANISMS (POPULATIONS)
SHOW VARIATIONS & EVOLVE, OR CHANGE,
TO ADAPT TO THEIR ENVIRONMENT
Idea of populations changing or evolving was first proposed by Charles Darwin in 1859 in his book On the Origin of Species by Means of Natural Selection and was counter to the generally accepted idea of Fixed Species.
Idea of Fixed Species = takes a literal interpretation of the book of Genesis which states that species were created and never changed.
For centuries, since at least the time of Aristotle (4th Century BC), people (including scientists) believed that simple living organisms could come into being by spontaneous generation. This was the idea that non-living objects can give rise to living organisms.
Observation: Every year in the spring, the Nile River flooded, leaving behind nutrient-rich mud that enabled the people to grow that year’s crop of food. However, along with the muddy soil, large numbers of frogs appeared that weren’t around in drier times.
Conclusion: It was perfectly obvious to people back then that muddy soil gave rise to the frogs.
Interesting recipe for bees: Kill a young bull, and bury it in an upright position so that its horns protrude from the ground. After a month, a swarm of bees will fly out of the corpse.
Darwin made two new, main points in The Origin of Species.
1) Modern species came from a succession of ancestors through a process of “descent with modification” or evolution.
2) Evolution occurs through a process called natural selection.
Natural SelectionIdea that nature, or the environment, will select the changes
that occur in a population over many generations by favoring those heritable traits that are best suited to that environment.
The result of natural selection is evolutionary adaptation, or an increase in the frequency of traits best suited for the environment.
Pattern?
Darwin’s Key Observations
1) Overproduction: More organisms are born than will survive to reproduce.
This creates a COMPETITION because individuals must compete for limited resources.
Any competition has winners and losers.
Darwin’s Key Observations2) Individual Variation: Members of a
population show variations, which can be inherited from the parents.
Some individuals have advantageous traits or adaptations to help them compete.
Adaptation = a genetically determined characteristic that increases an organism’s ability to survive and reproduce in its environment.
These adaptations can be:
Behavioral = actions, activities
Morphological = shape, form, structure
Physiological = function, or internal activities
Adaptation?
Darwin’s Inference fromHis Observations
Differential Reproductive Success
or Natural Selection
The survivors, with the adaptations, will live to reproduce & pass on these adaptations (DNA) to their offspring, more often than those without the adaptations.
Eventually the population will have more individuals with these adaptations which
equals a change in the population,
which equals evolution
Survivalof the Fittest
Survival of the Fittest = REPRODUCTIVE FITNESS
Sexual reproduction = produce sex cells or gametes, requires two parents
Eggs and Sperm are gametes
Each gamete has one half set of DNA or chromosomes
Fertilization unites the two gametes and creates a zygote with a full set of chromosomes
Asexual Reproduction = no sex cells or gametes produced, only requires one parent
Bacteria = binary fission
Yeast = budding
Plants = vegetative reproductionIncludes; rhizomes (underground stems)
Runners or stolons (e.g. strawberries, grasses)
Suckering (shoots that arise from existing root system)
Tubers (e.g. potatoes)
Bulbs (e.g. onions, tulips)
Many examples of asexual reproduction, that form clones.
A clone is a population created asexually from one parent or founder, where all individuals are genetically identical.
Populations show variations
Source of variations for;
Asexual Reproduction= mutations, which are changes in DNA
not done on purpose
Sexual Reproduction= mutations & new combinations from
fertilization
The Modern Synthesis:
Darwinism Meets Genetics
The modern synthesis is the combining of
Genetics with Evolutionary Biology
Genetics is the study of DNA and how it produces traits in organisms
Gene
= A segment of DNA
= A sequence of nucleotides
= A portion of the DNA that is the
instructions to build a specific protein
= A portion of the DNA that produces a
specific trait
Genome
Genome = a complete set of an organism’s genes; an organism’s genetic material
Genotype
Genotype = The genetic makeup of an organism; the type of genes an organism has in its cells
Alleles
Allele = a possible form of a gene
Dominant Allele = always expressed symbol is a capital letter, like “R”
Recessive allele = expression masked by dominant allele
symbol is a lower case letter, like “r”
Cells get TWO copiesof each different gene
What are the possible genotypes for a gene
with only one recessive & one dominant allele?
RR = Homozygous dominant
rr = Homozygous recessive
Rr = Heterozygous
Gene Pool
Gene Pool = all alleles ( possible forms of genes) in all the individuals making up a population
The Gene Pool is the reservoir from which the next generation draws its genes
Gene Pool
If only one allele exists for a loci in a population, that allele is said to be fixed in the gene pool, so all individuals would be homozygous.
Each allele has a frequency in a population.
Why is the following statement incorrect?
“Pesticides have created
pesticide resistance in insects”
Analyzing Gene Pools
Imagine a population of wildflowers with two varieties (called morphs), oneone redred and one and one white. white.
Allele for red flowers = R
Allele for white flowers = r
If we know that 70% or 0.70 of all genes in the population are “R” then what percent of all the genes in the population are “r”?
If we let p equal the frequency of the dominant allele (R) in a population of wildflowers and q the frequency of the recessive allele (r), then what would p plus q equal?
p + q = 1
So, if you know the frequency of one allele you can subtract it from 1 to get the
frequency of the other allele.
Remember that each body cell has two copies of each gene (RR, Rr, rr)
Each sex cell or gamete only has ½ the DNA of each parent so it only has one copy of each gene (R or r)
What copy of the gene for color would a flower with a homozygous dominant genotype donate to its egg?
What would a heterozygous flower donate?
Suppose the following five genotypes
represent an entire population of flowers
RR RR Rr RR Rr
What is p?
What is q?
What is the chance the pollen will have an R ?
What chance for an egg?
What is the chance the egg will have an r ?
Hardy-Weinberg formula
p2 + 2pq + q2 = 1
p2 = frequency of homozygous dominant
2pq = frequency of heterozygous
q2 = frequency of homozygous recessive
Phenylketonuria (PKU)
= inherited inability to
break down the amino
acid phenylalanine
Phenylketonuria (PKU)
Untreated it causes mental retardation
Occurs in about one out of 10,000 babies
born in the U.S.
Newborns routinely tested for PKU and
treated with strict diet
PKU is caused by a recessive allele
Phenylketonuria (PKU)
Genotype of RR = Phenotype?
Phenotype = Normal
Genotype of Rr = Phenotype?
Normal, but “carrier” of allele for PKU
Genotype of rr = Phenotype?
Phenotype = Phenylketonuria
Phenylketonuria (PKU)
What percentage of the U.S. population
who do not have PKU are carriers (Rr) ?
Phenylketonuria (PKU)
So if one out of 10,000 babies are born with PKU, what is the frequency of rr ?
frequency of rr = q2
q2 = 0.0001
What is q ?
q = square root of 0.0001 or 0.01
Phenylketonuria (PKU)
What is p ?
(or the frequency of the dominant allele?)
p = 1 minus q
p = 0.99
What percentage of the U.S. population
who do not have PKU are carriers (Rr) ?
= 2pq
= 2 X 0.99 X 0.01
= 0.0198 or 1.98% or about 2%
Main causes of evolution (mechanisms that change a gene pool)
1) Natural Selection
2) Genetic Drift
• Bottleneck Effect
• Founder Effect
3) Gene Flow
Genetic Drift = a change in the gene pool of a population
due to chance, usually occurring in a small population
A common result of genetic drift is the loss of all but one possible allele in a population, called genetic fixation
Genetic Drift
There are two main ways a population would
shrink down to a small size
1) Bottleneck Effect
2) Founder Effect
Bottleneck Effect
Disasters such as earthquakes, floods or fires may kill large numbers of organisms, producing a small surviving population.
Bottleneck Effect
Number of cheetahs fell drastically during last ice age
(about 10,000 years ago) and a second time during the
1800s, when framers hunted the cheetah to near extinction.
Founder Effect
Founder effect is the establishment of a small, new
population whose gene pool differs from that of the parent
population
For example a few individuals may colonize an isolated
island, lake, or other new habitat
Founder EffectIn 1814, a group of 15 people founded a British colony on Tristan da Cunha, a cluster of small islands in the middle of the Atlantic Ocean.
One colonist carried a recessive allele for retinitis pigmentosa, a form of blindness.
Recently the frequency of this allele was ten times higher on Tristan da Cunha than in the British population from which the founders came.
Gene Flow
Gene flow is when a population gains or loses alleles
when fertile individuals move into or out of the population
or when gametes (such as plant pollen) are transferred
between populations
Gene Flow
Gene flow tends to reduce
differences between populations.