How Populations Evolve

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 Selection Idea 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 Observations 2) 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 from His 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.

which equals evolution Eventually the population will have more individuals with these adaptations which equals a change in the population, which equals evolution

Survival of 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 reproduction Includes; 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 copies of 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), one red and one 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?

RR RR Rr RR Rr 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 Effect In 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.