BSCI160 - Ecology and Evolution

Key terms, people, and concepts

Evolution by natural selection

Natural selection: differential reproduction of individuals based on heritable variation in traits

Reproduction of individuals with favorable traits that survive environmental changes leading to evolutionary change

• inevitable outcome of trait inheritance, limited resources, competition for resources, and offspring variance

• only mechanism known for adaptive evolution

James Hutton (1726 - 1797)

• founder of geology

• concepts created

  • old earth - gathered evidence to prove that the earth is very old

  • gradualism - the idea of gradual change over long periods

Charles Lyell (1797 - 1875)

• expanded Hutton’s ideas of gradual change

• huge influence on Darwin

• formed Unitarianism

  • earth’s geological processes are slow and constant over time

    • provided time scale for species to evolve slowly

Jean Baptiste Lamarck (1744 - 1829)

• embraced alchemical view of chemistry

• became famous in his theory of change in structure according to use or dissuse

• believed organisms formed spontaneously

Darwin’s Expedition

~ 1831 - 1836

• traveled on H.M.S Beagle to South America, Australia, and South Africa

  • Wallace traveled to Brazil and Malay Archipelago (1854 - 1862)

• Galapagos Islands

  • how the study on natural selection was done

  • Darwin studied finches and how their beaks were unique to accomodate their different needs for survivial

Peter and Rosemary Grant

~ continue to study the finches and find new reserach concerning natural selection since 1976

Fitness

capacity of individuals to pass genes to reproducing offspring

• total number of offspring produced by individuals during their lifetime

• aspects

  • suvival to reproductive age

  • fecundity (# of offspring and their survival)

  • mating success

  • degree of parental care (will they survive with/without care)

Evolution

genetic change in a population

Adaption

heritable traits that increase an organism’s chance of survival and reproduction

• The result of natural selection is seen in individuals

Population

group of individuals of a single species that live and interbreed in a particular geographic area at the same time

What must be present for evolution to occur?

HERITABLE VARIATION

No variation = No evolution

variation must be present for natural selection to happen and the variation must have a genetic basis or it won’t lead to change

Carl Linnaeus (1707 - 1778)

• saw patterns in nature and created a system of classification for species

  • groups resembled each other: natural selection

  • groups resembled one another and are nested within groups: shared ancestry

William Paley (1743 - 1805)

• proposed natural theology as an argument against atheism

Lamarck’s Theory of Evolution

1. complex forces drive body plans towards higher levels, creating ladder of phyla

2. adaptive forces causes animals to adapt to circumstances, (use and disuse) creating specialization

Georges Cuvier (1769 -1832)

• founded field of paleontology (study of fossils)

• established extinction as a fact

Thomas Malthus (1766 -1834)

• used by Darwin as an economical view for natural selection

• multiplicate population growth lead to too many individuals

  • dN/dt = rN

  • struggle for survival

Darwin’s Basic Arguments for Natural Selection

1. variation - under-appreciated

2. heritable variation

3. struggle to exist

4. differential reproductive success

T.H. Huxley (1825-1895)

• good friend of Darwin

• firm advocate of Darwin’s evolution

Genetic Diversity

How much variation exists in the genetic variation

• good for prospects of long-term survival because they are subject to natural selection

• quantified by measuring the frequency of heterozygotes

• comes from mutation and sexual reproduction

Mutation

change in DNA

• ultimate source of new alleles

• affects on phenotypes = reduced fitness, increased fitness, or no fitness

• range varies —> small to great effects

Not based on environment, but based on equal likelihood (chance)

Sexual reproduction

unique allele combos produce unique genotypes and phenotypes in offspring

Favorabillity

depends on current environmental conditions (ever-changing)

• some traits are not always selected

Divergent Evolution

two species that evolve in diverse directions from a common point

• flowers share the same anatomy but look different

• result of selection in different physical environments and adaptions

Convergent Evolution

similar traits evolve independently in species that don’t share common ancestor

• bats and birds —> different ancestor but same trait of flight

Homologous Structures

structures that share basic form but have changes in shape and size

Vestigial Structures

structures from the past that have no use today

ex. the tailbone

Analogy/Homoplasty

characteristics occur because of environmental constraints and not close evolutionary relationships

• similarities can also occur because of similar selection pressure from environment

Phenotypes

observable characteristics in individuals resulting from genotype interactions with environment

Clone

derived from and genetically equivalent to another organism

Asexual reproduction

Genetic Drift

change in allele frequency across generations due to sampling error

• random, every population evolves differently

population may evolve when survival is not differentiable but heritable variation exists

Allele Frequency

• expressed as proportion or percentages

  • allele S = 0.75 or 75%

Sampling Error

The chance occurrence of a difference between the frequency of an allele in a large population vs. the frequency in a smaller sample

• smaller sample = more sampling error

Founder Effect

difference in composition between founding population (small sample) and source population (big pop.)

Random mating

occurs when an individual in a population has an equal chance of mating with any other individual in the population

Gene pool

collection of gene copies carried in gametes

Natural Selection vs. Genetic Drift

NS = alleles rise and fall because individuals survive and reproduce

GD = alleles rise and fall because individuals are lucky

Sampling Error occurs when…

• new population is established from source population

• very large population persists over time with constant births and deaths

• a population’s size is drastically reduced due to random events

Loss & Fixation

seen easiest in smaller populations

lost = frequency is 0

fixed = frequency is 1

Bottleneck effect

sudden and considerate reduction in population’s size,

result of natural disasters, disease, habitat loss, agriculture, or urbanization

Sewall Wright (1889 -1988)

create equation to predict expected loss of heterozygosity over time (result of genetic drift)

performs reasonably well despite population size

Non-random mating

individuals will mate with those nearby

the less random mating, the more quickly heterozygosity will be lost

Effective population

size of idealized population that loses heterozygosity at the same rate as the ideal population

• pure random mating, equal # of males and females, no migration, no mutation, no selection

Removing random mating will do what?

It will increase the chances that parents will be related and produce homozygous offspring

• inbreeding and genetic drift make heterozygosity decline faster

• the same inn large and small populations

Operational Sex Ratio

relative number of breeding females and breeding males in a population

• formula is usually off but accurate enough to capture that skewed sex ration reduces effective population size

The probability an allele will reach fixation…

is equal to its initial frequency

So if the allele C has a frequency of 0.3, the probability it will reach fixation is 0.3

Asexual Reproductionn

unicellular and few multicellular organisms produce genetically identical clones of themselves

Sexual Reproduction

Many single and multicellular organisms use this method through mating, which involves the production by parents of 2 haploid cells and the fusion of 2 haploid cells to form 1 genetically recombined diploid

• differ in size —> smaller = male, larger = female

• meiosis and fertilization

• introduces variation

Haploid

cells that contain 1 set of chromosomes

Ex. gametes

Diploid

cells that contain 2 sets of chromosomes

Ex. fertilized egg (zygote)

Gene pool

sum of all copies of all alleles at all loci in population

Allele

one of several alternative forms of a gene, located at same locus (position) on chromosomes

• different expressions of specific trait

Genotype frequency

proportion of each genotype among individuals in population

Different Reproductive Strategies

• large # of gametes

• retention/care of eggs

• bi-parental care (associated with monogamy)

• maternal care (associated with polygamy)

Stabilizing Selection

type of natural selection favors intermediate variants of trait, reduces variation and mainntainns status quo

• Ex. human birth weight

• acts against the extreme phenotypes

• more homogenous population

• preserves advantageous traits for survival and reproduction

• AKA purifying selection

Directional Selection

Individuals in the population have varying rates of survival and reproduction; individuals at 1 extreme contribute more offspring

• may favor a particular variant (positive selection)

  • depends on strength and variation of phenotype (genetic v. environment)

• evolutionary trend moves towards one extreme

Disruptive Selection

Individuals at opposite extremes of a character distribution contribute more offspring

• increased variation

• bimodal distribution

Heritability (h²)

proportion of phenotypic (observed)variance of a trait in a population due to additive genetic variance

• quantified by calculating (S) - selection differential

  • R = h² S

    • R = expected genetic gainn

Meiosis

nuclear division that forms haploids cells from diploid cells

• employs same cellular mechanisms as mitosis

Mitosis

nuclear division that produces daughter cells whose nuclei are genetically identical to original parent nucleus

Full process Meiosis I and II

LOOK AT PICTURE

Meiosis v. Mitosis

LOOK AT PICTURE

Fertilization

joining of two haploid gametes

• restores the diploid condition

• nearly all animals employ diploid-dominant life-cycle

Germ cells

specialized cell line that produces gamete, such eggs and sperm; produced within gonads

• capable of mitosis and meiosis

• once haploid gametes are formed, they can’t divide again

Gamete specific meiosis

LOOK AT PICTURE

Hardy Weinburg Equation

p + q = 1

p² + 2pq + q² = 1

p = frequency of dominant allele (allele #1)

q = frequency of recessive allele (allele #2)

Balancing selection

Frequency dependent and Heterozygous advantage

Frequency dependent: result of interactions between species or between genotypes (predatory, prey, sexual reproduction)

Heterozygous advantage: when an individual with different alleles have a better advantage in environments that don’t fully favor homozygous alleles (ex. sickle cell carriers (HBA/Hbs)

Cline

measurable gradient in single characteristics of species across geographical range —> smooth and continuous or abrupt changes

Polymorphism

Existence of two or more distinct forms of a traits within the population of a species —> can be morphological, behavioral, or physiological

Gene Flow

migration of individuals or movement of gametes between populations

ex. Pollen

Gregor Mendel (1822-1884)

Father of genetics, studied inheritance patterns in honeybees and plans

• demonstrated that traits are transmitted from parents to offspeing independently of other traits

  • dominant and recessive patterns

• revealed particulate, not blended inheritance and revealed dominance

  • 50% were true-breeding and 50% were hybrid

Blending Theory of Inheritance

original parental traits were lost or absorbed by blending in the offspring (only appeared correct because of continuous variation)

Continuous Variation

Character shows range of trait values with small gradations rather than large gaps between them

Discontinuous Variation

traits are distinct and are transmitted independently of one another

Hybridization

mating two true-breeding individuals that have different traits to produce an offspring with both genotypes

Trait

variation in physical apperance of heritable characteristics

Reciprocal Cross

a paired cross in which the respective traits of the male and female in one cross become the respective traits of the female and male in the other cross

Dominant v. Recessive traits

Dominant: traits inherited that are unchanged in hybridization

Recessive: traits inherited that disappear or become latent in offspring of hybridization

• reappears in progeny of hybrid offspring

Theoretical probabilities

come from knowing how events are produced and assuming that probabilities of individual outcomes are equal

Empirical probabilities

p = # of times event occurs / total # of oppurtunities event can occur

Product Rule

probability of two independent events occurring simultaneously can be calculated by multiplying individual properties of each event occurring alone

P_a x P_b

Sum Rule

probability of occurrence of at least one of two mutually exclusive events is the sum of their individual probabilities

P_a + P_b

Law of Dominance

in heterozygotes, one trait will conceal the prescence of anothor trait for the same characteristic

• dominant allele will be expressed exclusively and recessive allele will remain latent but transmitted to offspring

Law of Segregation

paired unit factors (genes) must segregate equally into gametes such that offspring have equal likelihood of inheriting either factor

• reason we apply the punnet square to accurately predict offspring of parents with known genotypes

Law of Independent Assortment

genes do not influence each other with regard to the sorting of alleles into gametes, and every possible combination of alleles for every gene is equally likely to occur

Dihybrid Cross

result of cross between two true-breeding parents that express different traits for two characrteristics

ex. YR x yr —→ YyRr

Test Cross

cross individual with unknown genotype with a homozygous recessive

Life History

Organism’s sequence, timing, and nature of events

• determined by its set of life-history traits

Population’s life history accounts for life histories of individual members and is represented as average age-specific rates (fecundity or survivorship)

Senescence

Natural death of an organism as a result of its deterioration with age

Evolutionary Trade-off

Investment in one life history trait increases, investment in another trait decreases