Evolution - A change in a population over

time.

Chapters 22-24

Pre-Darwin Theories:

1. “Idealism” (Essentialism) -States that there are 2 worlds: an ideal, real world and an illusionary world the humans perceive w/ senses. – Variations in plants & animals were imperfect

representations of ideal forms. – Evolution is counterproductive since ideal

organisms are already perfectly adapted to environments.

(Plato - who disagreed w/ gradual evolution)

Pre-Darwin Theories (Continued):

2. “Scala Natura” - Stated that organisms vary from simple to complex and that species are fixed and do not evolve. Prevailed for over 2000 years.

(Aristotle - Questioned Plato but excluded evolution)

Pre-Darwin Theories (Continued):

3. “Creationist - essentialist” - Based on the Old Testament account that species were individually created and fixed. Embedded in Western / Judeo-Christian culture. Fortified prejudice against evolution.– “Natural Theology” - the philosophy that the

Creator’s plan could be revealed by studying nature.

– Adaptations were thought of as being created for specific purposes.

Pre-Darwin Theories (Continued):

4. “Taxonomy” - Grouped organisms by categories and ranked categories by similarity. This lead to “binomial nomenclature” still used today.– Clustering species into groups did not imply

evolutionary relationships since he believed species were permanent fixtures.

– Was done only to reveal “God’s” plan.(Linneaus, 1707 - 1778, “Natural Theologian” - sought to order the diversity of life “for the greator glory of the creator”)

Charles Darwin

Published the 1st convincing case for evolution in a book “On the Origin of Species by Means of Natural Selection” in 1859 based on his voyage on the HMS Beagle.

Darwin (Continued):

The book:• Synthesized seemingly unrelated facts to

create a conceptual framework that accounts for both the unity & diversity of life.

• Discussed important biological issues about organisms, such as why there are so many different kinds of organisms, their organs and relationships, similarities & differences, geographic distribution, & adaptations to their environment.

Darwin (Continued):

The book made 2 major points:

– Species evolved from ancestral species & were not specially created.

– “Natural selection” is a mechanism that could result in this evolutionary change.

Natural Selection - "The survival of the fittest" the first proposed explanation of variation of life on Earth. It is now considered the mechanism of evolution. It states that survival is based on fitness of the organism.

– Fit is now defined as best able to pass on genetic material to offspring.

(Charles Darwin, 19th century)

Types of Natural Selection: 1. Directional - One phenotype is favored in the

environment. (ex. "dark" peppered moths have higher survival rates in soot covered forests)

2. Stabilizing - Organisms with extreme phenotypes are eliminated.

(ex. birth weight in humans, low survival rate for extremes)

3. Disruptive - Organisms with common traits are eliminated, extremes are favored.

(ex. small female and large male elephant seals)

4.Artificial - A breeder chooses which traits to favor. (ex. seedless grapes)

Types of Evolution:

1. Divergent - Closely related species have different

behaviors and traits. (ex. Color variations in peppered

moths or beak types in finches)

2. Convergent - Unrelated species come to have similar

traits. (ex. aardvarks, anteaters, and pangolins all have sharp teeth and claws, long snouts, and sticky tongues but evolved from 3 different mammals)

Fossil - Based Theories:

1. “Catastrophism” - The theory that major changes in the Earth’s crust are the result of catastrophic events rather than gradual change. This was the first to mention extinction.– Differences between fossil strata correspond to

catastrophic events– New species in younger rocks explained by periodic

local catastrophies. Foreign species would replace local extinct species.

(Couvier, 1769 - 1832, Who developed “palentology” - the study of fossils).

Fossil - Based Theories (Continued):

2. “Gradualism” - Theorized profound change is the cumulative product of slow, continuous processes. This competed with catastrophism. (James Hutton)

Fossil - Based Theories (Continued):

3. “Uniformitarianism” - Theorizes that geological processes are so uniform that their effects balance out over time. This was an expansion of gradualism that was rejected by Darwin but influential to him.

Ex. Mountains & erosionObservations:– Slow processes can cause substantial change.– Earth is ancient if geology is a slow process.

(Charles Lyell)

Fossil - Based Theories (Continued):

4. "The Law of Use & Disuse" - states that acquired traits are inheritable. Now proven incorrect - changes in somatic cells are not passed on through germ cells. Darwin also dismissed this theory.

(Jean-Baptiste de Lamarck, 18th century).

Modern Theories:

1. Punctuated Equilibrium - Long periods of stasis punctuated by episodes of relatively rapid speciation and change.

(In geologic time, a few thousand years for a species to evolve is small compared to the few millions of years a successful species may exist)

*Long periods of stasis may be the result of stabilizing selection in an unchanging environment.

Modern Theories (Cont’d):

2. Biological Species Concept - Defined species as a population or group of populations whose members have the potential to interbreed with one another in nature and produce viable, fertile offspring.(Earnst Mayr, 1942)

There’s a catch to the biological species concept!

It does not work in all situations.

It cannot be applied to:• Completely asexual organisms, ex. clones.• Extinct organisms represented only via fossils.

There are other possibilities:Morphological species concept

Recognition species concept

Cohesion species concept

Ecological species concept

Evolutionary species concept

Speciation - The origin of new species

Patterns of Speciation:

1. Anagenesis - (phyletic evolution) Involves the transformation of an entire species into a new species.

2. Cladogenesis - (branching evolution) New species arise from a parent species that continue to exist.

Modes of Speciation:

Allopatric - population becomes separated from rest of species by a geographic barrier.Leads to:

• Colonization of new areas• Founder effects - the loss of genetic variation that occurs when a

new population is established by a very small number of individuals from a larger population

• Genetic drift - the change in the relative frequency with which an allele occurs in a population that results from the fact that alleles in offspring are a random sample of those in the parents, and because of the role of chance in determining whether a given individual survives and reproduces. Genetic drift may reduce genetic variability.

• Differing selection pressures - due to different habitats.• Adaptive radiation on island chains - the evolution of many

adapted species from a common ancestor.

Modes of Speciation (Cont’d):

Sympatric - Formation of a new species without a geographic barrier.

• Reproductive isolation evolves w/o geographic isolation.

• Can occur quickly if genetic change results in reproductive barrier between mutants and parent population.

Varieties:

1. Autopolyploid - An organism w/ two or more complete sets of chromosome within each cell, all derived for a single species.Ex. 4n - Potatoes, 3n - Apples & Bananas

• Allopolyploid - n organism w/ two or more complete sets of chromosomes within each cell, derived from different species. (More common than autopolyploidy)Ex. 6n - Triticale, from 4n from wheat and 2n from rye

Barriers isolate the gene pools of biological species

Types of Barriers:

Prezygotic (Before Fertilization):1. Habitat Isolation - 2 species in different habitats.2. Behavioral Isolation - Signals/behaviors attract mates.3. Temporal Isolation - Species breed at different times,

day/season/years.4. Mechanical Isolation - Anatomical incompatibility.5. Gametic Isolation - Gametes of different species fail to

attract one another.

Types of Barriers (Cont’d)

Postzygotic (After Fertilization):1. Reduced Hybrid viability - hybrids lack vigor and

rarely, if ever, reach sexual maturity.2. Reduced Hybrid Fertility - hybrids are usually

sterile.3. Hybrid Breakdown - hybrids can reproduce but

their offspring have either reduced fertility or reduced viability.

Evolution is not goal oriented,

but trends do occur…

•It is possible to isolate a single evolutionary progression. Ex. Horse evolution

•Branching evolution can produce a trend even if some new species counter the trend.

•Species are analogous to individuals (Steven Stanley of John’s Hopkins)

•The presence of an evolutionary trend does not mean there is an intrinsic drive toward a preordained state.

Most evolutionary novelties are modified versions of older structures

Exaptation - a structure that evolves in one context and becomes co-opted for another function– Cannot be proven, but explains how novel designs

can arise gradually. – Higher taxa such as families and classes are defined

by evolutionary novelties (Ex. Birds are adapted to flight but ancestors are earthbound)

– Natural selection can not anticipate the future, but it can improve an existing structure.

– Reflects the Darwinian tradition of large changes as a result of natural selection through many small changes.

Genes that control development play a major role in evolutionary novelties.

Terms to Recognize:• Allometric growth - Differences in relative rates of growth

of various parts of the body. Ex. Human head size compared to body size, large in babies/small in adults

• Heterochrony - Evolutionary changes in the timing or rate of development. Ex. Head/brain development in humans Vs. chimps, same rate but chimp growth stops after birth,human growth continues.

• Paedomorphosis - Retention of ancestral juvenile structures in a sexually mature adult organism. Ex. 1. Salamanders retain gills in adulthood, unlike most amphibians. 2. Docility is a juvenile behavior retained in domesticated pets.

• Homeosis - Alteration in the placement of different body parts. Ex. Fruit fly legs develop in place of antennae.

Population GeneticsHardy - Weinberg Law - The relative genotypes of

a population remain constant over time if the following 5 conditions are met:

1. Large population

2. No mutations

3. No immigration or emigration

4. Random mating

5. No natural selection

Hardy-Weinberg Equations:The sum of the frequencies of each allele,

dominant (p) and recessive (q), in a population add up to 1.

p + q = 1The frequency of the genotypes in a population

may be determined using the following equation:

p2 + 2pq + q2 = 1

homozygous dominants (p), heterozygotes (pq), homozygous recessives (q)