General Biology (Bio107)

Chapter 13 – Populations evolve

• Cuvier documented the succession of fossil species in the Paris Basin; recognizes that extinction had been a common occurrence in the history of life.

• Cuvier advocates catastrophism instead of evolution; boundaries between strata were due to local flood or drought that destroyed the species then present. Vacated areas are repopulated by species immigrating from other unaffected areas.

• Geologist J. Hutton proposes the theory of gradualism, i.e. that the diversity of land forms (e.g., canyons) could be explained by mechanisms currently operating.

Theories of geological gradualism helped clear the path for evolutionary biologists, including Charles Darwin

Lamarck, Fossils & Early evolutionary ideas

• 1809: Jean Baptiste Lamarck publishes an early theory of evolution based on his observations of fossil invertebrates in the Natural History Museum of Paris.

• Lamarck thought that he saw what appeared to be several lines of descent in the collected fossils and current species.

• Each was a chronological series of older to younger fossils leading to a modern species.

• Central to Lamarck’s mechanism of evolution were the concepts of use and disuse of parts and of inheritance of acquired characteristics.- There is currently no hard scientific evidence that acquired characteristics can be inherited, but the new field of “epigenetics” and its discoveries of gene regulation by histone modification points in that direction

• Charles Darwin (1809-1882) was born in western England. Darwin had a consuming interest in nature as a boy, but his father sent him to the University of Edinburgh to study medicine.

• Darwin left Edinburgh without a degree and enrolled at Christ College at Cambridge University with the intent of becoming a clergyman.

• At that time, most naturalists and scientists belonged to the clergy and viewed the world in the context of natural theology.

• The main mission of the five-year voyage of the Beagle was to chart poorly known stretches of the South American coastline, including the Galapagos islands.

Field research helped Darwin frame his view of life

• Darwin made several important observations:

1. Plants and animals of South America were very distinct from those of Europe.

• Darwin noted that while most of the animal species on the Galapagos lived nowhere else, they resembled species living on the South American mainland.

• It seemed that the islands had been colonized by plants and animals from the mainland that had then diversified on the different islands.

2. Most of the animal species on the Galapagos lived nowhere else; they resembled species living on the South American mainland.

• It seemed that the islands had been colonized by plants and animals from the mainland and that they diversified on the different islands.

• Organisms from temperate regions of South America were more similar to those from the tropics of South America than to those from temperate regions of Europe.

• While on the HMS Beagle, Darwin read Lyell’s Principles of Geology.

• Lyell’s ideas and his observations on the voyage lead Darwin to doubt the church’s position that the Earth was static and only a few thousand years old.

• Instead, he was coming to the conclusion that the Earth was very old and constantly changing.

• Darwin’s views were influenced by the fossils he found and by Cuvier’s earlier ideas, including catastrophism.

• Cuvier recognized that extinction had been a common occurrence in the history of life.

• Fossils are relics or impressions of organisms from the past, mineralized in sedimentary rocks. Sedimentary rocks form when mud and sand settle to the bottom of seas, lakes, and marshes.

• New layers of sediment cover older ones, creating layers of rock called strata.

• Fossils within layers show that a succession of organisms have populated Earth throughout time.

• Paleontology, the study of fossils, was largely developed by Georges Cuvier, a French anatomist. In particular, Cuvier documented the succession of fossil species in the Paris Basin.

• After his return to Great Britain in 1836, Darwin began to perceive that the origin of new species and adaptation of species to the environment as closely related processes.

• For example, among the 13 types of finches that Darwin collected in the Galapagos, clear differences in the beak are adaptations to the foods available on their home islands.

• By early 1840’s Darwin had developed the major features of his theory of natural selection as the mechanism for evolution.

• 1844: writes long essay on the origin of species and natural selection, but does not publish.

• 1858: Alfred Wallace, a young naturalist working in the East Indies, sends Darwin a manuscript containing a theory of natural selection identical to his own.

• 1858: Wallace’s paper and extracts of Darwin’s essay are presented to the Linnaean Society of London.

• 1859: Ch. Darwin publishes his book On the Origin of Species by Means of Natural Selection which becomes bestseller.

• The Origin of Species challenged a worldview that had been accepted for centuries.

• The key classical Greek philosophers who influenced Western culture, Plato and Aristotle, opposed any concept of evolution.Aristotle believed that all living forms could be arranged on a ladder (scala naturae) of increasing complexity with every rung taken with perfect, permanent species.

• The Judae-Christian religious account of creation fortified the idea that species were individually designed by a divine creator and did not evolve.

• In the 1700’s, the dominant philosophy, natural theology, was dedicated toward studying the adaptations of organisms as evidence that the Creator had designed each species for a purpose.

Darwin’s The Origin of Species developed two main points:

1. The occurrence of evolutionary change in life forms on earth (“Descent with modification’)

2. Natural selection as its mechanism

• Darwin introduced the concept of natural selection as the cause of adaptive evolution.

• Central to Darwin’s view of the evolution of life is descent with modification.

• All present day organisms are related through descent from unknown ancestors in the past.• Descendants of these ancestors accumulated diverse modifications or adaptations that fit them to

specific ways of life and habitats.

• History of life is like a tree with multiple branches from a common trunk.

• Closely related species, the twigs of the tree, shared the same line of descent until their recent divergence from a common ancestor.

•Evolutionary tree of the elephant family based on evidence from fossils.

Last common ancestor

SARCOPTERYGIASARCOPTERYGIA(= lobe-finned fishes)(= lobe-finned fishes)

TETRAPODATETRAPODA

AMNIOTAAMNIOTA

Lungfish

Lungfish

Amphibians

Amphibians

Mammals

Mammals

Reptiles

Reptiles

& Bird

s

& Bird

s

SYNAPSIDSSYNAPSIDS SAUROPSIDSSAUROPSIDS

Dinosaurs

DinosaursToday

• Darwin’s theory of natural selection.

• Ernst Mayr, an evolutionary biologist, has dissected the logic of Darwin’s theory into three inferences based on five observations which are:

1. Tremendous fecundity2. Stable populations sizes3. Limited environmental resources4. Variation among individuals of a population5. Heritability of some of this variation.

• All species have such great potential fertility that their population size would increase exponentially if all individuals that are born reproduced equally successful.

• Production of more individuals than the environment can support leads to a struggle for existence among the individuals of a population, with only a fraction of the offspring survivingeach generation.

• Strongly influenced by a 1798 essay on human population by Thomas Malthus

• Capacity to overproduce seems to be a characteristic of all species; only a small fraction of eggs developing to leave offspring of their own.

•

1. Fecundity

2. Stable Population size

3. Limited Environmental/Nutrition Resources

• Individuals of a population of the same species vary extensively in their characteristics; no two individuals are exactly alike.

4. Variation

• Much of the variation observed in a species is heritable (genetic).- some traits are modified epigenetically

• Inference: Survival in the struggle for existence is not random, but depends in part on the hereditary constitution of the individuals.

• Those individuals whose inherited characteristics best fit them to their environment are likely to leave more offspring than less fit individuals.

• Inference: This unequal ability of individuals to survive and reproduce will lead to a gradual change in a population, with favorable characteristics accumulating over the generations ( adaptation).

• Darwin’s main ideas can be summarized in three points.

• Natural selection is differential success in reproduction (unequal ability of individuals to survive and reproduce).

• Natural selection occurs through an interaction between the environment and the genetic and morphological variability (variation) inherent among the individual organisms making up a population.

• The product of natural selection is the adaptation of populations of organisms to their environment.

• E.g. Related species of insects called mantids have diverse shapes and colors that evolved in different environments.

• In each generation, environmental factors filter heritable variations, favoring some over others.

• This increasing frequency of the favored traits in a population is evolution.

• Darwin’s views on the role of environmental factors in the selection of heritable variation was heavily influenced by artificial selection.

• Humans have modified a variety of domesticated plants and animals over many generations by selecting individuals with the desired traits as breeding stock.

• Examples:1. Dog varieties from wolf or coyote2. Corn varieties from teosinthe grass3. Cabbage varieties from wild mustard (Brassica)

Evolution of modern corn (Zea maize) by artificial selection & bottleneck effect

Teosinte(Zea mays

ssp. parviglumis)

6,300 yrs. ago

2,300 – 4,400yrs. ago today

Modern maize/Corn

(Zea maysssp. mays)

Primitive maize(Zea mays ssp.)

Morphological differences due to mutations in only five genomic regions

tga1 glume hardness

tb1 branched

Gene locus

su1 starch-debr. enzyme

pbf protein storage

Graphic©E.Schmid/2003

• If artificial selection can achieve such major changes in a relatively short time, then natural selection should be capable of major modifications of species over hundreds or thousands of generations.

• Darwin envisioned the diversity of life as evolving by a gradual accumulation of minute changes through the actions of natural selection operating over vast spans of time.

• While natural selection involves interactions between individual organisms and their environment, it is not individuals, but populations that evolve.

• Populations are defined as a group of successfully interbreeding individuals of a single species that share a common geographic area. no “reproductive barrier”

• Evolution is measured as the change in relative proportions of heritable variation in a population over a

succession of generations.

• Natural selection only amplifies or diminishes heritable variations, not variations that an individual acquires during its life.

• Natural selection is situational.

• Environmental factors vary in space and time.

• Adaptations for one set of environmental conditions may be useless or even detrimental under other circumstances.

Populations are the units of evolution

• One obstacle to understanding evolution is the common misconception that organisms evolve, in a Darwinian sense, in their lifetimes.– However, the evolutionary impact of natural

selection is only apparent in tracking how a population of organisms changes over time.

• It is the population, not its individual, that evolve.

• Evolution on the scale of populations, called microevolution, is defined as a change in the allele frequencies in a population.

• The Origin of the Species convinced most biologists that species are the products of evolution, but acceptance of natural selection as the main mechanism of natural selection was more difficult.

• Darwin’s explanation lacked an under-standing of inheritance that could explain how chance variations arise in a population while also accounting for the precise transmission of these variations from parents to offspring.

• This gap was filled in by the Synthetic Evolutionary Theory which as developed in the 1930s

• When Mendel’s research was rediscovered in the early twentieth century, many geneticists believed that the laws of inheritance conflicted with Darwin’s theory of natural selection.

– Darwin emphasized quantitative characters, those that vary along a continuum.

– Mendel and later geneticists investigated discrete “either-or” traits.

The modern evolutionary synthesis integrated Darwinian selection and Mendelian

inheritance

• A turning point for evolutionary theory was the birth of population genetics, which emphasizes the extensive genetic variation within populations and recognizes the importance of quantitative characters.

• In the early 1940’s, the modern synthesis of the evolutionary theory took form.– It integrated discoveries and ideas from paleontology, taxonomy,

biogeography, and population genetics.

• Its architects included geneticists Theodosius Dobzhansky and Sewall Wright, biogeographer and taxonomist Ernst Mayr, paleontologist George Gaylord Simpson, and botanist G. Ledyard Stebbins.

• A population is a localized group of individuals that belong to the same species.– Species = members of populations who are able to

successfully interbreed and produce fertile offspring in a nature.

• Populations of a species may be isolated from each other (= rare exchange of genetic material) or they may intergrade with low densities in an intermediate region.

• Members of a population are far more likely to breed with members of the same population than with members of other populations.

Gene pool

• Gene pool = the total aggregate of genes in a population at any one time.– It consists of all alleles at all gene loci in all

individuals of a population.– Each locus is represented twice in the genome

of a diploid individual.• Individuals can be homozygous or

heterozygous for these homologous loci.– Often, there are two or more alleles for a gene,

each contributing a relative frequency in the gene pool.

Gene pool & Hardy-Weinberg equation

• Hardy-Weinberg theorem describes the gene pool of a nonevolving population.

• E.g., a wildflower population with two flower colors. The allele for red flower color (R) is completely dominant to the allele for white flowers (r).

• An imaginary population of 500 plants, 20 have white flowers (homozygous recessive - rr). The other 480 plants have red flowers.

• Some are heterozygotes (Rr), others are homozygous (RR).

– Suppose that 320 are RR and 160 are Rr.

• Because these plants are diploid, in our population of 500 plants there are 1,000 copies of the gene for flower color.– The dominant allele (R) accounts for 800

copies (320 x 2 for RR + 160 x 1 for Rr).– The frequency of the R allele in the gene pool

of this population is 800/1000 = 0.8, or 80%.– The r allele must have a frequency of 1 - 0.8 =

0.2, or 20%.

• How will meiosis and sexual reproduction affect the frequencies of the two alleles R and r in the next generation of the imaginary wildflower population of 500 plants ? (80% (0.8) of the flower color alleles are R and 20% (0.2) are r).

• Assumptions:1. Fertilization is completely random2. All male-female mating combinations are equally likely.

• Gamete drawn from the gene pool at random has a 0.8 chance of bearing an R allele and a 0.2 chance of bearing an r allele.

• The processes of meiosis and random fertilization have maintained the same allele and genotype frequencies that existed in the previous generation.

• For the flower-color locus, the population’s genetic structure is in a state of equilibrium, Hardy-Weinberg equilibrium.– Theoretically, the allele frequencies should remain at 0.8

for R and 0.2 for r forever.

• The Hardy-Weinberg theorem states that the processes involved in a Mendelian system have no tendency to alter allele frequencies from one generation to another.– The repeated shuffling of a population’s gene pool over

generations cannot increase the frequency of one allele over another.

– Only true mutations do change allele frequencies

• In the wildflower example, p is frequency of red alleles (R) and q of white alleles (r).– The probability of generating an RR offspring is

p2 (rule of multiplication).• In our example, p = 0.8 and p2 = 0.64.

– The probability of generating an rr offspring is q2.• In our example, q = 0.2 and q2 = 0.04.

– The probability of generating Rr offspring is 2pq.• In our example, 2 x 0.8 x 0.2 = 0.32.

• The genotype frequencies should add to 1:

p2 + 2pq + q2 = 1

– For the wildflowers, 0.64 + 0.32 + 0.04 = 1.

• This general formula is the Hardy-Weinberg equation.

• Using this formula, one can calculate:1. Frequencies of alleles in a gene pool if one knows the frequency of genotypes or2. Frequency of genotypes if one knows the frequencies of alleles.

• Populations at Hardy-Weinberg equilibrium must satisfy five conditions.

(1) Very large population size. In small populations, chance fluctuations in the gene pool (= genetic drift), can cause genotype frequencies to change over time.

(2) No migrations. Gene flow, the transfer of alleles due to the movement of individuals or gametes into or out of our target population can change the proportions of alleles.

(3) No net mutations. If one allele can mutate into another, the gene pool will be altered.

(4) Random mating. If individuals pick mates with certain genotypes, then the mixing of gametes will not be random and the Hardy-Weinberg equilibrium does not occur.

(5) No natural selection. If there is differential survival or mating success among genotypes, then the frequencies of alleles in the next variation will deviate from the frequencies predicted by the Hardy-Weinberg equation.

• Evolution results when any of these five conditions are NOT met , i.e. when a population experiences deviations from the stability predicted by the Hardy-Weinberg theory.

Hardy-Weinberg is useful in public health science

• The Hardy-Weinberg theorem can estimate the percentage of the human population that carries the allele for a particular inherited disease, e.g. phenyketonuria (PKU). About 1 in 10,000 babies born in the United States is born with PKU, which results in mental retardation and other problems if left untreated. The disease is caused by a recessive allele.

• Q. What percentage of U.S. population carries the PKU allele?

• From the epidemiological data, we know that frequency of homozygous recessive individuals (q2 in the Hardy-Weinberg theorem) = 1 in 10,000 or 0.0001.– The frequency of the recessive allele (q) is the

square root of 0.0001 = 0.01.– The frequency of the dominant allele (p) is p = 1

- q or 1 - 0.01 = 0.99.– The frequency of carriers (heterozygous

individuals) is 2pq = 2 x 0.99 x 0.01 = 0.0198 or about 2%.

• A. About 2% of the U.S. population carries the PKU allele.

Microevolution

• The Hardy-Weinberg theory provides a baseline against which we can compare the allele and genotype frequencies of an evolving population.

• We can define microevolution as generation-to-generation change in a population’s frequencies of alleles.– Microevolution occurs even if the frequencies

of alleles are changing for only a single genetic locus in a population while the others are at equilibrium.

• Four factors can alter the allele frequencies in a population:

1. Genetic drift

2. Natural selection

3. Gene flow

4. Mutation

• All represent departures from the conditions required for the Hardy-Weinberg equilibrium.

• Genetic drift and natural selection are the two most important ones.

Genetic drift, gene flow & bottle neck effect

• Genetic drift occurs when changes in gene frequencies from one generation to another occur because of chance events (sampling errors) that occur when populations are finite in size (“small size phenomenon”)– The smaller the population, the greater the

chance of deviation from an idealized result.– Genetic drift at small population sizes often

occurs as a result of two situations: the bottleneck effect or the founder effect.

Genetic drift

• The gene pool of small population may not be accurately represented in next generation because of sampling errors.

• E.g. elimination of some alleles in small populations

Bottle neck effect

• Occurs when the numbers of individuals in a larger population are drastically reduced by a disaster, overhunting, flooding, overfishing, etc. By chance, some alleles may be overrepresented and others underrepresented among the survivors.

• Some alleles may beeliminated altogether.

Bottle neck & Conservation biology

• Bottlenecking is an important concept in conservation biology of endangered species.

• Populations that have suffered bottleneck incidents have lost at least some alleles from the gene pool.

• This reduces individual variation and adaptability.– E.g. the genetic variation

in the three small surviving wild populations ofcheetahs is very low whencompared to other mammals.

Gene flow (Migration)

• Gene flow is genetic exchange due to migration of fertile individuals or gametes between populations.

• Gene flow tends to reduce differences between populations.

• E.g., the migration of people throughout the world is transferring alleles between populations that were once isolated, increasing gene flow.

• If extensive enough, gene flow can amalgamate neighboring populations into a single population with a common genetic structure.

Variation is extensive in most populations

• Most populations show a great deal of variation, both, on the anatomical and molecular level.

• Many of the variable traits of the individuals of a population result fromcombined effect of several genes (“polygenic inheritance”).

• Populations are poly-morphic, i.e. two or more “morphs” are presentin noticeable numbers

• Not all variation is heritable (”epigenetics”)

• Evolutionary change of phenotypic characteristics and development of new species requires change or new assortment of the genetic material.

• Sequence of nucleotides on the DNA molecule has to be changed in order bring out a new gene product, i.e. a protein or enzyme, and (in a long term) to lead to a new phenotype of a species.

Sources of variation

• Two important principles are responsible for variation in biological life forms: 1. Mutations 2. Recombination

• Genetic variation due to mutation and/or recombination, is one of the pre-conditions of natural selection

• Natural selection acts on individuals, not their genes the natural forces and principles select from the offered arsenal of gene and allele variety within a population

Sources of variation

Mutations

• New alleles originate only by mutation.– Mutations are changes in the nucleotide

sequence of DNA.– Mutations of individual genes are rare and

random.– Mutations in somatic cells are lost when the

individual dies.– Only mutations in cell lines that produce

gametes can be passed along to offspring.

Mutations1. Point mutations• one nucleotide within the DNA molecule is changed• can be caused by different means, mostly by mutagens

and mistakes during DNA replication (lack of DNA repair)

e.g. certain chemicals, such as nitrosamines, benz(a)pyrene,

e.g. natural or artificial radiation (= radon, UV light, X-ray)

• most of point mutations either remain “silent” or are repaired

2. Deletion• a short piece of DNA gets lost or becomes actively excised out

of the DNA strand• has dramatic effects on protein structure and function• often caused as a consequence of the life cycle of certain

viruses which insert and excise DNA 

Mutations

3. Insertion• a short piece of DNA integrates into the DNA

molecule• it usually also leads to a massive stir-up of the flow of the

genetic information within a cell; altered protein shapes• many DNA insertions are caused by viruses which

randomly integrate their DNA nto the genetic material of their host cells

Impact of mutations

• Most point mutations, those affecting a single base of DNA, are probably harmless.– Most eukaryotic DNA does not code for proteins and

mutations in these areas are likely to have little impact on phenotype.

– Even mutations in genes that code for proteins may lead to little effect because of redundancy in the genetic code.

– However, some single point mutations can have a significant impact on phenotype.

• E.g. Sickle-cell disease is caused by a single point mutation.

Impact of mutations

• Chromosomal mutations, e.g. deletions, translocations, affect many genes and are likely to disrupt proper development of an organism.– However, occasionally, these dislocations link genes

together such that the phenotype is improved.

• Duplications of chromosome segments, whole chromosomes, or sets of chromosomes are nearly always harmful.– However, when they are not harmful, the duplicates

provide an expanded genome.– These extra genes can now mutate to take on new

functions.

Recombination Events

• Changes of the sequence of chromosomal DNA due to rearrangement of larger stretches of DNA (= chromosomal fragments including thousands of genes)

•  Can happen by twomajor cellular processes:

1. Crossing over during meiotic cell division in germ cells located in the gonads

2. Transposons (= movable genetic elements) with the help of enzymes called transposases

• Translocations (= movement of larger chromosomal fragments) are in most cases harmful to the organisms, but in rare cases they may bring out beneficial new characteristics in the affected individual

Impact of recombination events

• In organisms with sexual reproduction, most of the genetic differences among individuals are due to unique recombinations of the existing alleles from the population gene pool.– The ultimate origin of allelic variation is past

mutations.

• Random segregation of homologous chromosomes and random union of gametes creates a unique assortment of alleles in each individual.

• Sexual reproduction recombines old alleles into fresh assortments every generation.

Mutation rate

• Gene mutations and recombinations result in new alleles and allele combinations which are the ultimate source of variation within populations

Mutation Rate & Micro-Evolution

“Mutations happen all the time…this is why evolution is an unstoppable process on planet Earth!”

• Due to DNA replication and DNA repair mechanisms, mutation rates of individual genes are low, but since each organism has many genes, and a population has many individuals, new mutations arise in populations all the time

Microevolution

• A change in the frequency of alleles in the gene pool of a species over time is evolution in its smallest scale or also referred to as micro-evolution

• The driving force of natural selection is the unequal unequal success in reproductionsuccess in reproduction among members of a population.

• Natural selection leads to gradual change in the characteristics of a population of organisms which gets the favored characteristics of its most reproductive members.

• In most cases the physical environment (e.g. climate, seasonal changes, predators, etc.) screens for the most favored traits within a certain species.

• Natural selection tends to reduce the phenotypic variability in a population over time; but the individuals of a population do not become genetically uniform due to the existence of so-called recessive alleles

Diploidy and balancing selection preserve variation in populations

• The tendency for natural selection to reduce variation is countered by mechanisms that preserve or restore variation, including diploidy and balanced polymorphisms.

• Diploidy in eukaryotes prevents the elimination of recessive alleles via selection because they do not impact the phenotype in heterozygotes.– Even recessive alleles that are unfavorable

can persist in a population through their propagation by heterozygous individuals.

• Balanced polymorphism maintains genetic diversity in a population via natural selection.

• One mechanism in balance polymorphism is heterozygote advantage.

• In some situations individuals that are heterozygous at a particular locus have greater survivorship and reproductive success than homozygotes.

• E.g. heterozygous advantage maintains genetic diversity at the human locus for one chain of hemoglobin.

– A recessive allele causes sickle-cell disease in homozygous individuals, but heterozygous individuals are resistant to malaria and pass on their traits.

• The frequency of the recessive sickle-cell allele is highest in areas where the malarial parasite is common.

• The advantages of heterozygotes over homozygous recessive individuals who suffersickle-cell disease and homozygous dominant individuals who suffer malaria are greatest here.– Sickle-cell allele

may reach 20% of gene pool, with 32%heterozygotes resistant to malaria and 4% with sickle-cell disease.

Neutral variation

• Some genetic variations, neutral variation, have negligible impact on reproductive success.– E.g. diversity of human fingerprints seems to

confer no selectiveadvantage to some individuals over others.

– Much of the protein and DNA variation detectable may be neutral in their adaptive qualities.

• The relative frequencies of neural variations will not be affected by natural selection; some will increase and others will decrease by chance effects of genetic drift.

Variation & Natural Selection• Natural selection acts on the phenotypic variations of individuals of a

population by different means.• Important examples are:

1. Competition for resources and mates

2. Vulnerability to diseases, pathogens, etc.

3. Resistance to repellants, poisons or environmental toxins

4. Resistance to periods of malnutrition

5. Camouflage protection or other successful escape strategies from predators

• Some individuals of a population always turn out to be better adapted to their environment than other members

• Better adapted individuals have a higher chance to survive and are more likely to reproduce

3. Diversifying Selection

The total number of species in the world changed over long periods of time and phenotypes of species do not remain the same on Earth.

Early biologists of the 19th century suspected some common mechanism that is responsible for the appearance of new species over time; C. Darwin introduced the term speciation to explain the appearance of new species on Earth Definition: Speciation“Process by which new species of biological organisms evolve from a single population of parental species over long periods of time.”

Speciation

Speciation = responsible for the evolution of new species.

Definition: SpeciesA group of successfully interbreeding members of a population that are reproductively isolated from members of an original “parent population”; different species are biochemical and geographically separated from each other by a reproductive barrier.  Members of a “parent population” acquire and pass on new isolating traits while isolated from its parent isolation and ceasing to exchange genetic material with the “left-behind” population. Speciation is completed after establishment of a reproductive barrier between members of the separated populations

• Even though evolution is called a theory it does not mean that evolutionary theory is a mere guesswork. Its ideas are supported by a vast amount of evidences coming from many different scientific disciplines, such as:

1. Direct observation2. Fossil evidence3. Comparative anatomy4. Comparative embryology5. Biogeography6. Atavism7. Molecular biology

Evidence for Evolution

1. Direct observation

• The evolution of resistance to insecticides in hundreds of insect species and the rapid evolution of antibiotic resistance in bacteria are classic examples of natural selection in action.- Insecticides are poisons that kill insects that are pests in crops, swamps, backyards, and homes.

• The results of application of new insecticide are typically encouraging, killing 99% of the insects.

• However, the effectiveness of the insecticide becomes less effective in subsequent applications.

In wide areas of Asia where the insecticide DDT was intensively used in the past decades, some insects, i.e. AnophelesAnopheles, developed resistance to this environmentally very stable and ecologically critical chemical.

2. Fossil evidence

• Evidence of evolution by natural selection in the much grander changes in biological diversity is strongly documented by fossil records.– Evidence that the diversity of life is a produce

of evolution pervades every research field of biology.

– As biology progresses, new fossil discoveries, including the revelations of molecular biology, continue to validate the Darwinian view of life.

DickinsoniaDickinsonia- phylum ??

Original fossil Name

• Fossils of possibly first animalsfirst animals (650 – 544 Mio years ago), Vendian periodVendian period indicate first diversification of soft-bodied organisms

TribrachidiumTribrachidium-Cnidarian?

- Echinoderm?

Fossils of the Chengjiang site (China) 570 Mio years old

NaroiaNaroia- soft-bodied trilobite

LeanchoiliaLeanchoiliaillecebrosus illecebrosus

Original fossilReconstruction Name

Archaeopteryx lithographicaArchaeopteryx lithographica

Feathered reptilian (theropod-like) of the late Jurassic(200 – 144 Mio years ago)

- bony tail with reduced tail vertebrae (short for theropods)- reptilian teeth- clawed fingers- feathers

• Evolutionary transitions in life forms have left signs in the fossil record.– For example, a series of fossils documents the

changes in skull shape and size that occurred as mammals evolved from reptiles.

– Recent discoveries include fossilized whales that link these aquatic mammals to their terrestrialancestors.

Rodhocetus kasraniRodhocetus kasrani Fossil find: Pakistan, Age: 47 Mio yrs. old extinct whale ancestor of the Eocene sea

I. Reconstructed body features (Science 293(5538): 2239ff (2001)

II. Restoration of the found fossilized skeleton

Sahelanthropus tschadensisSahelanthropus tschadensis(6 – 7 Mio years ago)

almost complete cranium (skull) of “Toumai”

- Discovered in 2001 in today’s Chad (S. Sahara, Africa)- Small brain case (320 – 380 ccm)- the oldest known hominid after the evolutionary important split from chimpanzees (= Chimp- hominid split)- erect stance ?, bipedalism ?

• The succession of fossil records is compatible with what is known from other types of evidence about the major branches of descent in the tree of life.– For example, fossil fishes predate all other

vertebrates, with amphibians next, followed by reptiles, then mammals and birds; this is consistent many other types of evidence.

– In contrast, the idea that all species were individually created at about the same time predicts that all vertebrate classes would make their first appearance in the fossil record in rocks of the same age.• This is not what paleontologists actually

observe.

3. Comparative anatomy

• Descent with modification is evident in anatomical similarities between species grouped in the same taxonomic category.

• E.g., the forelimbs of human, cats, whales, and bats share the same skeletal elements, but different functions because they diverged from the ancestral tetrapod forelimb.

• They arehomologousstructures.

• Comparative anatomy confirms that evolution is a remodeling process via alteration of existing structures, rather than uniquely engineered for their existing function.– Historical constraints on this retrofitting are

evident in anatomical imperfections.– For example, the back and knee problems of

bipedal humans are an unsurprising outcome of adapting structures originally evolved to support four-legged mammals.

Homologous structures

•Another prominent example of homologous structures inanimals is the evolution of jaws by modification/re-modeling of gill arches

Vestigal Structures

Coccyx(‘tail bone”)

BlindCave crayfish

4. Comparative embryology

• Homologies that are not obvious in adult organisms become evident when we look at embryonic development.

• For example, all vertebrate embryos have structures called pharyngeal pouches in their throat and a notochord at some stage in their development.– These embryonic structures develop into very

different, but still homologous, adult structures, such as the gills of fish or the Eustacean tubes that connect the middle ear with the throat in mammals.

• Biogeography = geographical distribution of species.- first suggested as evidence for evolution by Ch. Darwin.

• Species tend to be more closely related to other species from the same area than to other species with the same way of life (environment), but living in different areas/continents.

5. Biogeography

Prominent examples for biogeography evidence are:1. Australian marsupial sugar gliders vs. the North American eutherian flying squirrels.

Marsupial animals only diversified in Australia, while all other continents are populated by dominating placental (eutherian) mammalians Marsupials evolved on Earth when South America, Antarctica, and Australia were still joined, but Australia separated from the other continents before the placental mammals arose.

2. Galápagos Islands are home to many more finch species than are found on the South American mainland3. Cacti are restricted to North American deserts, while drought-resistant Euphorbia plants dominate in Africa.

• Sugar glider and flying squirrel have adapted to the same mode of life, but they are not closely related.

• Instead, the sugar glider from Australia is more closely related to other marsupial mammals from Australia than to the flying squirrel, a placental mammal from North America.

• Resemblance between them is an example ofconvergent evolution.

Divergence of the Galapagos finches from ancestral colonizers from the South American main land

• In island chains, or archipelagos, individual islands may have different, but related species as the first mainland invaders reached one island and then evolved into several new species as they colonized other islands in the archipelago.

• A well-investigated example are the fruit flies (Drosophila) on the Hawaiian Archipelago.

• All of the 500 or so endemic species of Drosophila in the Hawaiian archipelago descended from a common ancestor that reached Kauai over 5 million years ago.

6. Atavism

• the very rare, sudden appearance of morphologies and phenotypic traits in living organisms that are usually found in distant still living or extinct life forms.

• Not completely understood what leads to the reappearance of these unusual body features in different life forms.

• Scientists suspect mutations of critical gene segments (i.e. gene promoters?, transcription factors?) that are crucial for the regulation of the genes that are responsible for the expression of morphology (“morphogenes?, Hox genes?”).

• Examples of atavism are:1. Extra fleshy hind flippers in dolphins

- observed occasionally in modern dolphins

- seen in fossil records of early dolphin ancestors which roamed the seas some 40 million years ago;

 

2. Lack of amphibian metamorphosis in Californian and Mexican mole salamanders (Axolotl)

- the dramatic morphological changes (metamorphosis) is not observed in

- reason(s) for this “stopped metamorphosis” is/are not known;

3. Extra nipples or even breast in humans - humans usually have one pair of nipples on their chest which develop along the milk line; -

- in other mammalian animal life forms the milk line gives rise to up to 5 pairs of nipples to suckle the offspring; - 1/20 human males develop at least one extra pair of nipples; some even more;

7. Molecular & biochemical evidence

• Further evidence for the process of evolution and descent from a common ancestor comes form biochemical analysis of proteins and analysis of sequence homologies of RNA/DNA molecules.

• Biological organisms use:1. the same basic biochemical molecules

e.g. DNA, RNA, pyruvate, acetyl-CoA and ATP

2. the same DNA triplet code to encode the genetic information of their enzymes

and proteins

3. many identical or nearly identical proteins and enzymes

• Common ancestry is experimentally substantiated by analysis of degree of similarity in:

1. Amino acids

e.g. cytochrome c, superoxide dismutase

2. Nucleotide sequences

e.g. ribosomal RNA (16S rRNA) or mtDNA

Species are not fixed but undergo changes in their inherited characteristics (variation) and gradually develop over long periods of time into new species (speciation) The understanding that organisms are not fixed but evolve culminated in the theory of evolution; evolution is the great unifying theme of modern Biology 

“Nothing in Biology makes sense except in the light of evolution.”T. Dobzhansky (1973)

 

• Darwin’s main ideas can be summarized in three points.

• Natural selection is differential success in reproduction (unequal ability of individuals to survive and reproduce).

• Natural selection occurs through an interaction between the environment and the variability inherent among the individual organisms making up a population.

• The product of natural selection is the adaptation of populations of organisms to their environment.

• By attributing the diversity of life to natural causes rather than to supernaturalcreation, Darwingave biology asound, scientificbasis.

• As Darwin said,“There is grandeur in this view of life.”