Evolution• Evolution – the process by which each type of organism

is descended from ancestors that were similar but not identical to it

• All life shares a common ancestry• Darwin (and independently, his contemporary Alfred

Wallace), proposed a mechanism for evolutionary change

• Many ideas about evolution pre-date Darwin• Jean Baptiste Lamarck (1744 – 1829) was the first to

propose a mechanism for evolution– Inheritance of acquired characteristics – living

organisms modify their bodies through the use or disuse of parts, and these modifications can be inherited by their offspring

Darwin and Wallace propose that evolution occurs by natural selection

• In 1858, Darwin and Wallace independently described a mechanism for evolution

• Darwin published On the Origin of Species by Means of Natural Selection the following year

• Darwin’s theory of natural selection included several important observations and conclusions:

• Observation #1 - Natural populations ( a population consists of all the individuals of one species in a particular area) of all organisms have the potential to increase rapidly – organisms produce far more offspring than can possibly survive

• Observation #2 - Nevertheless, the sizes of most natural populations and the resources available to support them remain relatively constant over time

• Conclusion #1 – Therefore, there is competition for survival and reproduction. Many individuals die young and fail to reproduce.

• Observation #3 – Individual members of a population differ from one another in their ability to obtain resources, withstand environmental extremes, escape predators etc. (Variation)

• Conclusion #2 – The most well-adapted (the “fittest”) individuals are the ones that leave the most offspring. Natural Selection – process by which the environment selects those individuals whose traits best adapt them to that particular environment.

• Observation #4 – At least some of the variation among individuals in traits that affect survival and reproduction is due to genetic differences that can be passed on from parent to offspring.

• Conclusion #3 – Over many generations, differential, or unequal, reproduction among individuals with different genetic makeup changes the overall genetic composition of the population. Evolution occurs as a result of natural selection.

Summary of Darwin’s Theory of Natural Selection

• Overproduction – each species has the capacity to produce more offspring than will survive to maturity

• Variation – individuals in a population exhibit variation (each has a unique combination of traits such as size, color, and ability to tolerate harsh environmental conditions)

• Limits on population growth – there is a limited amount of food, water, light, growing space, and other resources available to a population, therefore, organisms must compete for limited resources

• Differential reproductive success – individuals that possess the most favorable combinations of characteristics are more likely to survive and reproduce

• Natural selection leads to adaptation – evolutionary modification that improves the chances of survival and reproductive success of a population

Neo-Darwinism (Modern Synthesis)

• During the 1930’s and 40’s biologists combined the principles of genetics with Darwin’s theory of natural selection –

Neo-Darwinism (Modern Synthesis)

• Emphasizes the genetics of populations rather than individuals

Evidence for evolution

1. Fossil Record – remains or traces typically left in sedimentary rock (but also in bogs, tar, amber, and ice) by previous organisms

– shows a progression from the earliest, single-celled organisms to the many single-celled and multicellular organisms living today

– most fossils are dated by their relative position in sedimentary rock

Various methods are used to determine the age of fossils• Radioactive dating – radioisotopes decay in a

characteristic and known rate• Half-life is the amount of time required for 50% of the

radioisotope to decay – at the end of this period of time (any where from fractions of seconds to thousands of years, depending on isotope) radioactivity will be half of what it was before

each isotope has its own rate of decay and differ in their half-life (carbon-14, potassium-40, uranium-235)

• Carbon dating – excellent method of determining the age of organic material (relatively young, tens of thousands of years old)

• uses radioactive isotope of carbon, Carbon-14• 14C is unstable and spontaneously changes to 14N• all living organisms contain 14C in the same

proportion as found in the atmosphere• when organisms dies, process of incorporating

new carbon into body stops (ie, feeding)• half-life of carbon is 5730 years• Potassium-40 is used to date very old fossils

(hundreds of millions of years old)• 40K decays to form 40Ar • half-life of Potassium-40 is 1300 million years

Evidence from Paleontology – based on fossil record• fossils are difficult to explain without evolution• examples: evolution of horse, Archaeopteryx• fossil record provides direct evidence for evolution• fossils range from bone, teeth, shells, to actual body parts

preserved in bogs, tar, amber or ice• Fossils most commonly form in aquatic environments and

typically form in sedimentary rock

2. Comparative Anatomy – related species demonstrate similarities in their structure

– Homologous structures – body parts that have similar structure but may have different functions and appearance

– probably were derived from the same structure in a common ancestor (example: pentadactyl limb of vertebrates - human arm, cat forelimb, whale front flipper, and bat wing all have a similar arrangement of bones, muscles, and nerves)

– Vestigial structures – structures that have no apparent purpose

- often are homologous to structures that are found in and used by other organisms (ex: whales evolved from four-legged ancestral mammals – whales do not have hind legs yet they have small pelvic and leg bones embedded in their sides)

• Analogous structures – body parts that have the same function but very different internal anatomy (insect wing and bird wing)

– may have developed as a result of convergent evolution – unrelated species share the same environment (similar environmental pressures) and independently evolve similar structures

3.Embryological development – all vertebrates have similar patterns of embryological development indicating a common ancestor

4. Molecular comparisons – similarities and differences in biochemistry and molecular biology of various organisms provides evidence for evolution

- genetic code is universal – same bases make up DNA, same amino acids make up proteins, same use of mRNA, codons all code for amino acids the same way in all organisms- closely related species have similarities in DNA sequences

Artificial Selection • line of evidence that supports evolution by

natural selection

• the breeding of domestic plants and animals to produce specific desirable traits (ex. different breeds of dogs)

• people are doing the “selecting” rather than the environment

• people have bred very different dogs, all descendants of the wolf

Evidence for Evolution• Evidence from geographical distributions (biogeography)• if evolution did not occur, we would expect to find a given

species everywhere that it could survive – in reality this does not occur

• example – Australia (a separate land mass for millions of years) has distinctive animals (marsupials and monotremes) not found anywhere else

• 200 million years ago, Australia and other continents were joined together (Pangea)

• Australian continent gradually separated from others• monotremes and marsupials continued to thrive and

diversify in Australia• isolation of Australia prevented placental mammals

(which arose elsewhere at a leter time) from competing with monotremes and marsupials

• in other areas of the world, placental mammals outcompeted the marsupials and monotremes (which were eliminated)

Evolutionary Change in Populations • Evolution occurs in populations, not

individuals

• Individuals do not evolve in their lifetime

• Evolutionary changes are those that occur from generation to generation

Population – consists of all the individuals of the same species that live in a particular place at the same time

Population Genetics • branch of genetics dealing with the frequency,

distribution, and inheritance of alleles in populations • gene pool – sum of all the genes of all the individuals in

a population including all the alleles for all the genes present in the population

• allele frequency – the percentage of a specific allele of a given gene locus in the population

evolution of populations is best understood in terms of allele frequencies

if the allele frequencies remain constant from generation to generation, then the population is not undergoing any evolutionary change and is in genetic equilibrium

• evolution can be defined as changes in gene frequencies that occur in a gene pool over time (change in the genetic makeup of populations over time)

Natural Selection • mechanism of evolution in which members of a

population that possess more successful adaptations to the environment are more likely to survive and reproduce

• causes differential reproduction among organisms with different alleles – fitness of an organism is measured by its reproductive success (not just whether or not it survives)

• acts on the phenotype (although phenotype is an expression of genotype)

Three major types of Natural Selection 1. Directional Selection – favors phenotypes at one

of the extremes of the normal distribution– selects against both the average individuals and

individuals at the opposite extreme– over successive generations, one phenotype

gradually replaces the others

most common during periods of environmental change or when members of a population migrate to a new habitat with different environmental conditions

• Stabilizing Selection – selection against the phenotypic extremes, average is favored – associated with a population that is well adapted to its environment, tends to reduce variation

3. Disruptive Selection (Diversifying) – favors two or more different phenotypes at the expense of the mean

– special type of directional selection – trend is in several directions instead of just one

– results in divergence of groups of individuals within a population

• Natural selection works on genetic variations

• genetic variation originates with mutations – sexual reproduction also greatly contributes to genetic variation (crossing over, independent assortment, random union of gametes etc)

Coevolution • occurs when two species interact extensively and exert

strong selection pressures on each other • examples include predators and prey and organisms

that live in symbiotic relationships (mutualism, commensalism, parasitism)

• as one evolves a new feature or modifies an old one, the other typically evolves new adaptations in response

Natural Selection may lead to Extinction• the death of all the members of a species• occurs as a result of environmental change• some species may be predisposed to extinction as a

result of localized distribution and overspecialization• Three major environmental changes may drive a

species to extinction:– competition for limited resources with other species– introduction of new predators or parasites– habitat change and destruction

• greatest cause of extinctions• humans are rapidly destroying habitats• climate changes have caused many extinctions

(as a result of plate tectonics)

Mass extinctions • disappearances of many varied species in

a relatively short period of time over a wide area

• fossil record reveals episodes of extensive worldwide extinctions– may have resulted from enormous meteorites

hitting earth and kicking up enough dust to block out most of the sun’s rays

– most recent catastrophe occurred about 65 million years ago and coincided with the disappearance of dinosaurs

Origin of Species and Speciation • Species – definition for “species” has

changed over time– biological species concept – current definition:

groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups and capable of producing fertile offspring

– each species has a gene pool that is isolated from that of other species and each is restricted by reproductive barriers from interbreeding with other species

Speciation • process by which new species arise –

depends on two factors: – Isolation of populations – a population

becomes reproductively isolated from other members of the species – there is little gene flow between the different groups

– Genetic Divergence – during the period of isolation, the gene pools of the separated populations begin to diverge in genetic composition

– they evolve sufficiently large genetic differences so they can no longer interbreed and produce vigorous, fertile offspring

Two possible situations lead to speciation:1. allopatric speciation – speciation that occurs when

one population becomes geographically isolated from the rest of the species

– population evolves as a result of natural selection and/or genetic drift

– most common method of speciation– often results from physical isolation due to the

constant changing of the earth’s surface• river shifting courses, glaciers migrating, mountain

ranges forming, land bridges forming, etc.• may also result when a small population migrates and

colonizes a new area away from the range of the original species

• overall effect is to stop gene flow• speciation is more likely to occur if the isolated

population is small – genetic drift (including founder effect) has more effect on smaller populations

2. sympatric speciation – a new species develops within the same geographical region as the parental species – more common in plants than animals – usually results from:

ecological isolation chromosomal aberrations

1.ecological isolation – the same geographical area may contain two distinct types of habitats (distinct food sources, nesting places etc)– different members of a

single species may begin to specialize in one habitat or the other

– natural selection in the different habitats may result in speciation

2. changes in chromosome number may cause instantaneous speciation

– polyploidy – common speciation mechanism in plants – possession of more than two sets of chromosomes

– may occur when a fertilized egg duplicates its chromosomes but does not divide into two daughter cells – all subsequent divisions may be normal and all cells are now tetraploid

– most tetraploid plants are healthy and vigorous and can go through meiosis

– gametes produced can only fuse with other gametes from tetraploid plants – cannot fuse with gametes from original parents

– occurs in plants because plants can self-fertilize or reproduce asexually

• allopolyploidy – occurs when plants of two different species interbreed to form a hybrid

• following fertilization, if polyploidy occurs (doubling of chromosomes because fertilized egg does not separate into two cells) then proper synapsis and segregation of chromosomes can occur

Reproductive isolating mechanisms • structural and/or behavioral modifications

that prevent interbreeding between two different species

• prevents gene flow and preserves genetic integrity of each species

• Premating isolating mechanisms (prezygotic barriers) – prevent mating between species

1. temporal isolation – genetic exchange is prevented between two groups because they reproduce at different times of the day, season, or year

2. behavioral isolation – elaborate courtship behaviors are recognized only by a member of the same species – common in birds – courtship behaviors are only understood by females of the same species

3. mechanical incompatibility (mechanical isolation) – occurs when structural differences in male and female genital organs prevent successful mating – in animals, male and female sexual organs may not fit together – in plants, flower size or structure may prevent pollen transfer between species

4. gametic isolation – if mating takes place between two species, the gametes may fail to combine – may be due to molecular and chemical differences

• Postmating isolation mechanisms (postzygotic barriers)

1. hybrid inviability – if fertilization does occur, resulting hybrid may be weak or unable to survive (most spontaneous abort during embryonic development)

2. hybrid sterility – if the hybrid does survive, it will often be sterile (often because chromosomes fail to pair properly during meiosis) – ex. mules, ligers

• Speciation may occur rapidly or gradually – two models have been developed to explain evolution as observed in the fossil record:

1. Punctuated equilibrium – suggests that the fossil record accurately reflects evolution as it actually occurs in the history of a species• evolution occurs with long periods of stasis

(no evolutionary change) interrupted (“punctuated”) by short periods of rapid speciation

• speciation occurs in “spurts”• few transitional forms exist in the fossil

record because few transitional forms occur during speciation

2. Gradualism – more traditional view of evolution – evolution proceeds continuously over long periods of time

this model says that the fossil record is missing transitional forms because the fossil record in incomplete

Adaptive Radiation • when a species gives rise to many new

species in a relatively short period of time• typically occurs when populations of a single

species invade a variety of new habitats and evolve in response to the differing environmental selection pressures

• Darwin’s finches are a good example – encountered a wide variety of unoccupied habitats in the Galapagos Islands

• may occur after mass extinctions when the survivors fill empty habitats (ex – adaptive radiation of mammals after extinction of dinosaurs)

Modern examples of observed evolution• Darwin’s finches – finches of the Galapagos islands• all finches range in size, 10-20 cm long and are of

brownish or black coloration• shapes of beaks varied greatly from island to island

based on different feeding habits resulting from different selective pressures

• Rosemary and Peter Grant studied two species of finches on the Galapagos – Geospiza fortis and Geospiza scandens

• G. fortis had a short, wide beak for feeding on large, hard seeds - during 1982-83 there was a severe El Nino bringing heavy rain and changing the vegetation – by 1991, only 37% of the original population remained and those on average had longer more narrow beaks (compared to the original population) to eat the new smaller softer seeds being produced

• the population of G. scandens on the same island did not change over that time since its food and diet remained the same

Transient Polymorphism • a population may have more than one

common form – polymorphism• Example – Peppered moth

– Two forms – grey and black– Industrial melanism – natural selection resulting

from industrial processes– Proportions of black and grey have changed over

time (changes are temporary) – transient polymorphism

Balanced Polymorphism• Occurs when two or more alleles within a population

are not transient and changing but are stabilized by natural selection

• example – Sickle cell anemia– Condition is more common in areas where malaria is a

problem– Homozygous (HbSHbS) condition is lethal– Heterozygous condition (HbAHbS) gives person slight

resistance to malaria– Homozygous (HbA HbA) more likely to die from malaria– Allele for HbS remains relatively stable in population due to

heterozygote advantage

• Another example of natural selection:– Antibiotic resistance in bacteria– Read about in text

Patterns of Evolution

• Divergent evolution – related organisms become less alike – may result in speciation

– adaptive radiation – members of a species adapt to a variety of habitats – ex. Darwin’s finches

• Convergent evolution – two unrelated groups develop similar characteristics

– occurs when different species share the same environment and are subjected to similar selection pressures – ex. fish and dolphins