Lecture 22

Evolution

Evolution

• three key observations about life– 1. organisms are suited for life in their

environments– 2. many forms of life share characteristics– 3. life is diverse

• 150 yrs ago – Charles Darwin developed a scientific explanation for these observation

• published his theory as the Origin of Species– from his work categorizing species as a

member of the HMS Beagle• evolution = descent with modification

Evolution

• can also be defined as a change in the genetic composition of a population from generation to generation

• pattern of evolutionary change is revealed by data taken from biology, geology, physics and chemistry

History of Evolution

• greek philosophers suggested that life might have changed gradually over time– but Aristotle – viewed species as fixed and

said that life-forms could be arranged on a ladder of increasing complexity = scalae naturae

• Carolus Linneaus – developed the binomial system for naming species

• developed a nested classification system in contrast to Aristotle– grouped animals according to similar

characteristics– groups known as class, family, genus– thought the similarities were due to God’s

creation

Lamarck• many people proposed that life evolves as the

environment changes• Jean Baptiste de Lamarck proposed how

these changes happened• published his theory in 1809 – year Darwin

was born• he was wrong• proposed two principles at work: use and

disuse & inheritance of acquired characteristics– e.g. the giraffe stretching his neck to reach the

upper leaves of a tree produces a longer neck in subsequent generations

– also thought animals evolve because of a drive to become more complex

Evolution: Descent with Modification

• Charles Darwin – 1809-1882• naturalist trained at Cambridge• recommended after graduation to Captain

Robert Fitzroy – captain of the HMS Beagle• Beagle was embarking on a multi-year

voyage around the world to chart coastlines• Darwin observed plants and animals in

temperate regions of South America resembled species in the South American tropics more than they did species in the temperate regions of Europe

• also studied fossils and saw similarities with living species

Galapagos Islands• group of volcanic islands 900 km west

of South America• fascinated by the unusual animals and

plants• collected several kinds of birds• many were similar to each other but

were different species• some traits were unique to specific

islands• saw species on these islands that

resembled the SA mainland but were seen no where else

• used these specimens to formulate his theories on adaptations and descent

Natural Selection

• Darwin observed many examples of adaptations– inherited characteristics that enhance their

survival and reproduction in the environment• linked adaptation to the environment and the

origin of a new species

Descent with Modification

• Darwin never used the term evolution• used the term descent with modification• proposed that similarities between organisms was due

to descent from a common ancestor in the remote past

• the descendants lived in various habitats & developed adaptations to fit them to their habitat

• Linneaus grouped organisms based on similarities but never recognized these similarities were due to descent with modification

Artificial & Natural Selection

• so: Natural selection was proposed as the mechanism explaining evolution

• Darwin used artificial selection used by humans in breeding to explain his theory

• made two observations:– 1. members of a population often vary in their inherited traits

• SO: individuals whose inherited traits give them a higher probability of surviving and reproducing will leave more offspring

– 2. all species can produce more offspring than their environment can support – many fail to survive• SO: the ability to survive and reproduce will lead to an accumulation

of favorable inheritable traits• if these traits make your offspring more successful at coping with its

environment = traits will persist over time = NATURAL SELECTION

Natural Selection: a recap

• 1. NS is a process in which individuals with certain heritable traits survive and reproduce at a higher rate than other individuals who don’t have those traits

• 2. Over time, NS can increase the match between organisms and their environment

• 3. if an environment changes (or if an individual moves to a new environment) - NS may result in adaptations – sometimes giving rise to new species.

The science of evolution

• direct observations • homology• fossil record• biogeography

The science of evolution

• direct observations – evolution observed using scientific studies– introduced plant species – what happens when

herbivores begin to feed on a plant species with different characteristics than their usual food source?• soapberry bug – feed on fruits of various plants using a

hollow “beak”• beak length must match the depth at which seeds are

found within their fruit• introduce a plant with fruits closer to the surface –

evolution of shorter beaks within the bug population

– drug-resistant bacteria – e.g Staphylococcus aureus

The science of evolution

• homology – analyzing similarities among different organisms– similarity resulting from common ancestry = homology– several types:

• 1. anatomical • 2. molecular

The science of evolution

Humerus

RadiusUlna

CarpalsMetacarpalsPhalanges

Human Cat Whale Bat

• homology:• 1. anatomical – closely related species share similar features even though they may

have different functions– e.g. forelimbs of humans, cats, whales and bats– comparing early stages of development can reveal additional anatomical homologies –

e.g. pharyngeal pouches gills in fish and parts of the ears and throat in mammals– some “leftover” structures can give us important information about evolution = vestigial

structures• 2. molecular – similarities in DNA and RNA

– many genes have acquired new functions– but other genes – e.g. ribosomal proteins – remain remarkably similar from bacteria to

humans

Pharyngealpouches

Post-analtail

Chick embryo (LM) Human embryo

• although some organisms that are related share characteristics because of common descent – distantly related organisms can resemble one another due to convergent evolution– the independent evolution of similar features in different lineages

• i.e. different common ancestors!

– marsupials vs. eutherians– two lineages evolved separately but they experienced similar

environments and underwent similar adaptations– these shared features are said to be analogous NOT homologous

Sugarglider

Flyingsquirrel

NORTHAMERICA

AUSTRALIA

The science of evolution• fossil record – documents patterns of evolution• fossils also show how evolutionary changes have occurred in

various groups of organisms• can also shed light on the origins of new organisms

– e.g. cetaceans are closely related to ungulates

Most mammals Cetaceans and even-toed ungulates

(a) Canis (dog) (b) Pakicetus (c) Sus (pig) (d) Odocoileus (deer)

ankle bones between dogs (unrelated) and an early cetacean (pakicetus) and ungulates

The science of evolution• biogeography – the geographic distribution of species• species distribution is influenced by many factors• including continental drift

– 250 MYA – one land mass = Panagea– 200 MYA – Panagea began to break apart– scientists could predict where fossils might be found– horse evolution – based on fossils, present day horses originated about 5

MYA in north america– at that time north and south america were close together but not connected– scientist predicted that the oldest horse fossils should be found in north

america – found to be correct

Genetic Variation in Evolution

• smallest unit of evolution = microevolution– change in allele frequencies in a population over generations– one of the causes – natural selection– other causes: genetic drift & gene flow– only natural selection improves adaptation to the environment

Genetic Variation in Evolution

• genetic variation – seen in individual variations– genetic variation is the differences in composition of an

individual’s genes or other DNA segments (i.e. junk)– genetic variation produces variations in phenotypes– only the genetic component of a phenotype can have

evolutionary consequences• phenotypes are not necessarily passed on• e.g. body builder changes his phenotype but doesn’t pass on the bigger

muscles

Genetic Variation in Evolution

• variation within a population– characters that vary within a population: discrete or quantitative– quantitative characters – most heritable variation uses these

characters• may can be measured – e.g. height, weight, IQ• vary along a continuum within a population -e.g. hair color, eye color• usually results from the influence of 2 or more genes on a single

phenotypic characteristic = known as Polygenic Traits– discrete characters – classified on an “either-or basis”

• e.g. purple color of a flower or a white color• usually determined by a single gene with different allele forms producing

distinct phenotypes

Genetic Variation in Evolution

• variation between populations– species also exhibit geographic variation – differences in the genetic

composition of separate populations– geographic variation can be observed as a cline – a graded change in a

character along a geographic axis

Sources of Genetic Variation

• formation of new alleles – arise through mutation– must occur in a germ cell to be heritable– most mutations occur in somatic cells and are loss when the organism

dies

• altering gene number or position – chromosomal changes that delete, duplicate or rearrange genes– may not necessarily be bad – e.g. crossing over in meiosis

• rapid reproduction – increases the rate of mutations• sexual reproduction – produces genetic variability due to

combination of gametes

Hardy-Weinberg Principle

• used to test whether evolution is occurring in a population• population = group of individuals of the same species that live in the same area

and interbreed to produce fertile offspring• the population’s genetic make-up = gene pool

– all copies of every type of allele at every gene locus in all members of the population– if only one allele exists for a gene locus = fixed allele within the pool (all individuals are

homozygous – e.g. EE or ee)– if there is more than one allele – individuals may be homozygous (EE or ee) or

heterozygous (Ee)

Hardy-Weinberg Principle

• each allele has a frequency or proportion in a population– if a gene has two alleles (i.e. E or e) – p is used for the frequency of one allele (i.e. the

dominant), q is used for the other (i.e. the recessive allele)– Hardy-Weinberg principle states that the frequency of these alleles in a population

will remain constant from generation to generation– if the gene pool is in equilibrium = P+Q = 1 (NO EVOLUTION)– p2 + 2pq + q2 = 1 (i.e. 100%)

Hardy-Weinberg Principle• two alleles = E and e• p2 = expected frequency of the EE genotype in the population• q2 = expected frequency of the ee genotype• 2pq = expected frequency of the Ee genotype• p+q - must equal 1 (i.e. 100% of the population) – population

is NOT evolving• conditions for HW equilibrium

– 1. no mutations– 2. random mating– 3. no natural selection– 4. extremely large population size– 5. no gene flow in and out of populations

Applying the HW Principle

• HW principle can be used to measure the frequency of the heterozygote – often hard to measure due to its similarity to the dominant homozygote

• can be very helpful in diseases• disease – PKU; 1 in 10,000 births

– therefore q2 = 0.0001

• frequency of q allele = 0.01 (1% of population - square root of 0.0001)

• frequency of p allele = 1.0-0.01 = 0.99 (99% of population)• 2pq = 2x0.99x0.01 = 0.0198 or 2% of the population

Applying the HW Principle

• according to the HW principle - p2 + 2pq + q2 must equal 1 • does it?• p2 = 0.9801• q2 = 0.0001• 2pq = .0198• 0.9801 + 0.0198 + 0.0001 = 1

• so the frequency of PKU as a disease is stable within the population