21.0 Introduction

1. Many Reasons for Changes in Natural Populations
1. Best Documented Are Caused by Human Activities fig 21.1
2. Similar Changes in Untouched Populations

1. The Pace of Evolution
1. Evolution of Different Groups Proceed at Different Rates
1. Most mammal species evolve rather slowly
2. Lungfish exhibit little change over 150 million years

2. Evolution in Spurts?
1. Evidence in fossil record shows variability in evolutionary rates
2. Punctuated equilibria: Evolution proceeds in spurts fig 21.2
1. Proposed by Eldredge and Gould
2. Innovations occur, give rise to new lines
3. Lines persist unchanged for long periods of time

3. Contrasted with gradualism: Gradual evolutionary change
4. Stasis: Lack of evolutionary change
1. Occurs in large populations
2. Presence of diverse and conflicting selective pressures

5. Rapid evolution occurs when populations are small, isolated
1. Diverging from parental population via with founder effect
2. Combined with selective pressures results in rapid change

3. Distinctions Not Usually Clear Cut
1. African mammals have clearly evolved gradually
2. Marine bryozoa distinctly show irregular patterns supporting punctuated evolution
3. Gaps may also be due to incomplete fossil record

2. The Nature of Species
1. The Taxonomic Species Concept
1. Ray proposed first definition of species
1. Individuals breed, progeny still of that species
2. Dogs, pigeons each an individual species
3. Carp and goldfish in separate species

2. Species regarded as an important biological unit to be catalogued
1. All species individually created by Supreme Being
2. View held widely by all scientists of the time

3. Concept challenged by Darwin
1. Species distinction in terms of an individual's niche
2. Species exploits different resources in different ways
1. Occurs in particular location
2. Displays certain activities at certain times
3. Specific habitat

3. Natural selection favored changes to improve "fit" of organism to environment
4. Adaptation of populations to environments is key event in species formation

2. The Biological Species Concept
1. Definition of species in genetic terms by population geneticists
1. Early experimental work examined Drosophila
2. Concept of species shaped by those who studied them
3. Summarized by Mayr
4. Groups of interbreeding populations, reproductively isolated from others

2. Concept says that hybrids between species rare in nature
1. Individuals of same species interbreed freely
2. Erection of barriers is key event in species formation

3. Problems with the Biological Species Concept
1. Concept works well for most terrestrial mammals
2. Fish and plant species generally recognized via taxonomic species concept
1. No true barriers to hybridization between some groups
2. Strong barriers between other groups
3. Other groups reproduce asexually

3. Botanists generally utilize taxonomic species concept

4. Patterns of Variation Within a Species
1. Populations from separate areas posses groups of distinctive individuals
1. Groups informally called races
2. Classified taxonomically as subspecies or varieties

2. Populations of a species from same area have mixed appearance
1. Populations intergrade with one another
2. Possess features of both groups
3. Species occurring together do not intergrade, may hybridize

3. In some organisms local races are incapable of interbreeding
1. Example: Annual plants
2. Other groups form fertile hybrids, but not normally in nature

4. Biological species concept may not be appropriate for some organisms
5. Working definition of a species
1. Group of organisms unlike other such groups of organisms
2. Does not extensively hybridize with these groups in nature

1. The Divergence of Populations
1. Reducing Dispersal Can Increase Divergence
1. Geographical separation or dissimilar conditions promotes divergence
1. Allows natural selection to differentiate between populations
2. Reduced dispersal, increases isolation, favors increased divergence

2. Study by Cody and Overton
1. Examined weedy plants of daisy family with wind-dispersed seeds
1. Seeds are two-parted, actual seed (achene) and cottony fluff (pappus)
2. Larger puff, smaller seed promotes greater dispersal by the wind
3. Quantified by V_p/V_a (achene volume/pappus volume)

2. Sampled tiny islands off Vancouver Island
1. Compared dispersal of island plants to those of mainland
2. Island weeds had reduced dispersal ability compared to mainland plants
3. Populations transient, disappear in few years, recolonized from mainland
4. Selection operated quickly

3. Prediction confirmed by resampling over period of ten years
1. Achene size (V_a) increases with population age fig 21.4
2. Parachute size (V_p) decreases with population age
3. Ratio of V_p/V_a decreases sharply, as does time aloft and dispersal distance

4. Action of natural selection reduces dispersal
1. Seeds with larger puffs, smaller seeds get blown out to sea
2. Do not contribute to next generation

2. How Much Divergence Does It Take to Make a Species?
1. Traditional view: New species arise by accumulation of small genetic differences
2. Molecular biology suggests evolution of a species may involve very few genes
1. Example: Monkey flower species
1. Two species vary in only a few genes
2. Appear vastly different in appearance

2. Ecological Races
1. Divergence in Population Creates Races
1. Intermediate stage in evolution of species
2. Individuals from different areas appear different
3. Appearance of clones differs with their environment
4. Botanists thought environmental factors might account for differences

2. Ecotypes in Plants
1. Studied by Turesson in 1920-30s
1. Isolated races of plants from varying environments
2. Most unique features maintained when grown under common conditions
3. Most differences in appearance had genetic basis, few were environmental
4. Those with genetically based differences called ecotypes

2. Studies continued in California
1. Performed transplant experiments, distributed clones to three stations
2. Demonstrated existence of ecotypes in plants
3. Differences in physiology genetic as well

3. Led to erroneous conclusion that most differences due to genetics

3. Ecological Races in Animals
1. Morphological or physiological differences fig 21.5
2. Differences between subspecies may be striking
3. Almost exclusively genetically determined
4. Example: Variation in dogs
1. Russian wolfhound vs. chihuahua
2. Observer from another planet might consider them different genera

4. Human Races
1. Humans have differentiated in characteristics as they have spread through the world
1. Differences in hair, skin, eye color
2. Differences in proportions of ABO blood groups

2. Traits may better adapt inhabitants to certain environments
1. Blood groups may confer immunity to diseases
2. Dark skin shield s body from damaging effects of UV radiation

3. All humans capable of mating with one another, producing fertile offspring
4. Division into human races inconsistent, as many as 30 or a few as three
5. Characteristics defining races in organisms do not always correlate with each other
1. Difficult to delineate particular characteristics in humans
2. Variation patterns may correlate with geographical distribution
3. Intermingling of groups of people confound the distinctions
4. Differences among human races breaking down, homogenizing

6. Individual differences within a race greater than differences between such races

21.3 Species formation in completed by the evolution of reproductive barriers

1. Prezygotic Isolating Mechanisms
1. Retention of Species Identity
1. Prezygotic mechanisms: Prevent formation of zygotes
2. Postzygotic mechanisms: Prevent functioning and development of zygotes

2. Geographical Isolation
1. Species in different places adapt to local conditions
1. Individuals physically separate from one another
2. Do not hybridize in nature
3. Hybridization may occur in zoos, gardens

2. Examples
1. Species of oaks fig 21.6
2. Lion/tiger matings

3. Ecological Isolation
1. Same area, but different habits and habitats
1. Utilize different portions of habitat
2. Do not hybridize since they do not encounter each other

2. Example: Overlapped range of Indian lions and tigers
3. Ecological isolation of California oaks
1. Valley oak versus scrub oak
2. Valley oak habitat: Fertile soil, open grassland, gentle slopes and valley floors
3. Scrub oak habitat: Less fertile soil, steep slopes
4. Hybrids do occur and are fertile, no intermediate area where hybrids can flourish

4. Selecting for ecological preferences
1. Different insect species in same area use different food plants
2. Minimizes contact, promotes ecological isolation
3. Example: Checkerspot butterflies
1. Normally feed on Collinsia olant
2. Area invaded by Plantago plant
3. Within ten years most butterflies laid eggs on Plantago
4. Ecological preferences changed to meet new vegetation

4. Behavioral Isolation
1. Related species often differ in courtship rituals
2. Keeps species distinct even if same place inhabited fig 21.8
1. Example: Hawaiian Drosophila fig 21.7
1. 500 species of fruit flies found, diverse appearance and behavior
2. Flies are long-lived
3. Males have bizarre appearance, complex behaviors
4. Mating behaviors help maintain distinctiveness of species
5. Hybrids fully fertile but rarely occur

2. Example: Species of leopard frogs fig 21.10
1. Similar appearance
2. Different mating calls

5. Other Prezygotic Isolating Mechanisms
1. Temporal isolation
1. Breeding periods at different times
2. Example: Wild lettuce
3. Example: Rana species

2. Mechanical isolation
1. General structural differences
2. Specific differences in copulatory organs
3. Example: Arthropod groups
4. Example: Bees pollinating various plants

3. Prevention of gamete fusion
1. Sperm not attracted to eggs of other species
2. Sperm incapable of penetrating eggs
3. In plants growth of pollen tubes impeded

2. Postzygotic Isolating Mechanisms
1. Hybrids Form, Zygotes Produced, Offspring Do Not Occur fig 21.11
1. Hybrids develop improperly
1. Example: Sheep/goat hybrids
2. Example: Leopard frogs

2. Experimental manipulation of plant hybrids
1. Embryo removed from parent
2. Cultured with special growth supplements

3. Hybrids weaker than parents, are eliminated in nature

2. Strong Hybrids May Be Sterile
1. Abnormal development of sex organs
2. Fertility generally lower than normal
3. Example: Donkey horse = mule fig 21.12

3. Reproductive Isolation: An Overview tbl 22.1
1. Formation of Species Is a Continuous Process
1. Partially differentiated populations may interbreed
2. Difference may disappear over time
3. Selective factors may limit success of hybrids
4. Individuals that do not hybridize may be more fit than those that do

2. More Than One Factor Can Limit Frequency of Hybrids
1. Factors are by-products of adaptive change in populations
1. Factors may arise simultaneously
2. May characterize differentiated populations

2. Selection may strengthen isolating mechanisms already present

3. The Role of Polyploidy in Species Formation
1. In plants, fertile individuals may arise from sterile ones by polyploidy
1. Doubles chromosome number of original sterile individual
2. Results in more than two sets of chromosomes

2. Hybrid may be sterile because chromosomes do not pair
1. If chromosomes double, can then pair properly
2. Fertility restored

3. Half of plant species have experienced polyploid episode in their history
4. Includes many plants of economic importance
5. Advantages for natural selection are substantial

21.4 Clusters o species reflect rapid evolution

1. Darwin's Finches
1. Adaptive Radiation of Closely Related Species
1. Thirteen species inhabit Gal pagos Islands
1. New arrivals filled unoccupied niches
2. Ancestral finches rapidly split into series of diverse populations

2. Descendants occupy different kinds of habitats fig 21.13

2. Resulted in Four Primary Groups of Finches
1. Ground finches: Six species
1. Feed on seeds
2. Bill size related to seed size

2. Tree finches: Five species
1. Four feed on insects, related to bill size
2. One feeds on buds and fruit
3. One insect-eater uses twigs to probe like a woodpecker

3. Warbler finch: One species
4. Vegetarian finch: Heavy bill of bird used to feed on buds of branches

2. Hawaiian Drosophila
1. Cluster of Species Resulting from Behavioral Isolation
1. More than one-quarter of drosophila species found only in Hawaii fig 21.14
2. Species are unusual because of morphological and behavioral traits fig 21.9

2. Species Cluster of Scaptomyza flies in Hawaii
1. Closely related to Drosophila
2. Species intermediate between to genera exist only in Hawaii
3. Suggestion that all 800 species of two genera may have evolved from single ancestor

3. Rationale for Multitudes of Species
1. Species selective for host plants and plant parts
1. Older species invade new islands, new species evolve
2. Species highly selective in choice of plants for raising larvae

2. New islands arise from sea
1. Readily colonized by fly groups from older islands
2. New species evolved as new islands colonized

3. Lake Victoria Cichlid Fishes
1. Recent Radiation
1. Cluster evolved recently and rapidly
1. Sequence cytochrome b gene as molecular clock fig 21.15
2. First cichlids entered lake 200,000 years ago from Nile River

2. Changes in water level encourages species formation
1. New areas flooded, opened up new habitats
2. Species may have originated when lake dried down, isolating populations

2. Cichlid Diversity
1. Range from 2 to 10 inches, males a variety of colors
2. Over 300 closely related species described
3. Multitude of different eating habits fig 21.16
4. All species are mouth breeders, fosters pedopagic feeding habit

3. Abrupt Extinction
1. Diversity decreasing with recent introduction of Nile perch
1. All open-water species extinct by 1990
2. Over 70% of named species disappeared

2. Nile perch is food source for lake inhabitants
3. Isolation caused explosive species radiation
4. Nile perch ended bloom of speciation, isolation disappeared

4. New Zealand Alpine Buttercups
1. Role of Periodic Isolation in Species Formation
1. Previous examples all favored by periodic isolation
2. Invade new islands, new species form, reinvade original habitat
3. Falling water levels isolate species, promote diversification

2. Alpine Buttercups Also Exhibit Periodic Isolation
1. More species of Ranunculus exist on New Zealand islands that in all of Americas
2. Periodic glaciation is source of isolation
3. 14 species occupy five distinctive habitats
1. Snowfields
2. Snowline fringe
3. Stony debris
4. Sheltered situations
5. Boggy habitats

4. Species invade five habitats as glaciers formed
5. Glaciers recede to isolate species, reform to rejoin species again fig 21.17
6. Parallel set of "ecospecies" evolved
1. Evolved independently to occupy similar habitats
2. Pairs of distantly related species occur in close proximity

5. Sexual Selection and the Origin of Species
1. Darwin Considered Sexual Selection a Major Feature of Evolution
1. Subsequent writers denied its major significance
2. Denied distinctiveness of sexual selection

2. Sexual Selection
1. Differential reproduction resulting from variable success in obtaining mates
1. Contrasts with natural selection
2. Results from struggle between rivals of same sex for mates

2. Evolutionary change originates from characteristics used in sexual displays
1. Social factors more important than ecological factors
2. Leads to incompatibility of populations due to social and courtship incompatibility

3. Many animal species exhibit sexual dimorphism in males and females
1. Most commonly seen as difference in size between males and females
2. Includes male adornments, like male antlers, manes, colorful plumage fig 21.18a

4. Secondary sexual characteristics may be used in direct competition with other males
1. Example: Male mountain sheep use horns in butting contests
2. More common when single male maintains harem of females

5. Males with most impressive features may attract more females
1. Example: Evolution of tail of African long-tailed widow bird fig 21.18b
2. Females prefer long-tailed males more
3. Artificially caused changes in tail length
4. Changes had no direct effect in male-male competition
5. Females attracted four times as often to males with long tails