The Origin of Species

Chapter 24

Basic Patterns of Evolution Anagenesis one species

accumulates heritable changes, gradually the species becomes a different species

Cladogenesis branching evolution, one species to several w/ potential for interbreeding

Biological Concept of Species

A population or group of populations whose members have the potential to interbreed with each other in nature to produce viable, fertile offspring, but who cannot produce viable, fertile offspring with members of other species

Species are based on interfertility, not physical similarity

The eastern and western meadowlarks may have similar shapes and coloration, but differences in song help prevent interbreeding between the two species

Humans have considerable diversity,but we all belong to the same species because of our capacity to interbreed

How are Species kept separate? Reproductive Barriers prevents

populations belonging to different species from interbreeding, even if their ranges overlap

Reproductive barriers prezygotic postzygotic,

Pre zygotic Barriers

Impede mating between species or hinder fertilization of ova if members of different species attempt to mateHabitat isolationBehavioral IsolationTemporal IsolationMechanical IsolationGametic isolation

Post zygotic Barriers Prevents the hybrid zygote from

developing into a viable, fertile adultReduced hybrid viabilityReduced hybrid fertilityHybrid breakdown

Habitat Isolation Two organisms that use different

habitats even in the same geographic area are unlikely to encounter each other to even attempt matingTwo species of garter snakes, in the genus

Thamnophis, that occur in the same areas but because one lives mainly in water and the other is primarily terrestrial, they rarely encounter each other.

Behavioral Isolation Many species use elaborate behaviors

unique to a species to attract mates Visual Fireflies

Bird plumage Red Stickleback Fiddler Crabs Fruit Fly Blue Footed Booby

OlfactoryScandinavian mothPheromones

Auditory: Bird songs, frog calls

Temporal Isolation

Two species that breed during different times of day, different seasons, or different years cannot mix gametes

Flowers Skunk mating

seasons Frogs

Wood frog

44o

Leopard Frogs

55o

Green Frog

60o

Bull Frog Above 60o

Mechanical Isolation Closely related species may attempt

to mate but fail because they are anatomically incompatible and transfer of sperm is not possible

Flower structure of certain flowering plants prevents pollination by insects or other animals

With many insects the male and female copulatory organs of closely related species do not fit together, preventing sperm transfer

Gametic Isolation Gametes of two species do not form

a zygote because of incompatibilities preventing fusion or other mechanisms

Sperm/egg recognition Reproductive Tract unfavorable

Post Zygotic Barriers

Reduced hybrid viability Hybrids may be

frailhybrids between frogs in the genus Rana, which do not complete development and those that do are frail.

Reduced Hybrid Fertility Hybrids may be vigorous but may be infertile

and the hybrid cannot backbreed with either

parental species Horse (2n = 64) Donkey (2n = 62)

Reduced Hybrid Fertility

Mule (which is sterile)

Hence, donkeys and horses are separate species

Hybrid Breakdown

Some first generation hybrids are viable and fertile, but when the mate with one another, offspring are feeble

Species Concept Biological Concept“Species are groups of

actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups.” Ernst Mayr.

Morphological species concept, the oldest and still most practical, defines a species by a unique set of structural features

Ecological species defines a species in terms of its ecological niche, the set of environmental resources that a species uses and its role in a biological community

Biogeography of Species Two ways in which

speciation can occur. Allopatric

speciation occurs when a gene pool is divided into two

Sympatric speciation occurs without geographic separation

Allopatric Speciation

A geographic barrier isolates the population and species are maintainedSquirrels on the N/S of the Grand

Canyon

Allopatric speciation of squirrels in the Grand Canyon

Sympatric Speciation

Reproductive isolation without geographic isolationAutopoliploidyAllopolyploidyNon-random mating animals

Autopoliploidy Single parent doubles chromosome # Results in TETRAPLOIDS

Allopolyploidy 2 different species are involvedpolyploid

hybrid is formed This is how modern wheat, oats, cotton,

potatoes were developed More important in plant evolution

Non-random mating animals

Mate choice is based on coloration

Adaptive RadiationEvolution of many diversely adapted species from a

common ancestor upon introduction of new environmental opportunities

Tempo of Speciation Niles

Eldredge/Steven J. Gould

Punctuated Equilibrium

Gradualism

Macroevolution

Leads to new taxonomic groups Origin of mammals from reptiles Feathers and flight Increasing brain size of mammals Adaptive radiation of flowering plants

Macroevolution

Evolutionary novelties are modified versions of older structures

Exaptations: Preadaptations structures that evolve in one context but become

co-opted for another

Ex. Ancestral reptiles Hollow bones in birds

‘Evolution is like modifying a machine while it is running!’

Evo-Devo

The role evolution has in developmentEx: Shape of an organism depends on

relative growth rate of its parts Changes in Rate and Timing

Allometric GrowthHeterochronyPaedomorphosis

Changes in Spatial PatternsHomeotic Genes

Differential growth rate expanded time of brain development

Figure 24.19 Allometric growth

Allometric growth: proportional change

Heterochrony

Longer time for foot growth results in longer digits and less webbing

Foot growth ends sooner-shorter digits and more webbing

Paedomorphosis

Juvenile traits extended to adult Salamanders that retain gills don’t have to

leave the water

Homeotic Genes

Control the 3D placement of structures

HOX gene organize the embryo in space

Invertebrates 1 set Vertebrates several sets more

complex growth