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 mate  Habitat isolation  Behavioral Isolation  Temporal Isolation  Mechanical Isolation  Gametic isolation

Post zygotic Barriers Prevents the hybrid zygote from developing into a viable, fertile adult  Reduced hybrid viability  Reduced hybrid fertility  Hybrid breakdown

Habitat Isolation Two organisms that use different habitats even in the same geographic area are unlikely to encounter each other to even attempt mating  Two 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 Olfactory  Scandinavian moth  Pheromones 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 frog44 o Leopard Frogs 55 o Green Frog 60 o Bull FrogAbove 60 o

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 frail  hybrids 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 maintained  Squirrels on the N/S of the Grand Canyon

Allopatric speciation of squirrels in the Grand Canyon

Sympatric Speciation Reproductive isolation without geographic isolation  Autopoliploidy  Allopolyploidy  Non-random mating animals

Autopoliploidy Single parent doubles chromosome # Results in TETRAPLOIDS

Allopolyploidy 2 different species are involved  polyploid 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 Radiation Evolution 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 development  Ex: Shape of an organism depends on relative growth rate of its parts Changes in Rate and Timing  Allometric Growth  Heterochrony  Paedomorphosis Changes in Spatial Patterns  Homeotic Genes

Differential growth rate  expanded time of brain development

Figure 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

Figure A range of eye complexity among mollusks