Chapter 24 The Origin of Species

Concepts Speciation is the origin of new species and is at the focal point of evolutionary theory Evolutionary theory must explain how new species originate and how populations evolve

Biological Species Concept Species is a Latin word meaning “kind” or “appearance” Archaeopteryx lithographica Morphology, physiology, biochemistry, and DNA sequences are compared when grouping organisms

Definition of Species Species is a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring; they do not breed successfully with other populations

Concept of Species Biological species concept: reproductive barriers Ecological species concept: ecological niche Pluralistic species concept: unique factors depending on the species Genealogical species concept: genetic history

Biological Species Concept: Similarity and diversity

Reproductive Isolation Reproductive isolation is the existence of biological factors (barriers) that impede two species from producing viable, fertile offspring Hybrids are the offspring of crosses between different species Reproductive isolation can be classified by whether factors act before fertilization (prezygote) or after fertilization (postzygote).

Prezygotic (before the zygote) Prezygotic barriers block fertilization from occurring by: Impeding different species from attempting to mate Preventing the successful completion of mating Hindering fertilization if mating is successful

Types of Prezygotic Barriers Habitat isolation Behavioral isolation Temporal isolation Mechanical isolation Gametic isolation

Habitat isolation One lives in water the other on land

Behavioral isolation The dancing and singing are completely different

Temporal Isolation One diurnal and the other nocturnal

Mechanical isolation Spirals in opposite directions

Gametic Isolation Gametes are not compatible Timing of gamete release may be different Conditions in the female womb of a different species may not be ideal for the sperm Gamete recognition hampered due to molecules arround the ovum

Postzygotic Barriers Reduced hybrid viability Reduced hybrid fertility Hybrid brakedown

Reduced hybrid viability Ensantia hybrid shows to be frai, may not survive

Reduced hybrid fertility Mule: sterile hybrid of donkey and horse

Hybrid brakedown Hybrid (center sample) is stunt and may not survive or reproduce

Speciation Based on Gene Flow (a) Allopatric speciation (b) Sympatric speciation

Allopatric In allopatric speciation, gene flow is interrupted or reduced when a population is divided into geographically isolated subpopulations

The definition of barrier depends on the ability of a population to disperse Separate populations may evolve independently through mutation, natural selection, and genetic drift

Allopatric speciation in the Grand Canyon A. leucurus A. harrisi

Reproductive isolation between populations generally increases as the distance between them increases Barriers to reproduction are intrinsic; separation itself is not a biological barrier

Millions of years ago (mya) Mantellinae (Madagascar only): 100 species Rhacophorinae (India/Southeast Asia): 310 species Other Indian/ Southeast Asian frogs 100 80 60 40 20 1 2 3 Millions of years ago (mya) 1 2 3 India Madagascar 88 mya 65 mya 56 mya

Sympatric Speciation Sympatric means “same country” Speciation takes place in geographically overlapping populations Occurs because of Polyploidism Habitat differentiation Sexual selection

Polyploidism Polyploidy is the presence of extra sets of chromosomes due to accidents during cell division Autopolyploidy Allopolyploidy

Autopolyploidy in plants 2n=6 4n=12 Failure of cell division after chromosome duplication gives rise to tetrapliod cells which may be viable and self-fertile

Allopolyploid An allopolyploid is a species with multiple sets of chromosomes derived from different species Many important crops (oats, cotton, potatoes, tobacco, and wheat) are polyploids

Ancestral species: AA BB DD Triticum monococcum (2n = 14) Wild T. tauschii (2n = 14) Product: AA BB DD T. aestivum (bread wheat) (2n = 42)

Habitat differentiation Sympatric speciation can also result from the appearance of new ecological niches North American maggot fly can live on native hawthorn trees as well as more recently introduced apple trees, so food preference may isolate the two populations

Sexual selection Sexual selection can drive sympatric speciation Sexual selection for mates of different colors has likely contributed to the speciation in cichlid fish in Lake Victoria

Hybrid Zones region in which members of different species meet and mate Patterns abnormal, more fragile offspring, less fit (mule is sterile) more fit than the parents for both environments (paco vicuña)

Reinforced Barriers less fit hybrids might show a reinforcement in the barriers stronger in allopatric than sympatric species mule is sterile

Fusion barriers are not strong enough hybrids are more likely (red wolf= coyote x grey wolf)

Stability the hybrid production is stable barriers haven't happen or are too weak stil hybrids are not the norm because the location where they occur are the extremes of the normal habitat where the original species exist The End