Ch. 21 Warm-Up Use the following information to help you answer the question below: Population = 1000 people AA = 160 Aa = 480 aa = 360 What are the genotypic frequencies? Allele frequencies? Use directional, stabilizing or disruptive selection to answer the following: The mice in the Arizona desert have either dark or light fur. Birds produce 4-5 eggs per clutch Average human baby weighs 7 lbs. Darwin's finches and beak size during drought

Chapter 22 The Origin of Species

What You Need to Know: The biological concept of a species. Speciation may occur when two populations become reproductively isolated from each other. There are prezygotic and postzygotic barriers that maintain reproductive isolation in natural populations. How allopatric and sympatric speciation are similar and different. How a change in chromosome number can lead to sympatric speciation. Why speciation rates are often rapid in situations where adaptive radiation occurs or during times of ecological stress. The connection between speciation in a isolated population and a change in gene frequency, a change in the environment, natural selection, and/or genetic drift. How punctuated equilibrium and gradualism describe two different tempos of speciation.

Speciation = origin of species Microevolution: changes in allele frequencies within a single gene pool Macroevolution: evolutionary change above the species level cumulative effects of speciation over long periods of time

HHMI Video Clip: Reproductive Isolation and Speciation Running Time: 2:38 min

Biological Species Concept Species = population or group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring Reproductively compatible Reproductive isolation = barriers that prevent members of 2 species from producing viable, fertile hybrids

Types of Reproductive Barriers Prezygotic Barriers: Prevent mating or hinder fertilization Types: Habitat isolation Temporal isolation Behavioral isolation Mechanical isolation Gametic isolation Postzygotic Barriers: Prevent hybrid zygote from developing into fertile adult Types: Reduced hybrid viability Reduced hybrid fertility Hybrid breakdown

Types of Reproductive Barriers REDUCED HYBRID VIABILITY REDUCED HYBRID FERTILITY HYBRID BREAKDOWN

Types of Reproductive Barriers REDUCED HYBRID VIABILITY REDUCED HYBRID FERTILITY HYBRID BREAKDOWN

Types of Reproductive Barriers REDUCED HYBRID VIABILITY REDUCED HYBRID FERTILITY HYBRID BREAKDOWN

Types of Reproductive Barriers REDUCED HYBRID VIABILITY REDUCED HYBRID FERTILITY HYBRID BREAKDOWN

Other definitions of species: Morphological – by body shape, size, and other structural features Ecological – niche/role in community Phylogenetic – share a common ancestor, form one branch on tree of life

Two main modes of speciation

Two main modes of speciation: Allopatric Speciation “other” “homeland” Geographically isolated populations Caused by geologic events or processes Evolves by natural selection & genetic drift Eg. Squirrels on N/S rims of Grand Canyon Sympatric Speciation “together” “homeland” Overlapping populations within same geographic area Gene flow between subpopulations blocked by: polyploidy habitat differentiation sexual selection Eg. polyploidy in 80% of plants (oats, cotton, potatoes, wheat)

Allopatric speciation of antelope squirrels on opposite rims of the Grand Canyon

Sympatric Speciation by Polyploidy Autopolyploid: extra sets of chromosomes Failure of cell division (2n  4n) Eg. Strawberries are 4n, 6n, 8n, 10n (decaploid)!

Allopolyploid: 2 species produce a hybrid Species A (2n=6) + Species B (2n=4)  Hybrid (2n=10)

Hybrid Zones Incomplete reproductive barriers Possible outcomes: reinforcement, fusion, stability

Grizzly Polar “Grolar” or “Pizzly”

Bear Evolution

Fusion: The breakdown of reproductive barriers

Time Course of Speciation Gradualism Common ancestor Slow, constant change Punctuated Equilibium Eldridge & Gould Long periods of stasis punctuated by sudden change seen in fossil record

HHMI Short Film: Lizards in an Evolutionary Tree Topic: Adaptive Radiation Running Time: 17:50 min