Chapter 24: The Origin of Species

Essential Knowledge 1.c.1 – Speciation and extinction have occurred throughout the Earth’s history (24.3 & 24.4). 1.c.2 – Speciation may occur when two populations become reproductively isolated from each other (24.1). 1.c.3 – Populations of organisms continue to evolve (24.2). 2.e.2 – Timing and coordination of physiological events are regulated by multiple mechanisms (24.1).

Two Concepts of Species 1. Morphospecies 2. Biological Species

Morphospecies Organisms with very similar morphology

Two Schools 1. Splitters - Break apart species into new ones on the basis of small phenotype changes 2. Lumpers - Group many phenotype variants into one species

Biological Species A group of organisms that could interbreed in nature and produce fertile offspring Don’t identify species based upon similar likeness ALONE!

Key Points Could interbreed Fertile offspring May or may not be similar in phenotype

Morphospecies & Biological Species Often overlap Serve different purposes

Other Concepts of Species 1. Recognition Species Concept 2. Cohesion Species Concept 3. Ecological Species Concept 4. Evolutionary Species Concept

Ecological Species Concept Emphasizes a specie’s role or function in the environment. Called an organism’s niche

Hybridization has been successful in captivity (Added Slide ) Lions and tigers are ecologically isolated Live in forest Live in open grassland Tiglon Hybridization has been successful in captivity But it does not occur in the wild

Evolutionary Species Concept Emphasizes evolutionary lineages and ecological roles Used extensively in comparative biology Popular among paleontologists

Speciation Speciation = formation of a NEW species Caused by disruptive selection (see Ch 23) 3 Requirements for speciation: Variation in the population Selection Isolation

Reproductive Barriers Serve to isolate a populations from other gene pools Create and maintain “species”

Main Types of Reproductive Barriers Prezygotic – Prevent mating or fertilization Postzygotic – Prevent viable, fertile offspring

Prezygotic - Types 1. Habitat Isolation 2. Behavioral Isolation 3. Temporal Isolation 4. Mechanical Isolation 5. Gametic Isolation

Habitat Isolation Populations live in different habitats or ecological niches Ex – mountains vs. lowlands

Behavioral Isolation Mating or courtship behaviors different Different sexual attractions operating Ex – songs and dances in birds

Temporal Isolation Breeding seasons or time of day different Ex – flowers open in morning or evening

Mechanical Isolation Structural differences that prevent gamete transfer Ex – anthers of flower not positioned to put pollen on a bee, but will put pollen on a bird

Gametic Isolation Gametes fail to attract each other and fuse Ex – chemical markers on egg and sperm fail to match

Postzygotic - Types 1. Reduced Hybrid Viability 2. Reduced Hybrid Fertility 3. Hybrid Breakdown

Reduced Hybrid Viability Zygote fails to mature or develop Ex: When two different species of frogs hybridize Egg is fertilized (zygote IS formed) Zygote never develops properly (if at all)

Reduced Hybrid Fertility Hybrids/offspring are living, viable Offspring cannot reproduce sexually (nonfertile) Chromosome count often “odd” Meiosis doesn’t work out Ex: mules, ligers

Hybrid Breakdown Hybrid/offspring are living, viable AND fertile Offspring cannot compete successfully with the “pure breeds” Ex: Most plant hybrids Plants are “born” and can produce seeds Blooms/plants may lack certain color or size of petal

Their mating calls also differ substantially Hybrids between them produced defective embryos in the lab Their mating calls also differ substantially

Modes of Speciation 1. Allopatric Speciation 2. Sympatric Speciation Both work through a block of gene flow between two populations

Allopatric Speciation Allopatric = other homeland Ancestral population split by a geographical feature. Comment – the size of the geographical feature may be very large or small

Example Pupfish populations in Death Valley Generally happens when a species range shrinks for some reason

Another Example

Conditions Favoring Allopatric Speciation 1. Founder's Effect - with the peripheral isolate 2. Genetic Drift – gives the isolate population variation as compared to the original population 3. Selection pressure on the isolate differs from the parent population

Result Gene pool of isolate changes from the parent population New species can form

Comment Populations separated by geographical barriers may not evolve much Ex - Pacific and Atlantic Ocean populations separated by the Panama Isthmus Fish - 72 identical kinds Crabs - 25 identical kinds Echinoderms - 25 identical kinds

Adaptive Radiation Rapid emergence of several species from a common ancestor Common in island and mountain top populations or other “empty” environments Ex – Galapagos Finches

Sympatric Speciation Sympatric = same homeland New species arise within the range of parent populations Can occur In a single generation

Plants Polyploids may cause new species because the change in chromosome number creates postzygotic barriers Animals: Don't form polyploids and will use other mechanisms

Polyploid Types 1. Autopolyploid - when a species doubles its chromosome number from 2N to 4N 2. Allopolyploid - formed as a polyploid hybrid between two species Ex: wheat

Autopolyploid

Allopolyploid

Rate of Evolution??

Gradualism Evolution Darwinian style of evolution Small gradual changes over long periods of time

Gradualism Predicts: Long periods of time are needed for evolution Fossils should show continuous links Problem: Gaps in fossil record

Punctuated Evolution New theory on rate/pace of evolution Elridge and Gould – 1972

Punctuated Equilibrium Evolution has two speeds of change: Gradualism or slow change Rapid bursts of change/speciation

Predictions Speciation can occur over a very short period of time Fossil record will have gaps or missing links New species will appear in the fossil record Established species will show

Comment Punctuated Equilibrium is the newest, most widely accepted view Best explanation of timing

Summary Identify several concepts of "species". Recognize reproductive barriers that keep two species from reproducing with each other. Recognize various mechanisms of speciation and hybrid zones. Identify the Gradual and Punctuated interpretations of speciation. Identify genetic mechanisms of speciation.