Chapter 23 The Origin of Species

Question? u What is a species? u Comment - Evolution theory must also explain how species originate.

Two Concepts of Species 1. Morphospecies 2. Biological Species

Morphospecies u Organisms with very similar morphology or physical form.

Problem u Where does extensive phenotype variation fit?

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 u A group of organisms that could interbreed in nature and produce fertile offspring.

Key Points u Could interbreed. u Fertile offspring. Heaven Scent an F1 hybrid between 2 species, but sterile.

Morphospecies & Biological Species u Often overlap. u Serve different purposes.

African Violets u Originally ~20 species u 70,000 cultivars

Problem u What is a species? u Some plants didn’t fit placement. u Plants freely interbreed. u Answer – coming up later

Speciation Requires: 1. Variation in the population. 2. Selection. 3. Isolation.

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

Main Types of 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 u Populations live in different habitats or ecological niches. u Ex – mountains vs lowlands.

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

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

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

Gametic Isolation u Gametes fail to attract each other and fuse. u 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 u Zygote fails to develop or mature. u Ex – when different species of frogs hybridize.

Reduced Hybrid Fertility u Hybrids are viable, but can't reproduce sexually. u Chromosome count often “odd” so meiosis won’t work. u Ex - mules

Hybrid Breakdown u Offspring are fertile, but can't compete successfully with the “pure breeds”. u Ex – many plant hybrids

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

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

Example u Pupfish populations in Death Valley. u Generally happens when a specie’s range shrinks for some reason.

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.

Conditions Favoring Allopatric Speciation 3. Selection pressure on the isolate differs from the parent population.

Result u Gene pool of isolate changes from the parent population. u New Species can form.

Comment u Populations separated by geographical barriers may not evolve much. u Ex - Pacific and Atlantic Ocean populations separated by the Panama Isthmus.

Examples u Fish - 72 identical kinds. u Crabs - 25 identical kinds. u Echinoderms - 25 identical kinds.

Adaptive Radiation u Rapid emergence of several species from a common ancestor (Allopatric speciation) u Common in island and mountain top populations or other “empty” environments.

Mechanism u Resources are temporarily infinite. u Most offspring survive. u Result - little Natural Selection and the gene pool can become very diverse.

When the Environment Saturates u Natural Selection resumes. u New species form rapidly if isolation mechanisms work.

Examples u Galapagos – Finches u Usambaras Mountains – African violets

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

Plants u Polyploids may cause new species because the change in chromosome number creates postzygotic barriers.

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. u Ex: wheat

Autopolyploid

Allopolyploid

Animals u Don't form polyploids and will use other mechanisms.

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

Gradualism Predicts: u Long periods of time are needed for evolution. u Fossils should show continuous links.

Problem u Gradualism doesn’t fit the fossil record very well. (too many “gaps”).

Punctuated Evolution u theory that deals with the “pacing” of evolution. u Elridge and Gould – 1972.

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

Predictions u Speciation can occur over a very short period of time (1 to 1000 generations). u Fossil record will have gaps or missing links.

Predictions u New species will appear in the fossil record without connecting links or intermediate forms. u Established species will show gradual changes over long periods of time.

Possible Mechanism u Adaptive Radiation, especially after mass extinction events allow new species to originate. u Saturated environments favor gradual changes in the current species.

Comment u Punctuated Equilibrium is the newest ”Evolution Theory”. u Best explanation of fossil record evidence to date.

Origin of Evolutionary Novelty u How do macroevolution changes originate? u Several ideas discussed in textbook (read them) u Exaptation u Heterochrony u Homeosis

Another idea u Mutations in developmental or control genes (Chapter 21) u Looking very promising as a source of macroevolution

Exaptation u When a structure that was adapted for one context is co-opted for another function. u Ex. – feathers and flying

Heterochrony u Changes in the timing or rate of development. u Allometric Growth u Paedomorphsis

1. Allometric Growth – changes in the relative rates of growth of various parts of the body. u Ex. – skull growth in primates

2. Paedomorphosis – when an adult retains features that are present in the juvenile form. Ex. – gills in adult salamanders

Ex - Homeosis Changes in the basic body design or arrangement of body parts. Ex. – Hox gene clusters that gave rise to vertebrates from invertebrates.

Gene Duplications u Allow genes to be used for other functions such as in the previous slide. u Many other examples are known.

Future of Evolution ? u Look for new theories and ideas to be developed, especially from new fossil finds and from molecular (DNA) evidence.

Evolutionary Trends u Evolution is not goal oriented. It does not produce “perfect” species. u Remember – species survive because of their adaptations. They don’t adapt to survive.

Summary u Be able to discuss the main theories of what is a “species”. u Know various reproductive barriers and examples.

Summary u Know allopatric and sympatric speciation. u Be able to discuss gradualism and punctuated equilibrium theories.

Summary u Recognize various ideas about the origin of evolutionary novelties.