Chapter 24 The Origin of Species

Question? u What is a species? u Comment - Evolution theory must also explain how species originate. u Darwin’s “Mystery of mysteries”

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?

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

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

Reproductive Barriers u Serve to isolate a population 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. Temporal Isolation 3. Behavioral Isolation 4. Mechanical Isolation 5. Gametic Isolation

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

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

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

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 or salamanders 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 – donkeys and horses produce mules

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

How do species occur? Block gene flow between two populations. 1. Allopatric Speciation 2. Sympatric Speciation

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 species’s range shrinks for some reason.

Conditions Favoring Allopatric Speciation 1. Founder's Effect - with the peripheral isolate. 2. Genetic Drift – gives the isolated population variation as compared to the original population.

Conditions Favoring Allopatric Speciation 3. Selection pressure on the isolate differs from the parent population. (environment is different)

Result u Gene pool of isolate changes from the parent population and new species can form.

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

Adaptive Radiation u Rapid emergence of several species from a common ancestor ( often 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

Speed of speciation? u How long does it take a new species to form? u How many genes are involved?

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: 1. Gradualism or slow change 2. Rapid bursts of speciation

Predictions u Speciation can occur over a very short period of time (1 to 1000 generations). u Too fast for fossils to record. 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 Rapid: Adaptive Radiation, especially after mass extinction events u Gradual: Saturated environments favor gradual changes

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

How many genes? u Can be as little as 1 gene which prevents interbreeding. Ex – snails u Probably a larger number of genes in many cases.

1 Gene = 2 species