Biology 3201 Unit 4

EcologyCh. 21 - Adaptation and Speciation

Speciation

Speciation: the formation of a new species

Biological species: a group of organisms able to

interbreed and produce fertile offspring.

• Ex. Horses and donkeys are separate species.

They are able to interbreed but the offspring

produced are not fertile

How to species form?There are generally two pathways

in which species are formed:

1. Transformation: the formation of

a species because of a series of

accumulated changes over time.

One species changes into

another in this way

2. Divergence: the formation of

species from a parent species

Speciation

Speciation occurs when two groups in a population

become isolated from each other

The separation is caused by one or more

BARRIERS.

Barriers that cause speciation

Geographical barriers: When a population becomes

divided by a geographic boundary such as a canyon,

river, etc. This prevents interbreeding. Over time

natural selection causes genetic differences to

become so large two species form.

• Ex. Giraffes that have become separated by

mountains will eventually develop into separate

species.

Biological barriers: barriers that keep species

reproductively isolated; may be pre-zygotes

barriers or post-zygotic barriers

Pre-zygotic barriersThese are known as pre-fertilization barriers that

impede mating or prevent fertilization

Behavioral isolation: bird songs, courtship rituals,

pheromones, etc. are all species specific and

prevent fertilization

Temporal isolation: these are often timing barriers.

Several species mate at different times during the

year and as such are not able to mate.

Habitat isolation: some species live in the same area

but have different habitats; ex. North American

Garter snakes, one species prefers open areas while

the other prefers water

Pre-zygotic barriersMechanical isolation: some species are anatomically

incompatible thus not allowing them to exchange

sperm and egg

• Ex. The genitals on certain species of insect work

only on a lock-key system. If the lock does not fit

the key, no fertilization can occur.

Gametic isolation: sometimes the gametes from

species do not even meet. This prevents fertilization

• Ex. Sea urchins release their eggs into the water,

but chemicals on the surface of the eggs prevent

sperm from a different species from fertilizing them.

Post-zygotic barriers

These are barriers that prevent a zygote from developing into a

fertile offspring

Hybrid invariability: incompatibility of the two species may cause the

zygote to stop embryonic development

• Ex. Embryos of sheep/goats do not survive

Hybrid sterility: the production of an organism, but it is sterile

• Ex. Horse + donkey = mule (sterile)

Hybrid breakdown: sometimes the first generation of offspring are

viable, but subsequent generations are either sterile or wheat

(cotton plants)

Adaptive Radiation

Adaptive radiation: the diversifying of an ancestral

species into a variety of species.

This usually occurs after a novel characteristic has

evolved or there is a mass extinction

Ex. The finches of the Galapagos islands evolved

from one common ancestor.

Divergent evolution

Divergent evolution:

where species that were

once similar to an

ancestral species diverge

to become different

species

Adaptive radiation is an

example of divergent

evolution.

Convergent evolution

Convergent evolution: evolution where two

completely unrelated species have similar traits

Ex. Birds and bees have wings (similar trait)

Each species develops the same traits because they

adapt to the same type of environmental conditions.

The species DO NOT come from a common

ancestor

Coevolution

This is evolution where two species change together

where each species responds to the changes in the

other

Ex. Milkweed plants and Monarch butterflies. The

milkweed plant has toxins in their leaves. Monarch

butterflies eat the leaves and absorb the toxins

making them toxic. Most birds avoid Monarch

butterflies for this purpose.

The pace of evolution

How fast does evolution occur?

There are two theories/models that explain how fast

evolution occurs. Both examine the fossil record

1. Gradualism

2. Punctuated equilibrium

Gradualism

A model that says change occurs slowly and steadily

before and after a divergence

Proposed by Darwin

Fossils show a slow and repeated change through

the fossil record

Gradualism

Punctuated equilibriumA model proposed by Gould and Eldridge

A model that proposes evolution happens in "spurts"

The model says that species undergo long periods of

"stasis" where they remain unchanged, followed by

short periods of very rapid change (spurts)

The changes are usually brought about by sudden

environmental changes such as volcanoes,

earthquakes, etc.

Species previously disadvantaged could now be

advantaged and new species could develop quickly

Punctuated equilibrium

Gradualism vs

punctuated equilibrium

The origins of lifeMany theories exist that try to explain the origin and development of life on

Earth.

The following theories will be considered:

1. Chemical evolution

2. Panspermia

3. Gaia hypothesis

4. Heterotroph hypothesis

5. Symbiogenesis (Symbiotic theory)

6. Intelligent design

Chemical evolutionA theory of evolution created by Oparin-Haldane

They said that organic molecules (the building blocks of life) could

develop from inorganic compounds present on the surface of the

early earth

• The early earth had an atmosphere that consisted of no oxygen,

but plenty of hydrogen, ammonia, methane, and water vapor

(inorganic molecules)

• These gases condensed and formed a "primordial soup"

• Energy from lightning and UV radiation caused organic molecules

to develop from the inorganic molecules in the soup

• Over time, the organic molecules combined to become an early life

form

Chemical evolution -

Miller-UreyTwo scientists who designed an experiment to prove

Oparin-Haldane's theory:

• They combined methane, ammonia, water vapor,

and hydrogen in a flask and exposed the gases to

an energy source simulating lightning

• The liquid inside the flask changed color and when

examined contained several organic compounds

including amino acids

Panspermia theory

A theory that suggests life began "elsewhere" in the

universe and "migrated" to our planet

For example, it is believed that life originated from

bacterial cells elsewhere and travelled from outer

space to earth on meteorites.

Gaia hypothesisTheory put forth by James Lovelock

Idea that Earth is a "superorganism" called "Gaia"

The Earth has systems that keep a balance between

temperature and atmosphere

After life originated on earth, Gaia came "alive" and

began to regulate earth systems

The systems help provide an environment where life

could exist and survive

Heterotroph hypothesisTheory put forth by Oparin; said first "cells" on earth

had to be heterotrophs that eventually developed

into autotrophs

Primordial soup existed of organic molecules

The environment was oxygen poor

Heterotrophs such as anaerobic bacteria fed on the

organic molecules

The heterotrophs began to release carbon dioxide

into the atmosphere.

Heterotroph hypothesis

The heterotrophs developed into autotrophs and

began using the carbon dioxide

The autotrophs bean to release oxygen into the

atmosphere

This made the atmosphere oxygen rich that could

now support life

Symbiogenesis

Put forth by Lynn Margulis

Theory that attempts to explain the development of

mitochondria and chloroplasts as organelles that

appear in eukaryotic cells

Chloroplasts and mitochondria have their own DNA

and come from the symbiosis (working together) of

prokaryotic cells

Symbiogenesis - how it

happened

1. An anaerobic bacterium ate but did not digest an

aerobic bacterium (called a guest bacterium)

2. The guest provided oxygen to the bacterium. The

guest bacterium eventually became a mitochondrial

3. Other bacteria ate photosynthesizing bacteria. The

photosynthesizing bacteria became chloroplasts

Intelligent design

A theory that suggests life and mechanisms of life

are too complex to have evolved by chance

Believed that the generation and evolution of life

must have been directed by some unidentified

supernatural intelligence