Speciation 2013

Two Patterns of Speciation

1. Gradualism (Anagenesis)

A slow, gradual accumulation of

heritable changes (adaptations) in

a population, due to many small

episodes of natural selection. So,

one species changes slowly, step-

by-step, until it looks so different

that we call it a new species.

2. Branching (Cladogenesis)

A more rapid splitting of one or

more new species from an

original species that may or may

not continue to exist. So one

species branches into two or

more new ones. This process is

the basis for all biological

diversity.

How does one species evolve into two or more new

species by branching?

By a 2-step process…

1. Geographic isolation - A single population gets divided

into two (or more) populations.

The members of the different populations (which are

now separated from each other) are therefore no longer

able to interact with each other.

2. Evolution of reproductive barriers - Over time, as they

adapt to their different environments due to natural

selection, the two populations evolve one or more

reproductive barriers, which prevent interbreeding.

Step 1: Geographic Isolation

Gene flow between two populations is

interrupted (or reduced) between two

populations because they are geographically

separated. What causes this to happen?

a) Geological Change

New lava flows, the gradual formation of a river

valley or mountain range, or the slow movement s

of tectonic plates can isolate populations if the

organisms are unable to disperse/travel between

adjacent locations (eg., Harris’ antelope squirrel

and white-tailed antelope squirrel)

b) Geographic isolation can also result from

colonization of a new area (as in the founder effect)

The separation of a small “founding” population from the main

population is a crucial event in the formation of new species,

especially on islands.

The combination of natural selection and genetic drift increase

the likelihood that the population will change over time, making it

less and less similar to the main population.

Adaptive Radiation: many diversely-adapted species

evolved from a common ancestor

• Sometimes founder populations on island groups form

different species on each island; or sometimes different

species in each valley.



• 51 endemic species of honeycreepers

have evolved from 1 species of finch

that colonized Hawaii millions years

ago.

• More than 1/3 of the species are now

extinct (15 within historical times)

Hawaiian Honeycreepers



30 species of plants in Hawaiian Silversword alliance evolved from

one species that colonized Kauai about 5 million years years ago.

Closest non-Hawaiian relative is a tarweed that lives on the west coast

of N. America

Online Activity 15.1

Time-calibrated phylogeny of the

Hawaiian silversword alliance

and their closest continental relatives

Baldwin B G , Sanderson M J PNAS 1998;95:9402-9406

©1998 by The National Academy of Sciences

Important point to remember:

Geographic Isolation alone is not sufficient to

lead to speciation.

Even when changes in the

gene pool result in new

adaptations of an isolated

population to a local

environment, speciation may

not occur.

Speciation occurs only when

changes in their gene pools

result in the formation of

reproductive barriers

between the two (or more)

isolated populations.

Step 2. The formation of reproductive

barriers between species

• What are reproductive barriers?

Any morphological, physiological, or behavioral trait

that prevents different organisms from successfully

interbreeding (so they cannot produce viable, fertile

offspring).

• There are several types, which can be classified as

either pre-zygotic or post-zygotic barriers

Pre-zygotic Reproductive Barriers

Prevent the formation of a zygote by preventing

mating or fertilization

There are 5 types:


1. Habitat Isolation

Two species that occupy two different

habitats within the same geographical

area may never interact because they

never (or very rarely) encounter each

other.

e.g. two species of garter snakes that

live in the same area, but one is

primarily terrestrial and the other is

aquatic


•Prevent the formation of a zygote by preventing



2. Temporal Isolation (a matter of timing)

Species that breed during

different times of the day,

different seasons, or different

years cannot mate.

e.g. ranges of the eastern and

western spotted skunk overlap, but

the eastern species (c) mates in late

winter and the western one (d)

mates in the fall

Pre-zygotic Barriers




2. Temporal Isolation

3. Behavioral Isolation

Little or no sexual attraction between

males and females of different

species, perhaps due to unique

courtship behaviors

e.g. Eastern and western meadowlarks

are almost identical in shape, color and

habitat, but they remain separate species

because their courtship rituals differ







4. Mechanical Isolation

Reproductive structures are physically

incompatible (lack of “fit”). (e.g., The differently-

shaped penises of closely-related insects

prevent cross breeding)

In plants, the pollinators may be different. (e.g.,

The differently-shaped and colored blossoms of

flowers attract different types of pollinators;

hence cross-pollination is extremely rare.







4. Mechanical Isolation

5. Gametic incompatibility

Sperm from one species is unable to fertilize the eggs

of another species.

e.g. Two closely-related species of sea urchins may

breed at the same time on the same coral reef, but

their gametes are not compatible.

Post-zygotic Reproductive Barriers

• Occur after the formation of a zygote by preventing

the formation of healthy or fertile offspring

• There are 2 types:



the formation of a viable, fertile adult offspring

1. Reduced hybrid viability

A hybrid zygote fails to survive embryonic or juvenile

development.

e.g. Some salamanders in the genus Ensatina

live in the same regions and habitats, where

they occasionally interbreed; however, the

hybrid offspring rarely complete embryonic

development, and those that do are frail and

do not live long.

Same is true for sympatric frogs in genus

Rana.



the formation of a viable, fertile adult offspring

1. Reduced hybrid viability

2. Reduced hybrid fertility

Even if hybrids are healthy, they may be sterile.

If the chromosomes of the two parent species differ in structure or

number, meiosis in the hybrid offspring may fail to produce normal

gametes, preventing gene flow between the two species.

e.g. the hybrid offspring of a donkey and a horse, a mule, is healthy

but sterile

Concept Check

1. Why are donkeys and horses considered different species?

Their offspring (mules) are sterile (not fertile).

2. What is macroevolution?

Major evolutionary changes, often evident in the fossil record,

due to speciation and the evolution of major new adaptations

3. Give an example of a reproductive barrier that may separate

two similar species.

prezygotic: behavioral, habitat, temporal (timing), mechanical,

gametic,

postzygotic; reduced hybrid viability or fertility

Concept Check

4. Describe conditions that could make a new island a likely

place for adaptive radiation.

The island may consist of varied habitats with environmental

conditions that differ from one another and from other

landmasses.

Organisms may adapt to the varied conditions in different

ways, resulting in species diversity.

5. How does that branching model of evolution relate to

Darwin's theory of natural selection?

Suggests that natural selection causes most change as new

species begins. Major change is less common once a species

is established in an area.

Education

Speciation 2013