◦ Until recently, over 500 species of cichlid fishes lived in East Africa’s Lake Victoria...

◦ Until recently, over 500 species of cichlid fishes lived in East Africa’s Lake Victoria

Copyright © 2009 Pearson Education, Inc.

LakeTanganyika

Africa

Tanzania

KenyaUganda

LakeVictoria

LakeMalawi

◦ Lake Victoria’s cichlids diversified 100,000 years ago (young species!)

◦ Why??– Specialized mouthparts that allow them to

specialize on different food sources– Bright colors of the males vary with species,

as females chose males with specific colors

◦ Groups isolated by diet or female mate choice may have lost the ability to interbreed



Species: a group of organisms whose members can breed and produce fertile offspring, but who do not produce fertile offspring with members of other groups


◦ Over the last 30 years, 200 species of cichlids have disappeared from Lake Victoria

– Introduced predator (perch) eats them up

– Pollution makes it difficult for females to see colors


Speciation is the emergence of new species and increases diversity

Millions of species all arose from ancestor that lived 3.6 billion years ago


◦ Taxonomy is the field of biology that names and classifies species and groups them into broader categories

◦ Binomial system gives every organism a genus and species name

Example: Homo sapien, Escherichia coli

Order GenusFamily

Fe

lis

Fe

lida

e

Me

ph

itis

Mu

ste

lida

e Lu

tra

Ca

rniv

ora

Ca

nid

ae

Ca

nis

Species

Feliscatus

(domesticcat)

Mephitismephitis

(striped skunk)

Lutralutra

(Europeanotter)

Canislatrans

(coyote)

Canislupus(wolf)


But how do we decide what is actually a species??


◦ The biological species concept Species is a population or group of populations

whose members have the potential to interbreed in nature and produce fertile offspring

◦ Reproductive isolation prevents gene flow and maintains separate species


◦ Can the biological species concept always distinguish species from each

other?


◦ The morphological species concept :

classifies organisms based on observable phenotypic traits

It can be applied to asexual organisms, fossils, and when we don’t know about inbreeding


The ecological species concept:

Defines a species by its ecological role or niche– Consider the cichlids, which are similar in

appearance but feed at different depths in the lake


◦ The phylogenetic species concept:

Species as a set of organisms representing a specific evolutionary lineage

– Morphological or DNA similarities or differences can be used to define a species

– Defining the amount of difference required to distinguish separate species is a problem


◦ Reproductive barriers serve to isolate a species gene pool and prevent interbreeding

1. Prezygotic (before sex)

2. Postzygotic (after sex)

◦ Prezygotic Barriers: prevent mating or fertilization between species

1. Temporal isolation- two species breed at different times (seasons, times of day, years)



2. Habitat isolation: two species live in the same general area but not in the same kind of place



3. Behavioral isolation: there is little or no sexual attraction between species, due to specific behaviors

Reproductive barriers keep species separate



4. Mechanical isolation: female and male sex organs are not compatible

Reproductive barriers keep Reproductive barriers keep species separateseparate



5. Gametic isolation: female and male gametes are not compatible




◦ Postzygotic Barriers: operate after hybrid zygotes are formed

1. Zyogotic mortality

2. Reduced hybrid viability most hybrid offspring do not survive

3. Reduced hybrid fertility hybrid offspring are vigorous but sterile

4. Hybrid breakdown- first generation hybrid fertile, loss in second generation



◦ If two related species live in the same area, would natural selection favor the evolution of prezygotic or postzygotic reproductive isolating mechanisms?


MECHANISMS OF SPECIATION


◦ In allopatric speciation, populations of the same species are geographically separated, separating their gene pools

◦ Changes in the allele frequencies of each population may be caused by __________?????


◦ Gene flow between populations is initially prevented by a geographic barrier

– The Grand Canyon and Colorado River separate two species of antelope squirrels

A. leucurusA. harrisi

South North


◦ In sympatric speciation, new species may arise within the same geographic area as a parent species

◦ Gene flow between populations may be reduced by:1. Polyploidy: multiplication of the chromosome

number due to errors in cell division (plants)2. Habitat differentiation (animals)3. Sexual selection (animals)

In sympatric speciation, speciation takes place without geographic isolation


◦ Hybrid zones: regions in which members of different species meet and mate producing some hybrid offspring

How do you know if two species are reproductively isolated?

Ancestralspecies

21

Gene flow

Population(five individualsare shown)

Barrier togene flow

Newspecies

Hybridzone

Hybrid

Gene flow

3

4

Reinforcement Stability Fusion

Outcomes of hybrid zones


◦ What is the total length of time between speciation events (between formation of a species and subsequent divergence of that species)?

– In a survey of 84 groups of plants and animals, the time ranged from 4,000 to 40 million years

– Overall, the time between speciation events averaged 6.5 million years and rarely took less than 50,000 years

Prezygotic barriers• Temporal isolation• Habitat isolation• Behavioral isolation• Mechanical isolation• Gametic isolation

Zygote

Gametes Postzygotic barriers•

•

•

Viable,fertile

offspringReduced hybridviabilityReduced hybridfertilityHybrid breakdown

◦ Until recently, over 500 species of cichlid fishes lived in East Africa’s Lake Victoria...

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