Speciation

Introduction

• Species continue to evolve• Changes that occur within species

(microevolution) are easy to understand– Faster-running cheetahs– Colourful male songbirds– Larger brains in humans

• But what factors lead to speciation- the evolution of an entirely new species?

What is a Species?

• A species includes all members of a population that can interbreed under natural conditions

• Some species are quite distinct based on their morphology but some species are only distinguished by genetic comparisons

Modes of Speciation

• New species can evolve many ways

• Always includes the evolution of distinct features that isolate the new species reproductively, therefore genetically, from other species

Mechanisms of Reproductive Isolation• any biological factor that prevents the two

populations from interbreeding when living in the same region.

• Prezygotic mechanisms: prevents interspecies mating and fertilization– Different breeding seasons, physical or

behavioural traits, habitat preferences or the incompatibility of gametes

Mechanisms of Reproductive Isolation• any biological factor that prevents the two

populations from interbreeding when living in the same region.

• Postzygotic mechanisms: prevents maturation and reproduction in offspring from interspecies reproduction

Mechanisms of Reproductive IsolationPrezygoticTemporal Isolation: Species have different breeding

seasons.

Example: Western spotted skunks breed in the fall but the Eastern species breeds in late winter

Western Eastern

Eastern Western

Mechanisms of Reproductive IsolationPrezygoticBehavioural Isolation: Species may have different

courtship or mating behaviours.

Example: Eastern meadowlark and western meadowlark are almost identical in everything, except their courtship rituals and songs

Mechanisms of Reproductive IsolationPrezygotic Mechanical Isolation: Species may have reproductive

structures that are physically incompatible.

Example: Snails of the same species have reproductive structures that align, but snails from two different species part the other species

Mechanisms of Reproductive IsolationPrezygotic Ecological Isolation: very similar species may occupy

different habitats within a region

Example: The mountain bluebird lives at high elevations, while the eastern bluebird prefers lower elevations. The mountain bluebird and eastern blue bird do not encounter each other

Mechanisms of Reproductive IsolationPrezygoticGametic isolation: male gametes may not be able to

recognize and fertilize an egg of a different species

Example: corals, clams and sea cucumbers release their sperm and eggs into open water. The sperm recognize the egg of their own species through chemical markers on the egg surface.

Mechanisms of Reproductive IsolationPostzygoticZygotic Mortality: mating and fertilization are possible,

but genetic differences result in a zygote that is unable to develop properly

Example: If you have sheep and goats on a farm, some species of sheep and goat are able to mate, but the zygote is not viable.

Mechanisms of Reproductive IsolationPostzygoticHybrid Inviability: a hybrid dies early in development,

or lives to maturity but is not healthy and suffers an early death.

Example: Some salamanders species may mate with each other, but most of the offspring do not live to adulthood, or if they do they are weak.

Mechanisms of Reproductive IsolationPostzygoticHybrid Infertility: The hybrid offspring mature into

adults but are infertile.

• Example: A mule is bred from a female horse (left) and a male donkey (centre). Mules are physically healthy but they are infertile.

Allopatric Speciation• Most new species form when a

single species is separated into two geographically isolated populations

• Once physically separated, populations cannot share genetic information

• Over many generations the populations will gradually become less alike

• Mutations and genetic drift may have caused populations to change

• Reproductive isolating mechanisms may have developed

Allopatric Speciation

Harris’s antelope squirrelWhite-tailed antelope squirrel

Sympatric Speciation• The evolution of populations

within the same geographic area into separate species

• Individuals within a population become genetically isolated from the larger population