Lecture Notes: Speciation and Isolation

Introduction

Discussion on the idea of a hybrid animal, a "dog cat."
Cats and dogs are different species; species can interbreed and produce fertile offspring.
All dog breeds and all cat breeds, though varied, are the same species.

Species Definition: Organisms that can interbreed and produce fertile offspring.
Hybrid Example: Zonkey, a cross between a donkey and a zebra; usually sterile.
Different species can occasionally interbreed, but offspring like the zonkey are not fertile.

Speciation: Development of a new species; occurs when populations are reproductively isolated.
Mechanisms of Speciation:
- Natural selection as a mechanism of evolution.
- Isolation types leading to speciation.

Allopatric Speciation
- Geographical barrier separates populations (e.g., river, mountain).
- Natural selection and genetic drift act over time, leading to genetic differences.
- Eventually, populations unable to interbreed become different species.
Sympatric Speciation
- Speciation occurs in the same geographical area.
- Isolation occurs via other means.

Prezygotic Barriers (before zygote formation)
- Behavioral Isolation: Different behaviors (e.g., bird songs) prevent interbreeding.
- Temporal Isolation: Different breeding seasons/times.
- Habitat Isolation: Different habitat preferences within the same area.
Post-zygotic Barriers (after zygote formation)
- Hybrid Sterility: Offspring cannot reproduce (e.g., zonkey).
- Hybrid Viability: Offspring may be weak or not survive.
- Embryonic Development Failure: Genetic incompatibility prevents development.

Isolation is not the mechanism of change; it separates gene pools.
Change over time due to mechanisms like genetic drift or natural selection.
Variety exists in populations; genes enhancing fitness may increase in frequency.