Module 8 - speciation

genomes and evolution

What do fossil records reveal?

Anagenesis and Cladogenesis

Anagenesis

Single lineage into new species and original species goes extinct

Cladogenesis

a parent species splits into two or more distinct daughter species, creating new evolutionary branches (clades)

Morphological species concept

Grouping species that look similar into same group

Biological species concept

Group of organisms that can interbreed in nature to produce fertile offspring

Reproductive Barriers

Evolved traits that preclude production of fertile viable hybrid offspring

Pre-zygotic barriers

Reproductive isolating mechanisms that operate before fertilization, some before mating

Types of pre-zygotic barriers

Spatial, behavioral, temporal, mechanical, gametic

Spatial barrier

2 species live in different habitats with same geographic area

Behavioral barrier

Unique mating rivals or behavior prevent mate recognition between species

Temporal barrier

Species breed during different times of day, seasons, or years

Mechanical barrier

Incompatible reproductive structures prevent mating

Gametic barrier

Sperm of one species is unable to fertilize the eggs of another species because gametes are chemically incompatible

Types of post-zygotic barriers

Reduced hybrid viability, reduced hybrid fertility, hybrid breakdown

Reduced hybrid viability

Hybrid zygote formed and dies before it reaches productive age due to genetic incompatibility

Reduced hybrid fertility

Hybrid offspring survives but becomes sterile

Hybrid breakdown

First-generation hybrids are viable and fertile, but when they mate with each other or parent species, their offspring are weak, sterile, or non-existent

Speciation

One species splits into two or more distinct, new species, occurring when populations become reproductively isolated and accumulate genetic differences, preventing them from interbreeding

Allopatric speciation

new species forming from geographically isolated populations and can happen from dispersal or vicariance

Sympatric speciation

forms new species within the same area, driven by factors like polyploidy (common in plants) or niche differentiation, with no physical barrier preventing gene flow

Vicariance

the geographical separation of a population, typically by a physical barrier such as a mountain range or river, resulting in a pair of closely related species.

Dispersal

the action or process of distributing things or people over a wide area

What happens when reproductive isolation is incomplete

hybridization

Hybrid zones

an area where the ranges of two interbreeding species meet and interbreed

Reinforcement

Hybrids are less fit than either purebred species. The species continue to diverge until hybridization can no longer occur.

Fusion

Reproductive barriers weaken until two species fuse to become one.

Stability

Hybrids have reduced fitness but the hybrid zone persists because there is a continuous supply of new individuals from the parent species constantly migrating into the zone.