Speciation

What is a species? Biological species concept – a population or group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring. – Reproductive isolation – the existence of biological barriers that prevent reproduction. – These barriers either make it impossible to reproduce or make the offspring less viable.

Reproductive Barriers Prezygotic Barriers hinder mating or prevent fertilization Before the zygote Examples  Habitat isolation  Temporal isolation  Behavioral isolation  Mechanical isolation  Gametic isolation Postzygotic barriers hinder zygote development, reduce the viability of the offspring, or prevent the offspring from successfully reproducing. After the zygote Examples  Reduced hybrid viability  Reduced hybrid fertility  Hybrid breakdown

Habitat Isolation 2 species occupy different habitats in the same geographic area.

Temporal isolation Different breeding seasons or times prevent breeding.

Behavioral isolation Species have unique courtship rituals – Dances – Calls – Etc.

Mechanical isolation Morphological differences prevent mating – Pollinators are attracted to different flower colors and shapes

Gametic isolation Mechanisms prevent gametes from uniting even if mating occurs. – Sperm cannot penetrate egg. – Sperm cannot survive the reproductive tract.

Reduced hybrid viability Hybrid of 2 species is less likely to develop into mature adults.

Reduced hybrid fertility Hybrid of 2 species is infertile.

Hybrid breakdown Hybrid of 2 species is fertile and viable, but the next generation is sterile or feeble.

Other Species Definitions The biological species concept is not perfect – Example: can you use it for dinosaurs – Other definitions exist Morphological species concept Paleontological species concept Ecological species concept Phylogenetic species concept

Speciation Speciation – the origin of a new species

Allopatric speciation - occurs when 2 populations become separated geographically. Gene flow is interrupted by a physical barrier. Example: when a level of a lake lowers to make 2 small lakes. Mountain ranges, rivers, island colonization, etc.

Allopatric Speciation

Sympatric speciation Occurs when 2 species arise in the same geographic area. For some reason, organisms do not interact (reduced gene flow) Chromosomal changes and nonrandom mating.

Polyploidy Extra set of chromosomes (entire set). Common in plants

Autopolyploid Autopolyploid – has more than 2 sets of chromosomes that were derived from one species. – Mistake in meiosis – Result in tetraploid (which cannot reproduce with diploid original) – Instant new species (because some plants can reproduce by self fertilization or asexually)

Autopolyploid

Allopolyploidy 2 different species hybridize and form a new species. More common than autopolyploidy

Allopolyploidy

Habitat Differentiation Apple/Hawthorn maggot fly Apple Hawthorn

Adaptive Radiation Many species developing from one common ancestor – Usually occurs when a new area is colonized, or otherwise made available. – Example, when a group of islands are colonized – Example 2, when dinosaurs became extinct, new “areas” were now available for mammals.

Convergent Evolution Relatively unrelated organisms developing similar characteristics due to similar environmental characteristics

Small Genetic Changes Can Lead to Major Morphological Changes Genes responsible for timing of development and spatial patterns explain how major differences can occur quickly. Example, genes responsible for juvenile characteristics can be expressed as adults

Punctuated Equilibrium Long periods of stability “punctuated” by short speciation events. Fossil record supports this idea, rather than “gradualism” Species seem to appear suddenly in the fossil record They seem to stay relatively unchanged for a long period. They seem to suddenly disappear.

Punctuated Equilibrium The word “sudden” is different in evolutionary terms. – If a species survives for 5 million years – Most of its change happens in the first 50,000 years. – That is 1% of its existence – This looks like a sudden appearance in fossilized rock.