Bio II Speciation CH 18

Darwin’s Four Postulates ( the process of evolution by natural selection)

Summarization:

Evolution by natural selection occurs when heritable variation leads to differential success in survival and reproduction.

1. Individual Variation- Individuals in a population vary in their traits.

2. Heritability- some of these differences are heritable; they are passed on to offspring.

3. Differential Survival- survival and reproducton success are highly variable

4. survival of the fittest- a subset of individuals that survives best and produces the most offspring is not a random sample of the population.

Darwinian/ Biological Fitness

Ability of an individual to produce surviving, fertile offspring that is relative to the ability of others within a population.

What is an Adaptation

• A heritable trait that increases an individual's Darwininian fitness in a particular environment relative to individuals lacking the trait.

• Increase fitness — the ability to produce offspring

• NOT the same as acclimation

Galapagos Finch Study Conclusions

• Supports theory of evolution by natural selection

• beak form and body size are heritable in the birds

• Natural selection occured. The characteristics of the population have changed due to the drought

• Finches with larger beaks were better equiped to survive the drought. They were better able to crack open larger seeds that were more abundant

• When there was an abundance of rain, more smaller seeds were produced, so birds with smaller beaks were selected — because they could eat the smaller seeds.

Common Misunderstandings regarding evolution by natural selection

• evolution is just a theory

• evolution explains the origin of life

• adaptation is not the same as acclimation

• Individuals do not change — only the population does

• Evolution does not have a goal ( organisms do not evolve on purpose)

• Evolution is not progressive ( social dawinism leads to racism and Neo-nazism)

• individuals do not act for the good of the species

• natural selection operates within limits

What is speciation

• the process by which new species arise by either:

  • tranformation of one species into another

  • or by the splitting of one ancestral species into two descendant species

How can we define what a species is

• Biological species concept

• Morphospecies concept

• Phylogenetic species concept

Biological species concept

• species must be reproductively isololated from one another

  • Reproductively isolated- populations whose members do not mate with each other or who cannot produce fertile offspring

• Reproductive isolating mechanisms: barriers to successful reproduction

Morphospecies Concept

• Biologists identify evolutionarily independent lineages by differences in morphological feautures.

• Problems

  • cryptic species ( convergent evolution)

  • polymorphic species- exhibits multiple distinct heritable forms (morphs) within the same population, such as different colors, sizes, or behaviors, with these variations not solely due to the environment

Phylogenetic species concept

• For a population to be declared a separate species, it must be the smallest monophyletic group on the phylogenetic tree.

• Species identified based on evolutionary history

• Groups must be monophyletic (within clades)

  • Monophyletic groups: an ancestral population and all descendants

  • Synapomorphy: trait unique to a monophyletic group

Problems of the phylogenetic tree concept

• too many species distinguished

• geneitc analysis can be costly and impractical

• cannot be applied to extint species

How does reproductive isolation between species occur

• structural, functional, or behavioral characteristics can prevent successful reproduction between species from occurring

• categorized as prezygotic and postzygotic

Prezygotic barriers

• Individuals of different species

  • habitat isolation

    • ex. asian lions and tigers are ecologically separated

  • temporal isolation

    • ex. Mating at different times of a year/ day

  • behavioral isolation

    • ex. the male satin bowerbird builds a bower of twigs to attract females

• Mating

  • Mechanical isolation

    • ex. incompatible genital organs

    • ex. only hawkmoths can reach the nectar at the base of the madegascar orchid

  • Gametic isolation

    • ex. incompatible egg and sperm

    • ex. molecular recognition on the surface of the cells

Postzygotic

• Fertilization

  • Reduced hybrid viability

    • ex. hybrid embryos die when genetic regulation falls during development

  • Hyrbid sterility

    • problems during meiosis cause abnormal gametes

How do new species arise

• Allopatric speciation

• sympatric speciation

allopatric speciation

• when populations become physically separated

  • Causes of speciatian

    • dispersal

    • 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.

• most common type of speciation

• usually involving geographic isolation

• Examples

  • Darwin’s finches

  • snapping shrimp

  • squirrels separated by the Grand Canyon

Sympatric speciation

• Occurs without geographic isolation

  • External (extrinsic) events such as disruptive selection

  • Internal ( intrinsic) event such as chromosomal mutations

Sympatric speciation: Polyploidy

• individuals that have more than two sets of chromosomes

  • plants with four sets of chromosomes ( tetraploids) can survive, but not be fertilized by diploid individuals

  • major errors in meiosis or mitosis

  • can occur through autopolyploidy or allopolyploidy ( more common)

Autopolyploidy

• mutation results in doubling of chromosome number

• polyploid individual isolated from diploid individuals

• many plants can self-fertilize

• example- Maidenhair ferns

Allopolyploidy

• Two species hybridize

  • offspring have one copy of the chromosomes of each species

  • infertile: cannot reproduce with either species but may produce asexually

  • can become fertile if chromosomes spontaneously doubled ( polyploidy)

    • results in tetraploids that could interbreed

    • common in plants

Why you shouldnt have ALCOHOL at family reunions

• Fusion- species may interbreed

• Reinforcement may occur

• Hybrid zones may result

• New species through hybridization

• Extinction- loss of one species