Evolution of Populations

Chapter 15 recap Conditions for natural  variation in population, limited resources. selection  individuals are naturally selected

Conditions for  variation in population, natural selection, LOTS of time evolution  populations evolve (NOT individuals)

What Darwin  how traits are passed betw/ generations didn’t know  how variation in pop’n appeared

Biology  Mendel’s work was linked with Darwin’s theory 1930’s  genes control heritable traits (proteins!!)  Watson & Crick  showed molecular nature of DNA, mutations and genetic variation Genetic Variation  many genes have at least 2 alleles (versions)  individual can be heterozygous or homozygous  genes code for proteins which determine traits Sources of genetic  mutation: change in DNA sequence variation  may or may not cause change in trait  random  caused by chemicals or radiation in env’t  may or may not affect fitness of individual

 sexual reproduction: gene shuffling in gametes  independent assortment of chromosomes during meiosis  crossing over during meiosis  studied in a population  group of individuals (same species) that can interbreed  gene pool: all genes (all alleles) present in population  relative frequency (%) of alleles in population  number of times allele occurs in gene pool  e.g. Person A is RR: 2 ‘R’ alleles in population

Evolution in genetic terms

Natural  any change in the frequency of alleles in a population selection

Single gene  leads to changes in allele frequencies in a population trait  change in allele frequency = evolution

 controlled by 1 gene with 2 alleles  3 possible genotypes and 2 possible phenotypes  e.g. brown lizard with red and black forms present  red form more easily seen = less fit  black form move faster = more fit  change in allele frequency in population = evolution Polygenic trait

controlled by 2 or more genes each with 2 or more alleles  many possible genotypes and phenotypes  bell curve of phenotypes

 population phenotype distribution affected in 3 ways:  directional selection:  individuals at 1 end of curve have more fitness  e.g. bird beaks: shortage of small seeds . birds with larger beaks able to eat large seeds and survive

 stabilizing selection:  individuals near center of curve more fitness  e.g. birth mass of human babies

 disruptive selection:  individuals at ends of curve have more fitness  e.g. bird beaks: medium size seed shortage . birds with larger and smaller beaks have an advantage.  curve splits into 2 distinct phenotypes  new species can form

Genetic Drift  small populations: chance occurrences can change allele frequency.

Founder  small groups of individuals leave and colonize new habitat effect  diff’t allele frequencies than parent population  “founders” alleles determine new population frequencies.  given enough time: new species form.

Hardy-  how does evolutionary change operate? Weinberg  what are the conditions where there is NO evolution? principle

 genetic  no change in allele frequencies = no evolution equilibrium  5 conditions must be met to keep equilibrium from generation to generation.

1. mating must be random – all members of pop have equal opportunity to produce offspring (NO: males compete for females and females are picky)

2. populations must be large: >10,000 so no genetic drify (NO: not all populations are that large)

3. no movement into or out of the population: no mixing of gene pool with gene pool down the road ( NO: animals migrate)

4. no mutations: gene mutations = new alleles in the pop’n (NO: mutations can happen at any time)

5. no natural selection: all genotypes must survive and reproduce equally (no advantages) (NO: variation exists in every population)

 in some populations some conditions are met some of the time.

 if conditions not met = genetic equilibrium disrupted =change in allele frequencies = evolution

Formation of Species

Speciation  formation of new species.

Species  group of organisms that:  can breed and produce fertile offspring.  shares a gene pool.

How do new species form? Reproductive  groups of organisms become different enough that they: isolation  can no longer breed.  no longer share a gene pool.

Behavioral  2 species don’t interbreed because they have different isolation courtship rituals  e.g. bird songs

Geographic  2 species don’t interbreed because they are physically isolation separated  e.g. mountains, river, ocean, etc.

Temporal  2 species don’t interbreed because they reproduce at isolation different times  e.g. flowers pollinating at different times

Patterns of Evolution

Extinction  extinction is a normal part of life on earth  99% of all species that ever lived are extinct  natural selection: competition for limited resources  species that don’t adapt to env’t changes don’t survive  natural disasters  collapse of entire food webs

Adaptive  single species evolves into many different species radiation  ancient reptile evolves into dinosaur species  ancient mammal evolves into many mammal species

Convergent  unrelated organisms evolve with similar characteristics evolution  streamline body, flukes, tailfins  shark (fish)  penguin (bird)  dolphin (mammal)  seal (mammal)

Coevolution  change over time in 1 species causes change in another species  flowers and pollinators  plants and plant-eating insects

Punctuate  long time periods in earth’s history where little change in equilibrium species occurs (equilibrium)  brief time periods with lots of change in species (evolution)