M1 Clegg- Populations II: Move, adapt or die - how can species respond to different types of global change?

how does fitness vary with a single phenotypic traits?

w is ecological tolerance, the width of the fitness function- high w = weaker selection for this trait vs fitness

• there is stabilising selection towards the current phenotypic optimum (for all traits)

how and when can a population adapt to respond to environmental changes?

• if the environents changes to have a new optimum phenotype, it means the population is now maladapted and the relative fitness is low- there is a positive selection gradient for this new optimum trait

• a trait can evolve towards the new optimum, if there’s enough genetic variation and the rate of environmental change is low relative to the potential evolutionary rate eg. Brassica rapa adapted to flower earlier after 7 years of drought

  • predicted based on the breeders equation → R (response to selection) = h² (heritability, how similar offspring are to their parents) x S (strength of selection)

  • or based on quantitative genetics → delta X (change in mean trait value) = G (additive genetic variance of all alleles contributing to the trait, excluding dominance + epistasis) x beta (selection gradient)

    • the genetic variation is increased by high mutation rates, larger populations, more balanced sex ratios + more outbreeding

    • the selection gradient is increased by larger environmental changes and lower species tolerance to change

    • the evolutionary rate is also higher for species with short generation times

what factors affect whether a population can adapt to a changing evironment?

moving optima:

• the optimum can continue to move, so the population may not be able to ‘catch up’

• this is worsened because the population declines in a changing environment, so the genetic variation is further reduced and the evolutionary rate decreases- could wait for an advantageous mutation to ‘rescue’ the population but this is very unlikely (basis of breeding programmes)

correlated traits:

• genes do not always evolve separately and independently- they can interact to reinforce or constrain the response to selection based on their selection gradients and the genetic covariance between genes

phenotypic plasticity:

• shorter scale fluctuations (daily/seasonal) can select for phenotypic plasticity

• this allows the same genotype to produce a range of phenotypes matching the environment, based on development, gene expression or behaviour eg. egg laying date in great tits

• through genetic assimilation, a plastic change can become genetically based over time- but plasticity might weaken the selection pressure on genetic changes because it masks the phenotype

what are the possible responses of a population to environmental change?

• do nothing- the change could be within the ecological tolerance of the species so there is no major effect (particularly for species with broad ecological tolerance)

• move- dispersal to a favourable habitat/differential population growth or decline (particularly for species with good dispersal ability)

• adapt

  • phenotypic plasticity- one genotype produces variable phenotypes that can match the environment

  • genetic adaptation- mutations + changes in allele frequencies are selected for to evolve towards a new optimum phenotype (particularly for species with high mutation rates, large population sizes + short generation times)

• die- birth rate and/or survival rate declines to extinction